Dynamics and Evolution of SO2 Gas Condensation around Prometheus-like Volcanic Plumes on Io as Seen by the Near Infrared Mapping Spectrometer

Icarus 158, 460–482 (2002) doi:10.1006/icar.2002.6889

Dynamics and Evolution of SO2 Gas Condensation around Prometheus-like Volcanic Plumes on Io as Seen by the Near Infrared Mapping Spectrometer Sylvain Dout´e1 Laboratoire de Plan´etologie de Grenoble, Bˆat. D de Physique, B.P. 53, 38041 Grenoble Cedex 9, France E-mail: [email protected]

Rosaly Lopes, Lucas W. Kamp, and Robert Carlson Jet Propulsion Laboratory, 4800 Oak Grove, Pasadena, California 91109

Bernard Schmitt Laboratoire de Plan´etologie de Grenoble, Bˆat. D de Physique, B.P. 53, 38041 Grenoble Cedex 9, France

and The Galileo NIMS Team Received September 28, 2001; revised April 3, 2002

We analyze a series of spectral image cubes acquired by the Galileo Near Infrared Mapping Spectrometer (NIMS) over the Prometheus region of Io. We use SO2 frost, a volatile compound ubiquitous on the surface, as a tracer to understand various thermodynamic and volcanic processes acting in this equatorial region. Here we develop a new method to derive, from the 12-wavelength NIMS products, the distribution and physical properties of solid SO2 . This method is based on the inversion of a bidirectional reflectance model on two observed spectral ratios sensitive to (1) the areal abundance of SO2 and (2) its mean grain size. As a result, reliable and consistent maps of SO2 abundance and granularity are obtained which can be correlated to distinguish four different physical units. The distribution of these SO2 units indicates zones of condensation, metamorphism, and sublimation linked with the thermodynamic and volcanic processes of interest. Our maps depict equatorial plains undisturbed by any kind of vigorous volcanic activity over 35–40% of their surface. Elsewhere, 10–20% of the equatorial plains display abnormally low frost coverage which may imply the recent presence of positive thermal anomalies with temperatures in the range 110–200 K. Hot-spots such as Prometheus, Culann, Surya, and Tupan (to mention the most persistent) emit a great variety of gases, some of which will condense at Io’s surface near their source regions. Associated fields of freshly condensed SO2 are easily observed, and deposits of more refractory compounds with higher (e.g., S8 ) or lower (e.g., NaCl) molecular weight must also be

present (although their exact nature is unknown). Three different mechanisms of emission are proposed for the volatile compounds and supported by the distribution maps. These are (a) the interaction between flowing lava and preexisting volatile deposits on the surface, (b) direct degassing from the lava, and (c) the eruption of a liquid aquifer from underground. The geometric elongation of the Prometheus SO2 deposition ring being related to the development of a 95-km-long lava field is the best illustration of mechanism (a). Details of the progressive emplacement of the SO2 ring by the associated plume are examined by the development of a semiempirical model of material deposition based on a ballistic transfer from the sources to the surface. This model shows that lava emission may have been occuring at Prometheus at a fairly constant rate since Voyager. Mechanism (b) may operate at the hot-spot Surya, which presents a noticeable field of fresh SO2 frost but no extended lava flow. Finally, we have noted on the northwestern flank of the volcanic edifice Emakong the existence of an extremely deep ν1 + ν3 SO2 absorption which is indicative of abundant, pure, and perhaps icy SO2 deposits. These could be the result of the eruption of an SO2 liquid aquifer (mechanism (c)). c 2002 Elsevier Science (USA) Key Words: satellites of Jupiter; Io; infrared observations; ices; volcanism.

The close Io fly-bys of October 1999 (I24), November 1999 (I25), and February 2000 (I27) by Galileo represent a unique opportunity to investigate the volcanic activity of the satellite. Much valuable information has been obtained about the eruption

1

Initially at Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 405 Hilgard, Box 951567, Los Angeles, California 90095-1567. 460 0019-1035/02 $35.00 c 2002 Elsevier Science (USA)  All rights reserved.

1. INTRODUCTION

SO2 GAS CONDENSATION AROUND VOLCANIC PLUMES ON IO

of materials (liquids, pyroclastics, and gas) onto the surface or into the atmosphere. That information includes the physical conditions (temperature and pressure) prevailing at the source, the chemical nature of the emitted species, the emission rate and style, and the history of the eruptions. Indeed, the data can be exploited to refine and constrain key models of our understanding of Io’s interior, surface, and atmosphere: structure and dynamics of the upper mantle and lithosphere (Keszthelyi and McEwen 1997, Tackley et al. 2001), superficial geological processes such as tectonics, resurfacing, etc. (Turtle et al. 2002), and atmospheric composition and dynamics (Moses et al. 2002a,b, Austin and Goldstein 2000). The regions devoid of volcanic activity but displaying higher than average geothermal flux also constitute targets of great interest. These seem to cover quite large areas and contribute substantially to Io’s global thermal output, a strategic quantity when modeling the geophysics of the satellite (Veeder et al. 1994). The recently acquired data should allow the characterization (temperature) and mapping of these warm regions. Temperatures of silicate lava at different volcanic centers were inferred from the thermal emission of the hot-spots measured in the visible and infrared (IR) by the Solid State Imager (SSI) and the Near Infrared Mapping Spectrometer (NIMS) instruments (McEwen et al. 1998, Davies et al. 1997, 1999, 2000, LopesGautier et al. 2002). Magma temperatures have been seen to exceed 1700 K. Sometimes the eruption age, mass rate, and style of lava emplacement were also accessible from the same data, thanks to high-resolution images (Keszthelyi et al. 2002) and silicate cooling models (Davies 1996, Davies et al. 2000, Davies 2001). Assuming a temperature of 1760 ± 210 K for Pele magma, as evaluated by Lopes-Gautier et al. (2002) from NIMS data, Zolotov and Fegley (2001) calculated a vent pressure of 0.01 to 2 bars. For that purpose, they used thermochemical equations of volcanic gases at the equilibrium and at high temperatures. This set of equations is constrained by molecular ratios derived from Hubble Space Telescope (HST) measurements of SO2 , SO, and S2 abundances within the plume and gives in addition the oxidation state, the bulk O/S ratio, and the complete chemistry of the volcanic gases. In such a model, only two atomic elements (S and O) are considered, the chemical reactions are modelled as being quenched at the end of the eruption conduit, and photochemical processes in the aerial phase are supposed to have a limited effect on the three principal emitted gases (SO2 , SO, and S2 ). Thermochemical systems with a larger number of atomic elements (O–S–Na–K–Cl–H) were also explored with different imposed chemical ratios (constrained from plasma torus, extended atmosphere, and chondritic abundances) and a broader range of temperatures and pressures (Fegley and Zolotov 2000). Species like NaCl, KCl, KS, SCl, S2 Cl, Na2 SO4 , and SOCl are emitted in the plume along with SO2 , SO, and S2 and can condense at the surface. Other species (for example, S) can be rapidly processed by further chemical reactions activated by solar ultraviolet (UV) photons (Moses et al. 2002b).

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Unfortunately, the list of compounds directly identified in the atmosphere and on the surface is still quite limited. Pearl et al. (1979) detected SO2 in Loki plume with the Voyager Infrared Imaging Spectrometer (IRIS), and Spencer et al. (2000a) detected S2 , as well as SO2 , in Pele plume with the Hubble Space Telescope (HST). On the other hand, millimeter observations by Lellouch et al. (1992, 1996) showed that the dense and patchy component of Io’s atmosphere is formed of SO2 and SO gas. White, yellow, red, orange, and brown hues that give Io’s surface its colorful appearance suggest abundance of sulfurbearing compounds such as SO2 , native and short chain sulfur, sulfur oxides, and sodium sulfides (Geissler et al. 1999, Spencer et al. 1997). Nevertheless, none of these compounds has been clearly identified, except SO2 , thanks to its numerous and distinctive absorption bands in the NIR (near infrared) (Smythe et al. 1979, Howell et al. 1989, Schmitt et al. 1994, Carlson et al. 1997), and perhaps SO2 Cl2 (Schmitt et al. 2001). Since the beginning of Io’s monitoring by Galileo, description of the forms, modalities, and temporal evolution of volcanic eruptions has been the subject of numerous papers (see, for example, McEwen et al. 1998, Lopes-Gautier et al. 2000, McEwen et al. 2000, Spencer et al. 2000b, Keszthelyi et al. 20025, Davies 2001). Formation of plumes composed of gas and particles rising through the atmosphere is one of the most striking manifestations of this kind of activity. Kieffer (1982) theorized their internal thermodynamics and dynamics. Three opposing approaches are adopted to study their subsequent expansion into the nearvacuum of Io’s open space. First, the approach represented by the stochastic–ballistic models assumes that, before sticking to the surface at impact, the plume particles follow trajectories initially randomized near the vent and then governed by the classical equations of point mechanics under Io’s gravitation only (Cook et al. 1979, Glaze and Baloga 2000, Baloga and Glaze 2001). For certain conditions (energy distribution), these models satisfactorily reproduce the characteristics of some plumes (e.g., Prometheus) such as the regular, umbrella-like plume shape and the associated deposition ring. Second, is the approach represented by gas/particle dynamic models, which should be relevant if the fluid density remains sufficiently high along the flow (Moreno et al. 1991, Symthe et al. 2001). These models predict that after a nearly vertical expansion, gas falls back to the surface creating a shock front which deflects the flow horizontally. Then the gas experiences further expansion, cools, and condenses, possibly after one or two rebounds. According to Smythe et al. (2001), plumes can sometimes create their own atmosphere, and pyroclastic flows can be formed under certain conditions. If the gas mass fraction is on the order of 30%, the numerical modeling produces an umbrella-like shape as in the ballistic model. At present, the gas/particle dynamic models do not include proper treatment for gas condensation in the aerial phase. Finally, a third class of plume models was recently developed to overcome the main limitations of the fluid dynamic method: the direct simulation Monte Carlo models (DSMC) (Zhang et al. 2002). The DSMC models can handle

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rarefied flows, calculating the individual trajectories of millions of molecules undergoing gravitational and other forces, as well as statistical shocks by other molecules. The DSMC models, like the hydrodynamic models, predict that a reentry shock can form. The presence of noncondensible gas (such as O2 ) can also affect the flow behavior and thus the condensation patterns on the surface. New high-resolution observations of Prometheus by SSI and NIMS have renewed the interest in plume studies. This interest is fueled by the strange features of the Prometheus plume, which has wandered across the surface over the past 20 years while keeping constant geometric and optical properties. The most likely explanation is the violent vaporization of preexisting volatile deposits (SO2 and/or sulfur) by the advancing front of the Prometheus lava field (McEwen et al. 1998, Milazzo et al. 2002). Although they differ slightly in the thermodynamic treatment of SO2 –lava interaction and in the emplacement and number of gas sources, two models, respectively proposed by Keiffer et al. (2000) and Milazzo et al. (2002), manage to explain at least qualitatively the existence and characteristics of the Prometheus plume. In this paper, following concepts similar to those in Dout´e et al. (2001b), we use SO2 frost as a tracer to collect complementary information about the eruption of materials by Io’s volcanoes and to map the thermally active areas, with an emphasis on the Prometheus region. For this purpose, we have analyzed a series of NIMS spectral image cubes covering this region and acquired during the three close Io fly-bys (I24, I25, I27) by Galileo. We derive SO2 properties (areal abundance and granularity) from the inversion of a spectral bidirectional model on two SO2 -sensitive spectral ratios calculated for each pixel of the image cubes. As a result, reliable and consistent maps of SO2 abundance and grain size are obtained which can be correlated to distinguish four different physical units. The distribution of these SO2 units indicates zones of condensation, metamorphism, and sublimation linked with the thermodynamic, dynamic, and volcanic processes of interest (Dout´e et al. 2001b). Section 2 describes the NIMS data (2.1) as well as the assumptions and the method of their analysis (2.2). It also contains a discussion of the accuracy of the results. In Section 3, we present and interpret the maps of SO2 abundance, granularity, and physical state in order to study the general characteristics of the Prometheusregion volcanism and of the equatorial plains. Details of the dynamics and evolution of SO2 gas condensation directly linked with the Prometheus plume are examined in Section 4 by the development of a model. Finally, we summarize and discuss our results in Sections 4.5 and 5. 2. OBSERVATIONS AND THEIR ANALYSIS

2.1. Description of the Observations In October 1999, November 1999, and February 2000, Galileo accomplished three successful fly-bys of Io (I24, I25, I27), during which the NIMS made global and regional, as well as local,

TABLE I Principal Characteristics of the NIMS Image Cubes Analyzed

Observation

Target, gain state

I24INREGION02 Nearly global, GS 2

Resolution, size

Detections, comments

105 km · pixel−1 Thermal emission detected Nearly global from 13 main locations: Culann, Tupan, Malik, Prometheus, Zamama, Itzamna, Isum, Marduk, Pillan, Pele, Gabija, Ot, etc.

I24INREGION01 Prometheus 22.0–25.0 km · region pixel−1 GS2 972 × 168 km

Prometheus fallout ring from SO2 plume observed. Thermal emission detected from 8 main locations: Prometheus, Culann, Tupan, Camaxtli, (+14◦ , 150◦ W), (+22◦ , 146◦ W), (+15◦ , 139◦ W), and (+12◦ , 134◦ W).

I25INREGION01 Prometheus 14.0–25 km · region pixel−1 GS2 1760 × 770 km

Prometheus SO2 fallout ring observed. Thermal emission detected at 5 main locations: Prometheus, Chaac, (−5◦ , 132◦ W), (−2◦ , 144◦ W), and (+9.5◦ , 132◦ W). First detection of a hot-spot at Chaac.

I27INPROMTH01 Prometheus 6–7.5 km · region pixel−1 GS2 300 × 160 km

Mosaic of Prometheus lava flow and inner SO2 ring.

observations. Spectral image cubes of active volcanic centers (e.g., Prometheus, Amirani, etc.) and areas targeted for their distinct colorimetry (e.g., Tohil) were obtained with variable spatial resolution (from 0.5 km · pixel−1 to 25 km · pixel−1 ). LopesGautier et al. (2002) (see their Table I) give the complete list of acquired NIMS observations with additional information such as target name, spatial size and resolution of the scene, and detection of hot-spots. In Table I, we extract from the previous list some characteristics related to the image cubes on which we perform our study. To calibrate and test our method of analysis, we use I24INREGION02, a global observation (i.e., nearly covering a whole hemisphere of Io). The other cubes (I24INREGION01, I25REGION01, and I27INPROMTH01) cover, with increasing spatial resolution, the active volcanic center Prometheus and its associated ring of fallout, deposited from the plume. Two of them (I24INREGION01 and I25REGION01) also show additional hot-spots (e.g., Culann, Tupan, Camaxtli, Chaac, Emakong, etc.) and part of Bosphorus Regio, a region mostly devoid of hightemperature volcanism. NIMS normally produces spectra between 0.7 and 5.2 µm with 17 detectors in combination with a moving grating (Carlson et al. 1992). During the Galileo prime mission (06/96–11/97) and

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model, with global observations to determine SO2 abundance and granularity (Dout´e et al. 2001b). In this paper, we show that the inversion of the same reflectance model using two selected spectral ratios allows us to obtain a quantitative estimate of SO2 abundance and, to a lesser extent, SO2 granularity from the highspatial-resolution but low-spectral-resolution NIMS data. 2.2. Assumptions and Method of Analysis

FIG. 1. Effects of radiation-induced damages on NIMS detectors and grating illustrated by two unrelated spectra extracted in units of radiance factor from two observations (09/7/96 G2 and 10/11/99 I24). The spectral sampling is decreased from 408 (solid line) to 12 (stars and circles) wavelengths and the spectral range is reduced to 1.0–4.7 µm. The circles represent the three NIMS channels we use in this study to map and characterize the SO2 deposits.

the Galileo Europa Mission (12/97–12/99), two of the detectors malfunctioned. Additionally, just prior to the I24 fly-by, NIMS suffered a radiation-induced anomaly that stopped the movement of the grating and thus the scanning of all wavelengths. Consequently, the spectral sampling has been decreased from 408 to 12 wavelengths and the spectral range spanned by NIMS reduced to 1.0–4.7 µm, as shown in Fig. 1. The present correspondence between the NIMS detector numbers and the wavelengths they sample is also indicated. Nevertheless, the greater number of measurements at each remaining wavelength (24 instead of 1) increases the signal-to-noise ratio of the data. Fortunately, most NIMS spectra acquired over sunlit areas still sample the two most prominent and broad absorption features centered at 1.3 and 4.13 µm. The first feature is due to an absorption extending from below 1.0 to 1.5 µm which may have been revealed for the first time by Clark et al. (1986) but presents a quite different appearance in the NIMS global observations at full spectral resolution (Carlson et al. 1997). This absorption is still under investigation in order to determine the chemical nature and physical state of the responsible compound (Dout´e et al. 2001a). Comparisons with full-resolution NIMS spectra also demonstrate that the low signal at 4.13 µm relates to the strong ν1 + ν3 absorption band of solid SO2 . The latter is ubiquitous on Io’s surface where it appears as frosts of variable thickness and degrees of purity (Dout´e et al. 2001b). In addition to the ν1 + ν3 band, solid SO2 presents numerous absorptions in the range 1.0–4.5 µm. Among the bands, only three are sampled (2ν1 + ν2 , ν1 + ν3 , and 2ν1 ), unfortunately always in the wings, at 3.5659, (3.8489, 4.1326), and 4.4145 µm, respectively. The deepest absorption is measured within the ν1 + ν3 SO2 band. The insufficient sampling in the new instrument mode of operation prevents mapping of the frost abundance and granularity by direct modeling of each spectrum in the NIMS image cube. Previously, we used such a technique, based on the inversion of a bidirectional reflectance

2.2.1. Model of Io’s surface. The SO2 band widths and intensities depend on SO2 abundance, texture, and mean grain size, as well as on the presence of other materials. On Io, these materials may include different allotropes of sulfur, polysulfur oxides, sodium sulfides, and chlorine-bearing compounds like SO2 Cl2 (Geissler et al. 1999, Spencer et al. 1997, Schmitt et al. 2001). Most are neutral in the near infrared. The mixing mode of SO2 and these compounds (spatial segregation, intimate mixing at the scale of photon path lengths, vertical stratification, or a combination of these three possibilities) is also a key parameter and is thoroughly discussed by Dout´e et al. (2001b) for global-scale observations. In the next paragraph, we try to estimate how the change of spatial scale for the observations can affect this discussion and the spectral reflectance model we choose to analyze the data. At the global scale, efficient mechanisms of thermal differentiation acting on materials of different volatility (e.g., cycles of sublimation–transport–condensation) can be invoked to support the spatially segregated model (linear mixing model). Nevertheless, in the neighborhood of active volcanic centers, these processes of differentiation may not be fully expressed because of the short timescale of material emplacement. Although very high-resolution SSI images of the surrounding of Zamama and Prometheus lava flows display a relatively patchy appearance (Keszthelyi et al. 2002), intimate mixing is likely for fresh volcanic deposits. In that case, use of a linear mixing model instead of an intimate mixing model leads to overestimating the SO2 abundance. In some places, the optical thickness τ of SO2 frost may also be so small that the underlying substratum influences the reflectance spectrum of the surface and thus the SO2 bands. Spectral indicators (especially the 3.55-µm SO2 absorption) exist to evaluate τ , but they are no longer available for Io fly-by measurements with Galileo. However, the depth of light penetration at 4.13 µm, slightly shifted from the center of the strong ν1 + ν3 SO2 band, is limited to about 1 mm. As a consequence, thick frost deposits or thin layers display a comparable intensity level for the spectral channel at 4.13 µm. Even at τ as low as 1, the inversion of an infinitely thick model on the ν1 + ν3 absorption wing underestimates the areal abundance of SO2 by only 9% (Dout´e et al. 2001b). Based on the previous considerations, and because of its simplicity, we employ in this paper a model of spatially segregated, infinitely thick SO2 frost. We also assume that the frost has a granular texture. First, we think that this set of assumptions is still reasonable for a majority of pixels in the case of regional observations. Second, we can then test our method of analysis

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SO2 frost

Substratum

Unit USO2 : surface proportion f SO2

Unit U N : surface proportion f N = 1 − f SO2

Layer∞ [SO2 ] • optical index SO2 : lab. 125 K (Schmitt et al. 1994) • mean grain size: DSO2 • porosity: “mute” parameter • HG scattering parameter: gSO2

Layer ∞ [neutral components] • reflectance: R N

with an I24 global image cube by comparison with earlier results derived with the same kind of model (Dout´e et al. 2001b). Third, in the worst situation (presence of intimate mixtures), retrieved SO2 abundances will constitute an upper limit. This information is already of interest. Table II graphically represents the model we use for Io’s surface at NIMS pixel scale and lists the various related parameters: the areal abundance (or surface proportion) f SO2 , the mean grain size DSO2 , and the Henyey Greenstein (HG) scattering parameter gSO2 of SO2 frost on one side and the reflectance R N of the neutral components on the other side. A more precise description of these parameters can be found in Dout´e et al. (2001b). Under these conditions, the spectrum R of each pixel within a given image cube can be approximated by 







R = f SO2 RSO2 DSO2 ; gSO2 + 1 − f SO2 R N ,

1. Find at least two observables (observed reflectance values or a combination) primarily sensitive to f SO2 and DSO2 , respectively. This is a requirement to be able to invert Eq. (1) without encountering an ill-conditioned problem. 2. Modify our observables to improve their sensitivity with changes of f SO2 and DSO2 . 3. Reduce the effect of a possible nonnegligible absolute radiometric uncertainty (we assume the relative uncertainty to be better than 1%). Considering the behavior of the synthetic spectra with f SO2 and DSO2 at the wavelengths of NIMS channels, two specobs obs = Robs tral ratios, α obs = Robs p,q (3.28)/R p,q (4.13) and β p,q (3.28)/ obs R p,q (3.56), could fill our needs. They are the observed reflectance values at 4.13 (deep on the wing of the ν1 + ν3 absorption band) and 3.56 µm (in the 2ν1 + ν2 band wing) relative to the reflectance at 3.28 µm, an SO2 nonabsorbing wavelength. The 3.28-µm channel is among the channels falling in the continuum

1.2 1.0

Reflectance

TABLE II Model of Io’s Surface and Related Parameters Used for Our Analysis

0.8 0.6 DSO = 250 µm R N = 0.5 2

f SO =

0.4

2

1.0 0.8 0.6 0.4 0.2

0.2

(1)

with RSO2 (DSO2 ; gSO2 ) the spectral bidirectional reflectance of pure granular SO2 of mean grain size DSO2 for the incident, emission, and phase angles prevailing at (p,q). RSO2 is calculated by the radiative transfer algorithm of Dout´e and Schmitt (1998). The scattering parameter is fixed at gSO2 = −0.27, following the analysis of the mean photometry of SO2 frost by Dout´e et al. (2001b). 2.2.2. Seeking good observables. Using Eq. (1), we can assess the theoretical dependence of the 12 NIMS spectral channels with solid SO2 abundance and granularity. Figure 2 shows two series of high-resolution synthetic spectra where R N remains constant and equal to the mean reflectance of various kinds of sulfur (R N 0.5; Kargel et al. 1999) while f SO2 and DSO2 vary within realistic domains: 0.2 ≤ f SO2 ≤ 0.8 when DSO2 = 250 µm (top graph); 60 ≤ DSO2 ≤ 500 µm when f SO2 = 0.8 (bottom graph). Vertical lines indicate the position of the spectral channels. Most fall in the continuum of the spectrum, which is an increasing function of f SO2 but stays approximately constant with DSO2 . On the contrary, both f SO2 and DSO2 amplify the width and intensity of SO2 bands when they increase. At this point, in our data selection, we seek to fulfill three objectives:

0.0

2.44

2.0

2.72

3.28

2.5

3.56 3.85

3.0 3.5 Wavelength (µm)

4.13

4.41

4.13

4.41

4.0

1.2 1.0

Reflectance

464

0.8 0.6

fSO = 0.8 RN = 0.5 2

DSO = 2

0.4

500 380 300

0.2 0.0

2.44

2.0

220 140 60

2.72

2.5

3.28

3.0 3.5 Wavelength (µm)

3.56 3.85

4.0

FIG. 2. Theoretical dependence of NIMS reflectance spectra with solid SO2 areal abundance ( f SO2 ) and granularity (DSO2 ), for a fixed bidirectional geometry and for a fixed value of the neutral compound reflectance R N (=0.5). The vertical bars indicate the position of the NIMS spectral channels. The reflectance can be higher than 1 due to the anisotropic scattering behavior of SO2 frost. Top: DSO2 is kept constant at 250 µm whereas f SO2 varies. Bottom: f SO2 is constant at 0.8 whereas DSO2 varies.

SO2 GAS CONDENSATION AROUND VOLCANIC PLUMES ON IO

and is the one less susceptible to random noise. Rise of the continuum and simultaneous subsidence of the ν1 + ν3 band with f SO2 make the theoretical version of the first ratio α (calculated from Eq. (1)) particularly sensitive to SO2 areal abundance. The mean grain size DSO2 also determines the value of α, though with less strength, since the continuum is weakly DSO2 dependent. On the other hand, both the continuum and the reflectance of the 2ν1 + ν2 band wing increase with f SO2 , whereas only the latter quantity varies with DSO2 . One can thus expect a greater sensitivity of the theoretical version of the second ratio β with DSO2 than with f SO2 . Since the contrast between the 3.28- and 3.56-µm channels is much smaller (by a factor of 3) than that between the 3.28- and 4.13-µm channels, β is more affected by noise than α. Additionally, a limited optical thickness or a contamination of SO2 frost can modify the relationship between β and DSO2 . Despite these potential problems, β obs is the most sensitive parameter we found to estimate DSO2 . Systematic correlation between the I24INREGION02 global observation described in Table I and the reference SO2 abundance and granularity maps (Dout´e et al. 2001b) covering the same antijovian hemisphere of Io leads to an observational confirmation of the previous theoretical predictions. First, two spectral ratio maps α obs and β obs at 105 km · pixel−1 are calculated directly from the observed image cube. Then they are blurred to a resolution of 200 km · pixel−1 and reprojected to the reference geographical grid which corresponds to a cylindrical representation (latitude and longitude increment of 1◦ ). Second, values of α obs and β obs read at each grid point of the ratio maps are respecref ref tively connected with values of f SO2 and DSO2 extracted at the same position on the abundance and granularity maps. Original pixels displaying NULL values, being saturated, or showing a noticeable thermal emission are discarded from the process. In addition, we restrict the correlation to 50◦ S–50◦ N latitude and 150–240◦ W longitude in order to avoid the limb and the terminator of the I24INREGION02 observation. These limit regions are subjected to potential strong photometric effects which are only partially corrected by calculating ratios. Figure 3 repreref ref sents the resulting correspondence ( f SO2 , α obs ) and (DSO2 , β obs ) as clouds of black stars. The theoretical dependence of α on f SO2 , with DSO2 varying by increments of 20 µm between 20 and 500 µm, and the dependence of β on DSO2 , with f SO2 varying by increments of 0.01 between 0.2 and 0.8, are also expressed graphically (gray crosses). In both cases, R N takes various values from 0.3 to 0.8 with increments of 0.1 to take into account the natural spectral variability of the neutral compounds. Figure 3 (top) demonstrates that there is a linear correlation between the observed ratio α obs and SO2 abundance at the surface of Io with a correlation coefficient of 0.77. The remaining dispersion is likely due to the variability of SO2 grain size over the disk since the theoretical influence of R N on α is limited. A small part of the dispersion can also be explained by uncorrected photometric effects, variations from other interfering compounds, viewing angle variations from local topograref phy, etc. The observational cloud of points ( f SO2 , α obs ) is included and perfectly centered within the theoretical domain of

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FIG. 3. Relationship between SO2 areal abundance and granularity and two NIMS spectral ratios, α = R(3.28)/R(4.13) and β = R(3.28)/R(3.56). The black stars show the correspondence between spectral ratio maps calculated from the global observation I24INREGION02 and reference distribution and granularity SO2 mosaics obtained by Dout´e et al. (2001b). Top: correspondence ref ( f SO2 , α obs ). Additionally, the gray crosses express the theoretical dependence of α on f SO2 , with DSO2 varying between 20 and 500 µm. Bottom: correref spondence (DSO2 , β obs ). Additionally, the gray crosses express the theoretical dependence of β on DSO2 , with f SO2 varying between 0.2 and 0.8. In both cases, R N takes various values from 0.3 to 0.8.

α variation with f SO2 , DSO2 and R N . Their regression lines are nearly superimposed. The conclusion we can derive is: the ratio α obs is meaningful as an SO2 abundance indicator and is well calibrated. Figure 3 (bottom) shows that there is also a correlation beref tween the observed ratio β obs and SO2 mean grain size DSO2 , ref obs though with higher dispersion than for α and f SO2 . Variability of SO2 abundance over the disk of Io should account for this

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situation and suggests that DSO2 and f SO2 have nearly equal influences on β. This can be confirmed by a theoretical study of sensitivity using Eq. (1). The theoretical and observed cloud of points ref (DSO2 , β) and (DSO2 , β obs ) are not totally centered with each other, as shown by the corresponding regression lines (secondorder polynomials). As a conclusion, we can expect that the ratio β obs will give an estimation of DSO2 , but with limited accuracy. 2.2.3. Inversion method. Based on the previous results, we now propose a method of inversion of Eq. (1) that we apply to analyze the NIMS observations of Table I. This operation, carried out for each pixel of a NIMS image cube, leads to a quantitative estimate of SO2 abundance and, to a lesser extent, SO2 granularity over the area covered by the observation. For each observed spectrum Robs p,q , we first compute the ratios α

obs

β obs

 obs = R p,q (4.13)  obs = Robs p,q (3.28) R p,q (3.56). Robs p,q (3.28)

(2) (3)

Then we seek with a SIMPLEX optimization method the dous s blet of solution values ( f SO , DSO ), which minimizes the cost 2 2 function:      R(3.28)   obs R(3.28)  φ( f SO2 , DSO2 ) = α obs − + β . − R(4.13)   R(3.56) 

(4)

R(λ) is given by Eq. (1), taking into account the real local conditions of illumination and viewing, with R N kept fixed at 0.5. The absolute variability of α obs is 8 to 10 times higher than the variability of β obs . Thus, the minimization of relative dif-

ferences rather than absolute differences should be preferred a priori. However, such a method makes the solutions of Eq. (4) particularly sensitive to the noticeable noise which affects β obs (ratio of quite similar values). Pixels marked by the intense thermal emission of the hot-spots or by saturation cannot be treated properly by this process and are discarded. Once found, we cons s sider that f SO and DSO best characterize the local properties of 2 2 SO2 frost. The operation continues with all pixels of the treated image cubes and results in three primary maps with the same initial point-perspective geometry as the observation. The first two maps respectively describe the spatial distribution and the granularity of SO2 . The third expresses the discrepancy between the observed spectral ratios and the modeling, displaying for each s s pixel the value of the cost function φ( f SO , DSO ). 2 2 2.2.4. Evaluation of methodology. Before treating the series of image cubes presented in Table I, we must evaluate the uncertainty introduced by using only three spectral channels in the form of ratios instead of a complete high-resolution spectrum. We perform this task by qualitatively comparing maps of SO2 abundance and granularity. They are obtained, on the one hand, from the inversion of the global observation I24INREGION02 by the method described in the previous section, and, on the other hand, from the inversion of global NIMS images from the nominal Galileo mission by a method exploiting their full spectral resolution (Dout´e et al. 2001b). These two classes of maps display different spatial resolution: 105 km · pixel−1 for I24INREGION02 and 200 km · pixel−1 for the reference. Figure 4 allows comparison between the I24INREGION02 (right) and the reference (left) SO2 distribution maps over approximately the same geographical area. Both maps depict

FIG. 4. SO2 distribution maps respectively derived from the inversion of the image I24INREGION02 (right) and extracted from the global mosaic of Dout´e et al. (2001b) (left). The represented quantity is the frost coverage from 0 (black) to 1 (white).

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a similar situation with SO2 widespread on Io’s surface but very unevenly distributed, with the highest areal abundances at medium and high latitudes. Depleted regions in SO2 (less than 50% of the surface covered by the frost) or rich regions (coverage higher than 50%) correspond well in position and extension, except near the northern edge of the observations where the lighting conditions are poor. The map derived from I24INREGION02 seems have more contrast than the reference map, the mean value of frost coverage being higher within the frost-enriched regions (≈80–90% instead of ≈60–70%) and lower (≈20–30% instead of ≈30–40%) in the hot-spot neighborhoods. Part of this effect can be due to differences of spatial resolution, but not all of it. A possible complementary explanation is that the new method of analysis relies on a fixed value (0.5) for the reflectance R N of the neutral compounds present on Io’s surface. However, f SO2 and R N could be correlated, since SO2 -rich deposits (high values of f SO2 ) likely coexist with other IR-bright sulfur-bearing materials (high values of R N ). On the contrary, dark areas (low values of R N ) often correspond to volcanically active regions depleted in SO2 (low values of f SO2 ). Underestimating R N (0.5 instead of 0.75, for example) leads to overestimating f SO2 (80% instead of 60% in that case), and the converse is also true. We conclude that we likely overestimate the frost coverage of the SO2 -enriched regions and underestimate the coverage of the SO2 -depleted regions, thus increasing the contrast of the distribution maps. Figure 5 allows comparison between the I24INREGION02 (right) and the reference (left) SO2 granulometry maps. In this case, the correspondence is far from one to one, but similar general trends can be seen. South of Pele-Pillan and Marduk lie fields

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of fine-grained frost (50–200 µm) down to a latitude of ≈50◦ . At higher latitudes, SO2 mean grain size substantially increases to values greater than 300 µm. The equatorial region (±20◦ ) presents coarser grains than on average (
200 µm), except in the area immediately surrounding Pele and Pillan. Emerging from the equator at the longitude range of Pele and Pillan, a large area of coarse-grained frost extends into the northern hemisphere and encircles the hot-spots Isum and Mulungu. However, it stops at ≈200◦ W longitude on the I24INREGION02 map, whereas it continues east towards ≈150◦ W longitude on the reference map. Another substantial difference between the two maps occurs just south, southeast, and east of Culann and Prometheus, where SO2 grain size is close to the maximum value (500 µm) on the reference map but is on average 250 µm on the I24 map. In both maps though, the neighborhoods of the hot-spots Isum–Mulungu and Volund–Zamama are covered by fine grains of frost. The correlation of Figs. 4 (right) and 5 (right) shows no systematic reduction of grain size where solid SO2 is particularly abundant. In summary, our current method of analysis gives values of SO2 frost coverage similar to the values calculated from a more complete set of observables. Nevertheless, an accentuation of the extreme values found for regions well depleted or enriched in SO2 can be noted. Spatial variations of granularity are just sketched because the ratio β is very sensitive to noise in the measurements, to photometric effects that are not taken into account, and to situations departing from our model of Io’s surface. However, such a sketch, mostly derived from the use of β, improves the calculation of f SO2 from α compared to a situation where just α would be inverted with a fixed value for DSO2 ,

FIG. 5. SO2 granularity maps respectively derived from the inversion of the image I24INREGION02 (right) and extracted from the global mosaic of Dout´e et al. (2001b) (left). The represented quantity is the mean grain size of the frost from 20 µm (black) up to 500 µm (white).

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250 µm for example. We will see in the following section why we think that, at the regional and local scales, the SO2 granularity maps become more reliable and meaningful. 3. DISTRIBUTION AND PHYSICAL STATE OF SO2 FROST IN THE PROMETHEUS REGION

3.1. Introduction In the current section, we characterize SO2 frost distribution and physical state with a spatial resolution of ≈20 km · pixel−1 over an area equivalent to about 5% of Io’s surface. This area contains Prometheus, a dozen other hot-spots, and at least two plumes. Use of SO2 as a tracer is particularly relevant to studying the regional volcanic activity which has been previously detected and monitored by measurements of thermal emissions (LopesGautier et al. 2002) as well as by careful studies of geological features in high-resolution SSI images (Keszthelyi et al. 2002). Details of the dynamics and evolution of SO2 gas condensation directly linked with Prometheus are examined in the following section. We now apply the method of inversion presented in Section 2.2 to analyze the regional and local image cubes listed in Table I. We generate three sets of SO2 distribution and granulometry maps. For clarity, these maps are identified by a prefix, map fso2 for a distribution map and map Dso2 for a granulometry map, followed by the first letters/numbers of the original observation identifier. For example, map fso2 i24 stands for the SO2 distribution map derived from the observation I24INREGION01. The large geographical overlap of the observations I24INREGION01 and I25INREGION01 allows us to differentiate between real SO2 abundance or granularity variations and artifacts due to the limitations of our method. As shown by Dout´e et al. (2001b), the physical state of SO2 can be assessed over the observed geographical area by the correlation of corresponding distribution and granulometry maps. Here we use the same visual method of correlation, where four basic SO2 physical units can be distinguished by distinctive sets of colors which express relative variations of frost coverage and mean grain size. The units have been related at the global scale to processes of condensation, metamorphism, and sublimation. We assume that, at lower scales, this relation is maintained. Here, we recall briefly the principal characteristics of the basic physical units: • Unit (I): fields of abundant freshly condensed SO2 . This unit is characterized by high frost coverage and fine grains and is represented by green to olive colors. • Unit (II): the metamorphosed peripheries of the condensation fields. This unit, appearing gray to gold yellow, displays high frost coverage and coarse grains. • Unit (III): thermally active regions where SO2 frost scarcely condenses from place to place and is removed by sublimation on a regular basis. This unit, visible in blue to turquoise colors, is depleted in SO2 and is fine-grained.

• Unit (IV): principally the equatorial passive plains (see below for a description of their characteristics). They appear in violet to orange colors. Correlation maps segmenting NIMS images into SO2 physical units are named with the prefix map PUso2 . 3.2. Passive Equatorial Plains Maps map fso2 i24 and map fso2 i25 are placed side by side in Fig. 6 and depict basically the same situation. Over ≈35–40% of the region they cover, SO2 areal abundance shows little variation, staying close to a background value of approximately 45%. This constant level is often associated with large grain sizes, 300 µm and more, placing the concerned area into the category of the equatorial passive plains (coded in orange in Fig. 7). These plains are undisturbed by any kind of close, vigorous, recent volcanic activity. Continual and long-term condensation of the tenuous global atmosphere or of volcanic SO2 gases migrating from remote plumes may occur day or night, though with a low deposition rate. During the daytime, the condensed frost undergoes slow metamorphism under sunlight (see Section 3.3) and may even sublimate at some locations. The background value of 45% results from the geographical distribution of the net balance between condensation and sublimation. The condensation term basically depends on the mean surface temperature and the mean local flux of gaseous SO2 . The sublimation term varies with the mean incoming solar power and the distribution of slope orientation and surface albedos. Large grain sizes and a slow metamorphism rate indicate that SO2 has been accumulating over long timescales where it is stable (i.e., where condensation exceeds sublimation). The equatorial plains seem to achieve some kind of steady state between condensation, sublimation, and metamorphism. 3.3. SO2 Rich Deposits Maps map fso2 i24 and map fso2 i25 (Fig. 6) clearly illustrate that at some places regions well enriched in SO2 emerge from the background. The most striking example is a regular ring of abundant deposits circling the Prometheus lava field. On the two maps, the geometry of the deposits is identical as well as the relative variations of SO2 coverage within the ring. The only difference is that the absolute level in the eastern branch is slightly lower for map fso2 i24 than it is for map fso2 i25. We think that this is an artifact of spatial resolution rather than a temporal evolution between orbits I24 and I25, which took place only 1 month apart. Indeed, another extended zone of abundant deposits, which lies far from any known hot-spots (along the longitude 140◦ W between ≈−10 and 10◦ latitude) and should be stable over this period of time, is also less marked in map fso2 i24 than it is in map fso2 i25. Additional areas showing enhanced coverage by SO2 frost are visible on one or the other map. Two appear on map fso2 i24, one close to the location of the plume Culann and one centered at (22◦ N, 154◦ W) (northwest corner of the map). A third is off the limits of map fso2 i24 but can be

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FIG. 6. SO2 regional distribution maps (map fso2 i24 and map fso2 i25) derived respectively from the inversion of the images I24INREGION01 (top) and I25INREGION01 (bottom). The represented quantity is the frost coverage from 0 (black) to 1 (white).

distinguished on map fso2 i25 where it covers tens of degrees in latitude and longitude along the meridian 127◦ W, north of the equator. Close to its western boundary, one can note a quite confined zone of extremely high SO2 abundance (100%). This special spot is in fact located at the end of the northwestern slopes of

the great Emakong. Four of the enriched areas discussed above show a clear association with fine-grained frost (see Figs. 7 and 8) and thus, according to earlier studies (Dout´e et al. 2001b), they should be fields of freshly condensed SO2 (green to olive colors in the physical state map of Fig. 7). They are respectively

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genetically linked with Prometheus, Culann, Surya, and a little caldera on the flanks of Emakong. 3.3.1. Prometheus. For Prometheus the case seems clear: a very constant plume in terms of shape and activity has been reported for many years (McEwen et al. 2000). The SO2 distribution and physical maps, as well as an SSI image from C21 acquired at low phase angle and cropped to the dimension of the NIMS observations, all (Figs. 6, 7, and 8) show the result of such an activity: the ongoing deposition of materials (notably SO2 ) at some distance. The SO2 ring and its visible counterpart do not correspond exactly in terms of shape and size, the former being larger (12◦ instead of 10.5◦ ), wider, and more elongated than the latter. This suggests that Prometheus emits several different compounds in the gas phase (such as SO2 , S8 , NaCl, S2 Cl2 , etc.) that condense with a deposition distance from the source inversely related to their molecular weight or to their volatility. The gases and deposits may also evolve differently. The unique characteristic of the SO2 ring, i.e., the geometric elongation of its external edge coupled with higher SO2 concentration values within its eastern part confirms the existence of another mechanism: the displacement of the plume (80 km west since 1979 (McEwen et al. 1998)) linked with the development of the Prometheus lava field. Indeed, Lopes-Gautier et al. (2000) and McEwen et al. (2000) came to the conclusion that hot silicate magma, erupting from a vent situated at the eastern end of the lava field, moves west through lava tubes and emerges at an increasingly distant front. There, it interacts with underlying volatile materials which are violently vaporized, thus creating a moving plume (Milazzo et al. 2002). Now we assume that the freshly condensed SO2 deposits are relatively stable at the timescale of the 80-km displacement (≈20 years). While the “instantaneous” ring that we suppose circular is moving west, its eastern part is preserved for some time whereas its western part is buried by compounds less volatile than SO2 or is sublimated. Such an effect may explain the elongated shape of the deposition ring. Comparison between the SO2 distribution and physical unit maps from I24 and I25 supports this analysis. We see that the total field of deposited SO2 frost delineated on map fso2 i25, for example (Fig. 6), does not correspond exactly with the ring of “freshly” condensed SO2 coded in green on map PUso2 i25 (Fig. 8). The eastern branch is wider by 2.5◦ . As we progress through the branch from west to east, the grain size increases; hence the degree of metamorphism of the frost and consequently its age. 3.3.2. Culann. During the Galileo Prime Mission, a plume may have been observed erupting from Culann (McEwen et al.

1998). The SO2 physical unit map presented in Fig. 7 agrees with this view since fresh deposits can be seen at close distance from the hot-spot detected by Lopes-Gautier et al. (1999). Parts of the deposits are probably beyond the edge of the I24 observation, but the total area of fresh deposits is certainly smaller than the surface area occupied by the Prometheus ring. Such a difference is probably due to less intense activity at Culann in terms of emitted gas volume and/or in terms of temperature linked with a different eruption style. We also came to this conclusion from results obtained at the global scale. Prometheus and Culann deposits also differ in their geometry: they are quite symmetric around the source in the first case and irregular in the second. Freshly condensed SO2 covers the northeastern 5-by-5◦ grid cell adjacent to the hot-spot, but the northwestern cell is completely depleted. The presence in the latter cell of hot lava and pyroclastic flow, visible as black flows or diffuse dark materials, respectively, on the SSI image (Fig. 7), increases the level of thermal emission and prevents the condensation of the ice. The low albedo of the surface also retains more solar energy and should increase the local diurnal temperature, accelerating the sublimation of the frost. The SSI image also shows paleyellowish materials which form an arched structure approximately 7◦ away from the Culann volcanic fields to the east. They could be associated with the plume since they display a geometry very similar to that of the SO2 deposits and bound the latter quite exactly. In these conditions, we would have a reversed Prometheus situation with the bright material lighter (i.e., with a lower molecular weight) or more volatile than SO2 . The possibility of the bright material being more volatile than SO2 can be rejected because we see below that the bright deposits extend over an area belonging to the SO2 unit III. If SO2 is unstable in this place, then a compound more volatile is even more mobile. A lighter but more refractory molecule than SO2 could be NaCl, for example, as Fegley and Zolotov (2000) predict it could be emitted abundantly. We can summarize the previous results as follows: although it presents lower plume activity than Prometheus, Culann likely emits a great variety of gases (e.g., S2 , SO2 , NaCl, etc.) which may be transferred to Io’s surface according to a ballistic regime rather than according to a fluid dynamic regime. Asymmetry of the deposits could result from geometrical irregularities of the plume nozzle(s) or from the presence of warm materials (lava, pyroclastic flow, etc.) in some parts of the deposition area. 3.3.3. Emakong region. On the northwestern flank of the volcano Emakong, one can distinguish in the SSI visible image (Fig. 8) a small caldera situated at (2◦ N, 125◦ W). A gray

FIG. 7. Left: SO2 physical unit map derived from the observation I24INREGION01 (map PUso2 i24). The color variations reflect the relative changes in surface coverage and grain size. See Section 3.1 for a description of the four physical units. Right: Same area but observed by SSI during orbit C21 (July 1999) through three filters (NIR, GR, and VI) with a spatial resolution of 1.3 km · pixel−1 . FIG. 8. Top: SO2 physical unit map derived from the observation I25INREGION01 (map PUso2 i25). Bottom: Same area but observed by SSI during orbit C21 (July 1999) through three filters (NIR, GR, and VI) with a spatial resolution of 1.3 km · pixel−1 .

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7

8

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flow-like tongue extends northward. At exactly this location, NIMS detects one of the highest values for the spectral ratio α obs (see Section 2.2) ever observed during the I24, I25, and I27 fly-bys (the only other occurrence takes place near Chaac). Our inversion interprets such a high level as a spot 100% covered by SO2 frost. An intimate mixing model would also give an abundance close to 100%. No hot-spot or plume has been observed so far at this location by Voyager, Galileo, or from the ground, and this absence of high-temperature activity seems to be confirmed in the SSI image by the lack of red diffuse deposits. Along its longest dimension, the size of the spot apparently entirely covered by pure SO2 is ≈96 km. We now suppose that SO2 was condensed there from the gas phase after being vaporized by contact with some hot material and transferred ballistically through the atmosphere. Then the maximum ejection distance from the source rmax and the initial temperature T of the gas are crudely related by the formula: 3 1 kT = mgrmax , 2 2

(5)

where k is the Boltzman’s constant (k = 1.38 10−23 J K−1 ), g is the gravity at Io’s surface (g = 1.797 m s−2 ), and m is the molecular weight of SO2 (m = 1.0624 10−25 kg). If we now equate rmax to the SO2 deposit’s largest dimension, we can calculate an order of magnitude for the heat source maximum temperature: T ≈ 440 K. Such a upper limit is slightly above the liquidus temperature of sulfur (≈393 K) (Kieffer 1982) and is well below the fusion point of all silicates. As a consequence, a flow of hot molten sulfur from the small caldera at (2◦ N, 125◦ W) could be responsible for the vaporization of liquid aquifer SO2 or more superficial preexisting SO2 frost fields. Williams et al. (2002) suggest the possibility of sulfur flows from the flanks of the Emakong caldera. Thus, two studies, as well as evidence of low-temperature volcanism from Lopes-Gautier et al. (2002), support the possible existence of sulfur volcanism on Io, at least in the Emakong region. Nevertheless, one can invoke another mechanism to create a quite confined, highly concentrated deposit of solid SO2 . Smythe et al. (2000) demonstrate that, given sufficiently large quantities, liquid aquifer SO2 erupting to the surface can freeze in a layer of sulfur dioxide ice (at least a few mm thick) before completely sublimating under the very low pressure of Io’s atmosphere. 3.4. Thermally Active Regions Pixels in the immediate proximity of the hot-spots detected by NIMS and affected by a strong thermal component are not treated by our algorithm of inversion (see Section 2.2). However, in order to elaborate the SO2 distribution and granularity maps they have to be associated with arbitrary values of f SO2 and DSO2 , respectively, taken to be equal to 0 and 20 µm. This choice insures a certain continuity with the surrounding regions where the paucity and fine granularity of SO2 frost are real. Moreover, our choice is relevant since one does not expect the presence

of SO2 frost in locations where hot lava and pyroclastic flows are emitted. One exception seems to be the Emakong caldera which shows, in several NIMS pixels 25 km across, a noticeable thermal emission (T ≈ 300 K) in conjunction with marked SO2 absorption (Lopes-Gautier et al. 2002). It is not surprising to find very little SO2 at the locations of the hot-spots. It is more remarkable, though, to discover large regions in maps map fso2 i24 and map fso2 i25 (Fig. 6) that are depleted in SO2 when compared to the background (SO2 coverage
40%). The latter regions are most often associated with fine-grained SO2 and thus are coded in blue to violet colors (Unit III.) in the physical unit maps. Small grain sizes indicate little metamorphism, and therefore, associated with low abundances, short residence times for SO2 frost at these locations: SO2 is unstable. Unit (III.) first forms a large band visible on both map PUso2 i24 and map PUso2 i25 (Figs. 7 and 8) (case a). This runs northwest–southeast from (10◦ S, 157◦ W) to (25◦ S, 140◦ W) and possibly further (Prometheus ring is North, Culann is West). Two more limited irregular polygons, one just west of the Prometheus ring (case b) and the other southeast of Surya (22.0 N, 152.0 W)(case c), also belong to unit (III.). Finally, another large band (case d) strikingly extends northwest–southeast from (7◦ N, 140◦ W) to (15◦ S, 127◦ W). We can invoke three possibilities to explain the existence of such large areas where SO2 is not stable. 1. Recent deposition of hot pyroclastic flow materials (silicates, sulfur, etc.) from nearby plumes has buried and sublimated preexisting frost deposits. 2. The albedo of the substratum supporting the SO2 deposits is lower than the average of the equatorial plains. This increases the level of solar energy absorbed below the surface, thus raising the diurnal temperatures and accelerating the sublimation of the frost. 3. Io’s crust in these areas is thinner than the global average thickness. Hot materials from the mantle are closer to the surface, increasing the local geothermal flux and the surface temperatures. The first explanation is plausible for cases b and c since localized bright yellow or dark gray volcanic deposits can indeed be observed superimposed with unit (III.) in the SSI visible image (see Fig. 7). For case c, we have a hint of how recent the deposition is because of the detection of a hot-spot named Surya by Lopes-Gautier et al. (2002) during orbit I24. The mechanism of SO2 removal by hot pyroclastic materials can be discarded for cases a and d since the areas involved are considerably larger than what an eruption of solid or liquid material can cover with the thermal energy available in general. A gas plume can resurface a much larger area (e.g., Pele’s ring, ≈1200 km in diameter), but its considerable expansion and thermal loss into the tenuous atmosphere of Io lead to an efficient cooling of its constituents. The fallout reaching the surface is therefore likely to be very cold. One can imagine a dense series of eruptive sites quite regularly spaced along the geographical bands of cases a

SO2 GAS CONDENSATION AROUND VOLCANIC PLUMES ON IO

and d, all active within a relatively short span of time. However, SSI images only display a limited number of volcanic features (caldera, dark lava flow, etc.) in these areas. At first sight, the influence of the substratum albedo seems to be a satisfactory explanation for case a because the SSI image shows a region of reduced albedo corresponding nearly exactly with unit (III.) of Fig. 7. However, a closer examination reveals that the bright, whitish deposits associated with Culann (see Section 3.3) also fall within this region of enhanced temperatures. As a consequence, the albedo is not the main factor controlling the heating of the surface here. In fact, this role is held by the geothermal flux, likely higher than on average. This situation of a thermally active region (positive thermal anomalies) appears even more clearly for case d where unit (III.) coincides with a great diversity of albedo, from very dark lava fields to the very bright and white area characteristic of Bosphorus Regio. What could be the range of temperatures for unit (III.)? The detection limit of NIMS is a surface temperature of ≈180 K when the source fills a full pixel (Smythe et al. 1995). However, the reduced number of wavelengths obtained during the Io fly-by observations pushes this number to about 200 K. Except for the localized hot-spots (e.g., Tupan), no thermal emission is visible in the spectra related to unit (III). As a consequence, the upper limit on temperature for this unit is 200 K. Solid SO2 becomes unstable and sublimates above a mean diurnal temperature of 110 K for an atmospheric pressure of 0.1 nbar (tenuous component, upper limit (Ballester et al. 1994)). This temperature represents the lower limit for unit (III.). In summary, one can distinguish at the regional scale quite large areas displaying temperatures in the range 110–200 K, perceptibly above the mean diurnal temperature of SO2 frost (Schmitt et al. 1994). This kind of area has already been observed at the global scale by NIMS (Dout´e et al. 2001b) and by the Galileo Photopolarimeter Radiometer (PPR) (Spencer et al. 2000b), notably over the Jupiter-facing part of the trailing hemisphere. Such areas are likely positive thermal anomalies linked with variations of Io’s crustal properties. For example, the upwelling movement of molten rocks from a mushy convective mantle can cause thinning of the crust (Keszthelyi et al. 1999). One can then expect a greater level of volcanic activity in these areas. On the global scale, Loki, Pele, and Acala (all situated between 240 and 330◦ W) are very constant hot-spots and emit much energy (Davies et al. 2000, Davies 2001). In the Prometheus region, a high level of volcanic activity seems to be confirmed since case a of unit (III.) is surrounded by three of the most persistent hot-spots of Io: Prometheus, Culann, and Tupan. Case d of unit (III.) contains at least four or five structures of volcanic origin according to the SSI image, although only two nonpersistent or faint hot-spots were detected there by NIMS during the Galileo mission. Other regional observations should be conducted over regions susceptible to show unit (III.) areas, like Isum–Mulungu, to determine their exact distribution, extension, and link to geological features and hot-spots.

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4. EXPLAINING THE PROMETHEUS SO2 FIELD USING A STOCHASTIC–BALLISTIC MODEL OF PLUME ERUPTION

The field of solid SO2 observed around Prometheus by NIMS during Io’s fly-bys results from different factors: 1. the initial distribution of SO2 frost prior to the eruption; 2. the history of the lava flow emplacement (see Section 3.3); 3. the temporal and spatial distribution of the possibly numerous sources of erupting gas and particles within the lava flow and/or from its edge (Kieffer et al. 2000, Milazzo et al. 2002); 4. the “instantaneous” repartitioning of materials deposited on the surface around each individual source; 5. the expected processes of codeposition, overlapping, and sublimation affecting the previous deposits; 6. and finally, the possible chemical and thermodynamic evolutions of the deposits with time (e.g., metamorphism, sublimation, etc.) after deposition. Despite this complexity, we constrain qualitatively and quantitatively some aspects of these factors by modeling radial profiles of SO2 frost areal abundance extracted through the Prometheus ring. For that purpose, we have adapted a stochastic–ballistic model of plume eruption proposed by Glaze and Baloga (2000) to explain the observed Prometheus characteristics. 4.1. Extraction of SO2 Abundance Profiles In order to extract the abundance profiles, we use SO2 frost distribution maps derived from the observations I25REGION01 and I27INPROMTH01 (see Table I) using the method presented in Section 2.2. With a resolution of ≈20 km · pixel−1 , map fso2 i25 gives us a general view while map fso2 i27 at ≈7 km · pixel−1 refines the measurement of SO2 abundance variations close (
100 km) to the lava field where the first map loses precision because of thermal emissions. We define the origin of the profiles as the mean position of the different sources of SO2 gas at the time of our observations (November 1999 I25, and February 2000 I27). The profiles extend toward eight distinct directions: W (west), NW (northwest), N (north), NE (northeast), E (east), SE (southeast), S (south), and SW (southwest). Two steps are needed to determine their origin. First we assume that the origin corresponds to the geometrical center of the external western edge of the Prometheus ring, which basically forms half of a circle. We extract curves obs of SO2 coverage f SO (r ) versus the distance r from the center 2 to the S, SW, W, NW, and N directions. The curves generally show an increase from the depleted warm area of the lava field to the intersection with the ring medium perimeter where they reach their maximum. Beyond this point, f SO2 declines, eventually reaching the “continuum” level of the surrounding equatorial plains. However, these profiles display relative horizontal shifts of their maximum values indicating that our estimation of the mean location of the sources is somewhat inaccurate. The

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FIG. 9. Extraction of radial profiles of SO2 areal abundance through the Prometheus ring. Top: origin and direction of the different selected transects superimposed on the SO2 distribution map derived from I25INREGION01. Bottom: SO2 profiles.

second step uses these shifts to improve the calculation. An optimization algorithm navigates the origin of the profiles to align their maxima. The final location (1.6◦ S, 155.5◦ W) falls into the westernmost part of the Prometheus lava field where NIMS measures high brightness temperatures (Lopes-Gautier et al. 2002) and SSI observes lava breakouts and white streaks (possibly the result of SO2 gaseous jets) (Milazzo et al. 2002). Final profiles in the eight directions are then extracted from that point and these are presented in Fig. 9. Regarding the position of their maxima, we can distinguish two groups of profiles: the profiles of Prometheus’s western part (S, SW, W, NW, and N) and those of the eastern part (NE,

E, and SE). Calculation of the ring width across all the previous directions supports this classification. We set the internal (distance r1 ) and external (distance r2 ) limits of the ring along a given profile at the points where the gradient of frost coverage is maximum in absolute value. The width is then given by r2 − r1 . Table III summarizes the results for each profile direction. Widths calculated across the western part of the Prometheus ring are in relatively close agreement: three (W, N, and SW) out of five profiles give a value around 63 km. The other two lead to higher values (90 km for NW and 76 km for S) which are, nevertheless, of the same order of magnitude as the former.

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TABLE III Prometheus Ring Width across Eight Different Directions Direction

r1

r2

δr (km)

W NW N

110 97 97

173 187 161

63 90 64

NE E SE S SW

69 82 75 109 116

222 228 217 185 178

153 146 142 76 62

Notes

Two possible values for r1 . We take the closest to the source. same same same

Widths across the eastern part of the Prometheus ring show even less variability with a mean value of 146 km. 4.2. Conditions of Material Deposition and Evolution By exploiting these figures we can constrain some of the processes described in Section 3.3 and in the introduction of Section 4. The following discussion is based on an initial series of assumptions supported by the conclusions of Sections 3.2 and 3.3:

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and eastern edges if we assume that the latter is historic from the Voyager era (no frost sublimation). The justification for this assumption is given in the next paragraph. These assumptions yield a longitude of 154.1◦ for the longitude axis of symmetry. After extraction of two profiles (S and N) along this meridian, we find, by the derivation technique, a width of 60 km for the current field, and thus a similar value for the “instantaneous” ring. The latter value is very close to the mean width of the western part of the Prometheus ring (63 km). As a consequence, what we principally observe there is SO2 frost that is only just being deposited. In other words, most of the frost that accumulated west of the source from the Voyager era to Galileo must have been buried or sublimated by newer deposits. Determination of the latitude and longitude axis of symmetry also provides an independent way of estimating the current mean location of the plume sources. Indeed, the current (1999) and past (1979, when the emplacement of the Prometheus lava field started) mean location of the sources should be symmetric about the second axis (see Fig. 10). The latter position was pinpointed at 1.5◦ S, 152.3◦ W on the USGS maps derived from Voyager data. Assuming it also corresponds to the geometric center of the external eastern edge of the Prometheus ring, which basically forms half of the circle, we obtain quite similar coordinates:

1. The solid SO2 field results from the accumulation over time of fallout from a moving plume. The fallout is deposited on the initially uniform distribution of the equatorial plains. 2. The eruption pattern is perfectly symmetric and spherical about the source. 3. The eruption energy is relatively constant with time such that the debris radius remains the same. 4. The plume started moving around 1979 with a total displacement of Smean ≈ 80 km for a duration of τ ≈ 20 years. 5. The plume has been migrating perfectly westward (no north–south perturbations or trend). The figure we obtain (see Fig. 10) looks like a cat’s eye and admits a longitude as well as a latitude axis of symmetry. For Prometheus, the displacement of the source (Smean ≈ 80 km) is much shorter than the medium radius of the ring (rm ≈ 160 km). Consequently, the width of the deposition figure across its longitude axis of symmetry differs only slightly from the width of the instantaneous ring. The real field of SO2 displayed by map fso2 i25 (Fig. 6, bottom) is different from this theoretical distribution. Indeed, part of the frost that accumulated west of the source during 20 years must have been buried or sublimated, otherwise the widths of the western and eastern parts of Prometheus, respectively ≈63 km and ≈146 km, would be similar by symmetry. Moreover, some metamorphosed frost at the eastern edge of the field may be sublimating with time and disappearing. In these conditions, how can we determine the location of the former latitude and longitude axis of symmetry? The first axis lies at midway between the external northern and southern edge of the field: around 1.6◦ south latitude. Similarly, the second axis is equal distance from the external western

FIG. 10. Schematic accumulation of materials resulting from the deposition of a uniform ring of fallouts moving west along a parallel. The initial and final positions of the ring center are respectively indicated by a black star and a gray star. The figure admits two axes of symmetry that can be related to the real Prometheus SO2 field of Fig. 6. δri is the ring width of the instantaneous deposition ring; δr W and δr E are the widths of the deposition field’s western and eastern parts, respectively.

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DOUTE´ ET AL.

1.7◦ S, 153◦ W. Therefore, as the initial position of the sources, we take the intermediate position of 1.6◦ S, 152.6◦ W. Then, by symmetry, the current mean location of the plume sources can be estimated (1.6◦ S, 155.6◦ W), as well as the distance they covered in 20 years: ≈95 km. This location agrees well with the one we retrieved at the beginning of this section. Contrary to the discussion above, no assumption needs to be made about the preservation of SO2 in the eastern part of the Prometheus deposition field. This is an important clue supporting the stability of SO2 after deposition, but it is not the only one. The “instantaneous” ring width (δri ≈ 60 km) is less than the displacement of the mean position of the source (Smean ≈ 95 km). If no sublimation is taking place east of the sources, Fig. 10 shows that δr E = δri + Smean . Given the spatial resolution of the observation I25REGION01 (≈20 km · pixel−1 ), we can conclude that δri (60 km), Smean (95 km), and δr E (146 km) are compatible in the framework of our principal assumption: there was no SO2 sublimation when the deposition of materials from Prometheus ceased. We can summarize the conditions of material deposition and evolution as follows: sources of gas and particles are mainly distributed along the active lava front. During the past 20 years they have moved some 95 km west following the development of Prometheus’s lava field. All of the SO2 frost that accumulated east of the midcourse meridian has been preserved. This demonstrates its relative stability in this part of the equator. Most old SO2 frost that accumulated west of the midcourse meridian has been sublimated or buried by less-volatile and/or heavier material that erupted from Prometheus at a shorter distance from the vent(s). The transition between the area where the latter materials dominate and the area where SO2 can condense and persist is quite sharp and corresponds to the internal edge of the “instantaneous” ring where ri ≈ 103 km. 4.3. Model Now we present a semiempirical model simulating the distribution of solid SO2 in the immediate surroundings of Prometheus. Our goal is to further investigate the history and conditions of material deposition as well as evolution around the volcanic center. Let us see how we can express mathematically each factor listed in the beginning of Section 4 and involved in the buildup of the SO2 field. Neglecting Io’s rotundity at the scale of a few hundreds of km, we suppose that the Prometheus region can be described in a two-dimensional Cartesian space. The x-axis follows the local parallel and is oriented from east to west. The y-axis extends from north to south along a meridian. The origin O corresponds to the current mean location of the sources of gas and particles. Io’s fly-by by Voyager 20 years ago is chosen as t = 0. 4.3.1. Initial distribution of SO2 frost. We suppose that prior to Prometheus’s eruption, the distribution of SO2 was homogeneous and characterized by an areal abundance equal to bck ≈ 0.45. the background value of the equatorial plains: f SO 2

4.3.2. Areal abundance of SO2 emplaced on the surface by each individual point source. We use the stochastic–ballistic model of plume eruption proposed by Glaze and Baloga (2000). The motion of particles (i.e., SO2 molecules in our case) is controlled by stochastic processes near the vent and ballistic transport beyond. The particles are ejected isotropically from the stochastic region between 0 and some maximum zenith angle θo with a narrow Gaussian speed distribution η( f ) = √

1 2π σ

e−

(v f −¯v )2 2σ 2

,

(6)

with v f = v¯ (1 + f · RSD) and η( f ) fraction of particles emitted with a speed between v f and v f +d f . The parameter v¯ is the mean of the distribution, and RSD is its relative standard deviation (RSD = σ/¯v ). Equation 16 of Glaze and Baloga (2000) gives the normalized areal density ρ f (r ) of particles resulting from the ejection at a single speed v f . The variable r is the distance of emplacement from the point source. For 0 ≤ r ≤ ro , 1 ρ f (r ) = (1 − cos θo )4πr ξrmax



1−ξ . 2

(7)

For ro ≤ r ≤ rmax , the normalized areal density is equal to ρ f (r ) = 0

(8)

if θo ≤ 45◦ , and it is equal to 1 ρ f (r ) = (1 − cos θo )4πr ξrmax



1−ξ + 2



1+ξ 2

 (9)

if θo > 45◦ . Three intermediate quantities appear in the equations: rmax , ξ , and ro . They are defined as rmax (v f ) = v 2f /g = rmax (¯v )(1 + f · RSD), where g is acceleration due to gravity on Io (g is assumed to be constant withheight since Prometheus’s plume is
90 km high), ξ = 1 − (r/rmax )2 , and ro = 2rmax sin(θo ) cos(θo ). We obtain the total normalized areal density ρ(r ) of SO2 deposited around the point source by summation of the individual ρ f (r ) multiplied by the fraction of particles erupted with a velocity v f , ρ(r ; rmax (¯v ), RSD, θo )  +∞ = (erf ( f + d f ) − erf ( f ))ρ f (r ) d f,

(10)

−∞

where erf ( f ) is the error function derived from the Gaussian distribution. We refer the reader to Glaze and Baloga (2000) for a study of the behavior of ρ(r ) with the parameters rmax (¯v ), RSD, and θo . We now integrate these equations into a more complex model taking into account the multiplicity of the ponctual sources at Prometheus and their displacement over time.

SO2 GAS CONDENSATION AROUND VOLCANIC PLUMES ON IO

4.3.3. Temporal and spatial distribution of the point sources. At any time t, we suppose that the point sources are regularly dispersed around their mean location (xs (t), ys = 0) in a small rectangular domain of size x ×  y . The function xs (t) describes the movement of these sources as a whole since Voyager fly-bys, if we assume, for simplicity, that they continuously followed the 1.6◦ S parallel. Nevertheless, the morphology of Prometheus’s lava field, which reflects the trajectory of the active front and thus the trajectory of the sources, shows that the previous assumption is just an approximation. The real path, although mainly directed from east to west, may have departed from the theoretical one by as much as 20 km. According to Section 4.2 and to our conventions, we have xs (t = 0) = −95 km and xs (t = τ ) = 0 with τ ≈ 20 years. A uniform movement would be expressed by xs (t) = Smean (t − τ )/τ . At this point, we can calculate the distribution of solid SO2 that would result from the accumulation of fallouts during 20 years if we ignore any subsequent processes of sublimation and overlapping by other materials. We just sum for each point M(x, y) belonging to our 2D space the contribution of all the sources and integrate the result over time: acc f SO (x, y; rmax (¯v ), RSD, θo ) 2  T  +x /2  + y /2 = A(x , y , t ) 0

−x /2

χ looks like a step function rising rapidly from the level χo prevailing inside the inner edge of the ring of SO2 deposition (r
ri ) to 1 outside (r  ri ). The parameter dr controls the abruptness of the transition. When χo = 0, no SO2 frost persists. Within the entire surface swept by the refractory materials during the past 20 years (we note it Ar ), we can distinguish two crescent-shaped areas having experienced two different series of events due to the plume displacement (see Fig. 10). They are defined by the initial (t = 0) and final (t = τ ) positions of the internal edge of the SO2 deposition ring. The first area, SO2 -rich, lies on the eastern edge of the ring, whereas the second, which is SO2 -poor, is on the western edge. For convenience, we now change from rectangular coordinates (x, y) to polar coordinates (r , φ). Points belonging to the first area respect the conditions φ > φlim r
κ

simu acc bck f SO (r, φ) = f SO (r, φ) + f SO χ (r ). 2 2 2

(11)

The function A(x , y , t ) simply expresses the relative activity of the different sources localized in space in terms of gas emission at any time t . It also translates the normalized areal density ρ of Glaze and Baloga (2000) into SO2 coverage f SO2 . In reality, the function A(x , y , t ) displays a statistical behavior rather than a deterministic one. Prometheus’s plume has been apparently very stable in size, shape, and optical properties for 20 years (McEwen et al. 2000). First this fact shows that the statistical distribution of gaseous activity likely has not changed significantly with time (Milazzo et al. 2002). Secondly, it demonstrates that the cumulative effect of material deposition from many individual sources does not depend exactly on the repartition of activity in space. As a consequence, in the following, we will consider A(x , y , t ) as a constant. 4.3.4. Processes of codeposition, overlapping, and sublimation. According to the conclusions of Section 4.2, Prometheus’s plume constantly emits refractory materials (i.e., less volatile than SO2 ) which settle inside the “instantaneous” ring of SO2 deposition and cover or sublimate preexisting deposits. In order to treat these processes, we introduce an empirical function χ giving the fraction of SO2 frost remaining after an episode of overlapping and/or sublimation at a distance r from the sources: χ (r ) = 1/2[(1 − χo ) tanh((r − ri )/dr ) + χo + 1].

with φlim = arctan( S /ri ) + π/2  with κ = −cosφ S + ri2 − (sin φ S )2 .

(12)

(13)

There, all but a few traces of background SO2 primarily disappeared. Then, because the sources have been moving away ever since, SO2 has been able to condense and accumulate and keeps doing it. Such a sequence can be formulated by:

− y /2

 × ρ( (x − xs (t) − x )2 + (y − y )2 ) dy d x dt .

477

(14)

acc f SO (r, φ) can be calculated from Eq. (11) using the rectangular 2 coordinates of point (r, φ). Points belonging to the second area respect the conditions

φ ≤ φlim r
κ

with κ =ri /sin|φ| if φ >π/2 and κ =ri if φ ≤ π/2.

(15)

There, SO2 frost deposits first started to build up on top of the equatorial plain background but were later covered nearly entirely by the refractory components. Mathematically, this is written  acc simu bck f SO . (16) (r, φ) = χ (r ) f SO (r, φ) + f SO 2 2 2 For both area, the curve κ(φ) describes the boundary of Ar , and the expression of χ (r ) defined in Eq. (12) is slightly modified when appearing in Eqs. (14) and (16): ri is replaced by κ. The reason is that we take into account the cumulative effects of overlapping over 20 years and not just an episode a few months long. 4.4. Results First we have carried out a systematic study regarding the model sensitivity, against variations of its different parameters: • the maximum distance rmax reached by the particles, • the relative standard deviation of speed distribution RSD,

DOUTE´ ET AL.

TABLE IV Best Set of Parameter Values Reproducing the W and E SO2 Abundance Profiles Parameter

Value

rmax (¯v ) RSD θo A dr χo xs (t)

180 km 0.05 70◦ 550 10 km 0.3 uniform displacement xs (t) = Smean (t − τ )/τ

obs. E profile simulation

2

SO2 areal abundance (fSO )

Here we briefly summarize the results of this study. Each parameter has a distinctive and significant influence on the simsimu ulated field of solid SO2 f SO (r, φ). Such a situation makes us 2 confident when inverting the model on the different observed profiles of SO2 areal abundance extracted in Section 4.1. We should be able to well constrain the previous list of physical parameters, which characterize the history and the modalities of the long-lived Prometheus eruption. We first perform the inversion using a simple trial and error method to find the best set of paobs simu rameter values fitting f SO (r, φ) with f SO (r, φ), 0
r
300 km 2 2 ◦ and φ = 0, and 180 . This corresponds to the modeling of two profiles, respectively, along the W and E directions. Table IV lists the results and Fig. 11 illustrates the quality of the fit achieved when using these values to model the W and E profiles. In both cases, the position, amplitude, width, and shape of the main SO2 deposition ring of Prometheus are reproduced with minor discrepancies: the maxima of the E and W profiles are slightly (≈20 km) shifted toward the origin, and the main peak FWHM (full width at half maximum) of the W profile is overestimated a little. More pronounced differences between the observation and the model can be noted and explained. First, in the inner part of the simulated E profile, SO2 areal abundance levels off at ≈0.25 below ≈100 km, whereas the observed profile drops abruptly to 0. The point of divergence corresponds exactly to the Prometheus lava field boundary. Therefore, this discrepancy simply reflects the sublimation of SO2 frost by hot lava, a phenomenon not taken into account by the model. Second, the observed E and W profiles display a secondary peak respectively 90 and 120 km away from the main peak. Study of Fig. 9 (top) shows that such local extrema of SO2 abundance are localized and not signs of real, extensive processes. The most robust result of the simulation is the necessity of a quite uniform movement of the gas sources during the past

0.8

0.6

0.4

0.2 sublimation of SO2 by lava field not taken into account

0.0 50

100 150 200 Distance from origin (km)

250

300

0.8 obs. W profile simulation

local SO2 spot

2

• the maximum zenith angle of ejection θo , • the source activity A, • the transition distance dr between SO2 and other condensed materials on the surface, • the fraction of frost χo remaining after all the episodes of overlapping and/or sublimation, • and the kinetics of the source mean location xs (t).

SO2 areal abundance (fSO )

478

0.6

0.4

0.2

0.0 50

100 150 200 Distance from origin (km)

250

300

FIG. 11. Observed SO2 abundance profiles and their modeling using the best set of parameter values given by Table IV. Here we consider a uniform movement of the sources xs (t) = Smean (t − τ )/τ . Top: east profile. Bottom: west profile.

20 years to explain simultaneously the W and E abundance profiles. To illustrate this important point, Fig. 12 (top) shows the best modeling we obtain for the E profile when the sources sit at the Voyager position for some time, then migrate quickly to join the current position: xs (t) = Smean (t/τ )4 . The fit quality is comparable to that for the uniform solution. However, Fig. 12 (bottom) indicates that crucial characteristics of the W profile are not reproduced. In order to facilitate their interpretation, some of the parameters of Table IV can be expressed differently using a related physical quantity. For example, rmax is directly connected to the root mean square velocity v¯ and kinetic temperature T of SO2 gas erupting from the vent(s) by the relation noted from Glaze and Baloga (2000) (Eq. (21)), 1 2 3 1 m v¯ = kT = mgrmax (¯v ), 2 2 2

(17)

where m is the molecular mass of SO2 (m = 1.0624 10−25 kg), k is the Boltzmann constant (k = 1.38 10−23 J K−1 ), and g is the

479

SO2 GAS CONDENSATION AROUND VOLCANIC PLUMES ON IO

saturation arising from the overlapping of deposited SO2 frost by itself. In order to explain the reversal of asymmetry, we have performed a fine analysis of the SO2 deposition and accumulation processes as we treat them outside the surface Ar (see definition in Section 4.3). Yet it indicates that, indeed, their integrated intensity should be maximum in two areas situated perpendicular (north and south) to the trajectory of the gas sources and not along its axis, as observed. Possible explanations of this intriguing problem are proposed in the next section. 4.5. Discussion We now reexamine each hypothesis we have applied in Section 4.3, having attempted to express mathematically the different factors which determine the Prometheus SO2 field. Our goal is to propose clues to the reversal of asymmetry within the deposition ring noted between the SO2 distribution maps and our model. Conjugation, prior to the eruption, of two very peculiar situations—(a) the existence of enriched frost deposits just over

obs. S profile simulation

2

SO2 areal abundance (fSO )

1.0

0.6

0.4

0.2

0.0 50

1.0 2

acceleration due to Io’s gravity (g = 1.797 m s−2 ). rmax = 180 km corresponds to T = 830 K and v¯ = 568 m s−1 with a modelderived (based on RSD) standard deviation of σ = 28 m s−1 . Such parameters are also related to the theoretical maximum height reached by the plume: h max = v¯ 2 /2g = 90 ± 9 km. This range of altitudes is compatible with the measurement of Prometheus’s plume height performed by McEwen et al. (1998) on SSI images of Io’s limb: h obs = 50–100 km. We now apply the parameter values of Table IV to simulate two profiles extracted along the N and S directions (0
r
300 km, φ = 90, and 270◦ ). Unfortunately, Fig. 13 reveals that the model does not reproduce the observation well in terms of amplitude, and to a lesser extent, shape of the principal peak of SO2 deposition. The position of the maximum is correct though. Whereas map fso2 i25 of Fig. 6 (bottom) indicates higher concentrations of SO2 frost within the eastern and western area than within the northern and southern parts of the deposition ring, the model predicts the opposite. Additionally, the simulation shows values of f SO2 exceeding 1.0 at the peak. The latter artifact is due to the fact that the model does not properly treat the effects of

SO2 areal abundance (fSO )

FIG. 12. Observed SO2 abundance profiles and their modeling using the best set of parameters reproducing the east profile. Here we consider an accelerated movement of the sources xs (t) = Smean (t/τ )4 . Top: east profile. Bottom: west profile.

0.8

100 150 200 Distance from origin (km)

250

obs. N profile simulation

0.8 0.6 0.4 0.2 0.0 50

100 150 200 Distance from origin (km)

250

300

FIG. 13. Observed SO2 abundance profiles and their modeling using the best set of parameter values given by Table IV. Top: south profile. Bottom: north profile.

480

DOUTE´ ET AL.

areas currently occupied by the west and east parts of the ring and (b) depletion of SO2 over areas currently occupied by the north and south parts—is very unlikely. Indeed, the very homogeneous properties of the surrounding equatorial plains do not support such a possibility. The morphology of the Prometheus lava field indicates that latitudinal deviations from the purely longitudinal trajectory we have considered for the mean position of the sources occurred throughout the past 20 years (Keszthelyi et al. 2002). This could have some consequences for the morphology of both the north and south sides of the ring: for example, a covering of the SO2 deposits by less-volatile materials and an asymmetry if the sources have spent more time below the 1.6◦ parallel than above. However, the deviations (15 km at most) represent only one-quarter of the “instantaneous” width of the ring δri = 60 km. Moreover, when creating an average of map fso2 i24 and map fso2 i25, no north–south asymmetry is really visible. As a consequence, the description we choose for the displacement of the sources, and thus for their temporal and spatial distribution, does not seem to be too inaccurate and responsible for the relative abundance paradox. The fundamental assumption of the stochastic–ballistic model proposed by Glaze and Baloga (2000) is the axisymmetry of the plume and thus of the repartition of materials deposited on the surface. This assumption expresses the randomization of velocity components at the vent due to various processes such as collision between particles, thermalization, and geometrical irregularities of the vent nozzle. We can indeed expect it happens for a single isolated plume (even though vent conditions are likely very variable), but multiple plumes taking roots at short distances from each other compared to their overall dimensions could interact and modify their dynamical patterns. The large extent of the active front and the occurrence of numerous breakouts through the solidifying lava (Keszthelyi et al. 2002, LopesGautier et al. 2002) make likely the existence of numerous minor (Milazzo et al. 2002) or at least a few major (Kieffer et al. 2000) sources of gas (or a mixture of both). We then can imagine confinement and piping of the gas erupted from the interior of the source field by the gas erupted at its boundary. Study of this kind of plume interaction by the direct simulation Monte Carlo (DSMC) method (Zhang et al. 2002) would be necessary to explain the higher SO2 abundances on the west and east sides of the deposition ring. We can now complete the conclusions of Section 4.2: • The initial kinetic temperature of SO2 gas (T ≈ 830 K), as derived from our stochastic ballistic model, is compatible with frost–silicate interaction at the active lava front and with the height of the observed visible plume. • The conditions of this interaction are remarkably constant along the lava front, given the small value of the standard deviation of speed distribution (28 m s−1 ). • The maximum ejection angle of the SO2 molecules is on the order of 70◦ , depicting quite wide volcanic nozzles at the vents. • The development of the Prometheus lava field has been quite uniform during the past 20 years, indicating a fairly con-

stant eruption rate from the principal caldera (east hot-spot of Lopes-Gautier et al. (2000)). • A stochastic–ballistic model of material emplacement is plausible, but the resulting SO2 field is also shaped by complex processes of codeposition, overlapping, and sublimation. • Dynamical interaction between multiple plumes is possible since it could explain the reversal of the spatial SO2 frost accumulation pattern between the SO2 maps and the model. • SO2 dynamically deposited on the equatorial passive plains of the Prometheus region displays a noticeable stability. 5. CONCLUSIONS

We have analyzed a series of NIMS spectral image cubes acquired with spatial resolutions from 7 to 25 km · pixel−1 over the Prometheus region. We have used SO2 frost, a volatile compound that is ubiquitous on Io’s surface, as a tracer to understand various thermodynamic and volcanic processes acting in this part of the equator. The limited NIMS spectral sampling has required the development of a new method to derive, from the cubes, the distribution and physical properties of solid SO2 . This method is based on the inversion of a bidirectional reflectance model on obs two observed spectral ratios: α obs = Robs p,q (3.28)/R p,q (4.13) and obs obs obs β = R p,q (3.28)/R p,q (3.56). These two ratios show good correlation with, respectively, the areal abundance of SO2 ( f SO2 ) and its mean grain size (DSO2 ). As a result, reliable and consistent maps of SO2 abundance and to a lesser extent granularity have been obtained which can be correlated to distinguish four different physical units. The distribution of these SO2 units indicates zones of condensation, metamorphism, and sublimation linked with the thermodynamic and volcanic processes of interest. Our maps of SO2 distribution and physical state depict equatorial plains undisturbed by any kind of vigorous volcanic activity over 35–40% of their surface. There, the coverage and properties of frost deposits are controlled by the net balance between condensation of migrating plume gases and slow metamorphism– sublimation driven by sunlight. Elsewhere, 10–20% of the equatorial plains display abnormally low frost coverage and small grain sizes, which may imply temperatures in the range 110– 200 K, sometimes well above the mean diurnal temperature of SO2 (110 K, Kerton et al. 1996). Such areas, already detected by Veeder et al. (1994), are places of positive thermal anomalies, possibly linked with the upwelling of molten rock plumes through the mantle and with the consecutive thinning of Io’s crust. Hot-spots like Prometheus, Culann, Surya, and Tupan (to mention the most persistent) could be the superficial manifestation of these internal convective movements. Silicate lavas are generally erupted, as well as gases and pyroclastic materials. Prometheus and Culann emit a great variety of gases which may be ballistically transferred to Io’s surface. Associated fields of freshly condensed SO2 are easily observed, as well as deposits of more refractory compounds with higher (e.g., S8 ) or lower (e.g., NaCl) molecular weight. Different mechanisms of emission can be suspected for the volatile compounds and are actually

SO2 GAS CONDENSATION AROUND VOLCANIC PLUMES ON IO

supported by our maps: (a) the interaction between active lava and preexisting volatile deposits on the surface, (b) the direct degassing from the lava, and (c) the eruption of liquid aquifer from underground predicted by Kieffer (1982). The geometric elongation of the Prometheus SO2 deposition ring being related to the displacement of the plume during the past 20 years, which is itself due to the development of a 95-km-long lava field, is the best illustration of mechanism (a). This development results from the eruption of lava from the principal caldera with a fairly constant emission rate. The violent vaporization of preexisting SO2 frost and other volatile deposits mainly takes place along the active lava front and creates multiple plumes (Milazzo et al. 2002) which most likely interact dynamically. Such interaction may redirect part of the gas along a preferential east–west direction and determine, along with complex processes of codeposition, overlapping, and sublimation, the SO2 field observed around Prometheus. Mechanism (b) may operate at the hot-spot Surya, which presents a noticeable field of fresh SO2 frost but no extended lava field. Finally, we have noted on the northwestern flank of the volcanic edifice Emakong the existence of an extremely deep ν1 + ν3 SO2 absorption which is indicative of abundant, pure, and perhaps icy deposits. It seems to correspond on the SSI images to a gray flow that looks like a tongue, possibly the residue of the eruption of an SO2 liquid aquifer at low temperature (mechanism c). Our analysis and consecutive results suggest the following nonexhaustive list of tasks to be performed on the current set of data or on future observations:

481

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• complete mapping of the areas displaying low thermal emission and determination of their absolute temperatures; • monitoring of the temporal evolution of volcanic activity with an emphasis on Prometheus; • direct identification of the numerous gaseous compounds and pyroclastic materials emitted by the plumes; • modeling of the material ejection by the plumes and simulation of plume mutual dynamic interaction using a DSMC method (see Section 4.5); • consideration of possible intimate mixtures with an appropriate reflectance model; • determination of the age of the different SO2 deposits from their degree of metamorphism.

Dout´e, S., B. Schmitt, R. M. C. Lopes-Gautier, R. W. Carlson, L. Soderblom, and J. Shirley 2001b. Mapping SO2 frost on Io by the modeling of NIMS hyperspectral images. Icarus 149, 107–132.

ACKNOWLEDGMENTS

Fegley, B. J., and M. Y. Zolotov 2000. Chemistry of sodium, potassium, and chlorine in volcanic gases on Io. Icarus 148, 193–210.

We thank Lucas Kamp, Frank Leader, Robert Mehlman, Elias Barbinis, Marcia Segura, and the Galileo team at JPL for support with sequencing and data downlink activities. This work was begun at the Institute of Geophysics and Planetary Physics (UCLA) as well as at the Jet Propulsion Laboratory (California Institute of Technology), under contract with NASA through the Jupiter System Data Analysis Program. It continued at the Laboratoire de Plan´etologie de Grenoble with the support of the French Programme National de Plan´etologie and the CNES (Centre National d’Etudes Spatiales). Special thanks to Ashley Davies for his dedicated reading and corrections. Finally, we are particularly grateful to Julianne Moses for her patient review and her useful comments and suggestions which have greatly increased the quality of this paper.

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