Three decades of Loki Patera observations

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Three decades of Loki Patera observations Imke de Pater a,b,c,∗, Katherine de Kleer a, Ashley G. Davies d, Máté Ádámkovics a a

Astronomy Department, 501 Campbell Hall, University of California, Berkeley, CA 94720, USA Faculty of Aerospace Engineering, Delft University of Technology, NL-2629 HS Delft, The Netherlands c SRON Netherlands Institute for Space Research, 3584 CA Utrecht, The Netherlands d Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA b

a r t i c l e

i n f o

Article history: Received 28 November 2016 Revised 11 March 2017 Accepted 16 March 2017 Available online xxx Keywords: Io Infrared Observations Volcanism

a b s t r a c t We present observations of Io’s Loki Patera taken with the 10-m Keck telescopes between 1998 and 2016. Adding these data to those published by Rathbun and Spencer (2006) and the Gemini data of de Kleer and de Pater (2016a, 2017) results in a database of 3.5–3.8 μm emission from Loki Patera over almost 3 decades. Data presented here contain adaptive optics (AO) observations of Io’s sunlit hemisphere at wavelengths between 1.6 and 5 μm, AO observations of Io in eclipse at 2–5 μm, and non-AO observations of Io in eclipse at 1.6–12 μm. The non-AO data were taken in September of 1999, during the early phase of a brightening event that was documented by Howell et al. (2001). Dual-component Io Flow model (IFM) fits to our 1999 observations show a mostly cool lava crust over almost the entire patera floor, with a relatively small hotter component making up less than 1% of the total area, consistent with previous observations. The 30-year timeline of Loki Patera revealed that, after an apparent cessation of, or change in, brightening events in 2002, Loki Patera became active again in 2009. The more recent activity may have a slightly shorter periodicity than observed by Rathbun et al. (2002), and the direction of flow propagation appears to have reversed. Since 2009 the flow direction is in the clockwise direction, starting in the north or north-east corner and propagating along the patera towards the south-west. During the Galileo era the propagation was in the counter-clockwise direction, starting in the south-west and propagating towards the east. Both the 30-year timeline and the 1.6–12 μm spectrum that was obtained during the brightening event in 1999 agree well with Matson et al.’s (2006) overturning lava lake model, as modified by de Kleer and de Pater (2017). © 2017 Elsevier Inc. All rights reserved.

1. Introduction Io, with its tidally induced volcanism, remains one of the most fascinating bodies in our Solar System. Loki Patera, one of the most consistently prominent and persistently active volcanoes on Io, contributes on average 9% of the total accounted-for thermal emission (Veeder et al., 2012, 2015), and the observed distribution of energy output is rather skewed when Loki Patera is included. The very important question remains about what makes Loki Patera so unique, such that it can contribute so much of Io’s total heat loss. In this paper we focus on three decades of Loki Patera observations in an attempt to better understand the unique properties of this preternaturally prominent volcano. Debates regarding the exact style of Loki Patera’s volcanic activity are still ongoing, with some arguments in favor of a massive

∗ Corresponding author at: Astronomy Department, 501 Campbell Hall, University of California, Berkeley, CA 94720, USA. E-mail address: [email protected] (I. de Pater).

lava lake (e.g., Rathbun et al., 2002; Davies, 2003; de Pater et al., 2004; Matson et al., 2006), and others in favor of resurfacing by lava flows (Davies, 2003; Gregg and Lopes, 2008). Rathbun et al. (2002) demonstrated a 540-day periodicity in Loki Patera’s brightenings between 1987 and 2001, and modeled this feature as a periodically overturning lava lake. No brightenings were seen, however, from 2002 until (at least) 2007 (Rathbun and Spencer, 2010). Recent observations conducted with the Large Binocular Telescope Interferometer (LBTI; Conrad et al., 2015) revealed two hot spots in the horseshoe-shaped “lake” of Loki Patera, which those authors argued would favor the overturning lava lake scenario. We have observed Loki Patera semi-regularly with the W.M. Keck Observatory since 1998 (e.g., Macintosh et al., 2003; de Pater et al., 2004; Marchis et al., 2005; de Pater et al., 2014a, 2014b, 2016a; de Kleer et al., 2014; de Kleer and de Pater, 2016a, 2016b). In this paper we compile all our observations of Loki Patera taken with the Keck Observatory from 1998 through mid-2016; Table 1 provides a summary of these data. Our data reveal remarkably renewed activity at Loki Patera, starting in July 2009, after a 7-year hiatus (an early report was provided by de Pater et al., 2011). We

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I. de Pater et al. / Icarus 000 (2017) 1–17 Table 1 Observations of Loki Patera conducted with the Keck Telescopes 1998–2016. Date (UT) (year/month/day)

Time (h:min–h:min)

Filtera

1998/07/12 1998/07/28 1998/08/04 1999/09/24 1999/09/24 1999/09/24 2001/12/18 2001/12/18 2001/12/20–28 2002/11/12 2002/11/12 2002/02/21 2003/03/09 2004/05/28 2005/05/31 2007/04/03 2007/08/14 2008/05/11 2009/07/24 2009/08/16 2009/09/10 2010/07/27 2010/08/21 2011/07/28 2011/11/10 2011/11/12 2012/11/05 2013/01/20 2013/02/26 2013/02/28 2013/08/15 2013/08/20 2013/08/22 2013/11/18 2014/01/20 2014/02/08 2014/02/10 2014/10/31 2014/12/02 2015/01/12 2015/03/31 2015/04/02 2015/04/04 2015/11/23 2015/12/25 2016/01/22

12:18–12:31 10:46–10:53 12:32–12:42 ∼09:30–10:30 ∼10:50–11:30 ∼10:50–11:30 ∼07:30–08:50 ∼09:00–09:30

Kp Kp Kp L,M,8.9,11.9 L,M,8.9,11.9 N6a,N6b,N7 Kc,Lp,Ms Kc,Lp Kc,Lp,Ms Kp,H,Hn1,Hn2 Kp,H,Hn1,Hn2 Kc,Lp,Ms Lp Lp,Ms Kc,Lp,Ms Kc,Lp,Ms Kc,Lp,Ms Kp,Lp,Ms Jc,Hc,Kc,Lp,Ms Hc,Kc,Lp,Ms Kc,Lp,Ms Kc,Lp,Ms Hc,Kc,Lp,Ms Kc,Lp,Ms All All All Kc,Brα ,Lp,Ms All All Kc,Lp,Brα ,CH4 S Kc,Lp,Ms All All All Kc,Lp,Ms,Brα All Hc,Kc,Lp,Ms,Brα ,Brα c Kc,H2 O,Lp,Ms,Brα ,Brα c All All All Kc,Lp,Ms,Brα ,Brα c All All All

13:00–13:20 13:34–14:00 05:40–10:30 04:59 05:23–05.28 05:30–06:00 14:25–14:48 06:08–06:17 13:46–13:57 10:55–11:10 09:46–10:09 05:08–09:16 11:37–12:56 14:33–14:38 14:40–14:50 07:36–08:56 07:40–07:55 09:30–10:00 04:32–04:47 05:0 0–06:0 0 04:47–05:17 15:22–15:39 15:25–15:41 15:08–15:40 13:15–16:17 07:45–10:30 05:26–06:30 04:45–05:57 15:31–16:00 15:40–16:18 15:39–15:44 04:58–05:53 05:06–06:02 05:01–05:55 15:38–16:06 12:02–12:50 13:21–13:52



Obs-long (°)

Obs-lat (°)

r (AU)

(AU)

342 345 345 334–340 346–350 346–350 336 350 full maps 335 340 233–270 322.4 331 307–311 256–259 248–251 348 345 337–340 348–23 315–326 28–31 262 334–345 22–24 329–333 280 251–258 295–300 337–340 374–276 318–323 203–229 23–46 272–277 313.3 242–246 273–278 339 288–295 336–344 22–30 326–330 325–332 275–278

2.15 2.22 2.25 3.42 3.42 3.42 1.83 1.83 1.84 0.20 0.20 1.83 0.19 −1.31 −2.63 −2.94 −2.73 −1.51 0.52 0.47 0.38 2.35 2.35 3.23 3.33 3.32 3.12 2.94 2.80 2.79 2.06 2.03 2.02 1.66 1.67 1.14 1.14 0.93 1.02 1.13 −0.05 −0.05 −0.05 −1.59 −1.83 −1.96

4.921 4.797 4.977 4.958 4.958 4.958 5.169 5.169 5.17 5.288 5.288 5.189 5.327 5.434 5.458 5.339 5.295 5.197 5.050 5.040 5.036 4.967 4.963 4.953 4.967 4.968 5.049 5.070 5.080 5.083 5.14 5.14 5.15 5.174 5.202 5.206 5.209 5.300 5.312 5.327 5.348 5.351 5.352 5.411 5.417 5.421

4.511 4.297 4.215 4.083 4.083 4.083 4.215 4.215 4.21 5.148 5.148 4.581 4.521 5.295 4.893 4.858 4.880 4.618 4.099 4.032 4.130 4.367 4.089 4.910 4.001 4.009 4.175 4.427 4.988 5.023 5.85 5.79 5.78 4.529 4.251 4.392 4.414 5.420 4.926 4.443 4.745 4.774 4.802 5.653 5.152 4.746

Commentsb In eclipse; Ref. 8 In eclipse; Ref. 8 In eclipse; Ref. 8 In eclipse In eclipse; Ref. 2 Ref. 3 In eclipse; Ref. 3 Ref. 9 Ref. 4 In eclipse; Ref. 4 4 sets Ref. 7 Ref. 7

In eclipse

2 sets 2 sets; Ref. 5 Ref. 5

Ref. 6 Ref. 6 CH4 S replaced Hc; Ref. 6 2 sets; Ref. 1 Several sets; Ref. 1 2 sets; Ref. 1 2 sets of Lp; Ref. 1 2 sets; Ref. 1 2 sets; Ref. 1 Ref. 1 2 sets 2 sets 2 sets

All: The following filters were used: Hc, Kc, Lp, Ms, H2 O, PAH, Brα , Brα C . Filter characteristics are summarized in Table 2. References: 1: de Kleer et al. (2016a); 2: de Pater et al. (2002); 3: de Pater et al. (2004); 4: de Pater et al. (2007); 5: de Pater et al. (2014a); 6: de Pater et al. (2014b); 7: de Pater et al. (2016b); 8: Macintosh et al. (2003); 9: Marchis et al. (2005). a

b

extend our Keck time line with observations published by others, based on, for example, the InfraRed Telescope Facility (IRTF), the Galileo Near Infrared Mapping Spectrometer (NIMS), and Gemini telescope data (Rathbun et al., 2002; Rathbun and Spencer, 2006, 2010; Davies et al., 2012; de Kleer and de Pater, 2016a, 2017), so as to provide a most complete dataset over almost three decades. 2. Observations and data reduction 2.1. Keck LWS and NIRSPEC Data, UT 24 September 1999 On UT 24 September 1999 we conducted a unique experiment at the W.M. Keck Observatory on Maunakea, Hawaii, using both 10m telescopes simultaneously to observe Io while in eclipse. During this eclipse, we observed Io with the near-infrared spectrometer NIRSPEC on the Keck II telescope, and with the Long Wavelength Spectrometer (LWS) on the Keck I telescope (Table 1). The NIRSPEC observations, with the detection of the forbidden SO a1  → X3  − rovibronic transition at 1.7 μm, were discussed by de Pater et al. Please

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(2002), and are shown in Fig. 1. In this paper we are interested in the background intensity, i.e., the emission that is produced by hot spots on Io’s surface. The LWS detector is a 128 × 128 Boeing Si:As array. The pixel size of 0.0847 (Jones and Puetter, 1993) corresponds to 875 km at the sub-observer point at Io’s geocentric distance on UT 24 Sep. 1999. We used filters at 3.85 (L), 4.7 (M), 8.9, and 11.7 μm, the characteristics of which are summarized in Table 2. Together with the NIRSPEC data, this provided coverage from 1.6 to 11.7 μm. We used the standard chop-nod mode for the data acquisition, at an amplitude of 10 for all observations. We used typical integration times of 0.1–0.2 s per frame; at 8.9 and 11.7 μm this was usually obtained by co-adding 9 exposures of 0.01 s each, while at the shorter wavelengths we integrated for up to 0.2 s per frame. Chopping is performed to cancel out sky radiation. Nodding, in a direction opposite to that of chopping, is used to cancel out the telescope contribution to the background emission. The combined chop-nod mode is very efficient at canceling out all background radiation. This typically provided us with 50–150 Io frames per decades

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Fig. 2. Images of Io in eclipse as observed with LWS on the Keck I telescope on UT 24 September 1999. The intensity scale in the upper panel is linear, and in the lower panel logarithmic and stretched such as to bring out both bright and fainter emissions on the disk.

Fig. 1. NIRSPEC spectrum of Io while in eclipse (from de Pater et al., 2002). Io’s SO emission band complex is indicated, as well as unreliable parts of the spectrum due to Earth’s atmosphere (telluric lines). The background spectrum can be fit well with a blackbody curve with a temperature of 825 ± 25 K (see de Pater et al., 2002). The small vertical lines at the bottom of the graph indicate the wavelengths at which we specified the spectral intensities in Table 4. Table 2 Wavelength ranges of filters used. Filter

Wavelength (μm)

Wavelength range (μm)

Jc Hc CH4 S Hn1 Hn2 N6a N6b N7 Kp Kc H2 O PAH L Lp Brα c Brα M Ms 8.9 11.7

1.213 1.580 1.59 1.581 1.702 Spectrum Spectrum Spectrum 2.124 2.271 3.063 3.290 3.85 3.776 3.987 4.052 4.7 4.670 8.83 11.7

1.203–1.223 1.569–1.592 1.530–1.655 1.568–1.593 1.689–1.716 1.57–2.00 1.92–2.35 2.23–2.66 1.948–2.299 2.256–2.285 2.986–3.140 3.263–3.318 3.5–4.2 3.426–4.126 3.952–4.021 4.018–4.086 4.4 - 5.0 4.549–4.790 8.39 - 9.27 11.15–12.25

2.2. Keck NIRC2 observations

dataset. We took 2–3 datasets at each of the four wavelengths of Io while in eclipse, as indicated in Table 3. We reduced each individual Io frame in a dataset by subtracting the appropriate sky frames, and dividing the images by a flat field which was created from the sky images which were extracted from the chop-nod data-cubes. Bad pixels were replaced with the median of the neighboring pixels. Before co-adding the Io frames, each frame was visually inspected and bad frames were discarded. All remaining Io frames were then properly aligned so as to obtain the highest possible angular resolution, and co-added. The data were calibrated using the average of two infrared standard stars at each wavelength (Table 3). When Io was in eclipse, its airmass varied from 1.08 down to 1.05, which amounted into Please

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a potential correction in Io’s flux density of no more than 2–3%. Since no dependence in airmass was seen in the observed calibrator intensities, where the airmass varied from 1.12 to 1.27, we did not correct any fluxes for airmass differences. Our derived conversion factors from DN to flux agree with those derived by Roe et al. (2004) on the same night. Based upon the quality of the images and consistency in photometry for the two different stars, taken at different times during the night and at slightly different airmasses, we adopt an estimated absolute calibration uncertainty of 10% at all wavelengths; this uncertainty also encompasses potential differences in the actual column of water vapor and varying airmass above Maunakea during the observation. The results for Io in eclipse at all four wavelengths are shown in Fig. 2. The angular resolution, or full width at half maximum (FWHM), as derived from the calibrator sources, is 0.32 ± 0.08 at L and M bands, 0.23 ± 0.02 at 8.9 μm, and 0.28 ± 0.02 at 11.7 μm, respectively.

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In addition to listing the LWS and NIRSPEC observations (Section 2.1), Table 1 summarizes all of the observations that were carried out between 1998 and 2016 with the NIRC2 camera coupled to the Adaptive Optics (AO) system on the Keck II telescope at the W.M. Keck Observatory in Hawaii (Wizinowich et al., 20 0 0). NIRC2 is a 1024 × 1024 Aladdin-3 InSb array, which we used in its highest angular resolution mode, i.e., the NARROW camera at 9.94 ± 0.03 mas per pixel (de Pater et al., 2006), which translates roughly into 42 km/pixel at the center of Io’s disk when the satellite is at a geocentric distance of 5.8 AU, down to 29 km/pixel for a geocentric distance of 4.0 AU. All images were processed using standard near-infrared data reduction techniques (flat-fielded, sky-subtracted, with bad pixels replaced by the median of surrounding pixels). The geometric distortion in the Keck images was corrected using the “dewarp” routines provided by Brian Cameron of the California Institute of Technology.1 The individual images were aligned and co-added to increase signal to noise. Photometric calibration was performed on nearby standard stars (e.g., HD22686, HD129655, HD77281, HD201941, HD1160) from catalogues of Elias et al. (1982) and Leggett et al. (2003); on non-photometric nights or nights where no stars could be observed, the photometry was bootstrapped from wellcalibrated images (e.g., from de Pater et al., 2014b) while properly

1 http://www2.keck.hawaii.edu/inst/nirc2/forReDoc/post-observing/dewarp/ nirc2dewarp.pro.

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I. de Pater et al. / Icarus 000 (2017) 1–17 Table 3 UT 24 September 1999 LWS observations. Filter

Integr.a (s)

# Datasets on Io

Calibrator 1

Airmassb

Intensityb ergs s− 1 cm− 2 μm− 1

Calibrator 2

Airmassb

Intensityb ergs s− 1 cm− 2 μm− 1

Reference Calibrator

L M 8.9 11.7

22.4 30 21.07 22.7

2 2 2 3

HR0337 HR0337 HR0337 HR0337

1.17 1.16 1.19 1.20

3.098E-7 1.119E-7 1.157E-8 4.297E-9

HR1708 HR1708 HR6705 HR6705

1.12 1.12 1.25 1.27

2.698E-7 1.161E-7 6.910E-9 2.547E-9

Ref. Ref. Ref. Ref.

1 1 2 2

a Integration time per dataset on Io, while in eclipse. Integration times per frame are 0.09 s each at 8.9 and 11.7 μm (9 coadds of 0.01 s each), and 0.2 s in L and M band. b Airmasss and intensity of calibrator. Ref. 1: Sinton and Tittemore (1984); Ref. 2: http://www2.keck.hawaii.edu/inst/lws/lws_stds_ra.list.

accounting for the variations in Io’s heliocentric and geocentric distance using the JPL Horizons database for ephemerides at the time of the observations. When Io is observed in reflected sunlight, the satellite itself is used for wavefront sensing. However, when it is in eclipse, such as on UT 11 May 2008, we need to use another nearby Galilean satellite for wavefront sensing (see e.g., de Pater et al., 2004). On 11 May 2008 we used Ganymede, which was 27 to the (astronomical) west, and 7.5 to the north of Io at the beginning of the observations, increasing to 35.4 west and 5.9 north. The slight elongation in volcanic hot spots, and in particular in the hot spot halos, results from the anisoplanatic effect, caused by the reference star being ∼30 away from our target. This is explained in more detail by de Pater et al. (2004). Results for observations of Io-in-sunlight and of Io-in-eclipse are shown in Figs. 3 and 4, respectively. None of the images have been deconvolved, and the bright Airy rings are clearly visible around the brightest hot spots. In Fig. 3 we mostly show data that have not been featured in previous papers (previous publications that feature the images have been indicated in Tables 6 and 7). Data taken before Nov. 2011 were usually taken in Kc, Lp, and Ms bands only, while data taken on later dates were often observed in addition through narrow-band filters ranging from 1.6 μm up to 5 μm in wavelength. Many volcanoes are visible in the longer wavelength (>3.5 μm) images, in particular in those taken under the best seeing conditions (i.e., highest quality images). Most of the brightest and/or best-known volcanoes on the images are identified by name. Although this paper is focused on Loki Patera, we note a few interesting hot spots on Fig. 3: Pele, the only hot spot seen at 2.3 μm in Feb. 2002, has faded considerably in recent years, as discussed by de Pater et al. (2016a). The time evolution of Janus Patera and Kanehekili Fluctus through 2010 had been discussed by de Pater et al. (2014b); the present images show that Kanehekili was still (or has since 2010 become again) very active in late 2011. In Nov. 2012 an eruption was visible at Gibil Patera, clearly visible at all wavelengths between 3.29 and 5 μm, indicative of a relatively high temperature. These data will be analyzed in a future paper. Although the number of filters helps in fitting blackbody curves to the data (Section 4), the filters themselves were chosen such as to illuminate some compositional variations on the surface. In particular, the Brα (4.05 μm) filter coincides with a deep SO2 ice absorption band; indeed, at this wavelength we see clear dark regions on Io’s surface, attributed to patches of SO2 ice that absorb much of the incident sunlight, decreasing the amount of reflected light. Several prominent dark patches are visible to the west and south-west of Loki Patera, at the locations of Acala Fluctus and Ra Patera, respectively; these areas are relatively bright in the Kc (2.27 μm) band, as one might expect if indeed the area is covered by SO2 ice, which we expect it to be. Complete surface maps of Io in these and other narrow-band filters were presented by de Kleer and de Pater (2016b). It will be worthwhile in the future to conduct a detailed comparison of such maps. Please

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Table 4 Intensities of Loki Patera on UT 24 September 1999a . Wavelength (μm)

Intensity observed GW sr−1 μm−1

Emission angleb (°)

Intensity correctedc GW sr−1 μm−1

1.64 1.78 2.0 2.10 2.25 2.43 3.85 4.7 8.9 11.7

2.82 ± 0.07 4.22 ± 0.06 6.81 ± 0.13 7.74 ± 0.10 10.0 ± 0.14 13.3 ± 0.5 47.7 ± 4.8 70.1 ± 7.0 136.9 ± 13.7 130.6 ± 13.1

∼38 ∼38 ∼38 ∼38 ∼38 ∼38 35.7 36.7 37.4 38.3

3.6 ± 0.4 5.4 ± 0.6 8.7 ± 0.9 9.9 ± 1.0 12.7 ± 1.3 16.9 ± 1.8 59 ± 6 88 ± 9 172 ± 17 166 ± 17

a The data at 1.6–2.45 μm are derived from the NIRSPEC spectra (Fig. 1); the data at 3.8–11.7 are taken with LWS (Fig. 2). The uncertainty in the NIRSPEC data is based on the standard deviation in the spectrum. The uncertainty in the LWS data is the adopted 10% photometric error. b Emission angle was calculated assuming Loki Patera to be at 311° W longitude and 10° N latitude. c Intensity corrected for the emission angle effect. The uncertainty quoted for the NIRSPEC data includes also a 10% photometrc uncertainty.

The distribution of Io’s hot spots in the Io-in-eclipse images is shown in Fig. 4. Although quiescent in these data, the dark, central area in these images has been the location of both faint and highly energetic eruptions, in particular the extremely bright eruption at Ra Patera in Nov. 2002 (de Pater et al., 2007). 3. Results 3.1. Spectral intensities 3.1.1. LWS and NIRSPEC The spectral intensities of Loki Patera (in units of GW/sr/μm) as observed on UT 24 September 1999 with LWS while Io was in eclipse are listed in Table 4. Since only a fraction of the flux is contained in the central part of the PSF, we determined the total intensities based on the aperature photometry technique described by Gibbard et al. (2005) and de Pater et al. (2014b). We feel comfortable using this technique, since, like stars, Io’s hot spots are unresolved in our images. For the LWS measurements, we integrated the emission over the hot spot, and determined the background from an annulus immediately surrounding the hot spot. Since a fraction of flux is lost in the PSF halo, we determined this fraction by determining the spectral intensity of a star in the exact same way as for the hot spot (total flux within the same size circle centered on the star, minus the background as measured in the annulus around the star). Io’s hot spot intensity is then divided by the ratio of the stellar intensity to the total intensity from the star. We determined Loki Patera’s intensity from each dataset, and averaged the values thus obtained for each dataset at each wavelength (typically 2–3 datasets per wavelength; see Table 3). The individual values of the numbers varied by less than a few %. We adopted a decades

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Fig. 3. AO-corrected images of Io showing Loki Patera at 2.2, 3.8, and 4.7 μm obtained in various years with the Keck II telescope, as indicated in Tables 6 and 7. The brightness contrast in each image is optimized to show both faint and bright sources. The rings around bright sources are artifacts, produced by the PSF of the telescope (i.e., Airy ring). When Loki Patera is bright and emission can be seen at 2.2 μm, note that the location of emission within the patera can be approximated by eye.

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Fig. 3. Continued

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Fig. 4. AO-corrected images of Io while in eclipse, taken on UT 11 May 2008. The bright volcano at 3.8 and 4.7 μm is Loki Patera. The numbers on the 2.2 μm image refer to the sources in Table 5. The brightness contrast in each image is optimized to show both faint and bright sources.

10% uncertainty in our final numbers, consistent with the 10% absolute photometric uncertainty determined above. In order to facilitate comparison with previous flux estimates, the results were normalized using the distance to Io at the time of the observation. In addition to the LWS numbers, Table 4 also lists spectral intensities as derived from NIRSPEC spectra while Io was in eclipse. The intensities as listed in Column 2 were averaged over a spectral wavelength range of 0.08 μm, i.e., mimicking a bandwidth of 0.08 μm at the wavelengths listed (it was 0.16 μm at 2.43 μm), and the uncertainties quoted are the standard deviation over that part of the spectrum. We assume all emission can be attributed to Loki Patera, which was by far the most active volcano at the time (e.g., Fig. 2). Column 3 in Table 4 lists the emission angle θ (the angle between the center of the disk at the time of the observations and Loki Patera, which was assumed to be at 311° W longitude, and 10° N latitude), and Column 4 lists the spectral intensity after correction for the emission angle effect (i.e., divided by cos θ ). We use the cosine correction in the assumption that the thermal source is acting as a flat plate tilted away from the observer, an assumption supported by Galileo data of Loki Patera (Davies et al., 2011). The bulk of Loki Patera’s thermal emission exhibits this behavior as a function of viewing angle, and this supports the idea that the floor of Loki Patera is covered by the flat, relatively smooth crust on a lava lake. We note that parts of the patera floor may not exhibit this behavior. Thermal emission from cracks may be masked at high viewing angles. Lava breakouts are not flat. Lava fountains, should they be present, should not be corrected for emission angle. Large lava fountains with powerful thermal emission at short infrared wavelengths have not been seen at Loki Patera (Matson et al., 2006). At high emission angles the intensity drops faster than that expected from a cosine function, perhaps caused by topographic shadowing (de Kleer and de Pater, 2017). The error listed in Table 4 consists of the error in Column 2 and a 10% photometric error, added in quadrature. 3.1.2. NIRC2 The spectral intensities for the NIRC2 Io-in-eclipse data on UT 11 May 2008 were determined according to the method described by Macintosh et al. (2003). We first integrate the number of counts/sec at each wavelength over the entire source (Io), and convert this number to units of GW/sr/μm by using the photometric calibrator and the geocentric distance of Io at the time of the observations. We then determined the relative intensities of all volcanic sources, using the method described by Gibbard et al. (2005) and de Pater et al. (2014b), and which was summarized above. However, because Io was observed off-axis with the reference star (Ganymede) ∼30 away, much more flux was lost in the halos of the hot spots than can be corrected for by using a star that was observed with on-axis guiding. We can circumvent this issue using the fact that Io was in eclipse, which means that only volcanic hot spots contribute to Io’s thermal flux density and the total flux from all hot spots together should equal the total intenPlease

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sity from Io. It is thus relatively straightforward to calibrate the absolute spectral intensity for each hot spot. These values are listed in Table 5; we adopted a conservative estimate of 20% for the uncertainty on these values. Note that these numbers have not been corrected for the emission angle effect. The intensity of Loki Patera, as listed in Table 6, is the value reported in Table 5 after correction for the emission angle. The spectral intensities in the Kc, Lp, and Ms bands for the NIRC2 data of Io-in-sunlight as determined from Keck and Gemini data from 1998–2010 are listed in Table 6. Measurements obtained with NIRC2 at the Keck Observatory from 2011–2016 are listed in Table 7. These values were determined using the method described by Gibbard et al. (2005) and de Pater et al. (2014b) (Section 3.1.1). The intensities were corrected for the emission angle effect, using a location for Loki Patera as measured on the images (Section 3.2). 3.2. Location of Loki Patera In order to determine the location of Loki Patera on each image, all images were deprojected using the sub-earth longitude and latitude as obtained from standard ephemerides.2 As discussed in de Pater et al. (2014a), the center of the disk was found manually by centering a circle around the limb of Io in each image, and repeated for different image stretches and circle sizes. The uncertainty in the disk center is estimated to be smaller than 1/4 of a pixel. Longwards of 3 μm, however, it is harder to distinguish Io’s limb, which is reflected by a larger uncertainty in location. For the Io-in-eclipse data on 11 May 2008, where we cannot determine Io’s limb, we forced the position of Janus to be at 39° W longitude and 4.5° S latitude in the deprojected images. These locations are listed in Table 8, where Columns 8 and 9 show the locations of Loki Patera averaged over all images at all wavelengths taken that night. Although we will use the average position of Loki Patera in the remainder of this paper, we list both the average and several individual filter locations in Table 8, since different temperatures are sensed at different wavelengths and the positions as measured at the various wavelengths may therefore differ from one another. 4. Temperature fits to the data 4.1. Single-temperature (1-T) blackbody fits In Table 9 we list the temperature, area, and total power as derived from single-temperature (1-T) blackbody fits to all the data. In cases where we only had one datapoint we constrained the area to be equal to 21,500 km2 , the area of the horseshoe-shaped Loki Patera “lake” (Veeder et al., 2011). To determine the uncertainty of model fits to the data, the following approach was used (de Pater et al., 2014b). Each intensity datum has an uncertainty, expressed as a plus or minus value (Tables 4–7). To determine the range of model fits to the data, uncertainties were first added to short wavelength data and subtracted from long-wavelength data; and then model fits were made to data where uncertainties were subtracted from short wavelength data and added to long wavelength data. In this way the full range of model fits within the uncertainty limits was established. Because these curves correspond to the extreme errors on the data at the shortest and longest wavelengths, we divided the errors by sqrt(N) (like the standard deviation on the average of N data points is divided by the square-root of N) with N = 2. The uncertainties thus determined agree well with those determined via a full Markov Chain Monte Carlo (MCMC) method (as in de Kleer and de Pater, 2016a). Some representative 1-T fits are shown in Fig. 5. Note that in all cases the 1–5 μm data lie on the short-wavelength side of the peak 2

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Table 5 Intensities of all detected hot spots on UT 11 May 2008. Source #

Name

W longitudea (°)

Latitudea (°)

Intensityb GW sr−1 μm−1 2.27 μm

Intensityb GW sr−1 μm−1 3.78 μm

Intensityb GW sr−1 μm−1 4.67 μm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Dazhbog Patera Surt Fuchi Patera Loki Patera Daedalus Patera PV59 Rarog Patera 10° W. of Sengen Patera Heno Patera Paive Patera Uta Masubi Kanehekili Fluctus Janus Patera No Name No Name

300 ± 4 336 ± 0.5 327.8 ± 0.5 308.1 ± 0.5 279 ± 3 293.7 ± 2.0 306.9 ± 1.0 314.1 ± 1.0 313.5 ± 1.5 359.0 ± 1.5 22.3 ± 0.5 50.5 ± 2.0 34.3 ± 0.5 39.0 23.5 ± 0.5 12.1 ± 0.5

55.7 ± 2.0 46.0 ± 0.5 28.4 ± 0.5 12.6 ± 0.5 18.6 ± 1.0 −37.4 ± 1.0 −39.7 ± 1.0 −29.8 ± 1.0 −54.8 ± 1.5 −43.9 ± 1.5 −35.0 ± 0.5 −44.8 ± 2.0 −14.2 ± 0.5 −4.5 5.2 ± 0.5 36.5 ± 0.5

0.51 ± 0.10 0.40 ± 0.08 0.22 ± 0.04 0.29 ± 0.06 0.18 ± 0.03 0.26 ± 0.05 0.08 ± 0.02 0.06 ± 0.01 0.14 ± 0.03 0.13 ± 0.03 0.30 ± 0.06 0.11 ± 0.02 0.21 ± 0.04 0.48 ± 0.10 0.32 ± 0.06 0.22 ± 0.04

1.82 ± 0.36 0.70 ± 0.14 0.35 ± 0.07 12.6 ± 2.5 0.04 ± 0.01 1.86 ± 0.37 0.63 ± 0.13 0.46 ± 0.09 0.25 ± 0.05 0.42 ± 0.08 2.80 ± 0.56 0.70 ± 0.14 1.75 ± 0.35 2.45 ± 0.50 1.02 ± 0.20 0.35 ± 0.07

2.58 ± 0.51 0.90 ± 0.18 0.15 ± 0.03 39.0 ± 0.8 0.15 ± 0.03 1.50 ± 0.30 1.26 ± 0.23 0.99 ± 0.20 0.15 ± 0.03 0.90 ± 0.18 4.56 ± 0.90 2.58 ± 0.52 3.0 ± 0.60 2.70 ± 0.34 2.76 ± 0.35 0.66 ± 0.13

Comments

NE of Ulgen Patera; see Veeder et al. (2015)

Adopted position from USGS1 Grey irregular feature; Veeder et al. (2015) Small dark patera, USGS Io map

a We adopted a position of 39° W longitude and 4.5° S latitude for Janus Patera from the USGS map, and used this as a reference for all other sources on the map (http://planetarynames.wr.usgs.gov/). b Radiant Flux as observed, i.e., not corrected for emission angle. We adopted an uncertainty of 20%.

Table 6 Near-IR Intensities of Loki Patera from Keck and Gemini Data 1998–2010. Date (UT)

θ

(year/month/date/∼h)

(°)

1998/07/12/ 1998/07/28/ 1998/08/04/ 2001/12/18/∼7.5 2001/12/18/∼9 2001/12/20/∼7.5 2001/12/23/∼11 2001/12/28/∼12 2001/12/28/∼14 2002/02/21/∼6 2002/02/21/∼7.5 2002/02/21/∼9 2002/02/21/∼10 2002/11/12/∼14 2003/03/09/∼5 2004/05/28/∼5 2005/05/31/∼6 2007/04/03/∼14.5 2007/08/14/∼6.5 2008/05/11/∼14 2009/07/24/∼11 2009/07/26/∼10.5 2009/08/16/∼10 2009/09/10/∼5 2009/09/10/∼9 2010/07/27/∼12 2010/07/27/∼13 2010/08/21/∼15 2010/09/09/∼09 2010/09/18/∼09 2010/09/20/∼11

33.6 36.9 44.9 28 42 72 11 58 45 76 62 50 40 33 17 26 18 53 61 20 39 84 32 39 72 16 23 84 53 28 42

Intensitya Kc,Kpb GW/sr/μm

Intensitya Lp GW/sr/μm

Intensitya Ms GW/sr/μm

9.9 ± 1.5 4.9 ± 0.7 6.8 ± 1.0 1.22 ± 0.2

14.8 ± 2.2 16.6 ± 2.5 4.3 ± 0.6 23.4 ± 3.5 28.8 ± 4.3 21.9 ± 3.3 33 ± 5 53 ± 8 55 ± 9 55 ± 9

45 ± 7

55.7 ± 8.4 62.2 ± 9.3 67 ± 10 89 ± 14 77 ± 12 98 ± 15

18.5 ± 3.2 14.5 ± 2.2 10.7 ± 2.6 3.7 ± 0.6 2.5 ± 0.4 16.4 ± 3.3 97 ± 15 43 ± 7 95 ± 14 110 ± 17 75 ± 12 29.7 ± 4.5 34.7 ± 5.2 38 ± 6 45 ± 7 62 ± 9 97 ± 15

41 ± 6 49 ± 9 18 ± 3 11.3 ± 1.7 52 ± 10 260 ± 40 82 ± 12 195 ± 30 177 ± 26 110 ± 17 44.8 ± 6.7 55.4 ± 8.3 64 ± 10 130 ± 25 230 ± 35 143 ± 22

30 ± 5

0.44 ± 0.07

0.37 ± 0.08 2.9 ± 0.5 4.5 ± 0.7 4.4 ± 0.7

Comments/Referencec

Ref. 6 Ref. 6 Ref. 6 Ref. 1 In eclipse; Ref. 1 Ref. 7; E. limb Ref. 7 Ref. 7 Ref. 7 W. limb Fig. 3a in Ref. 4 Fig. 3a, this paper Eclipse; H band flux is 0.07 GW/sr/μm; Ref. 2 Fig. 1in Ref. 5 Fig. 1 in Ref. 5 Fig. 3a in Ref. 4 Fig. 3a in Ref. 4 In eclipse; Fig. 4 this paper Fig. 3a this paper Fig. 7 in Ref. 3; E. limb Fig. 3a this paper Fig. 3a this paper Fig. 7 in Ref. 3; E. limb Fig. 3 in Ref. 3 Ref. 3; Fig. 3a this paper Ref. 3; E. limb; Fig. 3a this paper Fig. 3b in Ref. 4 Ref. 3 Ref. 3

a All intensities have been corrected for the emission angle, θ , effect. These angles were calculated based on Loki Patera’s position as measured during that time period (Table 8). We adopted a 15% uncertainty on the absolute photometry of all intensities. For UT 11 May 2008 we adopted an uncertainty of 20%. Additional uncertainties were added in quadrature. b All measurements were obtained with the narrow-band Kc filter, except when Io was in eclipse, when the broad-band Kp filter was used. c All data from 1998 – Aug. 2010 are from the Keck telescope; the Sep. 2010 data are from the Gemini telescope. If the data were provided in another paper, a reference is provided. If only a figure was presented in another paper, without any measurements, that figure is listed. Ref. 1: de Pater et al. (2004); Ref. 2: de Pater et al. (2007); Ref. 3: de Pater et al. (2014b); Ref. 4: de Pater et al. (2016a); Ref. 5: de Pater et al. (2016b); Ref. 6: Macintosh et al. (2003); Ref. 7: Marchis et al. (2005).

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Table 7 Near-IR Intensities of Loki Patera from Keck Data 2011–2016.

θ

Intensitya Kc GW/sr/μm

Date (UT) Filter (Year/month/date/∼h)

(°)

2011/07/28/∼14.5 2011/11/10/∼08 2011/11/12/∼10 2012/11/05/∼10 2013/01/20/∼04.5 2013/02/26/∼05.5 2013/02/28/∼05 2013/08/15/∼15.5 2013/08/20/∼ 15.5 2013/08/22/∼15.5 2013/11/18/∼16 2014/01/20/∼08 2014/02/08/∼ 06 2014/02/10/∼05 2014/10/31/∼16 2014/12/02/∼16 2015/01/12/∼16 2015/03/31/∼05 2015/04/02/∼05 2015/04/04∼06 2015/11/23/∼15.5 2015/12/25/∼12.5 2016/01/22/∼13.5

48 ∼32 74 25 31 58 15 31 34 14 84 76 39 14 63 32 32 23 ∼ 32 65 20 28 33

ND ND ND ND ND ND 7.7 ± 1.4 13 ± 3.3 7.5 ± 1.3

Intensitya H2 O GW/sr/μm

Intensitya PAH GW/sr/μm

5±1

9.4 ± 1.5

ND

ND

2.5 ± 2.0 2.6 ± 0.5

4.8 ± 1.0 4.5 ± 0.7

Intensitya Lp GW/sr/μm 13.8 ± 2.1 20 ± 3 12.5 ± 2.5 7.7 ± 1.2 3.6 ± 0.5 11.3 ± 1.9 11.9 ± 1.8 >60 ± 10b 133 ± 27 136 ± 20

Intensitya Brα c GW/sr/μm

Intensitya Brα GW/sr/μm

32 ± 5 22.8 ± 4.3 10.0 ± 1.5

26 ± 4 19.3 ± 3.2 10.5 ± 1.5 9.3 ± 1.4 12 ± 2 16 ± 3 > 111 ± 19b

12.4 ± 2.0 15.0 ± 2.5

192 ± 26

15 ± 2 108 ± 15 54 ± 8 32.4 ± 5.0 16.3 ± 2.5 14.5 ± 2.2

231 ± 35 1.7 ± 0.5 6.2 ± 1.4 11.0 ± 1.7 21 ± 3 107 ± 15 50 ± 8 30 ± 5 15.5 ± 2.5 16.4 ± 2.7

5.8 ± 0.9 49 ± 7 172 ± 25

6.0 ± 0.9 54 ± 8 168 ± 25

8.1 ± 2.5 ND ND

2.1 ± 0.3

ND ND ND ND ND ND 1.1 ± 0.2 7.4 ± 1.1

12.3 ± 1.8 ND ND

7.7 ± 1.2 2.4 ± 0.4 1.2 ± 0.3

ND 17.5 ± 2.6 62 ± 9

ND 27.5 ± 4.1 94 ± 14

8.5 ± 1.3 11.1 ± 1.7 78 ± 12 47 ± 7 23.2 ± 3.5 9.8 ± 1.5 9.3 ± 1.4 ND 2.9 ± 0.4 38 ± 6 147 ± 22

Intensitya Ms GW/sr/μm

Comments/Referencec

41.6 ± 6.3 35 ± 5 20 ± 3 25 ± 4 17.4 ± 2.6 24.2 ± 3.7 26 ± 4

Fig. 3b in Ref. 5 Fig. 3a this paper Fig. 3a this paper; E. limb Fig. 3a this paper Fig. 3b in Ref. 5 Fig. 3b in Ref. 5 Fig. 3a this paper Ref. 3 Ref. 3 Fig. 3b this paper; Ref. 3 (corrected typo in Ref.3) Fig. 3c in Ref. 5; Ref. 1; W. limb Ref. 1; E. limb Fig. 3c in Ref. 5; Ref. 1 Fig. 3b this paper; Ref. 1 Fig. 3c in Ref. 5; Ref. 1; near W. limb Fig. 3c in Ref. 5; Ref. 1 Fig. 3b this paper; Ref. 1 Fig. 3c in Ref. 5 Fig. 3b this paper Ref. 1; on E. limb; very faint Fig. 3b this paper Fig. 3b this paper Fig. 3b this paper

200 ± 33 205 ± 34 28+8 10 17.4 ± 3.8 25 ± 4 32 ± 5 155 ± 23 122 ± 18 66 ± 10 30 ± 5 28 ± 4 15.4 ± 2.3 13.4 ± 2.0 85 ± 13 205 ± 30

a All intensities have been corrected for the emission angle, θ , effect. These angles were calculated based on the Loki Patera’s position as measured during that time period (Table 8). We adopted a 15% uncertainty on the absolute photometry of all intensities. Additional uncertainties were added in quadrature. ND stands for “not detected”. A blank space means no data were taken. b The intensities at Lp band may be too low by up to a factor of 2, and the Brα intensity may be too low by 20%, as detailed in Ref. 3. c Reference with published intensities for many of the filters. Ref.1: de Kleer et al. (2014); Ref.2: de Kleer et al. (2016a); Ref.3: de Pater et al. (2014a); Ref.4: de Pater et al. (2014b); Ref.5: de Pater et al. (2016a).

in the blackbody curve, and the data can be fit reasonably well with a single temperature blackbody curve. Temperatures typically range from ∼250 K up to ∼550 K, and there is some indication that the higher values correspond to periods of enhanced activity (see Section 4.2). Several observations (Table 9) yield areas that are greater than the dark area of Loki Patera (21,500 km2 – see Veeder et al., 2011). While it is possible that eruptions took place in the vicinity of Loki Patera generating flows or deposits on a larger scale than the dark floor area, there would be lingering effects that would be most likely identified in subsequent observations. The large areas generated here are most likely the result of problems with the data, as indicated in the Comments column, such as data taken close to the limb,3 non-photometric conditions, and the fact that Gemini Msband data were taken without adaptive optics. Also, fitting the data to the limits of uncertainty, and without the constraints imposed by data points at longer wavelengths, generates a large spread of possible areas, and similarly anomalous estimates of thermal emission. The resulting areas and total thermal emission numbers resulting from these temperatures and areas (the result of extrapolating thermal emission over all wavelengths) should therefore be regarded with some caution. 4.2. Dual-component fits to the 1999 LWS/NIRSPEC data Only in rare cases do we have data spanning wavelengths from 1.5 μm up to 12 μm, a range that is ideal for thermal modeling as it constrains both sides of the peak in thermal emission. Table

3 It may be possible for observations obtained at very high viewing angles that the time interval between observations, even if only a few minutes, could have led to a change in emitting area seen at different wavelengths, possibly due to topographic shielding. If this diminishes the short wavelength thermal emission, then the effect is to push the temperature fit to lower temperatures with an according increase in the emitting area and resulting total power output.

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10 shows the results of both single (1-T) and two-temperature, two-area (2-T), blackbody fits to the spectral intensities listed in Table 4; the results of these fits are shown in Fig. 6, panels a and b. Table 11 shows the results of fitting a more-sophisticated dual-component Io Flow Model (IFM) to the data (Davies, 1996), the result of which is shown in Fig. 6c (Fig. 6d is discussed below). Such a dual-IFM approach was previously used to interpret thermal emission Galileo NIMS data from Pillan and Pele (Davies et al., 2001), and most recently in the analysis of the 2004 eruption at Sui Jen Patera (de Pater et al., 2016b). The IFM utilizes a range of temperatures and areas, beginning with components at the lava eruption temperature. The model is described in detail in Davies (1996, 2007) and is briefly summarized as follows. The model is an attempt to reproduce the complex temperature distribution encountered on active lava surfaces. The utility of the model allows thermal emission from a cooling lava flow or lava lake crust to be modeled by one model component, and the higher temperature areas (the active vent area, hot areas visible through cracks in a developed, relatively cool, lava crust, new breakouts, etc.) by another component. The dual-component IFM fit not only gives an excellent fit to that data, but also yields considerably more information about the physics of the eruption, as the temperature ranges revealed are now directly related to the exposure times of the lava, the areal coverage rate, and the duration of the eruption. It is clear from Fig. 6, panels a and b, that the most accurate temperatures, areal coverage, and power output are obtained when data cover wavelengths on each side of the peak in the blackbody curve, and that a 2-T fit is superior to a 1-T fit. The 2-T fit shows a high-temperature component of 626 ± 9 K over an area of ∼130 ± 20 km2 , and a strongly-constrained low temperature component of 280 K (±2 K), covering almost the entire patera floor (17,300 ± 1100 km2 ). The dual-component IFM fit suggests that at least some parts of the patera floor are young and hot, since the oldest (i.e., lowest) surface temperature of these young hot areas decades

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Table 8 Locations of Loki Patera hot spots. Date (year/month/date/∼h) 1998/07/12 1998/07/28 1998/08/04 2001/12/18 2002/11/12 2002/02/21 2003/03/09 2004/05/28 2005/05/31 2008/05/11 2009/07/24 2009/08/16 2009/09/10 2010/07/27 2010/07/27 2010/09/09 2010/09/18 2010/09/20 2011/07/28 2011/11/10 2012/11/05 2013/01/20 2013/02/28 2013/08/15 2013/08/20 2013/08/22 2014/02/08/∼ 05.5 2014/02/08/∼ 06 2014/02/10/∼05 2014/02/10/∼06 2014/12/02/∼15.7 2014/12/02/∼16 2015/01/12/∼15.7 2015/03/31/∼5 2015/03/31/∼6 2015/04/02/∼5 2015/04/02/∼6 2015/11/23/∼15.5 2015/12/25/∼12.5 2016/01/22/∼13.5

W. Longitude (°) Kc

308.1 ± 0.5 304.6 ± 0.5 306.7 ± 0.5 310.6 ± 0.5 306.9 ± 0.5 307.2 ± 0.5

308.5 ± 2.0 307.2 ± 1.0 307.3 ± 1.0

Latitude (°) Kc

W. Longitude (°) Lp

12.6 ± 0.5 12.3 ± 0.5 10.9 ± 0.5 9.7 ± 0.5 19.0 ± 0.8 17.6 ± 0.8

9.7 ± 1.0 11.9 ± 1.0 9.9 ± 1.0

310.4 ± 0.5a

11.6 ± 0.5a

310.1 ± 0.5a

12.6 ± 0.5a

304.4 ± 0.5 305.7 ± 0.5

15.1 ± 0.5 14.3 ± 0.5

310.4 ± 0.5 311.8 ± 1.0 309.0 ± 1.0 311.0 ± 1.4 310.8 ± 1.0 305.9 ± 0.8 307.3 ± 0.8 310.4 ± 0.8 306.9 ± 0.5 306.4 ± 0.5 308.4 ± 1.4 308.1 ± 1.4 306.6 ± 2.0 308.9 ± 0.8 309.6 ± 0.5 307.9 ± 0.5 311.1 ± 0.5 310.6 ± 0.5 309.0 ± 2.0 309.6 ± 1.2 309.2 ± 1.2 310.1 ± 0.6 310.3 ± 0.6 310.2 ± 0.5 310.1 ± 0.5 305.2 ± 0.6 305.7 ± 0.6 306.7 ± 0.6 312.2 ± 0.8 311.5 ± 0.8 309.4 ± 0.8 311.2 ± 0.8 307.5 ± 0.5 306.0 ± 0.5 306.0 ± 0.8

Latitude (°) Lp

W. Longitude (°) Ms

10.4 ± 0.5 14.5 ± 1.0 13.0 ± 1.0 15.4 ± 1.4 12.2 ± 1.0 12.7 ± 0.8 11.8 ± 0.8 11.6 ± 0.8 15.2 ± 0.5 15.7 ± 0.5 15.2 ± 1.4 16.0 ± 1.4 16.3 ± 2.0 13.0 ± 0.8 15.1 ± 0.5 11.8 ± 0.5 11.7 ± 0.5 13.2 ± 0.5 12.0 ± 2.0 11.3 ± 1.2 10.9 ± 1.2 9.4 ± 0.5 9.0 ± 0.5 10.3 ± 0.5 11.0 ± 0.5 9.7 ± 0.5 9.6 ± 0.5 10.5 ± 0.5 12.3 ± 0.8 11.2 ± 0.8 13.7 ± 0.8 11.9 ± 0.8 11.2 ± 0.5 14.7 ± 0.5 13.7 ± 0.8

Latitude (°) Ms

311.0 ± 1.0

11.2 ± 1.0

309.8 ± 1.0 309.2 ± 1.4 309.5 ± 1.0 306.1 ± 1.2 307.1 ± 1.2 310.8 ± 1.2 307.4 ± 1.0 307.7 ± 1.0

15.0 ± 1.0 14.6 ± 1.4 12.7 ± 1.0 12.9 ± 1.2 12.5 ± 1.2 11.3 ± 1.2 16.8 ± 1.0 16.6 ± 1.0

W. Longitude (°) Mean

Latitude (°) Mean

comments

311.8 ± 0.9 311.9 ± 2.3 309 ± 1.6 311 ± 2 309 ± 2 310.6 ± 0.4 311.8 ± 1.0 309.4 ± 0.7 310 ± 1 309.5 ± 0.4 305.1 ± 0.4 306.9 ± 0.4 310.6 ± 0.4 307.0 ± 0.4

10.3 ± 0.8 10 ± 2 8.9 ± 1.3 9±1 12 ± 2 10.7 ± 0.5 14.5 ± 1.0 14.0 ± 0.7 15 ± 1 12.7 ± 0.4 12.5 ± 0.3 11.3 ± 0.4 10.4 ± 0.4 16.5 ± 1.4

Ref. 1 Ref. 1 Ref. 1 Ref. 2 Ref. 3 Pele at 255.7 W, 18.4 S

308.4 ± 1.4 308.1 ± 1.4 306.6 ± 2.0 308.6 ± 0.6 309.7 ± 0.3 307.9 ± 0.6 310.9 ± 0.4 310.7 ± 0.4 308.8 ± 1.2 307.9 ± 0.7 307.8 ± 0.7 310.6 ± 0.5

15.2 ± 1.4 16.0 ± 1.4 16.3 ± 2.0 12.8 ± 0.7 15.1 ± 0.5 12.7 ± 0.7 12.2 ± 1.0 13.3 ± 0.5 10.8 ± 1.2 12.0 ± 0.7 11.0 ± 0.7 10.1 ± 1.0

Pele at 255.7 W, 18.4 S; Ref. 4 Pele at 255.7 W, 18.4 S; Ref. 4 Ref. 4 Pele at 255.7°W, 18.4°S Allb filters averaged Allb filters averaged

310.1 ± 0.3

10.9 ± 0.6

Averaged all 3 values

Janus 40° W, 4.2° S

Averaged all 6 values

308.1 ± 1.2 309.8 ± 1.0 307.8 ± 0.8 310.5 ± 0.8 311.1 ± 1.0 309.0 ± 2.0 306.9 ± 1.4 307.0 ± 1.4 310.9 ± 0.7 311.0 ± 0.6 309.9 ± 0.5

12.6 ± 1.2 15.0 ± 1.0 12.6 ± 0.8 13.1 ± 0.8 13.8 ± 1.0 13.8 ± 2.0 13.2 ± 1.4 13.5 ± 1.4 11.0 ± 0.5 11.1 ± 0.5 11.5 ± 0.5

305.7 ± 0.6

12.2 ± 0.5

305.2 ± 0.7

9.7 ± 1.4

Allb filters averaged

305.6 ± 0.7 312.0 ± 1.2 311.3 ± 1.2 311.2 ± 1.2 310.6 ± 1.2 306.1 ± 0.8 307.2 ± 0.8 305.1 ± 1.0

11.8 ± 0.5 13.3 ± 1.2 12.8 ± 1.2 11.9 ± 1.2 14.6 ± 1.2 12.7 ± 0.8 13.8 ± 0.8 13.3 ± 1.0

306.7 ± 1.6 311.8 ± 0.6

11.8 ± 0.8 12.3 ± 0.8

Allb filters averaged Allb filters averaged

310.4 ± 0.4

12.9 ± 0.9

Averaged all 5 values

307.5 ± 1.5 305.7 ± 1.2 305.7 ± 0.6

12.0 ± 0.6 14.9 ± 0.7 13.6 ± 0.4

Allb filters averaged Allb filters averaged Allb filters averaged

Averaged all 4 values

a

Measured at 3.29 μm (PAH filter). All filters: see Table 1 for filter sets. Ref. 1: Macintosh et al. (2003); Ref. 2: de Pater et al. (2004); Ref. 3: de Pater et al. (2007); Ref. 4: de Pater et al. (2014b). b

are at 577 K, indicative of an exposure time of 6.3 h. These young hot regions cover a total surface area of 50 km2 . On the other hand, the low-temperature component, covering a large fraction of the patera floor, is ∼255 K, and much older at almost 1 year old. Given the spatial resolution of the data, we cannot say with any precision how the hot component is distributed within the much larger cooler area. The hot component makes up only 0.6% of the total cool area, and is likely a mixture of hot or warm cracks in the cooler crust, active margins, as well as newer crustal areas. In total, the thermal emission from the dual-IFM fit is ∼8 TW; the 2-μm radiant flux is 26 GW/μm; the 5-μm radiant flux is 300 GW/μm. The 2-μm:5-μm ratio is 0.088, a low value as expected from an area dominated by cool crust, and the radiant flux density is, accordingly, a low value of ∼0.45 kW/m2 , suggesting mostly quiescent lava surfaces and within the range of previous analyses of Loki Patera thermal emission data (Matson et al., 2006; Davies et al., 2010; Davies, 2007) which yielded radiant flux densities from 0.09 to 0.62 kW/m2 , much less than that from more active lava lakes; Pele typically exhibits a radiant flux density of ∼17 kW/m2 (Davies et al., 2010). The latter value is of the same order of magnitude as the radiant flux density from Loki Patera’s hot component, consistent with ongoing activity, albeit over a small fraction of the patera. Please

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5. Discussion 5.1. Loki Patera’s 1999 brightening event Howell et al. (2001) summarized data of a brightening event at Loki Patera, which started between August 25 and September 9, 1999 and continued through December of that year. We show their data at 3.39 and 3.81 μm in Fig. 7 (we use the same scaling between 3.81 and 3.39 μm as in Howell et al.’s Fig. 1), together with our L band LWS data point (in red). As mentioned by Howell et al. (2001), Loki Patera started to brighten around August 25, and reached its peak in early October. Its intensity remained high throughout December, after which it declined, to rise again in February 20 0 0. The scaling factor of 1/2.125 as used by the authors to match the longer wavelength data to the shorter wavelength data (see Fig. 7) implied a temperature of ∼350 K. The authors further report a constant resurfacing rate of 1160 m2 s−1 in late October through November. Our data were taken during the brightening phase of Loki Patera. Since we also have data at much shorter wavelengths than those presented by Howell et al., we are sensitive to higher temperatures; as shown above, we measure a hightemperature component of ∼625 K. The resurfacing rate we measured during the rising-intensity phase is about two times higher decades

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Table 9 1T Model Fits to Loki Patera eruptions. Date

1T fit

1T Fit

1T Fit

(Year/month/date/∼h)

Temperature K

Areaa 10 0 0 km2

Total Power TW

1998/07/12/

415 ± 4

21.5

36.2 ± 1.5

1998/07/28/

397 ± 4

21.5

30.2 ± 1.1

1998/08/04/

405 ± 4

21.5

32.8 ± 1.2

1999/09/24

370.3 ± 0.1

5.5 ± 0.5

5.9 ± 0.5

2001/12/18/∼8

344 ± 20

6.5±52..37

5.2±21..46

Combined data in-eclipse and in-sunlight

2001/12/20/∼7.5

227±20 15

,200 437±1250

66±100 30

E. limb

2001/12/23/∼11

318 ± 4

21.5

12.4 ± 0.7

2001/12/28/∼12

405±65 45

2.1±51..42

3.2±31..12

2001/12/28/∼14

.0 13.8±36 7.7 3.1±81..08 1.7±41..70 0.6±10..48 2.5±71..45

3.8±31..53

2002/02/21/∼10

328±40 30 394±60 40 444±85 55 508±120 70 426±80 50

2002/11/12/∼14

345 ± 3

21.5

17.2 ± 0.6

2003/03/09/∼5

311 ± 4

21.5

11.4 ± 0.7

2004/05/28/∼5

329±40 30

.8 8.9±22 4.9

5.9±72..05

2005/05/31/∼6

265±40 30

99±590 62

28±77 15

PSF problems; Ref.7 in Table 1

2007/04/03/∼14.5

Faint at Lp; near limb

2009/07/24/∼11

338 ± 20

2009/07/26/∼10.5

409±70 45

2009/08/16/∼10

392±30 20

2009/09/10/∼5

2013/08/20/∼15.5

456±35 30 488±105 65 478±95 60 459±85 55 443±85 55 325±50 35 290±30 25 486±100 65 337±40 30 443±25 20 559±110 65 313±30 20 287±25 20 375±40 20 385±20 15 514±40 60 499±45 35

48±150 28 22±60 12 19±23 10 44±39 19 2.9±81..06 9.4±84..40 2.9±21..31 1.1±30..73 0.5±10..34 0.8±20..52 1.3±30..76 31±110 19 176±448 98 1.5±40..92 7±16 4

12±20 6

2008/05/11/∼14

259±30 20 267±30 20 311±25 20

2013/08/22/∼15.5

540 ± 40

1.2±10..50

6.0±11..81

2013/11/18/∼16

–

–

–

2014/01/20/∼08

307±75 40

.4 7.4±65 4.9

.4 3.7±12 2.1

2014/02/08/∼06

326±40 25

.8 5.9±11 3.2

3.8±31..86

2014/02/10/∼05

351±20 15

4.0±31..06

3.4±11..50

2014/10/31/∼16

434±60 40

3.6±51..69

7.1±42..43

2014/12/02/∼16

335±30 20

.6 21.7±24 10.5

15.6±95..46

2015/01/12/∼16

338±25 20

.6 10.9±10 5.0

8.1±32..37

2002/02/21/∼6 2002/02/21/∼7.5 2002/02/21/∼9

2007/08/14/∼6.5

2009/09/10/∼9 2010/07/27/∼12 2010/07/27/∼13 2010/08/21/∼15 2010/09/09/∼09 2010/09/18/∼09 2010/09/20/∼11 2011/07/28/∼14.5 2011/11/10/∼08 2011/11/12/∼10 2012/11/05/∼10 2013/01/20/∼04.5 2013/02/26/∼05.5 2013/02/28/∼05 2013/08/15/∼15.5

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0.7 ± 0.5

.7 9.1±10 3.8

4.2±41..36 2.3±10..79 4.7±41..88

6±93 10±74 32±16 10

Not photometric

4.5±41..67 12.6±53..67 6.9±21..56 3.6±31..01 1.6±10..52 2.1±10..78 2.7±21..60 20±30 9

Gemini; Ms non-AO

70±90 30

Gemini; Ms non-AO

4.8±31..95

Gemini; Ms non-AO

5.2±52..41 1.6 ± 0.5

0.1±00..21

0.5±00..31

.0 8.8±15 4.7

4.8±42..20

.0 14.7±24 7.8

5.7±52..13

1.7±10..88

1.9±10..70

1.5±00..96

1.8±00..65

1.2±30..52

4.9±31..51

1.8±10..75

6.3±21..24

Adjusted intensities; see Table 7

Non-photometric and too close to limb

2015/03/31/∼05

344±25 20

4.4±42..10

345±20 15

4.1±21..75

3.5±11..81

2015/04/02/∼05 2015/04/04∼06

305 ± 2

21.5

10.6 ± 0.2

2015/11/23/∼15.5

307±25 20

5.7±72..48

2.9±21..01

2015/12/25/∼12.5 2016/01/22/∼13.5

421±30 20 505±35 25

2.3±11..70 1.8±10..71

4.1±11..52 6.5±11..94

a

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Comments

3.2±10..92

If only one data point was available, we set the area equal to Loki Patera’s area, 21,500 km2 .

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Fig. 5. Representative 1-T blackbody fits to the data; the results of all fits are listed in Table 9.

Table 10 1T and 2T model fits to Loki Patera’s 1999 eruption. Date

1T Fit Temperature A K

1T Fit Area 10 0 0 km2

1T Fit Total Power TW

2T Fit Temperature A K

2T Fit Area A km2

2T Fit Temperature B K

2T Fit Area B 10 0 0 km2

2T Fit Total Power TW

1999-Sep-24

370.3 ± 0.1

5.5 ± 0.5

5.9 ± 0.5

626 ± 9

130 ± 20

280 ± 2

17.3 ± 1.1

7.2 ± 0.5

Table 11 2-component IFM model fit to Loki Patera’s 1999 eruption. Component 1999-Sep-24 High T comp: Low T comp:

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Total Power

Total area

dA/dt

GW

km2

m2 /s

537 ± 9 7700 ± 350

50 ± 1 17,500 ± 800

2200 ± 80 645 ± 100

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Oldest Surface age 6.3 ± 0.2 hrs 315 ± 60 days

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Oldest Surface temp K

2-micron Radiant flux GW/micron

5-micron Radiant flux GW/micron

2:5 micron Radiant flux ratio

577 ± 2 254 ± 6

19.5 ± 0.5 6.8 ± 1.0

75 ± 1 225 ± 30

0.260 ± 0.004 0.030 ± 0.001

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Fig. 6. Blackbody (1-T and 2-T) and a dual-component IFM fit to the spectral intensity data from UT 24 September 1999. The dotted line in Panel (d) shows the Matson et al. (2006) overturning lake model as modified by de Kleer and de Pater (2017) superposed on the 1999 spectrum. The dashed line shows the same model, including a high temperature (1100 K) component, covering an area of 4 km2 , to fit the short-wavelength part of the spectrum.

than during the peak activity. At this high a resurfacing rate, the entire patera floor would be resurfaced in ∼125 days. Galileo’s PPR instrument obtained high-resolution images of Loki Patera on 11 October 1999 and 20 February 20 0 0 (Spencer et al., 20 0 0). The changes that were seen between these observations were attributed to flowing lava or to a front in a lava lake moving from the southwest corner of the patera, where the hottest area was seen on 11 October 1999, to the eastern portion of the patera. 5.2. Timeline of Loki Patera brightening events Loki Patera has been observed between 1987 and 2007 by Galileo/NIMS (Davies et al., 2012), and many observers using ground-based telescopes, including Rathbun et al. (2002), Rathbun and Spencer (2006, 2010), and Howell et al. (2001). Data obtained with the Gemini N. telescope in 2010 were published in de Pater et al. (2014b), and those obtained since mid-2013 were presented by de Kleer and de Pater (2016a, 2017). Fig. 8 shows all data, in units of GW/μm (i.e., the intensities given in units of GW/sr/μm were multiplied by a factor of π – see de Pater et al., 2016a), from 1987 through 2016. We omitted data taken when Loki Patera was near Io’s limb (Tables 6 and 7, at θ > 70°), since under those circumstances its intensity may be significantly decreased (for details see de Kleer and de Pater, 2017). When Rathbun et al. (2002) first analyzed the data taken between 1987 and 2001, they noticed a ∼540 day periodicity in their data. They considered periodic resurfacing by lava flows and periodic overturns of a lava lake. They favored the overturning lake scenario, where the lake starts to overturn in the south-west corner, and moves towards the east. No major Please

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brightenings at Loki Patera were detected between 2002 and the 2009 event reported in this paper. As shown, this eruption signaled a renewed periodic activity of Loki Patera, albeit with a period that might seem somewhat shorter. De Kleer and de Pater (2017) presented a model fit to the 2013– 2016 data of Loki Patera based upon Matson et al. (2006) magma sea model. Their preferred model shows a wave with an overturning front that propagates at a velocity of 1.2–1.5 km/day, corresponding to a resurfacing rate of 150 0–180 0 m2 s−1 . This number is in between the resurfacing rate of 2200 m2 s−1 derived from our 1999 data, and the resurfacing rate of 1160 m2 s−1 derived by Howell et al. (2001) during the later part of the 1999 eruption. De Kleer and de Pater’s set of input parameters to the Matson et al. (2006) model shows time intervals of ∼440–540 days between successive events, as observed. We show the de Kleer and de Pater model output (at 3.8 μm) superposed on the entire time sequence in Fig. 9; the only parameter that is changed between brightenings is the interval between the end of one overturn event and the start of the next. Even though there were large gaps in the data between 2004 and 2009 where, in principle, overturns could have been missed (indicated as dashed curves), it is clear that conditions changed: either all brightening events ceased, or the timing between them became very irregular. Since 2009 the patera became as active as before 2004; five brightening events were observed, and there is some evidence in the temperature and intensity timeline that a sixth occurred (in 2012) but went mainly undetected. The overturning lava lake model output of de Kleer and de Pater (2017) is also overplotted on the LWS/NIRSPEC data in Fig. 6d, decades

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Fig. 7. A time line of the 1999 event, using the data published by Howell et al. (2001) (open and filled black circles), and our 3.85-μm LWS data point (in red). The 3.39 μm data (open circles) were taken from WIRO (Wyoming Infrared Observatory) in-eclipse occultation disappearances and reappearances, and occultations and direct imaging results using the IRTF (InfraRed Telescope Facility). The 3.81 μm (solid black dots) were derived from adaptive optics images obtained at the 3.6-m ESO telescope in Chile. The 3.39 μm data (axis on the right) were scaled to the 3.8 μm data (axis on the left) using a factor of 2.125; this factor agrees with that expected based on blackbody curves for a temperature of 350 K (Howell et al., 2001). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to evaluate the wavelength dependence of the model. The model is not a fit to the data, but is produced by generating the predicted temperature distribution across the 21,500-km2 lake ∼24 days after the start of a brightening event, assuming an overturn wave propagating at a speed of 1.0 km/day (a speed consistent with the propagation speed derived from NIMS data by Davies, 2003). The model fits well at wavelengths >2 μm. However, a small area at a high temperature must be added to the model in order to fit the short-wavelength observations. The model shown includes an area of 4 km2 at 1100 K in addition to the emission predicted by the standard overturn model, and provides an excellent fit to the entire spectrum. The presence of such a high-temperature component is not surprising, since the eruption is clearly young and on-going. The dual-component IFM fit shows a continuum of temperatures with 1475 K at the upper end (melting temperature of basalt) and

Fig. 8. Timeline of Loki observations at a wavelength of 3.5–3.8 μm, in units of GW/μm. The black data points are from Rathbun et al. (2002), Rathbun and Spencer (2006); these data were taken with the IRTF at 3.5 and 3.8 μm and with the Wyoming Infrared Observatory (WIRO) at 3.39 and 4.8 μm, and translated to a wavelength of 3.5 μm assuming a color temperature of 355 K for the 4.8 μm data, and 500 K for the 3.39 and 3.8 μm data (see Rathbun et al., 2002, and Spencer et al., 1992, for details). The green data points have been derived from Galileo NIMS data at 3.5 μm (Davies et al., 2012). The red data are from the current paper and de Kleer and de Pater (2016a, 2017); these data are at a wavelength of 3.8 μm, and no wavelength conversion has been applied. The dotted lines connect data, to help guide the eye. Only intensities at emission angles less than ∼70° are plotted. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

∼575 K at the lower end, i.e., lava that had cooled for 6.5 h. This area is likely concentrated in the region of active resurfacing, but would include hot areas elsewhere in the patera floor, for example, where there are cracks in the relatively cool crust revealing hot material beneath. In Fig. 10 we compare the brightening events with the temperatures listed in Table 9; we also superpose the Matson et al. (2006) magma sea model, using the de Kleer and de Pater (2017) parameters. Although the measurements near Io’s limb at θ > 70° were omitted from the intensity data, we do plot the temperatures as measured near the limb; to distinguish these from other values, we plot them as open squares. As shown, these numbers typically agree well with the other temperature data. This is not too surprising, since we expect the intensity measurements near the limb to decrease by the same fraction at all wavelengths. Small differences may arise because there is a small time delay between the various measurements. It is also possible that, due to the near-limb viewing angle, the front of a lava flow or wave in an overturning lava lake is viewed in an optimal way, thereby increasing the observed temperature over what would have been measured if Loki Patera would be at the center of the disk. The presence of small lava fountains would also yield higher than

Fig. 9. Comparison of the timeline of Loki observations at a wavelength of 3.5–3.8 μm as in Fig. 8 (plotted here in units of GW/μm/sr) with superposed the overturn model of Matson et al. (2006), as modified by de Kleer and de Pater (2017). The only parameter that is changed to match the data is the time interval between the end of one overturn event and the start of the next. The dotted brightenings indicate events that could have happened between 20 04 and 20 09 without being observed. As in Fig. 8, the black data points are from Rathbun et al. (2002), Rathbun and Spencer (2006); the green data points from Galileo NIMS (Davies et al., 2012); the red data are from this paper and de Kleer and de Pater (2016a, 2017). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 10. Comparison of the timeline of Loki observations and the overturn model as shown in Fig. 9 with surface temperatures in blue derived from 1-T blackbody fits to the data, as listed in Table 9. The open squares correspond to temperatures derived from data when Loki Patera was close to the limb (θ > 70°). Note that the corresponding intensity values for these times have not been plotted. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Locations of eruptions at Loki Patera as tabulated in Table 8, Columns 8 and 9. Only locations with errors less or equal to 1.5° are shown.

expected short wavelength thermal emission at high emission angles than if the thermal source acted as a flat plate. As shown in the figure, there appears to be a reasonably good, though not perfect, correlation between the 3.8-μm intensity and surface temperature. To quantify this apparent correlation, we calculated the Pearson-R and Spearman-R coefficients for all dates where we have both intensity and temperature measurements (ignoring the dates where Loki Patera was near Io’s limb). The Pearson-R coefficient measures the degree of linear correlation between the datasets and ranges from −1 (inversely correlated) to +1 (directly correlated). The Spearman-R coefficient assesses monotonic relationships. The coefficients are between 0.70 and 0.75, i.e., there is indeed a good correlation. It is interesting that the high temperature observed just before the “gap” in 2012 might be indicative of a brightening event that took place during this period, but was missed because there were no observations. If true, the highest temperature measured in that period (open square) might be explained by viewing the event in an optimal way, such as the scenarios mentioned above. 5.3. Location of eruptions within the patera Fig. 11 shows the locations of eruptions at Loki Patera as determined from Keck observations (Columns 8 and 9 in Table 8). Only Please

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Fig. 12. Locations of various brightening events: (i) a sequence of three events in 2009, during the peak of the brightening event (white boxes 1, 2, and 3), (ii) the onset of a brightening event in 2010 (yellow boxes), (iii) the location during the peak of a brightening event in August 2013 (red box), and (iv) the onset of a brightening event, recorded in December 2015 and January 2016 (blue boxes). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

locations with uncertainties less than 1.5° in longitude and latitude are shown. Eruptions take place all around the horseshoe-shaped lake, although this figure together with Fig. 10 from de Kleer and de Pater (2017) featuring all 2013–2016 data (i.e., including Gemini data), is suggestive of an overall higher activity in the southern part of the patera than in the north. Some activity may come from the “island”, which may be related to the warm cracks observed by Galileo NIMS (Lopes-Gautier et al., 20 0 0). In 20 09, during the height of Loki Patera’s first eruption since it had quieted down in 2002, three observations were made at intervals of roughly 3 weeks: the first one on 24 July, the second one on 16 August, and the third one on 10 September. The location of these eruptions is indicated by white boxes on Fig. 12, and shows a clear clockwise progression, i.e., it moved from the east towards the west, along the south side of the patera. This progression in the patera can, in fact, be seen directly on the 2.27 μm images in Fig. 3. This movement is in the opposite direction to what was inferred in 1999–20 0 0 from Galileo PPR observations (Spencer et al., 20 0 0), and from that implied by the 1998 Keck data from Macintosh et al. (2003). In 2010 we observed the onset of a new eruption: we decades

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observed bright emission in the north-east part of the patera shortly after onset. These locations are indicated with yellow boxes on Fig. 12. We have no data of the eruption’s subsequent evolution. In August of 2013 we observed Loki Patera while it was at its height of an eruption (red box in Fig. 12); and in Dec. 2015/Jan. 2016 we again observed the onset of a new eruption (blue boxes on Fig. 12). The locations of these eruptions, some caught at their onset, some in the middle, and some over a 3-month period, are highly suggestive of eruptions starting in the north or north-east part of the patera, and propagating in the clockwise direction to and around the south side of the patera. Given their locations, the eruptions may be triggered near the edge of the horseshoe-shaped “lake”, perhaps by some event (such as an eruption) just outside the lake. The observations together are suggestive of a change in the propagation direction from counter-clockwise before 2004, to clockwise afterwards. De Kleer and de Pater (2017) present Keck and Gemini observations from 2013–2016, including the evolution of three brightening events. They overplotted the locations of their data on Loki Patera using different colors for the phase of the brightening event. Those data also show a clear progression from the east side of the patera early during the event, to the south-west corner towards the end of the event. Such a progression in the location of the eruption is highly suggestive of an overturning lava lake, with the front of the overturning wave propagating around the lake in a clockwise direction.

6. Conclusions We present observations of Loki Patera obtained with the Keck telescopes between 1998 and 2016. We augmented our data with observations from the Gemini N telescope in 2010 (de Pater et al., 2014b), and in 2013–2016 (de Kleer and de Pater, 2016a, 2017); these data, together with observations taken between 1987 and 2007 by Rathbun and Spencer (2006), Rathbun et al. (2002) provide a database covering 3 decades of time. The latter authors showed periodic brightening events at Loki Patera from 1987 until 2002 with a period of ∼540 days. By extending the previous dataset through 2016, we show that after the apparent cessation of brightening events in 2002, renewed activity commenced in 2009. The timeline of observations agrees well with the overturning lake model of Matson et al. (2006) as modified by de Kleer and de Pater (2017), when the intervals between overturning events are adjusted slightly to match the data. The more recent (2009–2016) activity may have a slightly shorter periodicity (∼420–480 days) compared to the ∼540 days found by Rathbun et al. (2002) for the pre-2002 activity. A comparison with this model further shows that three brightening events may have taken place (undetected) between 2004 and 2009. Although only five brightening events were detected from 2009–2016, the model comparison is strongly suggestive of an event in 2012, during a period without observations. Our blackbody temperature fits to the existing data just prior to the “undetected event”, show hints of such an event. Careful analysis of the precise location of the eruption events within the patera shows unambiguously that the propagation direction of eruptions has reversed compared to that seen by the Galileo spacecraft in 2001. Since 2009 resurfacing is in the clockwise direction, starting in the north or north-east corner and propagating along the patera towards the south-west. During the Galileo era the propagation was in the counter-clockwise direction, at least in the southern half of the patera, starting in the south-west and propagating towards the east. Hence, regardless of whether there was a cessation in events from 20 04–20 09, it is clear that something changed significantly at Loki Patera during that period to cause a reversal in resurfacing direction. Please

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We further present unique data taken during a brightening event that started in late August 1999 and continued through early January 20 0 0, an event that was recorded by Howell et al. (2001). Our data were taken on UT 24 September 1999 at wavelengths ranging from 1.6 μm up to 12 μm. These new data were analyzed with a dual-component Io Flow Model, and reveal the emergence of relatively hot magma, at ∼600 K, no older than 6–7 h, in addition to a ∼250 K temperature component over the entire patera floor, that is almost 1 year old. The hot magma is resurfacing Loki Patera at a rate of ∼2200 m2 s−1 . This is two times higher than the rate derived by Howell et al. during the peak of the brightening event. The 1.6–12 μm spectrum compares well with that produced by de Kleer and de Pater’s (2017) modifications to the Matson et al. (2006) overturning lake model, although the addition of a 1100 K temperature component, covering an area of ∼4 km2 , would provide a better fit to the short wavelength (NIRSPEC) data. Acknowledgments The data presented in this paper were obtained at the W.M. Keck Observatories. The Keck Telescopes are operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Our research was partially supported by the National Science Foundation, NSF grant AST-1313485 to UC Berkeley. Ashley Davies thanks the NASA Outer Planets Research and Planetary Geology and Geophysics Programs for support. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations of Ionian volcanoes from this Hawaiian volcano. References Conrad, A., et al., 2015. Spatially resolved M-band emission from Io’s Loki Patera— Fizeau imaging at the 22.8 m LBT. Astron. J. 149 (5), #175. Davies, A.G., 1996. Io’s volcanism: thermo-physical models of silicate lavas compared with observations of thermal emission. Icarus 124 (1), 45–61. Davies, A.G., et al., 2001. Thermal signature, eruption style, and eruption evolution at Pele and Pillan on Io. J. Geophys. Res. 106, 33079–33104. Davies, A.G., 2003. Temperature, age and crust thickness distributions of Loki Patera on Io from Galileo NIMS data: Implications for resurfacing mechanism. J. Geophys. Res. Lett. 30 (21), 2133. doi:10.1029/2003GL018371. Davies, A.G., 2007. Volcanism on Io: A Comparison with Earth. Cambridge University Press, p. 372. Davies, A.G., Keszthelyi, L.P., Harris, A.J.L., 2010. The thermal signature of volcanic eruptions on Io and Earth. J. Volcanol. Geotherm. Res. 194 75–99. doi:10.1016/j. jvolgeores.2010.04.009. Davies, A.G., Veeder, G.J., Matson, D.L., Johnson, T.V., 2011. Io: charting thermal emission variability with the Galileo NIMS Io Thermal Emission Database (NITED): Loki Patera. Geophys. Res. Lett. 39, L01201. doi:10.1029/2011GL049999. Davies, A.G., Veeder, G.J., Matson, D.L., Johnson, T.V., 2012. Io: charting thermal emission variability with the Galileo NIMS Io Thermal Emission Database (NITED): Loki Patera. Geophys. Res. Lett. 39, L01201. de Kleer, K., de Pater, I., Davies, A.G., Adamkovics, M., 2014. Near-infrared monitoring of Io and detection of a violent outburst on 29 August 2013. Icarus 242, 352–364. de Kleer, K., de Pater, I., 2016a. Time variability of Io’s volcanic activity from near-IR adaptive optics observations on 100 nights in 2013-2015. Icarus 280, 378–404. de Kleer, K., de Pater, I., 2016b. Spatial distribution of Io’s volcanic activity from near-IR adaptive optics observations on 100 nights in 2013-2015. Icarus 280, 405–414. de Kleer, K., de Pater, I., 2017. Io’s Loki Patera: modeling of three brightening events in 2013-2016. Icarus 289, 181–198. de Pater, I., Roe, H.G., Graham, J.R., Strobel, D.F., Bernath, P., 2002. Detection of the forbidden SO a1 →X3  − rovibronic transition on Io at 17 μm. Icarus 156, 296–301. de Pater, I., Marchis, F., Macintosh, B.A., Roe, H.G., Le Mignant, D., Graham, J.R., Davies, A.G., 2004. Keck AO observations of Io in and out of eclipse. Icarus 169, 250–263. de Pater, I., Gibbard, S.G., Hammel, H.B., 2006. Evolution of the dusty rings of Uranus. Icarus 180, 186–200.

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