STIS observations of Io’s dayside equatorial atmosphere

Icarus 248 (2015) 165–189 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Spatially resolved HST/...

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Icarus 248 (2015) 165–189

Contents lists available at ScienceDirect

Icarus journal homepage: www.elsevier.com/locate/icarus

Spatially resolved HST/STIS observations of Io’s dayside equatorial atmosphere Kandis Lea Jessup ⇑, John R. Spencer Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, United States

a r t i c l e

i n f o

Article history: Received 31 May 2014 Revised 7 October 2014 Accepted 13 October 2014 Available online 18 October 2014 Keywords: Ultraviolet observations Spectroscopy Atmospheres, composition Io

a b s t r a c t We report on an investigation of the spatial distribution of Io’s atmosphere, and its diurnal variability, using Hubble’s Space Telescope Imaging Spectrograph (HST/STIS). From December 2010 to January 2012, we obtained spatially resolved limb-to-limb 2100–3100 Å spectra at low latitudes (<30°), with the STIS 0.100 slit. Spectra taken at two central meridian longitudes (CMLs), 200 and 250W, over regions that are both bright and dark at near-UV (3000–4000 Å) wavelengths, allowed investigation of the variation in atmospheric density with terrain type and local time. The combined longitudinal coverage of these observations extends from 120 to 320W longitude, and observations of the 200–250W longitude region are obtained at 2 distinct times of day, differing by 50° of rotation of Io. Using primarily SO2 gas absorptions from 2100 to 2300 Å, we detect SO2 gas densities ranging from 0.3 to 2.2 1017 cm2, and 100–200 K gas temperatures. Because we avoided known persistent plume sites, and see little enhancement of SO2 density near known active volcanic centers, we conclude that SO2 gas densities 2 1017 cm2 can be obtained via sublimation alone. We correct column densities at each location to equatorial values by assuming vapor-pressure equilibrium with frost at temperatures that vary as cosine1/4(latitude), as inferred from earlier low latitude HST/STIS observations (Jessup, K.L., Spencer, J.R., Ballester, G.E., Howell, R.R., Roesler, F., Vigil, M., Yelle, R. [2004]. Icarus 169, 197–215). Inferred equatorial SO2 gas densities in the 120–320W longitude range show the following behavior: (i) rapid decrease from longitude 170W to 310W, consistent with previous disk-integrated 19 lm and 2100 Å spectroscopy (Spencer, J.R., Lellouch, E., Richter, M.J., López-Valverde, M.A., Jessup, K.L., Greathouse, T.K., Flaud, J.M. [2005]. Icarus 176, 283–304; Tsang, C.C.C., Spencer, J.R., Jessup, K.L. [2013]. Icarus 226, 604–616) and Ly-a imaging (Feaga, L.M., McGrath, M., Feldman, P.D. [2009]. Icarus 201, 570–584); (ii) at a given longitude, negligible variation with local time despite the 50° rotation of Io, consistent with the dawn-to-dusk presence of the atmosphere as derived previously from Ly-a images (Feaga, L.M., McGrath, M., Feldman, P.D. [2009]. Icarus 201, 570–584); and (iii) negligible dependence on the near-UV brightness of the underlying terrain. At longitudes 140–170W, inferred equatorial SO2 gas densities are a factor of 2 larger than those seen in 2001 (Jessup, K.L., Spencer, J.R., Ballester, G.E., Howell, R.R., Roesler, F., Vigil, M., Yelle, R. [2004]. Icarus 169, 197–215), consistent with the expected seasonal change due to variations in Io’s heliocentric distance (Tsang, C.C.C., Spencer, J.R., Lellouch, E., López-Valverde, M.A., Richter, M., Greathouse, T.K. [2012]. Icarus 217, 277–296). The inferred latitudinal and seasonal variations in atmospheric abundance, and the absolute SO2 gas abundances, are consistent with a sublimation dominated atmosphere with SO2 frost albedo 0.5 ± 0.09 and thermal inertia P300 MKS (Tsang, C.C.C., Spencer, J.R., Lellouch, E., López-Valverde, M.A., Richter, M., Greathouse, T.K. [2012]. Icarus 217, 277–296; Walker, A.C., Moore, C.H., Goldstein, D.B., Varghese, P.L., Trafton, L.M. [2012]. Icarus 220, 225–253), and an inferred volcanic component that is 20–30% of the sublimated gas density. However, the lack of diurnal temperature variations is difﬁcult to explain with vapor pressure equilibrium unless the frost thermal inertia is extremely high (2000 MKS), and the strong longitudinal atmospheric gradient cannot be explained by a vapor pressure equilibrium with frost that has uniform albedo and thermal inertia with longitude. We note, however, that there is evidence that frost properties are in fact highly spatially variable. Intriguingly, we ﬁnd that the longitude dependence of the zero-latitude SO2 gas density is correlated with the longitudinal variation in the low-latitude SO2 frost grain size derived from the near-IR spectrum (Douté, S., Schmitt, B., Lopes-Gautier, R., Carlson, R., Soderblom, L., Shirley, J., and the Galileo NIMS Team [2001]. Icarus 149, 107–132), decreasing as the SO2 grain size decreases – this may be most

⇑ Corresponding author. E-mail address: [email protected] (K.L. Jessup). http://dx.doi.org/10.1016/j.icarus.2014.10.020 0019-1035/Ó 2014 Elsevier Inc. All rights reserved.

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plausibly explained by an increase in frost albedo, and thus a reduction in temperature and vapor pressure, with decreasing grain size. We further note that the densest regions of Io’s atmosphere are correlated in both latitude and longitude with regions characterized by both large SO2 grain size and unusual photometric properties, and suggest that frost albedo may be particularly low in these regions. In addition to the gas densities, SO2 continuum emission was detected 0.28 lm ranging in brightness from 60.004 to 1.17 kRy/Å. The emission brightness was observed to increase as the ﬁtted SO2 gas density increased, but the variance in the emission brightness appears to be more strongly correlated with the relative proximity to the anti-jovian magnetic reconnection point. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction: motivations for continued observation of Io’s atmosphere Jupiter’s moon, Io, is the most volcanically active body in the Solar System. Io’s volcanic activity deﬁnes the composition of Io’s bound and extended atmosphere. The interaction of Io’s atmosphere with the jovian magnetosphere produces plasma that circulates in a torus around Jupiter and along magnetic ﬁeld lines to produce auroral behavior at Jupiter. It thus ﬁlls the jovian magnetosphere and contaminates the surfaces of the other jovian satellites. This makes Io’s tenuous atmosphere one of the most intriguing in the Solar System and deﬁnitely the most inﬂuential satellite atmosphere in the jovian system. The dominant gas in Io’s global atmosphere is SO2, and it has two primary sources. One source is the dominant release of that gas during volcanic eruptions; and the second is the sublimation of the resulting SO2 surface frost. Though it is understood that all the SO2 in Io’s atmosphere (and on the surface) ultimately originates from the volcanoes, it has been debated for years whether volcanism or sublimation controls Io’s dayside atmospheric density, which in turn determines the vertical and horizontal distribution of Io’s atmosphere from day to night. In an effort to fully understand the nature of Io’s atmosphere, Io has been observed both spectrally and through imaging on a near on-going basis for more than 3 decades. Over the past 16 years spectral observations of Io taken at multiple wavelengths have established that the atmospheric column density on Io varies spatially as a function of longitude, and that the gas density on the anti-jovian hemisphere is dramatically higher than that observed on the sub-jovian hemisphere (Jessup et al., 2004; Spencer et al., 2005; Feaga et al., 2009; Tsang et al., 2012). Additionally, spatially resolved near-UV (NUV) (Jessup et al., 2004; McGrath et al., 2000) and far-UV (FUV) (Feaga et al., 2009) spectral observations of Io’s anti- and sub-jovian hemispheres indicate that the atmospheric density peaks in the equatorial region, and decreases with increasing latitude. Altogether, these observations indicate that Io’s equatorial SO2 column density ranges from 1.5 to 22 1016 cm2 depending on the observed longitude and heliocentric distance of Io at the time of observation (McGrath et al., 2000; Jessup et al., 2004; Spencer et al., 2005; Feaga et al., 2009; Tsang et al., 2012). The longitudinal SO2 gas distribution derived from disk-averaged spectral observations (and from averaging the gas density across the disk from spatially resolved spectral data) maps closely with presumed indicators of frost abundance, and with the distribution of observed plume activity. Consequently, the nature and cause of the longitudinal distribution of Io’s dayside atmosphere has been and will remain under debate until the relative roles of volcanism and frost sublimation can be deﬁnitively constrained (Ingersoll, 1989; Strobel and Wolven, 2001; Jessup et al., 2004; Saur and Strobel, 2004; Moullet et al., 2008; Walker et al., 2010; Tsang et al., 2012). The key to resolving this issue is to observe atmospheric characteristics that are unique to each source. For example, the sensitivity of the sublimated SO2 gas density to surface tempera-

ture should make atmospheric behavior as a function of time of day a discriminator of the dominant atmospheric source. However, to use these properties to quantify the relative contribution of the volcanic and sublimated sources in maintaining Io’s dayside gas distribution requires observations with adequate time of day and spatial resolution. We present here new data that provides detailed measurement of Io’s dayside SO2 gas density, focusing on the high density equatorial region. These measurements provide absolute measurement of the SO2 gas density on Io at speciﬁc times of day and over speciﬁc terrain types (surface properties) that can be used in models designed to parameterize the physical processes and properties responsible for the gas density distributions observed on Io’s dayside. The new dataset was accumulated through multiple observations obtained in the time period extending from December 2010 to January 2012 (Table 1). Since the bulk of the observations were taken in 2011, for the sake of brevity we refer to this new dataset as the ‘‘2011’’ observations.

2. Detailed observation description 2.1. Motivation for target ﬁeld of view The newly acquired 2011 dataset is a follow-on to the Hubble Space Telescope Imaging Spectrograph (HST/STIS) NUV (2000– 3170 Å) long-slit limb-to-limb observations of Io taken in 2001 with the slit centered at 150W longitude near the equator (Jessup et al., 2004). In this conﬁguration, the slit completely encompassed the Prometheus plume vent and plume fallout region (see Fig. 1). For those earlier observations, because the slit was centered above the Prometheus plume near local noon, the gas from the plume prevented accurate determination of the sub-solar sublimation column density and corresponding equatorial surface frost temperature in the absence of a volcanic source; though nearby regions allowed reasonable limits to be placed on the non-Prometheus column density. In spite of the ambiguity surrounding the sub-solar gas source, analysis of the 2001 observations suggested that variations in the atmospheric gas density within 30° of the equator could be well ﬁt by assuming a sublimated atmosphere in vapor pressure equilibrium with the surface frost, for surface frost temperatures that vary as (cosine)1/4 of either the latitude or distance from the subsolar point, as expected for equilibrium with sunlight in high- and low-thermal inertia cases respectively. At mid-latitudes (30–50°), some of the observed gas density values were higher than predicted by a cosine1/4 dependent temperature model, and in some of these cases the high values were detected above regions coincident with volcanic hot spots which may have contributed to the observed gas density enhancement. However, this was not always the case; thus, the true source of the increased gas density at mid latitudes is not fully understood. To reduce the confusion regarding the inﬂuence of the volcanic/thermal sources on the observed sub-solar gas densities, and to investigate the inﬂuence of frost coverage and time of day

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K.L. Jessup, J.R. Spencer / Icarus 248 (2015) 165–189 Table 1 Observation summary. Observation

‘‘Targetted’’ CML

Observed CML

Date

Grating

Time (UT)

t_exp (s)

plate_scale (arcsec)

io_diam (arcsec)

Rsa (AU)

obgp07010_x2d.ﬁts obgp08010_x2d.ﬁts obgp09010_x2d.ﬁts obgp10010_x2d.ﬁts obgp11010_x2d.ﬁts

200W 200W 200W 200W 200W

198W 196W 197W 192W 192W

2010-12-15 2011-09-15 2011-12-07 2011-12-23 2012-01-01

G230L G230L G230L G230L G230L

09:32:36 19:44:15 22:23:28 19:58:26 16:18:02

2000 2000 2000 2000 2000

0.0248 0.0248 0.0248 0.0248 0.0248

1.05 1.20 1.20 1.14 1.11

4.95 4.96 4.97 4.97b 4.97b

obgp12010_x2d.ﬁts obgp13010_x2d.ﬁts obgp14010_x2d.ﬁts obgp15010_x2d.ﬁts obgp16010_x2d.ﬁts

250W 250W 250W 250W 250W

241W 247W 241W 243W 250W

2011-09-08 2011-10-05 2011-10-16 2011-10-24 2011-11-29

G230L G230L G230L G230L G230L

23:13:24 12:32:51 02:28:44 22:53:14 08:26:03

2000 2000 1950 1900 1900

0.0248 0.0248 0.0248 0.0248 0.0248

1.17 1.25 1.26 1.27 1.22

4.96 4.96 4.96 4.96 4.96

NOV–DEC 2001

G230L

1.09–1.20

5.15–5.17

2001: Prometheus observation quick summaryc 150W a b c

Heliocentric distance. Jupiter shine contaminated. See Jessup et al. (2004) for detailed observation list.

Fig. 1. Schematic of all known plume activity observed during the Voyager, Galileo and New Horizons visits to the jovian-system based on direct plume detection or the detection of circular ring deposits (cf. Lopes et al., 2004; Geissler et al., 2004a; Geissler and McMillan, 2008; Spencer et al., 2007) within the longitude ranges consistent with the 2011 (A) and 2001 (B) HST/STIS observations. For each plume detection we presume that the region of volcanic inﬂuence is ±5° from the plume center, with the exception of the Pele-type plumes (1, 4, 17, 19, 20) for which a region ±15° is assumed. Plumes that were detected during the New Horizons ﬂyby of the jovian system are identiﬁed by red triangles. Plumes that directly intersect the HST/STIS slit ﬁeld of view are circled in pink. The areal coverage of the HST/STIS slit for the 2011 CML200W (orange), CML250W (blue) and 2001 CML150W (olive) observations is outlined. The 2011 CML200W and CML250W observations are split into ﬁve 25° spatial bins, we indicate the central longitude of each of these bins with a diamond. We also show the 5 plume-free hotspots that intersect the 2011 HST/STIS FOV (orange asterisks); reading from left to right these hotspots are: Deadulus [19N, 275W], Girru [22N, 240W], Mulungu [17N, 218W], Haokah [21S, 187W] & Tupan [19S, 141W]. We also identify Loki, as a plume-less hot spot, as no plume has been detected there since Voyager. No plume/hotspot activity has ever been detected directly above the D2 or B2 regions, which correspond to local noon for the 2011 CML200W and CML250W observations, respectively. A detailed summary of the central latitude, longitude, and local time associated with spatial bins D0– D4 and B0–B4 is provided in Table 2. For the 2001 observations 6 15° 10° spatial bins are located between ±25° latitude, 3 of these bins were free of any known plume/hot spot activity, and the central longitude of each of these bins is indicated by a blue asterisk. For further details on the 2001 HST/STIS observations see Jessup et al. (2004). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

on the observed gas density the new HST/STIS spectral observations were speciﬁcally designed such that the regions observed near local noon would be free of any known persistent plume activity, as emphasized in Fig. 1. For these new observations we targeted the longitude region extending from 120 to 320W where disk-integrated data show a large drop in gas abundance with increasing longitude (Spencer et al., 2005; Feaga et al., 2009), and where the global average NUV reﬂectance, thought to correlate with SO2 frost distribution, also drops with increasing longitude. 2.2. Description of acquired observations Limb-to-limb observations of Io’s low latitude regions, i.e. at latitudes within 30° of the equator, were acquired on 10 different

occasions between December 2010 and January 2012, using the HST/STIS 5200 long 0.100 wide slit and the solar blind NUV MAMA detector. On ﬁve occasions the slit was centered at central meridian longitude (CML) 200W and on ﬁve separate occasions the slit was centered at CML 250W longitude (Table 1); where the observations obtained at 200W and 250W CML are centered over predominantly NUV bright and dark regions, respectively (Fig. 2). Each observation was obtained using the G230L grating which disperses light between 1700 and 3100 Å at a dispersion of 1.55 Å/pixel. For each observation the slit was oriented 23° from parallel to Io’s equator, so that only Io’s low-latitude regions were observed. We obtained repeat observations at each CML, thus covering the 130–260W and 190–310W longitude regions, and acquiring observations in the 200–250W longitude region at 2

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Io’s SO2 frost properties (Douté et al., 2001) indicates that at lowlatitudes Io’s SO2 frost abundance as a function of longitude is nearly ﬂat on the anti-jovian hemisphere, showing only minimal variations in the total frost abundance between 120 and 250W (Fig. 3), with no clear correlation to the NUV brightness of the surface (see Section 6.6 for in-depth discussion). We also point out that the heliocentric distance varied little during the observations (Table 1), thus seasonal variability in the sublimated SO2 gas density component is not expected. Therefore, we explore the possible correlation of the detected SO2 gas density with the SO2 frost abundance and the SO2 frost grain size in detail below, in addition to looking for correlations of the detected SO2 gas density with the observed latitude, longitude and local time, independent of any concern regarding seasonal variation in the sublimated SO2 gas density. 3. Data reduction 3.1. Flux calibration

Fig. 2. Map projections of Io’s surface derived from images taken at visible (top, Geissler et al., 2004a) and near-ultraviolet (bottom, Spencer et al., 1997) wavelengths. On each map the areal coverage of the HST/STIS slit for the CML200W (orange) and CML250W (blue) observations is outlined. Each observation is split into ﬁve spatial bins each covering 25° longitude as indicated on the maps. The spatial extent of these bins is speciﬁcally chosen to exclude the edge of the disk from the spatially co-added spectral bins in order to avoid foreshortening effects, i.e. the bins are deﬁned to cover the longitude regions extending ±70° from the CML longitude. On each map the bin located at the center of the slit is highlighted in lime-green. A summary of the central latitude and longitude of spatial bins B0–B4 and D0–D4 and the geologic features located in or near each bin is provided in Table 2. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

distinct times of day. We segregate each of the limb-to-limb observations into 5 speciﬁc spatial bins per observed CML (Fig. 1), where each bin covers 25° of longitude. The central longitude, local time and latitude associated with each spatial bin are listed in Table 2, notable geologic features falling within or near each bin are also listed. From this data we report our analysis of the variation of the SO2 gas density and temperature, as well as the magnitude of the SO2 continuum emission observed in the equatorial region from the morning to the evening terminators. Although the observations were obtained with the slit centered over CML 200 and 250W, our original goal was to target the slit to be centered at a single longitude so that by having the center of the slit move with the rotation of Io’s disk (not following the CML), time of day and latitude would be isolated as a function of the brightness of the surface. However, due to a targeting error, the slit was set to be constantly centered on the disk CML as Io rotated; thus time of day, NUV brightness, and latitude remained entangled. Though it has been historically presumed that NUV brightness maps to SO2 frost abundance (cf. McEwen et al., 1988; Spencer et al., 2005), the targeting error does not constrict our ability to uniquely deﬁne changes in the atmospheric density with latitude, longitude and time of day as a function of the expected SO2 frost abundance. This is because the most recent in-depth analysis of

The observations were initially reduced using the Space Telescope Science Data Analysis System (STDAS) CALSTIS reduction pipeline including ﬂat-ﬁelding, dark subtraction and absolute ﬂux calibration as well as wavelength calibration and geometric rectiﬁcation. For two of the 200W CML visits the acquired spectra included a signiﬁcant level of observable ﬂux from Jupiter because the observations were obtained when Io was close to Jupiter. As a result, we only utilize data from 3 of the 5 visits taken at 200W CML. Even for those observations not dominated by Jupiter light, the pipeline-reduced spectra retain some level of residual background ﬂux. To correct for this, the true ﬂux across Io’s disk is obtained by estimating and removing the sky background from the observations. Because the observations were obtained with the 5200 long slit, the sky background was straightforwardly estimated from the ﬂux levels recorded in regions of the slit sufﬁciently far from the edge of Io’s 1.15 ± 0.100 diameter disk. 3.2. Determination of the geographical coordinates along the slit After establishing the absolute ﬂux calibration we determined the absolute geographical coordinates in latitude and longitude for each 0.02400 spatial pixel along the 5200 slit that intersected the disk of Io, utilizing the methods described in Jessup et al. (2004). Thus, the exact slit locations for each STIS spectrum are deﬁned based on the 3400 ± 100 Å WFPC/2 intensity proﬁles that best matched the 3120–3160 Å along-slit intensity proﬁle recorded within each STIS image (Fig. 4). Although Io’s reﬂectance levels at 3100–3200 Å are not identical to the 3400 Å reﬂectance levels observed in the F336W images, the 3155 ± 25 Å STIS and 3400 ± 100 Å WFPC/2 spatial proﬁles are well matched for each observation, as was true for the 2001 HST/STIS observations (see Jessup et al., 2004). Once the latitude and longitude of the center of Io within the slit is deﬁned, the latitude and longitude of each 0.02400 spatial pixel along the slit that intersected the disk of Io is also straightforwardly determined based on the diameter and orientation of Io’s disk on each date of observation. 3.3. Wavelength calibration To verify the wavelength calibration of the observations, we compared the average ﬂux level obtained per wavelength across the disk of Io with the known solar ﬂux measured by SOLSTICE at (0.33 Å sampling, 1.0 Å FWHM) for each of the 8 usable dates of observation (as provided by Marty Snow, personal communication). To make an accurate comparison between the two spectra,

169

K.L. Jessup, J.R. Spencer / Icarus 248 (2015) 165–189 Table 2 Summary of ﬁt properties relative to the observed geology. (LON, LAT, SZAa)

TODb (h)

NSO2/1016 (cm2)

NSO/1015 (cm2)

TSO2 (K)

0.28 lm emission (kRy/Å)

0.31 lm reﬂ.

SO2 frost

Geo. featurec

In/ neard

Plume evidence (VO|GA|NH)e

Results for 200W CML observations B0. (142.5, 21.0, 52.9°) 14.9 B1. (167.5,12.0, 29.2°) 13.4

13.6 ± 4.4 22.5 ± 1.5

66.8 6.3 ± 1.7

100. ± 12.5 200. ± 12.5

0.362 0.950

0.036 0.043

0.36 0.44

B2. (192.5, 0.0, 6.18°) B3. (217.5, 12.0, 26.5°) B4. (242.5, 22.0, 51.5°)

21.0 ± 2.0 14.0 ± 1.0 6.5 ± 1.9

5.5 ± 2.5 61.4 61.2

200. ± 12.5 175. ± 12.5 175. ± 62.5

1.17 0.914 0.352

0.042 0.046 0.041

0.46 0.43 0.52

Tupan Culann Calderaf Calderaf Mulungu P. Girru P.

In In In In In In

N|N|N N|Y|N N|N|N N|N|N N|N|N N|N|N

8.7 ± 0.9

66.0

200. ± 12.5

0.728

0.040

0.47

Marduk Haokah Reiden Kaminari Peleg Pillan Reiden Kaminari S. Deadulus N. Loki

Near Near Near In Near Near Near Near Near Near

Y|Y|Y N|N|N N|Y|N N|Y|N Y|Y|Y N|Y|Y N|Y|N N|Y|N N|N|N Y|N|N

11.8 10.2 8.7

Results for 250W CML observations D0. (197.5, 21.5, 50.5°) 15.0 D1. (222.5, 12.0, 24.7°)

13.4

11.5 ± 0.5

62.6

175. ± 12.5

0.595

0.041

0.51

D2. (247.5, 0.0, 7.78°)

11.8

9.8 ± 1.2

64.2

200. ± 12.5

0.231

0.034

0.47

D3. (272.5, 11.0, 31.7°) D4. (297.5, 19.0, 55.6°)

10.2 8.6

6.3 ± 1.1 2.9 ± 0.5

60.8 1.2 ± 0.6

100. ± 12.5 175. ± 37.5

0.158 60.004

0.036 0.039

0.42 0.36

a

SZA: solar zenith angle. TOD: local time of day in Io hours. Geologic feature within 10° of slit ﬁeld of view; the center of each of the listed features is considered to be a region of thermal activity, except the caldera located in spatial bin B2. d Relative position of geologic feature (in: denotes that the feature is in the ﬁeld of view; near: denotes that the feature is within 10° of the outside edge of the slit ﬁeld of view). e Era in which plume activity may have been detected. VO, GA, NH: denotes the Voyager era, the Galileo era; and the 2007 New Horizons ﬂyby of the jovian system; detection of plume activity above any of the identiﬁed geologic features during each of these eras is indicated by a yes (Y) or no (N). f Unnamed calderas, there is no record of thermal activity being detected above either of these calderas. g The slit was within 10° of the NE edge of the Pele plume deposit region, but was 15–20° from the hotspot at the epicenter of the Pele plume. b

c

Fig. 3. Comparison of Io’s average equatorial/low-latitude NUV surface reﬂectance levels (cyan line) and average equatorial SO2 frost coverage (lilac triangles) per longitude. The frost abundance is derived from the Galileo near IR observations of Io (Douté et al., 2001), and the NUV reﬂectance is derived from HST/WFPC2 images of Io obtained at 3400 Å (see Fig. 2). In each case the average equatorial values are derived based on measurements made ±20° of the equator, per longitude. The average SO2 frost coverage encountered per spatial bin for the observations obtained at CML 200W (orange diamonds) and CML 250W (blue diamonds) is also shown; and, we overlay the NUV surface reﬂectance recorded in the HST/WFPC2 images directly at the equator (black line) and at the longitudes that map directly to spatial path of the HST/STIS slit during the CML 200W (orange dashed) and CML 250W (blue dashed) HST/STIS observations. In all cases the NUV reﬂectance variance with longitude (shown as lines) does not match the SO2 frost abundance variance (shown as symbols). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

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Fig. 4. In the top panels we show the slit orientation on Io against images of Io taken with the Wide Field and Planetary Camera 2 (WFPC2) by Spencer et al. (1997), in the F336W (3400 Å) ﬁlter, for each of the targeted CMLs (dotted line). In the bottom panels we show the near-ultraviolet (NUV) spatial proﬁle predicted to be within the slit based on the WFPC2 images (thick black) vs. the average 3100–3170 Å brightness detected in the HST/STIS slit across the disk of Io. We deﬁne the latitude and longitude encountered by the slit at each pixel based on the correlation of the STIS and WFPC2 NUV spatial proﬁles.

we re-sampled the SOLSTICE spectra to match that of the STIS data; we also convolved the SOLSTICE spectra with a Gaussian comparable to the complex 6.2 Å line-spread function (LSF) proﬁle expected from the convolution of the STIS point-spread function (Woodgate, 1998) with the 0.100 slit. I.e., we found that the best cancellation of solar Fraunhofer lines was obtained using a Gaussian LSF of 6.1 Å FWHM, boxcar smoothed to a 6.4 Å FWHM LSF; thus, we adopted this LSF as the effective spectral resolution of our data. We made small corrections to the wavelength of each STIS spectrum by aligning the Fraunhofer absorption lines with those in the solar spectrum. We found that at the most adjustments of 1.1 Å were required to achieve alignment with the solar spectrum. 3.4. Spatial binning Having deﬁned the absolute wavelength scale and the precise latitude and longitude extent of each spatial pixel we co-added each of the long-slit spectra into spatial bins with a width of 25° of Io longitude at each wavelength. The central latitude and longitude of the slit for each observation was not identical between visits for either the 200W or 250W datasets (see Table 2). Because of the slight variability of the central longitude and disk diameter between observations, the co-addition of the data by longitude was preferred to binning by pixel position. In order to avoid foreshortening effects at the edge of the disk, we chose to co-add the CML 200W observations into the ﬁve 25° longitude bins found between 130 and 255W, this ensured that the data analyzed incorporated longitudes that were 20° or more from the eastern and western edge of the disk for each of the usable 200W spectral observations. Similarly, we co-added the CML 250W observations into the ﬁve 25° longitude bins between 185 and 310W. Variations in geometry between visits also resulted in some variability in the latitude values at each longitude within each of our two CML observation sets. Since the variation in the longitude between observations was 7° or less, and the north-pole position angle of the slit was constant between observations, by careful mapping of the coincident longitude ranges between observations, the variation in central latitude of each of the co-added spectra was 6° or less within each spatial bin. 3.5. Albedo derivation and observational trends The albedo for each of the longitude bins is derived using the standard reﬂectivity relationship; i.e., the geometric albedo (p)

per longitude bin is deﬁned as the ratio of the observed intensity (FIo) multiplied by p and divided by the solar ﬂux at the Earth (Fs) that is scaled to Io’s heliocentric distance (F s =R2s ), thus:

p ¼ ½p F Io =½F s =R2s ; where FIo is the fully calibrated and corrected ﬂux recorded by HST/ STIS, and Rs is the heliocentric distance to Io, and Fs is obtained from the SOLSTICE spectra, matched to the spectral resolution of the STIS spectra, as described above. Because the SO2 gas absorption signature is best revealed between 1900 and 3100 Å and the highest signal-to-noise (S/N) levels are achieved at wavelengths longward of 2100 Å, we present the 2100–3100 Å range of the albedo spectra in Fig. 5. In the absence of atmospheric emission, the observationally derived albedo spectra would include the signatures of the incident solar ﬂux, atmospheric absorption, and Io’s surface reﬂectance. However, as was true of the 2001 spectra of Io’s anti-jovian hemisphere (Jessup et al., 2004), the observationally derived albedos also include emission like features at wavelengths longward of 2600 Å at locations and relative magnitudes consistent with additional continuum emission, probably from the ﬂuorescence of the SO2 gas, which reduces the Fraunhofer line depth in the Io spectra, thus preventing complete cancellation of the solar Fraunhofer lines. These features are most prominent between 140 and 220W longitude. Because the observations were obtained utilizing the NUV MAMA detector, which is insensitive to solar radiation redder than the 1600–3100 Å detector band-pass, we are conﬁdent that the inferred continuum emission is Iogenic and not an instrumental artifact due to contamination of the spectra by scattered longer-wavelength radiation, i.e. the instrumental throughput efﬁciency is 0.004 at 3150 Å and <104 at longer wavelengths. In each of the spectra presented in Fig. 5 the SO2 gas absorption is evident in the strong SO2 bands below 2300 Å and for a few cases in the weaker bands above 2800 Å (although the long wavelength signature is frequently obscured by the continuum emission signature). The depth of the strong SO2 gas absorption bands appears to be lowest between longitudes 270 and 290W. Discrete SO lines are not evident in the spectra; however, irregularities in the shape of the gas absorption signature shortward of 2300 Å are present in some of the observations, suggestive of the presence of SO gas (Fig. 6).

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Fig. 5. Plots of the albedo signature derived per longitude bin, as observed (black) and with the ﬁt continuum emission levels removed (orange). The continuum emission model adequately removes the emission like features at 2800 Å; however, our current continuum emission model fails to fully remove the emission signatures located above 2950 Å in the 200W CML dataset (see text for details). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

4. Analysis description The observed albedo can be modeled as a function of wavelength k as:

GAðkÞ ¼ ½RðkÞ Sðk; uðh; u; NÞÞ; where h is co-latitude (90° latitude), u is longitude, R(k) is surface reﬂectance, and S(k, u(h, u, N)) is the atmospheric transmission, which is a function of the absorption path, u. u is deﬁned as 2 N/(cos du sin h), where N is the vertical atmospheric column density, and the factor of two results from the fact that the solar light passes through Io’s atmosphere twice; cos du sin h, is the cosine of the solar zenith angle; and, du is the difference between u and the central meridian longitude (CML). This formulation assumes that the Sun and observer are in Io’s equator, and that solar phase angle is small, which are good approximations in the case for our observations (subsolar latitude = 2–3°, phase angle = 1–11°). 4.1. Atmospheric transmission For the current observations, we assume that the atmospheric column density will be a function of SO2 and SO, the two most prominent gases in Io’s atmosphere. Thus the full atmospheric transmission equation is a product of the SSO2 and SSO. We use the Malkmus band transmission model (cf. Jessup et al., 2004; Zhu, 2004) to determine the most accurate deﬁnition of the

SO2 gas transmission utilizing the best available high resolution SO2 absorption cross-section data, thus:

SSO2 ¼ expðpyðkÞ=2 ½ð1 þ 4rSO2 ðk; TÞ=py uSO2 Þ1=2 1Þ; where rSO2(k), the SO2 absorption cross-section, is both wavelength, k, and temperature, T, dependent; additionally, the SO2 mass path, uSO2 is deﬁned as uSO2 = 2 NSO2/(cos du sin h), where NSO2 is the vertical SO2 column density, and y is a parameter that assesses the saturation effects of the gas absorption in the wings of the bands as a function of wavelength. As discussed in Tsang et al. (2013), the model used in our current analysis improves upon the work presented in Jessup et al. (2004) by more precisely determining the value of the y-factor in the 2100–2300 Å region. This increased precision is achieved by utilizing the higher 5 mÅ spectral resolution SO2 cross-section measurements taken at multiple temperatures (160 K, 198 K, and 295 K), by the same instrument and with near identical spectral sampling and resolution (Rufus et al., 2003, 2009; Blackie et al., 2011; Stark et al., 1999). To assess the impact of SO gas absorption on the newly acquired HST/STIS spectra we also built a matrix of SO transmission spectra which is calculated from a grid of SO column densities ranging from 0 to 3.5 1016 cm2. The available SO absorption cross-section data are of medium (1.0 Å) resolution. At this resolution the line structure of the absorption bands is not resolved and only the broad shape of the cross-section is observed; because the lines are not resolved the observed band shape only estimates the absorption

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Fig. 6. The best ﬁt to the B2 spatial bin (0N, 192W) albedo spectrum is deﬁned by an SO2 gas column density = 2.1 1017 cm2 and an SO gas density that is 3% of that density (see Table 2). To show the impact of the SO gas absorption on the observed albedo signature we show in the left panel the B2 spatial bin (0N, 192W) albedo spectrum (black) vs. model albedoes generated with an SO2 gas column density = 2.1 1017 cm2, assuming an SO/SO2 gas density ratio of either 0 (blue) or 10 % (orange); where the reﬂectance level of the model albedoes has been offset by 0.014. These plots show that the inclusion of the SO gas density modiﬁes the relative intensity of the dominant SO2 gas absorption signature causing the SO2 band located at 2170 Å to appear deeper than the neighboring bands. On the right we repeat the plots shown in the left panel, overlaying on each spectrum the best ﬁt model albedo spectrum (pink). Comparison of each of these spectra indicates that the inclusion of the SO gas column is needed to replicate the depression of the SO2 bands located shortward of 2170 Å; however, the relative depth of the SO2 bands longward of 2170 Å is not sufﬁcient to warrant an SO/SO2 gas density percent ratio as high as 10%. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

behavior in the wings of the band (cf. Jessup et al., 2004). Thus, rather than using the Malkmus band model that relies on apriori knowledge of the detailed line structure (cf. Zhu, 1994), the SO transmission spectra are calculated using Beer’s Law; thus: SSO = exp(rSO(k) uSO), where rSO(k) is the wavelength dependent SO absorption cross-section, uSO = 2 NSO/(cos du sin h), and NSO is the vertical SO column density. The SO transmission calculations are completed at a single gas temperature of 300 K, as the cross-sections have only been measured at this temperature (Phillips, 1981). 4.2. Surface reﬂectance We treat the surface reﬂectance term R(k) as a free parameter, deﬁning its value at 50 Å wavelength increments and interpolating between these wavelengths by a cubic spline. This allows us to separate the broad spectral structure of the surface reﬂectance from the 10 Å-wide ﬁne structure of the SO2 and SO gas absorption bands. Although the gas absorption also produces broad-scale structure due to overlap of absorption bands, ﬁtting of the shape and depth of the band cores allows estimation of the broad-scale gas absorption independently of the surface reﬂectance (cf. Jessup et al., 2004). The resulting surface reﬂectance compares favorably to the spectrum of SO2 frost, though the match is not exact, indicating the probable presence of additional surface components (Fig. 7). 4.3. Continuum emission estimate The continuum emission that is revealed by the mis-cancelled Fraunhofer lines in our albedo spectra is modeled on the assumption that it is due to electron impact induced emission from SO2 gas, which is also the probable source of diffuse 4040 Å emission seen in Galileo eclipse images (Geissler et al., 1999, 2001). We follow the same methods for ﬁtting the continuum emission signature outlined in Jessup et al. (2004), thus we assume the spectral distribution measured by Ajello et al. (2002) for an electron impact energy of 9 eV (see Fig. 5, Jessup et al., 2004). We ﬁt the data using this electron energy because ﬂuorescence spectra for higher electron energies show an SO emission feature at 2550 Å that is not evident in our data, and spectra for lower electron energies are not available. 4.4. Least squares ﬁtting procedure Using the relationships described above, we ﬁt the gas absorption and emission signatures observed in the 2100–3100 Å region

in two steps. First, we ﬁt the 2050–2300 Å gas absorption signature as a function of vertical SO2 gas column density (NSO2), temperature and vertical SO gas column density, since the SO2 and SO gas absorption bands are the strongest at these wavelengths. Second we ﬁt the long wavelength end (above 2600 Å) of the spectrum. Although there is a weak SO gas band absorption system located between 2500 and 2600 Å, absorption cross-section data for these bands are currently not available, neither is there any evidence of absorption from this weaker band system in the current dataset. Thus, the region of the spectrum above 2600 Å contains only the weak SO2 gas band absorption signature, the continuum emission signature, and the surface reﬂectance signature. For each spectrum we use the SO2 gas densities ﬁt to short-wavelength end of the spectrum to predict the SO2 absorption spectrum at longer wavelengths, and assume this spectrum when ﬁtting the longwavelength continuum emission levels, while retaining R(k) as a free parameter. After the continuum emission levels are constrained we remove that signature from the observationally derived albedos to generate the ﬁnal corrected geometric albedo spectrum per longitude bin (Fig. 5). Each of the parameters ﬁt in our model are constrained from a grid search through a pre-deﬁned parameter space. The SO2 parameter space includes column densities in the range of 0.2 1016–3.5 1017 cm2, at increments of 0.2 1016 cm2 for NSO2 < 1 1017, and 0.1 1017 cm2 for NSO2 > 1 1017 and temperatures in the range 110–500 K, at increments of 25 K. The SO parameter space includes column densities of 0.0 cm2, and in the range of 1014–3.5 1016 cm2, at increments of at increments of 0.2 1015 cm2 for NSO2 < 1 1016, and 0.1 1016 cm2 for NSO2 > 1 1016. We ﬁt continuum emission ﬂuxes in the range of 0.001–11 kRy Å1 at 2800 Å, at increments of 0.001 kRy Å1. The best-ﬁt parameter values are derived from the least squares ﬁt to the albedo weighted by the error in the albedos, which is estimated from the photon statistics of the Io spectra and the published uncertainties of the SOLSTICE solar spectrum (McClintock et al., 2005a, 2005b). Because the solar ﬂux intensity between 1700 and 3170 Å decreases with decreasing wavelength, the overall S/N of the observations decreases with decreasing wavelength.

5. Results We present in Fig. 8 the model albedo spectra generated by the ﬁtted gas densities compared to the observed geometric albedo spectra corrected for continuum emission. All of the observed spectra are well ﬁt in the 2000–2950 Å region using the SO2 gas density inferred from the ﬁt of the gas bands observed between 2000 and

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Fig. 7. We show examples of the continuum surface reﬂectance derived from the ﬁt of the albedo spectra as function of the observed CML vs. the known SO2 frost reﬂectance spectrum for medium grained SO2 frost (Hapke et al., 1981; Nash et al., 1980), highlighting the results derived from observations taken at noon (blue) and near 9 a.m. (pink). The ﬁt reﬂectance maps well with the broad behavior of the SO2 frost; however, when the continuum emission signature longward of 2950 Å is higher than can be accounted for by the known SO2 gas continuum emission spectrum (e.g., near noon at CML 200W) compensation is made in the ﬁt reﬂectance between 2950 and 3050 Å to accommodate for the incomplete removal of the emission signatures located at those wavelengths (see text for details). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 8. The model albedo (pink) ﬁt to the corrected albedo signature derived per longitude bin (black) is shown. While the continuum emission model adequately removes the emission like features at 2800 Å, in the 200W CML dataset the emission signatures located above 2950 Å cannot be fully removed based on a model that only includes the SO2 continuum emission spectrum. As a result the SO2 gas density ﬁt to the shortwave region of the spectrum appears to over-estimate the gas absorption bands longward of 2950 Å in the 200W CML dataset, this issue is not evident in the 250W dataset (see text for discussion). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

2350 Å. However, the ﬁtted SO2 gas density seems to over-estimate the gas absorption depth above 2950 Å in those spectra taken between 168 and 218W longitude at latitudes ±12° from the equator (spatial bins B1–B3), and in the spectrum taken at 143W longitude (spatial bin B0). Notably, all of these spectra are from the CML 200W visits, and correspond to the strongest continuum emission observed during those visits (see Fig. 8, Table 2). Comparing to the

observed spectra without emission removal (Fig. 5), it would appear that for the B0–B3 CML 200W spatial bins the emission ﬁlling in the Fraunhofer lines wavelengths longward of 2950 Å are not well replicated by the SO2 continuum emission variance with wavelength. Given that the albedo model (Fig. 8) well replicates the observed gas absorption signature up to 2950 Å in these observations, within the uncertainty of the data, we conclude that in

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Fig. 9. The 0.28 lm continuum emission brightness levels detected in the CML200W (black) and CML250W (green) observations is plotted vs. the ﬁt SO2 gas density (left panel) and vs. longitude (right panel). The continuum emission brightness tends to increase as the ﬁt SO2 gas density increase, and it also seems to vary symmetrically about 190W longitude. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

every case in Fig. 8 where the SO2 gas absorption is over-estimated, it is due to the obscuring of the absorption signature by the mis-cancelled Fraunhofer lines. The simplest explanation for the incongruence between the ‘‘corrected’’ albedo spectra (i.e. the spectra with the ﬁtted level of SO2 continuum emission removed) and the modeled albedo spectra is that an additional emission source was contributing to the spectral signature observed longward of 2950 Å. That an additional emitter would be contributing to the signature near 3000 Å is consistent with the 2001 HST/STIS results, as well as emission detections made by Cassini in the 3000–5000 Å region from which Geissler et al. (2004b) speciﬁcally found that the ratio of the intensities detected in the BL1 (3909–5000 Å) ﬁlter and UV3 (3000– 2800 Å) ﬁlters was larger than expected for excitation of SO2 alone. In contrast to the CML200W observation behavior, the continuum emission signature that was evident in the CML250W spectra in spatial bins D0–D3 is completely removed by the SO2 only continuum emission model (see Fig. 5). Comparison of the observational details of the two datasets reveals no obvious pattern relative to the geometry of the anti-jovian magnetic connection point (located at 180W ± 5° and ±10° latitude, Dessler, 1983) that would explain why the additional emission signature was detected in the B0–B4 spatial bins, but not in the D0–D3 bins. For example, the B1–B3 spatial bins are all located near the longitude of the anti-jovian magnetic connection point, but the detection of the additional emission signature in the CML 200W spectra is not a function of the proximity of the observation to the longitude of the anti-jovian magnetic connection point since the additional emission is also detected in spatial bin B0 located 40° from 180W longitude. Although the same longitude region associated with the B2 and B3 spatial bins is also included in the 250W CML D0 and D1 spatial bins, the CML200W observations are obtained 23° higher in latitude than the CML250W observations. Since, this is the only real distinction geographically between these observations it is possible that the additional signatures are produced because there is a compositional difference in the surface components located at these latitudes. However, deﬁnitive assessment of this or any other possible explanation requires additional analysis which is beyond the scope of this paper. In spite of our inability to completely remove the continuum emission signature at wavelengths longward of 2950 Å in the B0–B3 spectra, the SO2 continuum emission shape is able to adequately replicate the mis-cancelled solar absorption features observable between 2650 and 2950 Å in every spectra taken during the CML 200W and CML 250W observations. Based on the ﬁts to that wavelength region, the magnitude of the inferred of SO2 continuum emission brightness varies with longitude from 60.004 to 1.17 kRy/Å. And, as was seen in 2001, the ﬁt SO2 gas density and 2800 Å (0.28 lm) continuum emission brightness levels appear to increase in parallel (Fig. 9).

A full summary of the vertical SO2 gas column density, SO2 gas temperature and vertical SO gas column density ﬁt per longitude bin is presented in Table 2, along with the continuum emission brightness inferred from each of the spectral ﬁts. Also listed in this table are the central latitude and longitude of each bin, the observed 3100 Å (0.31 lm) reﬂectance and associated geologic features, including the average SO2 frost coverage observed by Galileo NIMS (Douté et al., 2001), within the latitude and longitude range associated with each spatial bin. As Table 2 indicates, the ﬁtted SO2 gas temperatures are in the range of 100–200 K, with no clear pattern in terms of latitude, time of day or longitude. Combining the ﬁt SO2 and SO gas densities, we infer that an SO/SO2 ratio 1–4% was detected at mid-day and early afternoon in the NUV bright regions centered above spatial bins B1 and B2 during the CML200W observations; likewise an SO/SO2 ratio 2–5% was detected mid-morning in observations obtained near 20N, 300W, just north of the Loki lava lake during the CML250W observations. For all other regions only an upper limit to the SO gas density could be ﬁt, and these results suggest an SO/SO2 ratio upper limit in the range of 1–7%, consistent with previous observations (cf. Lellouch et al., 1996). Notably, the SO detections made on the anti-jovian hemisphere are obtained in those spatial bins where the highest SO2 continuum emission brightness levels were detected. The coincidence of the SO detections with these signatures is intriguing, since it is known that a by product of the excitation of the SO2 by electrons is the formation of SO and radiation in the B–X SO band system (cf. Ajello et al., 2008). The proximity of the CML250W SO detection to Loki may suggest that Loki was venting SO gas. However, detailed analysis of sub-mm observations (Moullet et al., 2010) suggests that photolysis is a key source of the gas and that the lifetime of the SO gas in Io’s atmosphere is longer on the trailing hemisphere. Thus, the CML250W detection may also/alternatively suggest an enhancement of the SO gas on the trailing hemisphere towards the morning terminator due to the onset of SO2 photolysis. For both datasets (i.e., observations obtained with the slit centered at 200W above predominantly NUV bright regions and centered at 250W above predominantly NUV dark regions) the maximum SO2 gas density is detected at latitude 10S rather than directly above the equator. Based on Table 2 and the overlay of the ﬁt SO2 column density onto maps of Io’s NUV surface reﬂectance (Fig. 10), we see that the maximum SO2 gas density corresponds to an NUV bright region located near 170W longitude and that this gas density is a factor of 2 higher than the maximum density observed over the low-latitude NUV dark regions centered near 220W. For our chosen observing geometry, 10S latitude is always 1.4 h past local noon. If there were no variation in latitude within the slit for these observations, this result would suggest that the variation in gas density is primarily a time of day (TOD) effect. However, the signiﬁcant latitudinal extent of the

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Fig. 10. Pictorial representation of the ﬁt SO2 gas density vs. the relative NUV brightness, time of day, latitude and longitude of each spatial bin for the 2011 HST/STIS CML200W (top panel) and CML 250W (bottom panel) observations.

Fig. 11. The SO2 gas density ﬁt per spatial bin for observations obtained at CML 200W (black triangles) and at CML 250W (green asterisks) is plotted vs. W. Longitude, (A) latitude (B) and local time (C). The time of day and latitude effects are intermingled in each plot. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

observations results in SO2 gas density detections for which the TOD, longitude, and latitude dependences are entangled. For example, Fig. 11a shows that the SO2 gas density tends to increase with decreasing longitude; at the same time, a decrease in gas density near the limbs is also evident, this latter trend could result from

either the difference in the latitude or time of day associated with each of the detections (Fig. 11b and c). Similarly, though both datasets include SO2 gas detections made near 200W longitude, the SO2 gas density inferred at that longitude from each dataset is dramatically different; without an a priori understanding of the latitude or

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Fig. 12. (A) The inferred zero-latitude SO2 gas density ﬁt per spatial bin for observations obtained at CML 200W (black diamond) and at CML 250W (gray x) is plotted vs. W. Longitude. Spatial bins that include known hotspot and plume regions are highlighted in orange and purple, respectively. The inferred zero-latitude gas density is nearly linearly correlated with the observed longitude, independent of the presence of the hotspots. (B) The inferred zero-latitude frost temperatures are plotted vs. W. Longitude. These values are derived assuming that the ﬁt SO2 gas densities are related to the frost temperature as a function of the cosine(latitude)1/4, revealing a linear correlation in the zero-latitude gas densities with longitude. The line that best represents the correlation between inferred zero-latitude frost temperatures and the longitude (purple line) and as well as a 0.4 K deviation from that line (gray dashed) is plotted. In panel (C) the gas density curve predicted by the frost temperature correlation lines is plotted on top of the zero-latitude gas densities inferred for each spatial bin assuming a cosine(latitude)1/4 dependence. In every case where the observed longitudes are nearly identical, the variation in the inferred zero-latitude gas density values is consistent with the deviation in the gas density expected for a 0.4 K frost temperature uncertainty. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

time of day dependencies of the gas density distributions, there is no obvious way to sort the true source of the observed gas density variance. Fortunately, previous observations have provided us with some insight into the plausible latitudinal dependence of Io’s low-latitude gas density behavior (see Jessup et al., 2004); we explore the implications of our previously inferred SO2 gas density latitudinal dependence on our interpretation of the observed SO2 gas density variances in detail in Section 6. Independent of the intermingling of the latitude and time of day variation within our spatial bins, neither of the zero-latitude, subsolar observations (i.e. spectral-pixel bin B2 or D2) included in the two datasets presented here were made directly above regions that included any known persistent plume source or any known active hot spot. Thus the 2 1017 cm2 SO2 gas density detected above zero-latitude near 200W longitude clariﬁes that an SO2 column density 2 1017 cm2 are possible in the equatorial region independent of an active volcanic source. Because the current observations were obtained close to perihelion, the observed gas densities likely represent the maximum sublimated SO2 gas density observable on Io’s anti-jovian hemisphere.

6. Discussion and interpretation 6.1. Zero-latitude SO2 gas density correlations with longitude As described above, we obtained repeat observations of the 200–250W longitude region at low-latitudes (i.e., at latitudes within 30° of the equator) at 2 distinct times of day, but at different latitudes at each time of day. Jessup et al. (2004) showed that an excellent description of the latitude dependence of SO2 density at low latitudes near longitudes 140–170W can be obtained by

assuming Io’s atmosphere is in vapor pressure equilibrium with surface frost, where the frost temperature varies as the cosine1/4(latitude) as expected from equilibrium with sunlight. Therefore, in order to estimate the longitude variation independent of the latitude variation we assume that the same latitude dependence also pertains to the these new observations obtained between 120 and 320W at latitudes <30°, and derive the zero-latitude SO2 gas density expected at the central longitude of each of our spatial bins. We ﬁnd that the inferred zero-latitude SO2 gas density is strongly correlated with the observed longitude (Fig. 12). The correlation with longitude is nearly linear in this longitude range. The fact that correction for latitude results in essentially perfect correlation of equatorial gas density with longitude provides evidence that our assumed latitude dependence is indeed correct. We will proceed on that assumption, though we acknowledge that it is also conceivable that there is a more complex dependence on both latitude and time of day, and that the perfect correlation seen in Fig. 12 is coincidental. In spite of the fact that 3 of the 5 spatial bins included in the 200W CML observations include known hotspot regions (orange diamonds in Fig. 12, see Fig. 1, Table 2), and that regions of known plume deposits such as at Kaminari (9S, 234W, spatial bin D1, purple ‘‘x’’ in Fig. 12, see Fig. 1, Table 2) are also included in the observations, the gas density detected in the majority of these bins does not deviate from the longitude trend deﬁned by bins without hot spots. Only one point, at 167W (spatial bin B1), which includes the Culann hotspot/plume deposition region centered at 20S, 161W, is slightly elevated above the longitude trend (purple diamond, Fig. 12a). Unfortunately, the frequency and expected output of the eruption activity associated with the plumes/plume deposit regions that intersect the STIS slit ﬁeld of view (FOV) is unknown. For example, Culann had an active plume in 1998 (Geissler and

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Fig. 13. (A) Plots of the inferred zero-latitude SO2 gas density for the 200W and 250W CML observations obtained in 2011 and the 150W CML observations obtained in 2001 (dashed lines). Observations taken at longitudes where plume or hot spot activity has been observed are highlighted in purple and orange, respectively. The gas densities inferred from the 2011 observations are strongly correlated with the observed longitude. In both 2001 and 2011, independent of what terrain was observed, the inferred zerolatitude gas density is statistically equivalent in the 140–190W region. Only at the longitude of the Prometheus plume is the inferred zero-latitude density enhanced signiﬁcantly above the range of gas densities deﬁned by the detections made above volcanically inactive regions. As a reference Io’s heliocentric distance (Rs) is listed for each CML observed (see text for details). (B) Plot of the subsolar gas density inferred from disk-integrated observations of Io taken in the near ultraviolet (NUV) and at mid-infrared wavelengths. (Left axis) The maximum of the 4-year average dayside pole-to-pole gas density detected every 10° longitude at Ly-a wavelengths is also plotted vs. longitude (Feaga et al., 2009); these values have been scaled so that the longitudinal variance of the Ly-a results can be easily compared to the NUV results. The proper magnitude scale for the 4-year averaged Ly-a SO2 gas densities is plotted on the right axis. Similar to the 2011 HST/STIS results, the inferred gas densities are statistically equivalent in the 140–190W region, and the gas density is observed to increase with decreasing longitude from near 300W to near 170 ± 20°W longitude. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

McMillan, 2008), but was not active in 2007 (Spencer et al., 2007). Likewise, activity at Kaminari and Reiden was detected during the Galileo era via the presence of new surface deposits (Geissler et al., 2004a), but no new surface deposits or plumes were detected by New Horizons (Spencer et al., 2007). The current observations imply that with the possible exception of Culann, any gas released from the thermal sources included within the FOV of the slit was insufﬁcient to produce a signiﬁcant enhancement in gas density above the values predicted by our latitudinally dependent vaporpressure equilibrium model. The inferred 120–300W zero-latitude SO2 gas density variation agrees with the longitudinal variation in the subsolar gas densities inferred from previous disk-integrated near-UV and IR observations (Tsang et al., 2012, 2013; Spencer et al., 2005) and the longitudinal variation in the maximum gas density derived from spatially resolved Ly-a observations (Feaga et al., 2009). Notably, in the case of the Ly-a observations, the plotted value corresponds to maximum gas density encountered from pole-to-pole within each 10° longitude bin; and in each case this value was always detected within 10° of the equator. Therefore, all of these observations track the near-equatorial SO2 gas density behavior. In each of the previous observations described above the near zero-latitude gas density was basically constant in the longitude region extending from 190 to 140W. For the mid-IR and mid-UV observations this ﬂat region also corresponded to the peak equatorial SO2 gas density observed between 100 and 300W longitude, while for the Ly-a observations the peak gas density was observed just east of the ﬂat region at 130W longitude. In all cases, the SO2 gas density was observed to decrease to the east of 130W and to the west of 190W longitude (cf. Tsang et al., 2013; Feaga et al., 2009); and in each case, the relative increase between the peak gas density and the gas density detected near 250W longitude is 60%. Our current observations show these same trends, i.e. there is a 60% increase in the SO2 gas density between 250W and 190W, and given the uncertainty in the gas density ﬁt at 140W longitude, the inferred 140W zero-latitude frost temperatures and gas density is equivalent to the values detected at 190 and 170W longitude (see Figs. 12 and 13). The consistency in the longitudinal variability of the atmosphere between these observations indicates that the observed pattern in the longitudinal variability has remained stable over more than a decade.

6.2. Zero-latitude SO2 gas density correlations with time of day As Table 2 and Fig. 11a show, the CML200W and CML250W spatial bins overlap in longitude between 200 and 250W, and the separation in local time for the overlapping observations is 3 Io hours (corresponding to 45° of Io rotation relative to the Sun). In this overlapping region, the inferred zero-latitude SO2 gas densities (Fig. 14A) at a given longitude are roughly constant with local time. We can interpolate linearly between our bins to obtain the gas density at speciﬁc longitudes at each of the two local times (Fig. 14B). In this case, due to the geometry of the observations, at each of the longitudes detected during both the CML200W and CML250W observations, the local Io time is always earlier in the CML200W observations than for the CML250W dataset. Surprisingly, there is a modest decrease in the inferred zero-latitude gas densities (and corresponding inferred SO2 frost temperatures) as local time increases—suggesting that the gas density and frost temperature decreased between the morning and the afternoon hours. While the implied SO2 frost temperature decrease is small, 0.6 ± 0.1 K, it is physically implausible that the SO2 frost temperature at 10 a.m. is higher than that observed at 1 p.m. local Io time. Thus, this behavior highlights that our zero-order latitude dependence model does not capture all of the physics controlling the SO2 frost temperature at each longitude (see Section 6.4). Independent of these small anomalies, the results imply that the zero-latitude SO2 gas density varies primarily with longitude and not time of day. Thus, we emphasize that our inference of the zero-latitude gas densities at each longitude implies that the sub-solar frost temperature varies with sub-solar longitude, assuming vapor-pressure equilibrium. For example, the gas densities detected at CML 200W and 250W at noon on Io imply that the sub-solar frost temperature at those longitudes differs by 2.4 K (i.e., comparing the frost temperature consistent with bins B2 and D2 in Table 3). At the same time, the fact that the sub-solar frost temperature change is less than 3 K over this 50° difference in longitude indicates that the change in the frost temperature as a function of the change in sub-solar longitude is slow. Additionally, comparison of the inferred zero-latitude gas densities obtained at a single longitude at variable times of day indicates that the dawn-to-dusk frost temperature at zero-latitude varies by less than 1 K (Fig. 14). Thus,

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Fig. 14. The inferred zero-latitude gas density is plotted as a function of time of day in panels (A and B), revealing that the inferred zero-latitude SO2 gas densities fall into two nearly parallel lines when plotted vs. time of day in Io hours. Comparing the gas density inferred from observations made at near identical longitudes indicates that for those observations that were taken at times that are separated by 45° in Sun angle (SZA), the change in the zero-latitude gas density is negligible; and even for a 50° Change in SZA, the difference in the zero-latitude gas density from morning to late afternoon is less than 10%. However, panel (B) shows that when identical longitudes are compared, a decrease in the gas density (corresponding to 0.6 K decrease in frost temperature) from morning to late afternoon is implied. In panel (C) the results derived from the 2011 observations are compared with model results derived by Walker et al. (2012) for the anti-jovian hemisphere assuming that the subsolar-longitude (sslong) is 195W, and the thermal inertia 200 MKS; the Walker et al. model predicts a change in gas density with time of day that is much faster than what is inferred from our observations.

Table 3 Inferred frost temperature per spatial bin. CML 200W

a b c d

CML 250W

Bin #

TODa (h)

Latitude (longitude)

NSO2b

Tf (K)c

DTf_eqd (K)

Bin #

TODa (h)

Latitude (longitude)

NSO2b

Tf (K)c

DTf_eqd (K)

B4 B3 B2 B1 B0

8.7 10.2 11.8 13.4 14.9

22.0 (242.5W) 12.0 (217.5W) 0.0 (192.5W) 12.0 (167.5W) 21.0 (142.5W)

6.5 ± 1.9 14.0 ± 1.0 21.0 ± 2.0 22.5 ± 1.5 13.6 ± 4.4

114.64 116.92 118.16 118.37 116.83

3.5 1.2 0 0.2 1.3

D4 D3 D2 D1 D0

8.6 10.2 11.8 13.4 15.0

19.0 (297.5W) 11.0 (272.5W) 0.0 (247.5W) 12.0 (222.5W) 21.5 (197.5W)

2.9 ± 0.5 6.3 ± 1.1 9.8 ± 1.2 11.5 ± 0.5 8.7 ± 0.9

112.33 114.54 115.84 116.32 115.50

3.5 1.3 0 0.5 0.3

Local time on Io in hours. Fit SO2 gas column density. Inferred SO2 frost temperature, assuming vapor pressure equilibrium. Difference between the frost temperature at equator and the latitude of the spatial bin, Tf(Bi) Tf(B2) or Tf(Di) Tf(D2), where i ranges from 0 to 4.

plots of the inferred zero-latitude SO2 gas density vs. longitude (Fig. 12) show no evidence of the decrease of the SO2 gas density towards the terminators that is apparent in Fig. 11 based on the ﬁt of the SO2 gas densities at the observed latitude and longitude. The insensitivity of the zero-latitude gas density to local time is consistent with the dawn-to-dusk absorption recorded in previous Lyman-a observations of Io (cf. Feaga et al., 2009; Feldman et al., 2000). However, these results do not corroborate the simulations published by Walker et al. (2012) which predict a more rapid fall off in the equatorial SO2 gas density from the afternoon peak to the morning terminator (Fig. 14C), when the atmosphere is supported by SO2 frost with albedo P0.55, and thermal inertia <200 J m2 K1 s1/2 (hereafter, MKS). Walker et al. was best able to ﬁt the 2001 HST/STIS data using a model where the TI was equal to 300 MKS and the SO2 frost albedo was equal to 0.5, and even with this high a thermal inertia and low frost albedo, dawn and dusk column densities are underestimated by their model. In fact,

even a thermal inertia of 600 MKS results in an equatorial diurnal temperature variation of 7 K, and extremely high thermal inertias of 2000 MKS (the thermal inertia of solid water ice) still produce diurnal amplitudes of 2 K, higher than we infer from our data. Whether such high thermal inertias are plausible is unclear, and it is possible that other physical processes, not yet included in our models, will need to be invoked to explain the lack of diurnal variability. The 0.55 minimum SO2 frost albedo adopted by Walker et al. (2012) was based on (i) the correlation of SO2 rich regions with high bolometric albedo values, based on the analysis of Galileo NIMS and SSI data, completed by Douté et al. (2001) and Simonelli et al. (2001), respectively, and (ii) the assumption that any variance in the observed average surface bolometric albedo (which is a function of both SO2 and non-SO2 components) is solely a function of changes in the relative fractional coverage of the SO2 and non-SO2 frost components. However, in Io’s low latitude

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Fig. 15. Io’s SO2 frost abundance increases with latitude away from the equator. In order to show an example of the richest SO2 frost regions detectable at low latitudes, we show Io’s average surface bolometric albedo derived by Simonelli et al. from Galileo SSI data vs. the SO2 frost distribution derived from the NIMS data by Douté et al., for longitudes extending from 120 to 230W, at 30N and 30S latitude. Bond albedo values ranging from 0.4 to 0.75 are observed in regions where the SO2 frost percentage is high (>60%). Variability in bolometric albedo value independent of change in SO2 frost percentage is evident, and may be due to changes in the composition (thus spectral properties) of the non-frost surface components, and/or changes in the SO2 frost grain size/shape. In regions where the highest SO2 frost abundance was observed the bond albedo is 0.75. But this value is also obtained if the SO2 frost abundance is 14% lower.

region there are multiple locations that are rich in SO2 with lower albedo values (ranging from 0.52 to 0.4); additionally, as Fig. 15 shows there are several places where the SO2 frost abundance is high and yet the observed average bond albedo of the surface varies with no change in the SO2 frost abundance. If the argument holds that regions richest in SO2 frost are composed of nearly pure SO2 frost, the plots shown in Fig. 15 would suggest that it is possible to have a pure/near-pure SO2 frost region with a bolometric albedo <0.55. In reality, for those cases where the bolometric albedo changed independent of any change in the percentage of SO2 frost surface coverage, it is equally plausible that a change in the composition of the non-SO2 frost (leading to different spectral characteristics) component, or a change in the size and shape of the SO2 frost grains (leading to different reﬂective properties, cf. Grundy et al., 2000), or some combination of the two is responsible for the observed change in the average surface bond albedo. Thus, models that include SO2 frost albedo values within the 0.45–0.55 range should not be excluded carte blanche from investigations that focus on Io’s low-latitude gas density and frost behavior, particularly for spatially resolved observations that may center directly over regions rich in SO2 frost, but have variable bolometric albedo values. 6.3. Arguments for a two-component sublimation dominated atmosphere As pointed out above, if any SO2 gas was released from the thermal sources near or intersecting the CML 200W or CML 250W observations, it did not result in a signiﬁcant enhancement in gas density above the regional trend, except for a possible small enhancement near Culann. Additionally, none of the plumes that were in the neighborhood of the CML 250W observations are known to be persistently active; thus, it is not likely that any of those eruption centers contributed to the observed/inferred SO2 gas densities. Similar insights regarding the impact of Io’s known thermal sources on Io’s zero-latitude gas density behavior as a function of longitude can be derived from the 2001 STIS observations obtained at CML 150W (Jessup et al., 2004). Thus, in Fig. 13A we use the same vapor pressure equilibrium cosine1/4 latitude frost temperature dependence model to derive the zerolatitude gas density at the central longitude of each spatial bin

deﬁned for the 2001 observations obtained at low-latitude. Though the FOV of the 2001 observations encountered multiple hotspots and known plume centers in the region ±25° of the equator (Fig. 1B), we ﬁnd that the inferred 2001 zero-latitude SO2 gas densities, which cover the longitude range 140–190W, are equivalent within the uncertainty of the ﬁt, independent of what terrain was observed. The zero-latitude SO2 density inferred from the bin containing the Prometheus plume is signiﬁcantly higher than that associated with the other plumes observed in 2001; additionally, only in the Prometheus plume bin does the inferred zero-latitude gas density range extend signiﬁcantly higher than that associated with the detections made above the thermally inactive regions. This would suggest that in 2001 only the Prometheus plume was releasing enough gas to support an enhancement in the gas density above the background sublimated gas density level evident at low-latitudes. Based on these two datasets we conclude that the volcanic contribution is either equal to or less than the sublimation component at most longitudes. Comparison of the 2001 zero-latitude SO2 gas densities detected in those regions where no thermal activity or active plume sources have been observed (green circles, Fig. 13) to the zero-latitude SO2 gas density values inferred from the 2011 data at similar longitudes, indicates that there is a consistent factor of 2 increase in the gas densities inferred from the 2011 data over that inferred from the 2001 observations (Fig. 13). I.e., we can compute the gas density expected in 2011 independent of volcanic sources at each longitude by the linear correlation between the gas densities observed in 2011 (olive line, Fig. 13). Based on these values, the ratio of the gas density predicted for 2011 to that observed in 2001 in the non-volcanic active regions is always in the range 1.7–2.6. For example, if we assume that the gas density detected near 167W in 2011 is enhanced by Culann plume activity, the ratio of the gas density expected in 2011 at 167W to that observed at 167W in 2001 ranges from 1.3 to 1.9 (if the 2001 error bars are included). Likewise, the ratio of the gas density expected at 147W longitude in 2011 to that observed in 2001 is in the range of 1.7–2.8. And similar values are found if the gas density observed at 147W in 2001 is compared to either gas density predicted for 147W or observed at 142W in 2011. This behavior can be readily explained by seasonal sublimation effects, driven by changes in Io’s heliocentric distance (Rs) between the two years of

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Nss_2001 =2.3 Nss_2011

Nss_2001 =2.0 Nss_2011

Nss_2001 =2.1 Nss_2011

Nss_2001 =1.6 Nss_2011

Fig. 16. Best chi-square surface model ﬁts to mid-IR disk-integrated observations of Io from Tsang et al. (2012) are shown. The best ﬁt surface models assume bond albedo values in the range of 0.42–0.51 (coincident with the 0.52 ± 0.09 value reported by Simonelli); and thermal inertia (TI) values in the range of 400–1250, where the highest TI values approach the values expected for non-porous SO2 ice. The ﬁts to the mid-IR data require both a volcanic + sublimation component (red dash-dot). The gas density produced solely by the sublimation component is shown as a dotted red line. We use dotted green lines to emphasize that the ratio in the sublimated sub-solar gas density component (Nss) between late 2001 and late 2011 is always a factor of 2. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

observations. In particular, the CML 150W observations obtained in 2001 and the CML200W and CML250W 2011 observations were all taken in the later part of the year of observation; for these time periods the heliocentric distance (Rs) of Io was equal to 5.16 and 4.96 in late 2001 and late 2011, respectively. Tsang et al. (2012) produced multiple models of Io’s expected subsolar SO2 gas column density as a function of heliocentric distance, for a range of SO2 surface albedos, and thermal inertias. In every case, independent of the assumed albedo or thermal inertia rate, the ratio of the gas density expected in late 2011 to late 2001 based on the known heliocentric distances was in the range of 1.6–2.3 (Fig. 16), consistent with the observationally derived gas density ratios. At the same time, the Tsang et al. models that predict a 2011/ 2001 ratio of the SO2 gas density in the range of 1.6–2.3, require that an additional volcanic SO2 gas density component, corresponding to 5–30% of the total expected subsolar gas density is included in the atmospheric model. So, we see that whether Io is observed at the NUV or mid-IR wavelengths, the observed SO2 gas densities are best reproduced by an atmosphere that may have some volcanic contribution, but is sublimation dominated. Notably, Tsang et al. found that the best ﬁts to their mid-IR observations of Io’s seasonal atmospheric variation assumed an SO2 frost bond albedo of 0.42–0.51 (consistent with the 0.52 ± 0.09 global average surface bond albedo reported by Simonelli et al. (2001), which is a function of both SO2 frost and other surface components), and thermal inertia equal to 400– 1250 MKS, where the higher bond albedo values correspond to

thermal inertias at the lower end of this range, and vice versa. Since, as discussed above, it is possible that SO2 frost on Io may have bolometric albedos that vary spatially and are as low as 0.4 in some locations, each of Tsang et al. ﬁts are plausible. Interestingly, for our low-latitude observations, the regions richest in SO2 frost have an SO2 frost abundance 47–48% (based on averaging over the 25° longitude span of our spatial bins), and in these regions the corresponding average bolometric albedo ranges from 0.48 to 0.56. If we assume that any pure SO2 frost found in these regions has the same range of bond albedo values then the Tsang et al. (2012) models of the seasonal sublimation behavior that include a volcanic component 20–30% of the sublimation component are favored (Fig. 16). 6.4. Implied frost temperature variance with latitude As discussed above, we ﬁnd that the gas densities detected in 2001 are well replicated by assuming Io’s SO2 frost temperature displays a cosine1/4(latitude) dependence, and have assumed that same relationship in the analysis of the 2011 data. It is also interesting to consider how the observed gas densities (and corresponding frost temperatures) compare to other models designed to predict the average surface temperature as a function of latitude based on the average albedo of the surface. For example, comparison of the gas densities detected at each latitude per observed CML indicates that the frost temperature decreased by 3.5 K between 0 and 22N latitude independent of which CML was observed (Table 3). Models of Io’s latitudinal frost temperature

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NOONTIME SURFACE TEMPERATURE (K)

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Fig. 17. On the left we insert Fig. 6 from Simonelli et al. (2001) which shows the noon-time average surface temperatures predicted by Simonelli et al. based on assuming Io’s bolometric bond albedo is either constant with latitude at a value of 0.52 (ﬁlled circles), or that it varies rapidly with latitude as inferred from Voyager visible-wavelength photometry (open circles). On the right we plot the SO2 frost temperatures inferred from the 2011 CML200W (top-right) and CML250W (bottom-right) HST observations of Io’s low-latitude SO2 gas density as a function of latitude (with time of day entangled; note however, if our models are correct, the entanglement of the time of day would produce less than 1 K difference in the frost temperatures expected at local noon vs. that inferred at the time of day at which each latitude was observed by HST). Although the absolute value of the frost temperatures inferred from the Simonelli et al. model and the HST observations are different, it is clear that the variance in the SO2 frost temperatures inferred from the HST data looks more like what is expected for minimal change in the bond albedo with latitude, than what is expected for bond albedo that peaks at the equator and then varies rapidly with latitude. The fact that the SO2 frost temperatures that are inferred from the SO2 gas densities detected by HST are cooler than the temperatures predicted by the Simonelli model for the average surface behavior highlights that the average surface temperature is predicted as function of the assumed average surface albedo and average surface conductive properties, thus the thermal inertia of the SO2 frost and non-frost surface components are convolved together, while the inferred SO2 frost temperatures are only a function of the albedo and thermal inertia of the SO2 frost.

variance that assume, based on Voyager visible-wavelength photometry, Io’s bond albedo varies with latitude and is strongly peaked at the equator predict an increase in the surface temperature between 0° and 30° latitude, before declining at higher latitudes (Fig. 17). Whereas models produced assuming the average bond albedo value is equal to a single value (0.52) at all latitudes (similar to the albedo variance inferred from the Galileo observations) predict the frost temperature should drop by 5–6 K between 0° and 30° latitude, and by 3 K between 0° and 20° latitude (Simonelli et al., 2001). Thus, it is the latter rather than the former trend that is mimicked by the SO2 frost temperatures inferred from the HST/STIS observations obtained at northern low-latitudes. Notably, SO2 frost temperatures listed in Table 3 are dependent only on the SO2 frost properties, such as albedo and thermal inertia, while the magnitude of the predicted noon-time surface temperatures presented in Fig. 17 is dependent on both the assumed bond albedo and the conductive properties of the surface (where the conductive properties of the SO2 frost and non-SO2 frost are convolved, but assumed to be constant) (see Simonelli and Veverka, 1988). Thus, the similarity in the predicted rate of change of the average surface temperatures assuming limited variance in the average surface albedo and the observationally inferred SO2 frost temperature changes suggests that if the frosts temperatures can be replicated from a model that varies primarily as a function

of the change in the albedo of the conducting medium then, like the average surface albedo, the SO2 frost albedo does not vary signiﬁcantly with latitude at low-latitude on Io’s northern hemisphere. On the other hand, the SO2 detections obtained at southern longitudes in 2011 (Table 3) indicate that the rate at which the frost temperature changed with latitude is signiﬁcantly slower, corresponding to <2 K between 0 and 22S latitude. In fact, the frost temperatures appear to increase slightly between 0 and 12S, before decreasing between 12 and 22S latitude. Additionally the inferred change in frost temperature between the equator and the southern most observed latitude is not equivalent between the two CMLs. These differences may be an indication of differences in frost properties between the two hemispheres. For example, differences in the macroscopic roughness purity, or grain size of the SO2 frost between the northern and southern latitudes and even as function of longitude in the south, could lead to differences in the surface frost albedo, which in turn could lead to differences in the corresponding SO2 frost temperatures. In fact, as shown in Fig. 18, analysis of the Galileo data (Simonelli et al., 2001) indicates that the average bolometric albedo (and thus average surface temperature) values at northern and southern latitudes differ. Thus, qualitatively, it is plausible that the SO2 frost albedo values differ in the north and the south and these differences may be responsible for

Fig. 18. Plots of the bolometric albedo derived from the Galileo SSI data by Simonelli et al., for select longitude regions. It is quite apparent that the average surface bolometric albedo at low-latitudes is variant relative to north and south of the equator.

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the differences in the rate at which the frost temperatures inferred from the SO2 gas detections made at low-latitudes on the northern and southern hemispheres change as the distance from the equator increases. (Of course, quantitative deﬁnition of the value of the bond albedo of the individual SO2 and non-SO2 frost components based on the average surface bond albedo cannot be made without further analysis.) In spite of the inferred differences in the rate of frost temperature variance on the northern and southern hemispheres, similar to north, the variance in the inferred southern latitude SO2 frost temperatures does not show a smooth increase in the frost temperature between 0° and 20° latitude. Thus, the inferred southern low-latitude SO2 frost temperature behavior is also inconsistent with the low-latitude temperature variance predicted by models that assume that the average surface bond albedo is strongly peaked at the equator and varies rapidly with latitude, so again, it is not likely that the southern low-latitude SO2 frost albedo varies in this way either. Although Galileo observed differences in Io’s average surface bond albedo at low latitudes on the northern and southern hemispheres, which may point to variances in the macroscopic roughness, purity, or grain size of the SO2 frost and/or the composition of the non-SO2 frost at these latitudes, these variances do not appear to be signiﬁcant enough to offset the monotonic increase in gas density (frost temperature) with longitude that is revealed in the 2010–2012 data by assuming the frost temperatures vary with a cosine1/4 latitude dependence. On the other hand, inclusion of the potential differences in the SO2 frost albedo (and potentially in the macroscopic roughness of the SO2 frost) as a function of latitude on the northern and southern hemispheres in a detailed model of the surface frost temperature behavior may eliminate/reverse the slight decrease in gas density (frost temperature) at a single longitude with time of day that is depicted in Fig. 14 based on our zeroorder vapor pressure equilibrium model.

6.5. Frost properties derived from multi-spectral atmospheric and surface reﬂectance observations Given the consistency between the low-latitude SO2 gas density longitudinal variance inferred from the spatially resolved near-UV observations and the mid-IR and Ly-a observations (Figs. 13 and 19), it is logical to expect that the same physical processes/properties that support the gas density distributions derived from the NUV data are applicable to the gas density behavior derived from observations obtained at other wavelengths. Assuming that the atmosphere is sublimation supported, all of the Ly-a and NUV observations require high (300 MKS or greater) frost thermal inertia values to replicate the substantial SO2 gas density detected along the terminators in those observations (cf. Strobel and Wolven, 2001; Feaga et al., 2009; Walker et al., 2012). Likewise, as discussed above the seasonal variability indicated by both the NUV and mid-IR observations is best replicated by thermal inertial values greater than 300 MKS, for average SO2 bond albedo values 0.5. Thus, if it is assumed that Io’s atmosphere is sublimation dominated, deﬁnitive agreement exists in the thermal inertia ranges inferred from a large subset of Io atmosphere observations obtained within the last twenty years, independent of the wavelength of the observation. Intriguingly, the high (300 MKS or greater) frost thermal inertia range needed to replicate the low-latitude gas density ranges inferred from mid-IR (Tsang et al., 2012), FUV (Feaga et al., 2009) and NUV (Jessup et al., 2004; Tsang et al., 2013) observations (cf. Tsang et al., 2012; Walker et al., 2012), also suggests that Io’s low-latitude frosts may be partially annealed or coarse-grained SO2 ‘‘ice’’ (Walker et al., 2012). In this case Io’s surface should be populated by large grain SO2 frost at low-latitudes.

Interestingly, Carlson et al. (1997) reported that absorption within the 3.35 lm weak band was primarily equatorial (i.e. ranging from 20N to 25S) between 150 and 240W longitude with signiﬁcantly shallower absorption depths detected above ±30° latitude. Because the 3.35 lm band is a weak band that requires absorption along a long path, frost grain sizes 200–500 lm were inferred. Likewise, analysis of the (0.7–5.23 lm) NIMS spectra by Douté et al indicated that SO2 frost grains of 300–500 lm are located prominently on Io at low-latitudes between 120 and 270W longitude, and that the 3.35 lm band becomes less prominent with increasing latitude. Using the relative depth of the 2.79 and 3.78 lm SO2 absorption signatures in the NIMS spectra as a function of latitude as additional constraints, Douté et al. concluded that it is likely Io’s coarse grained SO2 frost component is of spatially variable thickness; i.e., that optically thick 200– 500 lm frost is located at low-latitude while optically thinner 200–500 lm frosts can be found within the remaining, and much larger percentage of the surface that extends from medium to high latitudes. At the same time, Simonelli et al. (2001) showed that scattering behavior of Io’s low-latitude equatorial band is uniquely and unusually phase dependent (brightening at low phase). This result is also consistent with the properties of the Io’s low-latitude SO2 frost being distinct from that observed at higher latitudes. Since the scattering behavior of the SO2 frost is highly sensitive to grain shape/size (Douté, 1998; Grundy et al., 2000), it is plausible that the latitudinal gradation in the optical thickness and abundance of the large size SO2 frost grains contributes to and/or sustains the unique photometric properties of Io’s low-latitude SO2 frost. While it is not clear if it is the thermal inertia of the low-latitude 200–500 lm SO2 frost that supports the gas density behavior observed at low-latitudes, it is interesting to note that optically thick patches of large grain SO2 frost are found at low latitudes on Io, and that the latitudinal extent of the optically thick large grain SO2 frost coverage coincides with the latitudinal extent of Io’s dense atmosphere. 6.6. Comparisons of the SO2 gas density variability with SO2 frost properties One caveat yet to be well understood for the sublimation component of the atmosphere is what causes the atmospheric density, and thus the low-latitude, sub-solar frost temperature assuming sublimation support, to decrease with increasing west longitude in the 150–300W longitude range. If the inferred zero-latitude gas densities are truly sublimation supported then it is likely that the gas densities are correlated with one or more properties of the local SO2 frost, such as particle size or frost purity (see Appendix A), both of which can impact the temperature of the frost. Over 20 years of data exists documenting the longitudinal distribution of Io’s SO2 gas density based on disk integrated observations of Io (cf. Ballester et al., 1994; Trafton et al., 1996; Spencer et al., 2005; Tsang et al., 2013). Strong correlations between the gas distributions inferred from those observations and the hemispherically averaged NUV brightness, and SO2 frost distributions derived from the disk-resolved NUV brightness, have also been documented based on disk integrated values derived from HST (Spencer et al., 1997) and Voyager (McEwen et al., 1988) observations, respectively (cf. Spencer et al., 2005; Tsang et al., 2012). Since the low-latitude SO2 gas density derived from the spatially resolved 2001 and 2011 HST/STIS observations mimics the behavior derived from previous disk-integrated observations, it is not surprising that those densities also are well correlated with the globally averaged NUV brightness and SO2 frost distributions derived from the NUV brightness (Fig. 19). However, for simple vapor pressure equilibrium with surface frost, gas abundance should depend only on

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Fig. 19. The longitudinal variance of Io’s zero-latitude SO2 gas density as inferred from the 2011 (top panel) and 2001 (bottom panel) observations is compared to the variance of Io’s average low (±20°) latitude NUV brightness (cyan), the disk-integrated NUV brightness variance (dash dot), and the globally averaged SO2 frost abundance derived by McEwen et al. (1988) from Io’s NUV brightness as observed by Voyager (plus marks). The disk-integrated SO2 frost abundance values have been offset by 15 for ease of comparison. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

the temperature of the frost, not on its abundance. Additionally, the correlation between the SO2 gas density and the NUV brightness breaks down when local NUV brightness levels are compared to gas densities (Fig. 19). The fact that the gas density appears to correlate with the disk-integrated NUV brightness, but not with the local brightness, would suggest a physical process that allows the surface properties averaged over a large regional scale to deﬁne the frost temperatures and ultimately the gas density behavior at low latitudes, independent of the local surface properties. It is not clear how such a process would operate.

6.6.1. Correlation of zero latitude gas density with NIMS derived SO2 frost properties To further investigate the possibility of correlations between the SO2 gas and frost properties, rather than relying on the NUV brightness distribution (which does not fully characterize the SO2 frost abundance, see Appendix A), we use the frost property analysis inferred from the multi-wavelength (0.7–5.23 lm) reﬂectance observations of Io obtained by Galileo NIMS between 80 and 360W (Douté et al., 2001). We utilize the results derived from this dataset to investigate the SO2 gas and SO2 frost relationship because Galileo NIMS dataset represents the most extensive spectrally resolved data on Io’s frost reﬂectance spectrum. Additionally, the retrieval methods utilized in the Douté et al. analysis improve over that utilized previously. For a brief summary of how the Douté et al. work compares to previous studies of Io’s SO2 frost properties the reader is referred to Appendix A. In Fig. 20, we compare the longitudinal variation of the SO2 frost properties derived from the Douté et al. (2001) analysis of the Galileo NIMS data and the zero-latitude SO2 gas densities inferred from the 2011 HST/STIS observations of Io. We consider the correlation of the low-latitude gas densities with the disk-integrated averaged SO2 frost abundance, as well as the SO2 frost abundance and grain size averaged over latitudes extending ±20° of the

equator and longitudes extending ±12.5° of the central longitude of each of the spatial bins included in the 2011 HST/STIS data set. As Fig. 20A shows, the longitudinal variance in the diskintegrated SO2 frost abundance mimics the overall trend of increase eastward of 270W longitude towards 220W longitude; however, the decline in the disk-integrated SO2 abundance inferred from the NIMS data eastward of 220W longitude towards 170W longitude is not consistent with the longitudinal variance of the low-latitude SO2 gas density. If the frost properties inferred from the NIMS data analysis are correct, the inconsistency with the disk-integrated frost abundance distribution derived from that dataset further indicates that the SO2 frost abundance at locations far from the low-latitude region is not the primary factor deﬁning/ regulating the sublimated gas densities in Io’s low-latitude regions. In terms of the localized frost properties, Fig. 20A also shows that the low-latitude SO2 frost abundance increases rapidly from 300W towards 250W, and then slowly declines as the longitude decreases from 250 to 150W. This behavior is quite different from the monotonic increase in low-latitude SO2 gas density from 300W to 170W longitude; in general the frost abundance does not correlate well to the inferred zero-latitude gas density (Fig. 20B). On the other hand, low-latitude SO2 grain size increases monotonically towards the east through most of this longitude range, and shows a better correlation with gas density than does frost abundance, though the relationship, if real, is not consistently linear (Fig. 20C and D). 6.6.2. Discussion of spatial correlation of SO2 grain size and SO2 gas density The combination of the results presented in Fig. 20 with the analysis of previous observations suggests that the densest regions of Io’s atmosphere (cf. Feldman et al., 2000; Jessup et al., 2004; Feaga et al., 2009) and the regions of coarse-grained low-latitude frost are spatially correlated in both latitude and longitude. If we assume that the atmosphere is sublimation dominated, then the

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Fig. 20. Comparison of the longitudinal variance of the SO2 frost properties and the zero-latitude SO2 gas densities inferred from the 2011 HST/STIS observations of Io. These plots show the SO2 frost abundance (A: lilac triangle, right axis) and grain size (C: brown triangle, right axis) averaged between ±20° of the equator and ±12.5° of the central longitude of each of the spatial bins observed in the 2011 HST/STIS data set, and the disk integrated SO2 frost abundance between 80 and 270W (A: blue line). In each case the SO2 frost properties are derived based on the Douté et al. (2001) analysis of the Galileo NIMS spectral observations of Io’s surface. In panels (B and D) we show the quality of the correlation between the low-latitude frost properties averaged as a function of the 2011 spatial longitude bins and the zero-latitude gas density inferred from the 2011 observations. In each of these plots the gray line represents the best possible linear correlation between the points. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

positive correlation of atmospheric abundance with grain size shown in Fig. 20C suggests that higher frost temperatures are found in regions that have higher grain size. If this is the case then, longitudinal variations in frost properties may explain the longitudinal variations in atmospheric abundance, or vice versa. Notably, the SO2 frost grain size impacts several of the key properties that directly affect the frost temperature, such as thermal inertia, macroscopic surface roughness, and the bolometric albedo. If we are assuming that the atmosphere is sublimation dominated then it is important to deﬁne and compare what the SO2 frost albedo and grain size variation should be in order to adequately support the observed atmospheric density variance and

Fig. 21. Plot of the longitudinal variance of the SO2 frost grain size and the average surface bolometric albedo derived from the Galileo NIMS (Douté et al., 2001) and SSI (Simonelli et al.) observations of Io. These values are averaged between ±20° of the equator and ±12.5° of the central longitude of each of the spatial bins observed in the 2011 HST/STIS data set.

the observed average surface properties. In Fig. 21 we compare the variations in SO2 grain size and average surface bolometric albedo (which is a function of both the SO2 and non-SO2 frost components) at low latitudes from 140 to 300W longitude. On the anti-jovian hemisphere between 140 and 220W longitude the average surface bolometric albedo and the SO2 frost grain size are anti-correlated, but, the SO2 frost abundance is fairly constant, corresponding to 45 ± 3% (Fig. 20A). Thus, the observed variation in the average surface bond albedo should be due primarily to variations in the magnitude of the albedo of the surface components (SO2 and/or non-SO2 frost), rather than the fractional coverage of these components. If the overall change in average bolometric albedo in the 140–220W range is dominated by changes in the SO2 frost albedo, then it is implied that the SO2 frost albedo is increasing as the grain size is decreasing, and vice versa. In this case, the inferred variance in the SO2 frost albedo could lead to changes in the SO2 frost temperature that are consistent with the zero-latitudes gas density behavior inferred for those longitudes (cf. Fig. 20A, Table 2). In particular, in this region the minimum albedo (corresponding to the maximum absorbed sunlight, leading to the highest frost temperatures) and the maximum gas density are observed in spatial bin B1. Additionally, there is a relative increase in bolometric albedo (implying a decrease in frost temperatures) within the spatial bins located east and west of bin B1 between 120 and 220W longitude, which would be consistent with the relative decrease in gas density also observed above those spatial bins. However, from longitude 220W to 300W, the mean bolometric albedo, SO2 frost grain size and SO2 gas abundance decrease together (Fig. 21). If we are assuming that the relationship between the grain size and the SO2 frost albedo are the key factors impacting the SO2 frost temperature (and sublimated gas density),

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Fig. 22. In the top panels we plot the ﬁt SO2 gas density vs. relative distance from 190W longitude (top-left) and local Io time (top-right). In the bottom panels we plot the continuum brightness levels vs. relative distance from 190W longitude (bottom-left) and local Io time (bottom-right). In every case the CML200W and CML250W results are plotted in black and green, respectively. The SO2 gas density shows no clear relationship to relative distance from 190W (outside of time of day effects) (top-left), while the emission brightness is strongly dependent on relative distance from 190W independent of the local time (bottom-left). Additionally, while the SO2 gas density decreases towards evening terminator for both datasets (top-right), for the CML 250W (green) observations the continuum emission continues to increase independent of the local time (bottom-right). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

then to replicate the observed SO2 gas density variance with longitude, it is necessary to assume that the SO2 frost albedo is anti-correlated with the observed decrease in SO2 frost grain size in this longitude range; thus, SO2 frost albedo would be increasing westwards. In this case, the observed decrease in the average surface albedo would have to be maintained by a decrease in the albedo of the non-SO2 surface components (perhaps via a compositional change) that occurs at a rate higher than the (expected) increase in the albedo of the SO2-frost component with increasing west longitude (presuming anti-correlation with the SO2 frost grain size). Notably, the SO2 frost abundance is also observed to decrease signiﬁcantly over this longitude range (Fig. 20A), thus while SO2 frost abundance has no direct impact on SO2 frost temperature (and resulting sublimated SO2 gas density), the lower percentage of the SO2 frost (implying a higher percentage of nonSO2 frost components) may allow the presumed decrease in the albedo of the non-SO2 frost component to lead to an overall decrease in the average surface albedo values. Alternatively, the westward decrease in SO2 frost temperature inferred from the westward decrease in gas abundance might be due to a westward increase in frost thermal inertia. However, it seems unlikely that increased thermal inertia would correlate with the observed westward decrease in frost grain size since smaller ice grain sizes are expected to result in lower, not higher, thermal inertias. Therefore, we conclude that the most likely way to produce the observed correlation is if the SO2 grain size and SO2 frost albedo are anti-correlated. To verify these scenarios, a more detailed (and quantitative) look into the relationship between the SO2 grain size, SO2 frost albedo and SO2 frost temperatures is needed. If the scenarios described above are conﬁrmed, then the two-component, sublimation dominated nature of Io’s atmosphere would continue to be viable, and would be consistent with that fact that Io’s seasonal variation (Tsang et al., 2012), and longitudinal asymmetry behavior (Tsang et al., 2012; Walker et al., 2012; Feaga et al., 2009; Jessup et al., 2004) are also best explained by a two-component, sublimation dominated atmosphere. This type of study is beyond the scope of this paper, nevertheless, the observations presented here provide the parameters

needed to complete that work. Keeping in mind the implied correlation of the SO2 gas density with SO2 grain size, and the potential for the albedo/reﬂective properties to change as a function of grain size, we suggest that new surface models be developed that address these key new questions (i) how does the grain size/shape (and associated surface roughness) of the SO2 frost have to change to support the albedo and thermal inertia values inferred from current and previous observations of Io’s photometric surface properties and SO2 gas density as function of longitude, and (ii) is the implied variance in grain size/surface roughness physical, i.e. can it map to the NIMS derived results. 6.7. SO2 continuum emission correlations with longitude Finally, we summarize the physical implications of the detected dayside SO2 continuum emission. Fig. 9 shows that the 0.28 lm continuum emission brightness is positively correlated with the SO2 gas density, and that it varies with longitude. However, when we look at the relationship between the SO2 gas density, the observed longitude and the detected continuum brightness levels in greater detail other trends become apparent. For example, for the CML200W observations the longitudinal variance in the 0.28 lm emission brightness is symmetrical about a peak near 190W longitude (Fig. 22, bottom-left); neither the ﬁt SO2 gas densities (top-left Fig. 22) nor the inferred zero-latitude SO2 gas densities (Fig. 13) show this type of longitudinal variance. Fig. 22 additionally emphasizes that while the detected SO2 gas density decreases in the afternoon hours for both the CML200W and CML250W observations (top-right panel), the CML250W emission brightness increases steadily in the afternoon hours (bottom-right panel) independent of local time. When the CML250W and CML200W observations are plotted simultaneously as a function of the relative distance from 190W longitude (Fig. 22, bottom-left), it becomes clear that all of the data show the same trend, that is the emission brightness increases as the relative distance from 190W longitude decreases, independent of which CML was observed. Noting that the Io’s anti-jovian magnetic connection point is at 180 ± 5°, the observed longitudinal variance suggests that the

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0.28 lm emission brightness is strongly related to relative distance from the longitude of the magnetic connection point. The sensitivity of the emission brightness to its proximity to the anti-jovian magnetic connection point is not surprising given that, in addition to gas density, electron density and electron temperature are the main factors controlling the emission brightness (Geissler et al., 1999; Saur et al., 2000), and both of these latter factors are directly related to the proximity to the magnetic connection point. E.g., the electron density from the torus is highest at the magnetic connection points, and the electron temperature is hottest at the anti-jovian magnetic connection point (Saur et al., 2000) which while stable in longitude wobbles ±10° about Io’s equator (Dessler, 1983). At the same time, the comparison of the emission brightness derived from both the CML200W and the CML250W datasets (Fig. 22) indicates that while the peak brightness was obtained near 190W longitude for both datasets, the magnitude of the emission was higher for the CML200W observation. Notably, previous observations of Io’s near-UV and visible aurora obtained by both Galileo and Cassini indicate that the location and relative brightness of Io’s auroral emissions follows the location of the magnetic connection point in both latitude and longitude (Geissler et al., 1999, 2004a, 2004b). Although the relative distance from the anti-jovian magnetic connection point was the same for the CML200W and CML250W observations in longitude, the relative distance of the two observations from the magnetic latitude was not. For the CML200W observations, the magnetic latitude was located at 8N and 8S depending on the date of observation. Since the data that was obtained closest to 190W longitude during these observations, was obtained at 195W, 0N latitude, for this dataset the observations taken closest to anti-jovian connection point were always obtained within 10° of the latitude of the magnetic connection point. For the CML250W observations, the observations taken closest to the longitude of the anti-jovian magnetic connection point were obtained at 22S latitude. However, on these dates the latitude of the magnetic connection point fell in the range of 3S ± 1.5 on every day except the ﬁrst day, at which time the magnetic latitude was equal to 9.5S. Thus, on average the auroral emission detected closest to the anti-jovian magnetic connection point during the CML250W observations were obtained 19° south of the latitude of the magnetic connection point. Thus it is likely that the increased brightness at 190W for the CML200W observation occurred because the spatial separation between those observations and the anti-jovian magnetic connection point was lower for those observations than for the CML250W observation. Interestingly, for the CML200W dataset the observation taken closest to the anti-jovian magnetic longitude was obtained at local noon, whereas for the CML250W dataset the comparable observation was obtained near 15 h local Io time. Given that Io’s electron population is also supplied by solar photo-ionization (Summers and Strobel, 1996; Saur et al., 2000), there also would have been a relative increase in electron population from photoelectrons during the CML200W observations. Although in the absence of electron acceleration the impact of photoelectrons on Io’s FUV auroral signature is expected to be small (cf. Bhardwaj and Michael, 1999), these observations suggest that a study of the contribution of the photoelectrons to the production of the emissions responsible for Io’s NUV auroral signature as a function of local time may be warranted.

7. Summary and conclusion We used the HST/STIS 5200 long 0.100 wide slit to obtain medium spectral resolution limb-to-limb 2100–3100 A spectra of Io’s low-latitude region (<30° latitude) in the time period extending

from late 2010 to early 2012 (Table 1). These observations were centered at local noon at CMLs 200 and 250W over regions that were NUV bright and dark, respectively. From these data we derived the variation of the SO2 gas density and temperature, SO gas density, and the magnitude of the SO2 continuum emission at low latitudes from the morning to the evening terminators. Though the data were accumulated over an extended period of time, the heliocentric distance for each of the observations was nearly identical (Table 1), thus interpretation of the SO2 gas density behavior independent of any confusion regarding seasonal variability in the sublimated SO2 gas density component is possible. Our analysis of the CML200W and CML250W data led us to the following conclusions: SO2 gas densities ranging from 0.3 to 2.2 1017 cm2 were detected, at gas temperatures ranging from 100 to 200 K. The highest SO2 gas density was detected above a region centered at (12S, 170W) that also included the Culann plume center. However, gas densities only 5% lower, were detected in the region just west and north of Culann at (0N, 200W) above a region lacking any active volcanic sources, clarifying that SO2 gas densities 2 1017 cm2 can be obtained via sublimation independent of any discrete volcanic contribution. Because the observations were obtained close to perihelion, and the SO2 gas density is known to be highest on the anti-jovian hemisphere (cf. Jessup et al., 2004; Feaga et al., 2009; Tsang et al., 2012), the SO2 gas densities retrieved from these observations likely represent the maximum sub-solar sublimated SO2 gas densities observable on Io. The SO2 gas density on Io is variable as a function of latitude and longitude. If we assume that the latitude dependence of atmospheric density follows the same simple vapor-pressure equilibrium model inferred from our analysis of the low latitude 2001 HST/STIS observations, then we can infer the zero-latitude atmospheric density at each longitude observed in our 2011 HST/STIS observations. The inferred zero-latitude gas densities depend strongly on longitude. Comparison of the zero-latitude gas densities derived from observations taken at the same longitude but different local times show limited (<3 1016 cm2) variation with time of day. If the atmosphere is in vapor pressure equilibrium with surface frost, the constancy of the SO2 gas density with time of day implies a very small (<2 K) variation of the low-latitude SO2 frost temperature with time of day which would imply high frost thermal inertia (P300 MKS) (cf. Walker et al., 2012). The longitudinal variation of the zero-latitude SO2 gas density inferred from the 2011 HST/STIS observations is consistent with that derived previously from Ly-a (Feaga et al., 2009), mid-IR (Spencer et al., 2005; Tsang et al., 2012) and near-UV (Tsang et al., 2013) observations of Io’s atmosphere; conﬁrming that the longitudinal variability of Io’s atmosphere has remained stable over more than a decade. Zero-latitude SO2 gas density inferred from the late 2011/early 2012 observations obtained near 167W longitude was 2 higher than that observed in 2001 at the same longitude. This increase is consistent with the expected seasonal increase in the sublimated gas density between the two time periods, based on the decrease in Io’s heliocentric distance from 5.15 AU in late 2001 to 4.96 AU in late 2011 (Tsang et al., 2012). The models produced by Tsang et al. (2012) that best account for the seasonal variance in Io’s sublimated gas density inferred from the 2001 and 2011 observations, utilizing the most plausible bolometric albedo ranges for Io, imply that Io’s atmosphere is sublimation dominated, but includes a volcanic component producing 20–30% of the total SO2 gas density observed on Io’s dayside.

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Replication of the substantial SO2 gas density detected along the terminators in spatially resolved HST/STIS Ly-a and NUV observations (cf. Feaga et al., 2009; Jessup et al., 2004) as well as the seasonal variability indicated by both the NUV and mid-IR observations requires thermal inertial values greater than 300 MKS, for average SO2 bond albedo values 0.5 (Strobel and Wolven, 2001; Feaga et al., 2009; Walker et al., 2012; Tsang et al., 2012). Thus, deﬁnitive agreement exists between a large subset of Io atmosphere observations obtained within the last twenty years regarding the thermal inertia ranges needed to replicate Io’s dayside atmospheric behavior via a sublimation dominated atmosphere, independent of the wavelength of the observation. The required thermal inertia range is consistent with that expected for a partially annealed or coarse-grained SO2 ‘‘ice’’ (Walker et al., 2012). This inference is consistent with the characteristics of the low-latitude SO2 frost derived from observations of Io made at near IR wavelengths by Carlson et al. (1997) and Douté et al. (2001). The zero-latitude SO2 gas longitude dependence parallels the disk integrated surface NUV brightness, but does not mimic the longitudinal variance in the NUV brightness of the surface at the locations of the observations. This suggests that the gas density is not linked to the physical properties that determine the local NUV brightness. The zero-latitude SO2 gas longitude dependence is also different from the longitude dependence of the low-latitude SO2 frost density as derived from the 0.5 to 7.0 lm Galileo NIMS observations by Douté et al. (2001). The zero-latitude SO2 gas longitude dependence most closely mimics the longitudinal variance in the low-latitude SO2 frost grain size derived from the near-IR spectrum (Douté et al., 2001), decreasing as the SO2 grain size decreases. Attempts to understand the gas density-grain size correlation within the constraints of a sublimation dominated atmosphere requires understanding the impact of grain size on the factors that can regulate the frost temperature. To achieve the expected impact on frost temperature would require that the SO2 frost bond albedo increases with increasing westward longitude, and that the SO2 frost bond albedo increases with increasing grain size. These scenarios are possible, but they also require that the non-SO2 frost bond albedo in the 220–300W longitude region is decreasing with increasing westward longitude, for consistency with the average surface bolometric albedo observed by Galileo SSI (Simonelli et al., 2001). The Fraunhofer lines recorded in the 2011 HST/STIS data at wavelengths longward of 2650 Å are shallower than in the solar spectrum. As was true for the 2001 HST/STIS observations (Jessup et al., 2004), the 2650–2950 Å behavior is consistent with the continuum emission signature produced by the ﬂuorescence of the SO2 gas. The 0.28 lm continuum emission brightness levels inferred from the 2011 HST/STIS observations are in the range of <0.004–1.17 kRy/Å, which is comparable to the 0.004–1.5 kRy/Å brightness levels inferred from the 2001 observations. Though the 0.28 lm SO2 continuum emission brightness tends to increase as the ﬁt SO2 gas density increases, a stronger correlation with distance from the magnetic reconnection point near 190W is observed. In particular, peak emission brightness levels were detected near 190W and observed to decrease symmetrically with distance from this longitude. Neither the ﬁt SO2 gas densities nor the inferred zero-latitude SO2 gas densities show this type of longitudinal variance. SO gas was positively detected near the latitude and longitude of the anti-jovian magnetic connection point (or at midday in the equatorial north near 200W) in the spatial bins where the highest SO2 continuum emission brightness levels were

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detected, and just north of the Loki lava lake. For these observations an SO/SO2 ratio 1–5% was detected. Given the location of these observations it is plausible that the primary source of the detected SO gas was electron excitation of SO2 and direct volcanic venting on the anti-jovian and trailing hemispheres, respectively.

Acknowledgments The authors are grateful to Marty Snow of the Laboratory for Atmospheric and Space Physics at University of Colorado, Boulder for providing SOLSTICE data at high spectral (0.33 Å) sampling. This work was funded through Space Telescope Science Institute grants. Appendix A. A brief review of the models used for SO2 frost property retrieval There is a long history of using the 3100–3400 Å NUV brightness as a proxy for the SO2 frost abundance (cf. Spencer et al., 1997). However, the NUV brightness does not uniquely (or precisely) identify the SO2 frost distribution on Io’s surface. I.e., it should not be assumed that whatever is bright in the NUV-VIS (3200–4400 Å) is pure SO2, or that the NUV-bright regions are the only locations of SO2 frost on Io. McEwen (1988) and McEwen et al. (1988) associated the SO2 frost abundance with the high reﬂectivity regions of the NUV-VIS (3200–4400 Å) Voyager mosaics (unit 5 in McEwen, 1988, unit 1 in McEwen et al., 1988); however, McEwen (1988) acknowledged that ‘‘unit 5’’ was probably not pure SO2, stating ‘‘this unit is probably largely SO2, with up to 30% mixing’’ with other materials. Further, Douté et al. (2001) showed that the level of mixing of yellow sulfur (S8) and white SO2 frost needed to replicate the SO2 absorption signatures recorded by Galileo NIMS between 2 and 5 lm, was also able to replicate the high UV (3200–3800 Å) reﬂectance values reported by McEwen et al. above the low-latitude ‘‘unit 5’’ regions. In addition, Lopes-Gautier et al. (2000) showed that regions on Io that appear pink at visible wavelengths can have a high SO2 frost abundance. Likewise, Douté et al. (2001) showed that if there is molecular mixing of frosts (i.e., if volatile sulfurous gases like SO2 and H2S mix in the atmosphere and then condense as molecules on the surface) that high abundances of SO2 frost would be present and detectable at NIR wavelengths in regions that would be dark at NUV wavelengths. Thus, this type of mixing impacts the purity and reﬂectance properties of the SO2 frost, but does not deﬁne the abundance of the SO2 frost. The reﬂectance properties of Io’s surface (at any wavelength) are a function of the SO2 abundance, the optical depth of the frost, the grain size of the frost and the type of mixing (linear or nonlinear, stratiﬁed or not) of each of the surface components. Recognizing this, Douté et al. uses the efﬁcient radiative transfer algorithm developed by Douté and Schmitt (1998) to determine the geographic distribution and the physical characteristics of the SO2 frost on Io. By calculating the spectral bidirectional reﬂectance of a local homogeneous surface unit composed of ices and minerals, with a granular or a compact texture, they determined for arbitrary illumination and viewing conditions, the link between the physical, chemical, structural, and stratiﬁcation properties of the underlying medium with its spectral signature. To complete this work Douté et al. assumes geographic (or linear) mixing of surface components, such that SO2 is areally mixed with thick patches of a non-SO2 component; the non-SO2 components may also be overlain by SO2 or free of SO2. In this model the grain size, fractional coverage, angular scattering property and porosity of the SO2 frost are all considered to be free parameters; likewise the

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Fig. A.1. The disk-integrated SO2 frost abundance derived from NIMS data analysis (solid blue line, Douté et al., 2001) and the Voyager imaging analysis (dash-dot, McEwen et al., 1988) as well as the abundance values inferred from the available 170–270W longitude disk-integrated 4.08 lm ground based near-IR (red asterisks, Howell et al., 1984) and 0.33 lm IUE near-UV (sky-blue asterisks, Nelson et al., 1980) observations assuming intimate (non-linear) mixing of surface components (McEwen et al., 1988) are plotted. The overall trend in the longitudinal variance of the abundance values derived from the 0.33 lm and 4.08 lm observations is outlined in gray. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

reﬂectance of the non-SO2 unit is considered to be a free parameter. In their base model the SO2 component is assumed to be in a single optically thick layer, however, several test cases are also run where the optical thickness of the SO2 frost component is allowed to be variable. Using this model the SO2 frost grain size, optical thickness and abundance are derived based on the best ﬁt of the multi-wavelength reﬂectance signature of Io’s surface. A reliable ﬁt requires that both the continuum reﬂectance level and multiple SO2 absorption bands are ﬁt simultaneously; and, the ﬁt frost parameters are also validated by cross-correlating the bestﬁt model spectra with 0.4–1.0 lm reﬂectance spectra obtained by Galileo SSI. Thus, their analysis improves over the broad spectrum analysis completed by McEwen et al. (1988), which did not include any spectrally resolved SO2 absorption bands. Since different SO2 reﬂectance inversion techniques give different SO2 abundances, for the sake of completeness, we compare and contextualize the disk integrated SO2 frost abundances derived from the NIMS data to previous estimates of the disk-integrated SO2 frost abundance. In Fig. A.1 we show the disk-integrated SO2 abundance derived from NIMS data with the previously published disk-integrated (previously, erroneously referred to as ‘‘global average’’) SO2 abundance derived from the Voyager UV/VIS dataset (McEwen et al., 1988), the ground-based near-IR (4.08 lm) (Howell et al., 1984) and International Ultraviolet Explorer (IUE) NUV (0.33 lm) observations (Nelson et al., 1980), where for the latter two datasets the reﬂectance inversion model used to derive the frost abundances assume intimate (non-linear) mixing of surface components. Because the NIMS data were obtained between 80 and 360W longitude we can only deﬁne the disk-integrated SO2 frost abundance at longitudes extending from 170 to 270W longitude. Summarizing these comparisons, we note ﬁrst that the disk-integrated SO2 abundance range derived from the NIMS and Voyager observations between 170 and 270W is similar, corresponding to 30–40% and 20–40%, respectively. However, as Fig. A.1 shows, direct comparison of the derived abundances at each longitude rarely overlaps in magnitude. The general trend implied by each of the represented observations is that the SO2 frost abundance increases as the longitude decreases between 270 and 230W longitude. However, at longitudes increasingly eastward of 230W longitude, the results derived from the NIMS data and the IUE data show a trend that differs from that inferred from the other datasets. Notably, the frost abundance values derived from the near-infrared NIMS data tend to be higher at most longitudes than the values inferred from either the IUE or Voyager NUV/VIS observations, particularly between 270 and 220W longitude.

Since it is understood that the assumptions made about the mixing of the SO2 frost component with other non-white surface materials in the analysis of the near-IR SO2 absorption signature may lead to an estimate of the frost abundance that is higher than what would be obtained from the analysis of the SO2 UV/VIS reﬂectance properties (cf. McEwen et al., 1988; Douté et al., 2001), the differences between the NIMS and NUV/VIS observations between 270 and 220W are somewhat expected. However, explanation of the discrepancy between the longitudinal variance in the SO2 frost abundance inferred from the NIMS and the 4.08 lm observations at longitudes eastward of 220W, the convergence of the SO2 frost abundance values derived from the NIMS and NUV/VIS observations at 170W, and the fact that the frost abundance derived from the NIMS data falls between the values derived from the 4.08 lm ground-based observations and 0.33 lm NUV observations does not ﬁt neatly into the agreement/disagreement expected for observations made at the given wavelengths. Thus, we conclude that these behaviors highlight the intricate dependence of the inferred frost abundance values on the way in which the frost properties are represented in the reﬂectance inversion models (including, but not limited to the way in which the SO2 and non-SO2 surface components are mixed) (cf. Howell et al., 1984; McEwen et al., 1988; Douté et al., 2001). Given that Douté et al. used the most sophisticated inversion model derived to date in the analysis of the NIMS data, the cross-validation of the NIMS and SSI data, and that the NIMS data set provides the most extensive spectrally resolved data on Io’s frost reﬂectance spectrum, it is likely that this most recent analysis is the most reliable.

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STIS observations of Io’s dayside equatorial atmosphere

STIS observations of Io’s dayside equatorial atmosphere

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