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The SAR imaging mode of the Cassini RADAR instrument enables us to map the surface of Titan through its thick atmosphere. The first Cassini close flyby Ta, acquired on 26 October 2004, revealed a complex surface, with areas of low relief and dome-like volcanic constructs, flows and sinuous channels. In particular, fan-like features with strong radar backscattering were observed. Such structures, extending from tens of kilometers to more than 200 km in length, could be the result of cryovolcanism. Several radar-dark spots, up to 30 km across, were also observed: they may correspond to smooth hydrocarbon deposits. We present here a first modeling of these radar-bright and radar-dark features, based on classical radar backscattering models. We considered two main materials which could constitute the surface of Titan, tholins and water-ammonia ice, and modeled both single and two- layer cases, taking into account volume and sub-surface scattering. Our results show that SAR-bright regions could better be explained by the effect of a thin layer of water-ammonia ice covering a tholin substratum. Radar-dark spots can be modeled two-ways: a rough tholin surface or a smooth one with some volume scattering. We show that multi-incidence SAR data could help discriminate between the various scenarios proposed.

Tholins are brownish, sticky residues formed by the energetic processing of mixtures of gases abundant in the cosmos, such as CH4, N2, and H2O, either with ultraviolet light or electrical discharge. This thesis describes investigations of the tholins produced by the processing of mixtures of N2 and CH4, as a model of photochemical processes occurring in the atmosphere of Titan. These compounds are mixed with liquid water during impacts or cryovolcanism, in melt pools which eventually freeze. The melt pools are interesting sites for astrobiological research, containing a wealth of organic material interacting over long time periods in a liquid water solvent. Studies of the near infrared reflectance spectrum and the fluorescent properties of the tholins are presented, yielding information on the composition of the tholins and a means of finding organic deposits on the surface of Titan. Tholin decomposition on heating is extensively investigated, partly to ascertain the utility of pyrolysis as a technique for tholin characterization. Tholins are found to undergo significant chemical change at temperatures as low as 100 °C, releasing large quantities of ammonia, while at higher temperatures, cyclizing and aromatizing reactions occur. Pyrolysis is thus not a good technique for the characterization of tholins. The development of two instruments for tholin characterization are also discussed: a gas chromatograph with an ultraviolet absorption detector, for functional group analysis of the tholins, and sensors designed for the determination of enantiomeric excess.

The synthetic aperture radar (SAR) imaging mode of the Cassini RADAR instrument enables us to map the surface of Titan through its thick atmosphere. The first Cassini close flyby, acquired on 26 October 2004, revealed a complex surface, with areas of low relief and dome-like volcanic constructs, flows, and sinuous channels. In particular, fan-like features with strong radar backscattering were observed. Such structures, extending from tens of kilometers to more than 200 km in length, could be the result of cryovolcanism. Several radar dark spots, up to 30 km across, were also observed; they may correspond to smooth hydrocarbon deposits. We present here a first modeling of these radar-bright and radar-dark features on the basis of classical radar backscattering models. We considered two main materials which could constitute the surface of Titan, tholins and water-ammonia ice, and modeled both single- and two-layer cases, taking into account volume and subsurface scattering. Our results show that SAR-bright regions could better be explained by the effect of a thin layer of water-ammonia ice covering a tholin substratum. Radar-dark spots can be modeled in two ways: a rough tholin surface or a smooth one with some volume scattering. We show that multi-incidence SAR data could help discriminate between the various scenarios proposed.

The first two swaths collected by Cassini's Titan Radar Mapper were obtained in October of 2004 (Ta) and February of 2005 (T3). The Ta swath provides evidence for cryovolcanic processes, the possible occurrence of fluvial channels and lakes, and some tectonic activity. The T3 swath has extensive areas of dunes and two large impact craters. We interpret the brightness variations in much of the swaths to result from roughness variations caused by fracturing and erosion of Titan's icy surface, with additional contributions from a combination of volume scattering and compositional variations. Despite the small amount of Titan mapped to date, the significant differences between the terrains of the two swaths suggest that Titan is geologically complex. The overall scarcity of impact craters provides evidence that the surface imaged to date is relatively young, with resurfacing by cryovolcanism, fluvial erosion, aeolian erosion, and likely atmospheric deposition of materials. Future radar swaths will help to further define the nature of and extent to which internal and external processes have shaped Titan's surface.

Observations of Titan's mid-latitude clouds from the W. M. Keck and Gemini Observatories show that they cluster near 350°W longitude, 40°S latitude. These clouds cannot be explained by a seasonal shift in global circulation and thus presumably reflect a mechanism on Titan such as geysering or cryovolcanism in this region. The rate of volatile release necessary to trigger cloud formation could easily supply enough methane to balance the loss to photolysis in the upper atmosphere.

Martian gullies are found on steep slopes of all origins, on all sorts of terrains of all ages, scattered across nearly all of Mars. Gullies are observed on all manner of substrates (layered, massive, shattered, rubble), with or without nearby mantling deposits. Gullies are most common in the southern midlatitudes but also occur in the northern hemisphere, in near polar terrain, on equatorial volcanoes, and on northern plains. Most gullies in the southern hemisphere are on south-facing slopes, but they occur on slopes of all orientations. Gullies are among the youngest features on Mars but locally are overlain by eolian deposits and cut by faults. Old or eroded gullies are rare, and those found have been partially stripped from slopes, leaving no rock debris behind. Most gully deposits contain no detectable rocks. These data are inconsistent with published hypotheses of gully formation, including seeps and breakouts of water or brine, hydrothermal activity, cryovolcanism, and breakouts from liquid carbon dioxide. The data are consistent with gullies being dry flows of eolian material (dust and silt), comparable to climax snow avalanches on Earth. Eolian sedimentation should be correlated little with underlying geology: cause of slope, age of terrain, type of terrain, or the nature of the rocks. Eolian sedimentation should be correlated with wind deceleration (which will cause suspended sediment to drop), and areas with common gullies are those with strong wind deceleration (predicted by global circulation model). In such areas, sediment will be deposited preferentially in the lee of obstacles; for the gully-rich areas of the southern midlatitudes, winds blow from the NNW, so that sediment is deposited on SSE-facing slopes (i.e., poleward). These predictions are in accord with observations.

We have considered a wide array of scenarios for Europa's chem-ical evolution in an attempt to explain the presence of ice and hy-drated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution and in-tracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface al-teration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous dif-ferentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatiliza-tion of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e.g., an ocean/crust of sodium carbonate or sulfuric acid, or a crust con-taining abundant clathrate hydrates. Realistic chemical–physical 226 0019-1035/00 $35.00 All rights of reproduction in any form reserved. EUROPA'S OCEAN 227 evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical hetero-geneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical–chemical structures predicted from these scenarios. These predicted conse-quences and observed aspects of Europa's geology may serve as a basis for further analysis and discrimination among several al-ternative scenarios. Most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms.

The major icy satellites of the outer planets are recognized as complex planet-like objects with unique geologic histories and their own cryogenic versions of the igneous and tectonic processes that shape Earth and other rocky planets. The volcanological nature, surface compositions, internal structures, and overall geologic histories of icy satellites depend considerably on the satellites’icy compositions and the physical properties and phase behavior of these ices. Selected data pertinent to cryovolcanism and melting processes of icy satellites are reviewed here.

The Cassini spacecraft has been touring Saturn's neighborhood since it entered orbit there on 1 July 2004. As well as interrogating the giant planet and its filigree rings, understanding the formation of Saturn's moons is one of the key goals of the mission. Saturn has 35 named moons in its family, as well as a plethora of smaller objects hidden in the rings. The variety of moons is staggering. Some consist of exposed rock; others are enveloped in thick ice. Some are pock-marked with billion-year-old craters; others varnished with fresh snow. Cassini's busy schedule included more than 20 encounters with Saturn's moons in the past year, offering but a sampling of these different worlds. One of the most curious of all Saturn's moons is Enceladus. The sixth largest saturnian satellite—although at a mere 504 km across, its width is smaller than that of Arizona or Spain—Enceladus rides in the middle of Saturn's E ring, a wide and diffuse blue ring of fine particles. Because this ring lacks the bigger shards characteristic of ancient satellite smash-ups, astronomers have long suspected that the E-ring particles emanate from Enceladus itself. Earlier Earth-based and Voyager space probe images revealed that Enceladus' surface is icy and complex. Old cratered terrains butt up against newly resurfaced smooth ice flows. So Cassini scientists hoped to see signs of recent activity and possibly ice volcanism. As the papers in this special issue show, their anticipation was rewarded. In three flybys, in February, March, and July 2005, Cassini trained its instruments on Enceladus. The first two cruises descended to about 1000 and 500 km above the moon's equator. Onboard cameras snapped images of lines of folded mountain ridges and cracked white ice plains streaked with dark green organic material. The magnetometer saw signs of ions leaking out from Enceladus'atmosphere, and it localized, in the second flyby, a strong outflow from the south pole. The trajectory of the third flyby on 14 July 2005 was then adjusted to fly through the emergent gas just 168 km above the south pole. The flybys show that Enceladus' south polar landscape is still active today and is being resurfaced by cryovolcanism and fresh snowfall. Tidal forces have twisted and buckled the surface ice, producing long ridges and fractures. At infrared wavelengths, the south pole actually glows. An underground heat source lies beneath a surface grid of “tiger stripes”: a parallel set of linear trenches stained with dark organic material. From these warm vents, water vapor, ice, and dust particles are lofted in a spectacular plume, like spray from a Yellowstone geyser. Ions are driven further into the atmosphere and out into the magnetosphere of Saturn itself and the E ring. The rate at which water is being blown out is enough to replenish not only the E ring but also oxygen throughout the whole saturnian system. So is the mystery of Enceladus solved? Not at all. Finding such active geology on such a tiny moon is a big surprise. Neptune's Triton and Jupiter's Io have ice and sulfur volcanoes, respectively, but are larger bodies; Europa has experienced plate tectonics but no current activity is seen there. Yet tiny Enceladus produces a plume large enough to drench the whole Saturn system. The origin of Enceladus' internal heating is also still a major puzzle. To produce the plume, water needs to be boiled off or sublimated. Ammonia could act as antifreeze but was not detected by Cassini. Does liquid water, or locked-up ammonia, lie beneath the surface? We may need to wait for future missions to this enigmatic moon to find out for sure, but Enceladus is definitely on the map.

[1] Very high-resolution imaging and stereo topographic data obtained during the Galileo G28 encounter with Ganymede show 1) evidence for Europa-like, crustal spreading and resurfacing to form portions of the bright terrain, and 2) bright terrain that appears smooth at Voyager resolution (and thus a strong candidate for cryovolcanism) but instead is tectonically deformed and lacks embayment relationships when viewed at high resolution. In contrast to previous views, these new data show that tectonism has been the dominant process in shaping some very smooth areas and that Ganymede appears to have experienced Europa-like crustal spreading during its previous history.

The JUpiter Icy moons Explorer (JUICE) will investigate Ganymede's and Callisto's surfaces and subsurfaces from orbit to explore the geologic processes that have shaped and altered their surfaces by impact, tectonics, possible cryovolcanism, space weathering due to micrometeorites, radiation and charged particles as well as explore the structure and properties of the icy crust and liquid shell (Grasset et al., 2013). The best possible synergy of the JUICE instruments is required to answer the major science objective of this mission and to fully exploit the potential of the JUICE mission. Therefore, the JUICE team is aiming to define high priority targets on both Ganymede's and Callisto's surfaces to support the coordination of the planning activities by the individual instrument teams. Based on the science objectives of the JUICE mission and the most recent knowledge of Ganymede's and Callisto's geologic evolution we propose a collection of Regions of Interest (RoIs), which characterize surface features and terrain types representing important traces of geologic processes, from past and/or present cryovolcanic and tectonic activity to space weathering processes, which are crucial to understand the geologic evolution of Ganymede and Callisto. The proposed evaluation of RoIs is based on their scientific importance as well as on the opportunities and conditions to observe them during the currently discussed mission profile.

The JUpiter Icy moons Explorer (JUICE) will investigate Ganymede's and Callisto's surfaces and subsurfaces from orbit to explore the geologic processes that have shaped and altered their surfaces by impact, tectonics, possible cryovolcanism, space weathering due to micrometeorites, radiation and charged particles as well as explore the structure and properties of the icy crust and liquid shell (Grasset et al., 2013). The best possible synergy of the JUICE instruments is required to answer the major science objective of this mission and to fully exploit the potential of the JUICE mission. Therefore, the JUICE team is aiming to define high priority targets on both Ganymede's and Callisto's surfaces to support the coordination of the planning activities by the individual instrument teams. Based on the science objectives of the JUICE mission and the most recent knowledge of Ganymede's and Callisto's geologic evolution we propose a collection of Regions of Interest (RoIs), which characterize surface features and terrain types representing important traces of geologic processes, from past and/or present cryovolcanic and tectonic activity to space weathering processes, which are crucial to understand the geologic evolution of Ganymede and Callisto. The proposed evaluation of RoIs is based on their scientific importance as well as on the opportunities and conditions to observe them during the currently discussed mission profile.

The JUpiter Icy moons Explorer (JUICE) will investigate Ganymede's and Callisto's surfaces and subsurfaces from orbit to explore the geologic processes that have shaped and altered their surfaces by impact, tectonics, possible cryovolcanism, space weathering due to micrometeorites, radiation and charged particles as well as explore the structure and properties of the icy crust and liquid shell (Grasset et al., 2013). The best possible synergy of the JUICE instruments is required to answer the major science objective of this mission and to fully exploit the potential of the JUICE mission. Therefore, the JUICE team is aiming to define high priority targets on both Ganymede's and Callisto's surfaces to support the coordination of the planning activities by the individual instrument teams. Based on the science objectives of the JUICE mission and the most recent knowledge of Ganymede's and Callisto's geologic evolution we propose a collection of Regions of Interest (RoIs), which characterize surface features and terrain types representing important traces of geologic processes, from past and/or present cryovolcanic and tectonic activity to space weathering processes, which are crucial to understand the geologic evolution of Ganymede and Callisto. The proposed evaluation of RoIs is based on their scientific importance as well as on the opportunities and conditions to observe them during the currently discussed mission profile.