tomographic imaging of thermally induced fractures in granite using bayesian inversion the internal structure of rock samples studied in laboratory experiments can be described by a variety of physical parameters some of them like the velocity of acoustic waves enhanced velocity or quality factor can be reconstructed by means of ultrasonic tomography this article presents the results of classical velocity tomography imaging accompanied by the results of attenuation tomography and recently introduced enhanced velocity tomography obtained for a lac du bonnet granite sample subjected to thermal stresses to invert acoustic data recorded during six heating cycles a bayesian inversion scheme accompanied by a genetic algorithm optimization approach and the robust cauchy norm have been used to obtain the highest possible spatial resolution of images the inversion was performed in two steps in the first step a crude parameterization of the sample was used the result of this stage was next taken as an a priori model for a final inversion with refined parameterization the choice of parameterization cell sizes and damping parameters at both stages was based on an analysis of the resolution operator both velocity and enhanced velocity tomography accurately imaged changes in the rock microstructure caused by thermal stresses however enhanced velocity tomography gave a much better spatial resolution than velocity tomography on the other hand attenuation tomography based on inversion of pulse rise times was able to image only a rough structure of the sample and it has difficulty with reasonable imaging of the crack formed in the sixth heating cycle

ridges and tidal stress on io abstract sets of ridges of uncertain origin are seen in twenty nine high resolution galileo images which sample seven locales on io these ridges are on the order of a few kilometers in length with a spacing of about a kilometer within each locale the ridges have a consistent orientation but the orientations vary from place to place we investigate whether these ridges could be a result of tidal flexing of io by comparing their orientations with the peak tidal stress orientations at the same locations we find that ridges grouped near the equator are aligned either north south or east west as are the predicted principal stress orientations there it is not clear why particular groups run north south and others east west the one set of ridges observed far from the equator 52 s has an oblique azimuth as do the tidal stresses at those latitudes therefore all observed ridges have similar orientations to the tidal stress in their region this correlation is consistent with the hypothesis that tidal flexing of io plays an important role in ridge formation

mapping of the culann tohil region of io from galileo imaging data abstract we have used galileo spacecraft data to produce a geomorphologic map of the culann tohil region of io s antijovian hemisphere this region includes a newly discovered shield volcano ts i goab tholus and a neighboring bright flow field ts i goab fluctus the active culann patera and the enigmatic tohil mons radegast patera tohil patera complex analysis of voyager global color and galileo solid state imaging ssi high resolution regional 50 330 m pixel and global color 1 4 km pixel images along with available galileo near infrared mapping spectrometer nims data suggests that 16 distinct geologic units can be defined and characterized in this region including 5 types of diffuse deposits ts i goab fluctus is the center of a low temperature hotspot detected by nims late during the galileo mission and could represent the best case for active effusive sulfur volcanism detected by galileo the culann volcanic center has produced a range of explosive and effusive deposits including an outer yellowish ring of enhanced sulfur dioxide so2 an inner red ring of so2 with short chain sulfur s3 s4 contaminants and two irregular green diffuse deposits one in tohil patera apparently produced by the interaction of dark silicate lava flows with sulfurous contaminants ballistically emplaced from culann s eruption plume s fresh and red mantled dark lava flows west of the culann vent can be contrasted with unusual red brown flows east of the vent these red brown flows have a distinct color that is suggestive of a compositional difference although whether this is due to surface alteration or distinct lava compositions cannot be determined the main massif of tohil mons is covered with ridges and grooves defining a unit of tectonically disrupted crustal materials tohil mons also contains a younger unit of mottled crustal materials that were displaced by mass wasting processes neighboring radegast patera contains a nims hotspot and a young lava lake of dark silicate flows whereas the southwest portion of tohil patera contains white flow like units perhaps consisting of ponds of effusively emplaced so2 from 0 15 s the hummocky bright plains unit away from volcanic centers contains scarps grooves pits graben and channel like features some of which have been modified by erosion although the most active volcanic centers appear to be found in structural lows as indicated by mapping of scarps dems derived from stereo images show that with the exception of tohil mons there is less than 1 km of relief in the culann tohil region there is no discernable correlation between centers of active volcanism and topography

global color variations on io abstract visible and near infrared images of io from the galileo spacecraft reveal a surface more colorful than previously thought red yellow green white and black hues decorate the satellite presumably caused by a varied composition of sulfur compounds and silicates almost a third of io is covered by red and orange materials particularly at polar latitudes above 30 these red regions were scarcely distinguishable in the shorter wavelength voyager observations bright red pyroclastic deposits mark the locations of many hot spots plumes and visible surface changes providing a prominent flag of recent volcanic activity io s equatorial regions are dominated by yellow materials which occupy about 40 of the satellite s surface white and gray materials cover about 27 of io primarily in equatorial areas and in localized deposits at high latitudes these are identified with moderate to coarse grained so2 as mapped by the nims instrument on galileo r w carlson et al 1997 geophys res lett 24 2479 2482 greenish yellow materials in small isolated spots on io s anti jupiter hemisphere were recently discovered in 3 km pixel color imaging from orbit 14 unlike other ionian terrains these regions have a negative near infrared spectral slope suggesting contamination by a nonsulfur component only about 1 4 of io s surface is occupied by dark materials which display a variety of visible colors ranging from black to red and green most dark spots have a shallow spectral absorption feature at 0 9 m suggesting magnesium rich silicates rather than black sulfur little large scale alteration in the global color and albedo pattern has occurred between the voyager and galileo eras 90 of the surface appears unchanged despite the vigorous volcanic activity which must have taken place in the intervening 17 years this suggests that over a time scale of decades the bulk of io s resurfacing is restricted to a few small but persistently active areas

lava lakes on io observations of io s volcanic activity from galileo nims during the 2001 fly bys abstract galileo s near infrared mapping spectrometer nims obtained its final observations of io during the spacecraft s fly bys in august i31 and october 2001 i32 we present a summary of the observations and results from these last two fly bys focusing on the distribution of thermal emission from io s many volcanic regions that give insights into the eruption styles of individual hot spots we include a compilation of hot spot data obtained from galileo voyager and ground based observations at least 152 active volcanic centers are now known on io 104 of which were discovered or confirmed by galileo observations including 23 from the i31 and i32 io fly by observations presented here we modify the classification scheme of keszthelyi et al 2001 j geophys res 106 e12 33 025 33 052 of io eruption styles to include three primary types promethean lava flow fields emplaced as compound pahoehoe flows with small plumes 200 km high plumes and rapidly emplaced flow fields and a new style we call lokian that includes all eruptions confined within paterae with or without associated plume eruptions thermal maps of active paterae from nims data reveal hot edges that are characteristic of lava lakes comparisons with terrestrial analogs show that io s lava lakes have thermal properties consistent with relatively inactive lava lakes the majority of activity on io based on locations and longevity of hot spots appears to be of this third type this finding has implications for how io is being resurfaced as our results imply that eruptions of lava are predominantly confined within paterae thus making it unlikely that resurfacing is done primarily by extensive lava flows our conclusion is consistent with the findings of geissler et al 2004 icarus this issue that plume eruptions and deposits rather than the eruption of copious amounts of effusive lavas are responsible for io s high resurfacing rates the origin and longevity of islands within ionian lava lakes remains enigmatic

orogenic tectonism on io 1 we catalog 143 ionian mountains montes and mountain like features mensae tholi plana and small peaks in order to investigate orogenic tectonism on io from this comprehensive list we select 96 mountains for which there are sufficient coverage and resolution to discern spatial relationships with surrounding geologic features three of the 96 mountains are probably volcanoes 92 appear to be tectonic massifs and 1 is ambiguous of the 92 tectonic mountains 38 abut paterae volcanic or volcano tectonic craters with irregular or scalloped margins this juxtaposition is unlikely to be a coincidence as the probability of it occurring by chance is 0 1 we propose instead that orogenic faults may act as conduits for magma ascent thus fueling patera formation near mountains as resurfacing buries a shell of material from io s surface to the base of the lithosphere its effective radius is reduced and it heats up we calculate the lithospheric volume change due to subsidence and thermal expansion as a function of lithospheric thickness conservation of volume dictates that this material must be uplifted at io s surface by estimating the total volume of the mountains we are able to place a lower limit of 12 km on io s lithospheric thickness we hypothesize that in some cases mountain formation may be facilitated by asthenospheric diapirs impinging on the base of the lithosphere the resulting lithospheric swell could focus the compressive stresses that drive orogenic tectonism this model is one of several possible mechanisms for uplifting isolated mountains such as are observed on io

large scale topography of io implications for internal structure and heat transfer new stereographic techniques were applied to voyager 1 images to determine the shape and large scale topography of io the best fit triaxial figure has semiaxes of 1830 0 1818 7 and 1815 3 kilometers with random errors of about 200 meters systematic navigation and optical system errors are comparable the resulting figure is consistent with a differentiated satellite in hydrostatic equilibrium the deviation from homogeneity could be explained by the presence of an iron core with a radius of about 800 km most elevations over areas larger than 500 km diameter deviate from the triaxial figure by no more than 1 km we suggest that many of these broad topographic swells and basins may be due to isostatic responses to thermal changes in the lithosphere asthenosphere system in the simplest picture increasing the heat flow converts the basal lithosphere into lower density asthenosphere resulting in isostatic uplift lithospheric thicknesses ranging from 5 to 100 km may result in elevation variations of up to 1 km if the large scale topography is positively correlated with heat flow then the global pattern of basins and swells suggests tidal dissipation in a molten or partially molten asthenosphere rather than in the mantle areas higher than 1 km may consist of low density crustal materials media regio the largest such region is centered near the equator at longitude 70

observations and initial modeling of lava so2 interactions at prometheus io we present observations and initial modeling of the lava so2 interactions at the flow fronts in the prometheus region of io recent high resolution observations of prometheus reveal a compound flow field with many active flow lobes many of the flow lobes are associated with bright streaks of what is interpreted to be volatilized and recondensed so2 radiating away from the hot lava lower resolution color data show diffuse blue to violet areas also near the active flow front perhaps from active venting of so2 not clearly visible in any of the images is a single source vent for the active plume while the size of the proposed vent is probably near the limit of the resolution we expected to see radial or concentric albedo patterns or other evidence for gas and entrained particles above the flow field the lack of an obvious plume vent earlier suggestions that the prometheus type plumes may originate from the advancing flow lobes and the high resolution images showing evidence for large scale volatilization of the so2 rich substrate at prometheus encouraged us to develop a model to quantify the heat transfer between a basaltic lava flow and a substrate of so2 snow we calculate that the vaporization rate of so2 snow is 2 5 10 6 m s 1 per unit area using an estimated 5 m2 s 1 lava coverage rate from change detection images we show that the gas production rate of so2 at the flow fronts is enough to produce a resurfacing rate of 0 24 cm yr 1 at the annulus of prometheus this is much less than other estimates of resurfacing by the prometheus plume while not easily explaining the main prometheus plume our model readily accounts for the bright streaks

mountains on io high resolution galileo observations initial interpretations and formation models during three close flybys in late 1999 and early 2000 the galileo spacecraft acquired new observations of the mountains that tower above io s surface these images have revealed surprising variety in the mountains morphologies they range from jagged peaks several kilometers high to lower rounded structures some are very smooth others are covered by numerous parallel ridges many mountains have margins that are collapsing outward in large landslides or series of slump blocks but a few have steep scalloped scarps from these observations we can gain insight into the structure and material properties of io s crust as well as into the erosional processes acting on io we have also investigated formation mechanisms proposed for these structures using finite element analysis mountain formation might be initiated by global compression due to the high rate of global subsidence associated with io s high resurfacing rate however our models demonstrate that this hypothesis lacks a mechanism for isolating the mountains the large fraction 40 of mountains that are associated with paterae suggests that in some cases these features are tectonically related therefore we have also simulated the stresses induced in io s crust by a combination of a thermal upwelling in the mantle with global lithospheric compression and have shown that this can focus compressional stresses if this mechanism is responsible for some of io s mountains it could also explain the common association of mountains with paterae

mountains and calderas on io possible implications for lithosphere structure and magma generation abstract the combination of voyager images and newly acquired galileo images with low illumination and resolutions ranging from 2 to 6 km pixel now allows determination of the global distribution of mountains and volcanic centers on io the mountains generally do not have characteristics typical of terrestrial volcanic landforms they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes relative elevations determined by shadow measurements and stereoscopy indicate that mountains in the newly imaged area range in elevation up to at least 7 6 km the origin of the mountains remains uncertain some appear to be multitiered volcanic constructs others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes yet others show extensive tectonic disruption volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes the low latitudes have one volcanic center per 7 10 4 km 2 and on average the centers are spaced roughly 250 km apart the global distribution of high mountains suggests that the lithosphere over most of io is thick although the thickness cannot be calculated the previously suggested 30 km appears reasonable as a lower limit the high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere the slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on io coupled with the high rate of magma production will likely result in larger melt fractions than is typical for source regions on earth the variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface this mixture will be carried downward through the lithosphere as a consequence of the 0 5 1 5 cm yr resurfacing rates during descent the more volatile components will tend to be driven off early but complete or near complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains

active volcanism on io io one of jupiter9s moons has hundreds of active volcanoes many of which are erupting high temperature silicate lavas in his perspective mcewen reviews how data returned by the galileo spacecraft have helped to resolve some questions regarding io9s volcanism but have raised others especially regarding the moon9s internal structure

landform degradation and slope processes on io the galileo view the galileo mission has revealed remarkable evidence of mass movement and landform degradation on io we recognize four major slope types observed on a number of intermediate resolution 250 m pixel 1 images and several additional textures on very high resolution 10 m pixel 1 images slopes and scarps on io often show evidence of erosion seen in the simplest form as alcove carving slumps and slides at all scales many of the mass movement deposits on io are probably mostly the consequence of block release and brittle slope failure sputtering plays no significant role sapping as envisioned by mccauley et al 1979 remains viable we speculate that alcove lined canyons seen in one observation and lobed deposits seen along the bases of scarps in several locations may reflect the plastic deformation and glacial flow of interstitial volatiles e g so2 heated by locally high geothermal energy to mobilize the volatile the appearance of some slopes and near slope surface textures seen in very high resolution images is consistent with erosion from sublimation degradation however a suitable volatile e g h2s that can sublimate fast enough to alter io s youthful surface has not been identified disaggregation from chemical decomposition of solid s2o and other polysulfur oxides may conceivably operate on io this mechanism could degrade landforms in a manner that resembles degradation from sublimation and at a rate that can compete with resurfacing

a komatiite analog to potential ultramafic materials on io ultramafic volcanism is one model which fits currently available data for some eruptions on jupiter s moon io assuming that such activity does occur it is possible to apply komatiitic analogs and terrestrial ultramafic flow models to io we have used such analogs and models along with galileo data to investigate the nature of ultramafic materials that are assumed to occur at high temperature hotspots on io the unusual komatiites of the commondale greenstone belt south africa are consistent with available galileo data on the temperatures and composition of potential ionian ultramafic materials these komatiites had high sio2 50 wt and mgo 31 wt contents which are inferred to result in high liquidus temperatures 1610 c low dynamic viscosities 0 2 pa s and low densities 2680 kg m and in the crystallization of orthopyroxene phenocrysts not found in other komatiites we used the commondale composition and the model of williams et al 1998 to investigate potential ultramafic flow behavior on io compared to their terrestrial counterparts ionian ultramafic lava flows would have less potential for turbulent flow lower maximum emplacement distances and lower thermal erosion rates because of the lower ionian gravity and their more silicic composition shallow 10 m thermal erosion channels seem less likely on io unless 1 lavas are superheated 2 lavas are more ultramafic 31 mgo 3 eruption rates eruption durations or flow volumes are high or 4 substrates are unconsolidated partly consolidated or volatile rich and thus more erodable if volatiles are present in the lava as vesicles then increasing lava vesicularity results in decreasing maximum emplacement distances decreasing thermal erosion rates and decreasing erosion channel depths for given eruption durations

a model for large scale volcanic plumes on io implications for eruption rates and interactions between magmas and near surface volatiles 1 volcanic plumes deposit magmatic pyroclasts and so2 frost on the surface of io we model the plume activity detected by galileo at the pillan and pele sites from 1996 to 1997 assuming that magmatic eruptions incorporate liquid so2 from near surface aquifers intersecting the conduit system and that the so2 eventually forms a solid condensate on the ground the temperature and pressure at which deposition of solid so2 commences in the ionian environment and the radial distance from the volcanic vent at which this process appears to occur on the surface are used together with observed vertical heights of plumes to constrain eruption conditions the temperature pressure and density of the gas magma mixtures are related to distance from the vent using continuity and conservation of energy similar eruption mass fluxes of order 5 107 kg s 1 are found for both the pillan and the pele plumes the pele plume requires a larger amount of incorporated so2 29 34 mass than the pillan plume up to 6 mass implied vent diameters range from 90 m at pillan to 500 m at pele the radial extents of the optically dense isothermal incandescent parts of the eruption plumes immediately above the vents are 100 m at pillan and 1300 m at pele gas pressures in the vents are 20 kpa at pillan and 2 kpa at pele and the eruption conditions appear to be supersonic in both cases though only just so at pele

a possible 20th century slowdown of southern ocean deep water formation chlorofluorocarbon 11 inventories for the deep southern ocean appear to confirm physical oceanographic and geochemical studies in the southern ocean which suggest that no more than 5 x 10 6 cubic meters per second of ventilated deep water is currently being produced this result conflicts with conclusions based on the distributions of the carbon 14 carbon ratio and a quasi conservative property po 4 in the deep sea which seem to require an average of about 15 x 10 6 cubic meters per second of southern ocean deep ventilation over about the past 800 years a major reduction in southern ocean deep water production during the 20th century from high rates during the little ice age may explain this apparent discordance if this is true a seesawing of deep water production between the northern atlantic and southern oceans may lie at the heart of the 1500 year ice rafting cycle

topographic evidence for shield volcanism on io abstract a shield volcano has been identified on io based on photoclinometrically determined slope values and planimetric presentation the slope values typically 10 and 2 5 km height of the volcano imply that it is composed of a material with the mechanical properties of basaltic rock the height of the volcano may also indicate a minimum value of 40 km for the thickness of the local lithosphere

observations and temperatures of io s pele patera from cassini and galileo spacecraft images abstract pele has been the most intense high temperature hotspot on io to be continuously active during the galileo monitoring from 1996 2001 a suite of characteristics suggests that pele is an active lava lake inside a volcanic depression in 2000 2001 pele was observed by two spacecraft cassini and galileo the cassini observations revealed that pele is variable in activity over timescales of minutes typical of active lava lakes in hawaii and ethiopia these observations also revealed that the short wavelength thermal emission from pele decreases with rotation of io by a factor significantly greater than the cosine of the emission angle and that the color temperature becomes more variable and hotter at high emission angles this behavior suggests that a significant portion of the visible thermal emission from pele comes from lava fountains within a topographically confined lava body high spatial resolution nightside images from a galileo flyby in october 2001 revealed a large relatively cool

origin of mountains on io by thrust faulting and large scale mass movements voyager stereoimages of euboea montes io indicate that this mountain formed when a large crustal block was uplifted 10 5 kilometers and tilted by approximately 6 degrees uplift triggered a massive slope failure on the northwest flank forming one of the largest debris aprons in the solar system this slope failure probably involved relatively unconsolidated layers totaling approximately 2 kilometers in thickness overlying a rigid crust or lithosphere at least 11 kilometers thick mountain formation on io may involve localized deep rooted thrust faulting and block rotation due to compression at depth induced during vertical recycling of io s crust

mapping so2 frost on io by the modeling of nims hyperspectral images abstract we analyze a collection of hyperspectral images of io acquired by the near infrared mapping spectrometer nims of galileo during the g2 to e16 orbits of jupiter this analysis leads to the geographical distribution and physical characterization of so 2 frost deposits over about three fourths of io s surface these deposits are excellent tracers of various phenomena including volcanic production and emission atmospheric transportation condensation metamorphism irradiation and sublimation that occur throughout the so 2 cycle we assume that the deposits of solid so 2 are optically thick and are geographically mixed with other sulfur bearing compounds we first assess the mean moderate backscattering behavior of the so 2 frost henyey greenstein phase function parameter g 0 27 0 05 using a sequence of spectra at two different locations over a large range of phase angles this behavior may indicate a granular texture with many defects or a fluffy texture second a more systematic inversion of the hyperspectral images is achieved based on a linear spectral model of pure so 2 with variable grain size mixed with a spectrally neutral unit as a result we produce two global mosaics that map so 2 frost coverage and mean grain size so 2 deposits are omnipresent on io s surface at the spatial scale of this study 200 km but the so 2 frost is concentrated within several large areas centered at medium latitudes these so 2 rich regions surface coverage higher than 60 show a longitudinal correlation with plumes located lower in latitude suggesting that these plumes are the principal sources of so 2 gas after a possible dynamic condensation around the plumes or at the equator the gas is remobilized by the solar or thermal fluxes and flows mostly latitudinally toward the coldest and nearest regions devoid of hot spots third the correlation of the distribution and grain size mosaics distinguishes four different so 2 physical units that indicate relative regional variations of condensation metamorphism and sublimation finally comparisons with voyager ultraviolet a s mcewen t v johnson d l matson and l a soderblom 1988 icarus vol 75 pp 450 478 and galileo visible p geissler a s mcewen l keszthelyi r m c lopes gautier j granahan and d p simonelli 1999 icarus vol 140 pp 256 282 observations demonstrate molecular contamination of so 2 at medium and high latitudes and that these contaminated so 2 deposits may be optically thin

io galileo evidence for major variations in regolith properties galileo ssi images taken at different times early in the jupiter orbital tour show striking widespread changes in io s color and albedo patterns that result from observing the satellite s surface at different lighting and viewing geometries such changes indicate significant variations in the photometric or light scattering behavior of the ionian surface from region to region and thereby provide strong evidence for major lateral variations in regolith properties in particular the regolith particles in io s bright equatorial band appear to be larger than or have a higher density of internal scatterers than those in the satellite s higher latitude regions such heterogeneity is not surprising on io given the rapid rate of volcanic resurfacing

emplacement and erosion by archean komatiite lava flows at kambalda revisited we have developed a mathematical model to evaluate the flow and erosional potential of submarine channelized komatiite lavas at kambalda western australia field data from kambalda were used to constrain the choice of important input parameters and model results were compared with data from field studies and geochemical analyses our results suggest that thermal erosion is strongly dependent upon the nature and behavior of the substrate if the substrate is treated as an unconsolidated hydrous sediment that can be fluidized by vaporized seawater then our model predicts that an initially 10 m thick basal kambalda komatiite lava could have produced very high thermo mechanical erosion rates 23 10 m day crustal thicknesses of 5 20 cm at distances of 5 35 km from the source and a high degree of lava contamination 3 12 in contrast if the substrate is treated as a more consolidated anhydrous sediment that could not be fluidized then our model predicts that a kambalda komatiite flow would have had much lower thermal erosion rates 1 2 0 4 m day and degrees of contamination 1 3 and would have had crustal thicknesses of 5 20 cm at longer flow distances of 30 165 km field constraints are generally consistent with our predictions for a non fluidized substrate the reentrant embayments at kambalda are thought to form from either erosion of deep channels in a flat basaltic seafioor huppert et al 1984 or erosion of a thin 5 m sediment with minor undercutting of basalt in pre existing topography lesher et al 1984 our modeling indicates that the former was possible for long eruption durations months whereas the latter was possible for short eruption durations 2 weeks as the latter hypothesis is more consistent with the existing field evidence for thermal erosion at kambalda we believe it is the preferred interpretation

prometheus io s wandering plume unlike any volcanic behavior ever observed on earth the plume from prometheus on io has wandered 75 to 95 kilometers west over the last 20 years since it was first discovered by voyager and more recently observed by galileo despite the source motion the geometric and optical properties of the plume have remained constant we propose that this can be explained by vaporization of a sulfur dioxide and or sulfur snowfield over which a lava flow is moving eruption of a boundary layer slurry through a rootless conduit with sonic conditions at the intake of the melted snow can account for the constancy of plume properties

paterae on io a new type of volcanic caldera paterae defined by the international astronomical union as irregular crater s or complex one s with scalloped edges are some of the most prominent topographic features on io paterae on io are unique yet in some aspects they resemble calderas known and studied on earth mars and venus they have steep walls flat floors and arcuate margins and sometimes exhibit nesting all typical of terrestrial and martian basalt shield calderas however they are much larger many are irregular in shape and they typically lack shields their great sizes some 200 km diameter and lack of associated volcanic edifices beg comparison with terrestrial ash flow calderas however there is no convincing evidence on io for the high silica erupted products or dome resurgence associated with this type of caldera ionian paterae seem to be linked with the eruption of large amounts of mafic to ultramafic lavas and colorful sulfur rich materials that cover the floors and sometimes flow great distances away from patera margins they are often angular in shape or are found adjacent to mountains or plateaus indicating tectonic influences on their formation a database of 417 paterae on io measured from images with 3 2 km pixel 1 resolution 80 of its surface reveals that their mean diameter of 41 0 km is close to that for calderas of mars 47 7 km is smaller than that for venus 68 km but dwarfs those for terrestrial basalt shield calderas 6 6 km and ash flow calderas 18 7 km thirteen percent of all paterae are found adjacent to mountains 42 have straight or irregular margins and 8 are found atop low shields abundant smaller paterae with more continuously active lava eruptions are found between 25 s and 25 n latitude whereas fewer and larger paterae are found poleward of these latitudes patera distribution shows peaks at 330 w and 150 w longitude likely related to the direction of greatest tidal massaging by jupiter ionian patera formation may be explained by portions or combinations of models considered for formation of terrestrial calderas yet their unusual characteristics may require new models with a greater role for tectonic processes

io in the near infrared near infrared mapping spectrometer nims results from the galileo flybys in 1999 and 2000 galileo s near infrared mapping spectrometer nims observed io during the spacecraft s three flybys in october 1999 november 1999 and february 2000 the observations which are summarized here were used to map the detailed thermal structure of active volcanic regions and the surface distribution of so2 and to investigate the origin of a yet unidentified compound showing an absorption feature at 1 m we present a summary of the observations and results focusing on the distribution of thermal emission and of so2 deposits we find high eruption temperatures consistent with ultramafic volcanism at pele such temperatures may be present at other hot spots but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations loki is the site of frequent eruptions and the low thermal emission may represent lavas cooling on the caldera s surface or the cooling crust of a lava lake high resolution spectral observations of emakong caldera show thermal emission and so2 within the same pixels implying that patches of so2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another thermal maps of prometheus and amirani show that these two hot spots are characterized by long lava flows the thermal profiles of flows at both locations are consistent with insulated flows with the amirani flow field having more breakouts of fresh lava along its length prometheus and amirani each show a white ring at visible wavelengths while so2 distribution maps show that the highest concentration of so2 in both ring deposits lies outside the white portion visible measurements at high phase angles show that the white deposit around prometheus extends into the so2 ring this suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction so2 mapping of the chaac region shows that the interior of a caldera adjacent to chaac has almost pure so2 the deposit appears to be topographically controlled suggesting a possible origin by liquid flow

dynamics and evolution of so2 gas condensation around prometheus like volcanic plumes on io as seen by the near infrared mapping spectrometer abstract 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 an d 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 1 3 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