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The great lakes of Europa The Galileo spacecraft revealed a number of 'chaos' regions on Jupiter's moon Europa, where the surface terrain appears to have been disrupted from below. In many places, the surface contains sharp-edged blocks or rafts of ice that have at some point been flipped or rotated. Some characteristics of these regions have been hard to explain, such as the fact that the archetypal Conamara Chaos stands above its surroundings and contains matrix domes. Schmidt et al . apply lessons learned from analogous processes within Earth's subglacial volcanoes and ice shelves to an analysis of archival data that suggests chaos terrain forms above liquid water 'lenses' that are perched only 3 kilometres deep within the ice shell. The data suggest that ice–water interactions and freeze-out give rise to the varied morphology of chaos terrains, implying that more water is involved than has been previously appreciated — for instance, the sunken topography of Thera Macula, a large chaos area, may indicate that Europa is actively resurfacing over a lens comparable in volume to North America's Great Lakes. Europa, the innermost icy satellite of Jupiter, has a tortured young surface 1 , 2 , 3 , 4 and sustains a liquid water ocean 1 , 2 , 3 , 4 , 5 , 6 below an ice shell of highly debated thickness 1 , 2 , 3 , 4 , 5 , 7 , 8 , 9 , 10 . Quasi-circular areas of ice disruption called chaos terrains are unique to Europa, and both their formation and the ice-shell thickness depend on Europa's thermal state 1 , 2 , 3 , 4 , 5 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 . No model so far has been able to explain why features such as Conamara Chaos stand above surrounding terrain and contain matrix domes 10 , 18 . Melt-through of a thin (few-kilometre) shell 3 , 7 , 8 is thermodynamically improbable and cannot raise the ice 10 , 18 . The buoyancy of material rising as either plumes of warm, pure ice called diapirs 1 , 9 , 10 , 11 , 12 , 13 , 14 , 15 or convective cells 16 , 17 in a thick (>10 kilometres) shell is insufficient to produce the observed chaos heights, and no single plume can create matrix domes 10 , 18 . Here we report an analysis of archival data from Europa, guided by processes observed within Earth's subglacial volcanoes and ice shelves. The data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres. Our results suggest that ice–water interactions and freeze-out give rise to the diverse morphologies and topography of chaos terrains. The sunken topography of Thera Macula indicates that Europa is actively resurfacing over a lens comparable in volume to the Great Lakes in North America.

X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets. Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust. The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles. Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance.

The MESSENGER Gamma-Ray Spectrometer measured the average surface abundances of the radioactive elements potassium (K, 1150 ± 220 parts per million), thorium (Th, 220 ± 60 parts per billion), and uranium (U, 90 ± 20 parts per billion) in Mercury's northern hemisphere. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursor materials, and supports formation from volatile-containing material comparable to chondritic meteorites. Abundances of K, Th, and U indicate that internal heat production has declined substantially since Mercury's formation, consistent with widespread volcanism shortly after the end of late heavy bombardment 3.8 billion years ago and limited, isolated volcanic activity since.

Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation.

The Mars Global Surveyor Mars Orbiter Camera has acquired data that establish the present-day impact cratering rate and document new deposits formed by downslope movement of material in mid-latitude gullies on Mars. Twenty impacts created craters 2 to 150 meters in diameter within an area of 21.5 x 10⁶ square kilometers between May 1999 and March 2006. The values predicted by models that scale the lunar cratering rate to Mars are close to the observed rate, implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating. The new gully deposits, formed since August 1999, are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water: relatively long, extended, digitate distal and marginal branches, diversion around obstacles, and low relief. The observations suggest that liquid water flowed on the surface of Mars during the past decade.

The rover Opportunity has investigated the rim of Endeavour Crater, a large ancient impact crater on Mars. Basaltic breccias produced by the impact form the rim deposits, with stratigraphy similar to that observed at similar-sized craters on Earth. Highly localized zinc enrichments in some breccia materials suggest hydrothermal alteration of rim deposits. Gypsum-rich veins cut sedimentary rocks adjacent to the crater rim. The gypsum was precipitated from low-temperature aqueous fluids flowing upward from the ancient materials of the rim, leading temporarily to potentially habitable conditions and providing some of the waters involved in formation of the ubiquitous sulfate-rich sandstones of the Meridiani region.

Lobate features abutting massifs and escarpments in the middle latitudes of Mars have been recognized in images for decades, but their true nature has been controversial, with hypotheses of origin such as ice-lubricated debris flows or glaciers covered by a layer of surface debris. These models imply an ice content ranging from minor and interstitial to massive and relatively pure. Soundings of these deposits in the eastern Hellas region by the Shallow Radar on the Mars Reconnaissance Orbiter reveal radar properties entirely consistent with massive water ice, supporting the debris-covered glacier hypothesis. The results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes. Such features may collectively represent the most extensive nonpolar ice yet recognized on Mars.

Titan's lakes revealed The existence of oceans or lakes of liquid methane on Saturn's moon Titan was predicted more than twenty years ago. But with a dense haze preventing a closer look, it has not been possible to confirm their presence. Until the Cassini flyby of 22 July 2006 that is. Radar imaging data from the flyby, published this week, provide convincing evidence for large bodies of liquid. The cover gives a taste of what Cassini saw. Intensity in this image (colorized — though not as a representation of what the human eye would see) is proportional to the logarithm of radar backscatter cross-section. The lakes, darker than the surrounding terrain, are emphasized here by tinting regions of low backscatter blue and radar-brighter regions tan. The strip of radar imagery is about 140 km wide, and is foreshortened to simulate an oblique view of the highest latitude region, seen from a point to its west. The surface of Saturn’s haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface 1 , 2 . Initial visible 3 and radar 4 , 5 imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface 5 , 6 . Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T 16 ). The radar imaging polewards of 70° north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface 2 , 7 . The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan’s surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface ‘liquid methane’ table.

High-resolution images of Mercury's surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features support the inference that Mercury's interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system's innermost planet.

An ethane lake on Titan Saturn's moon Titan was once thought to have global oceans of light hydrocarbons on its surface, but the Cassini spacecraft's 40 close flybys of Titan have shown that no such oceans exist. Yet there are surface features similar to terrestrial lakes and seas. Infrared spectroscopic data from Cassini's 38th flyby reveal a possible explanation. The spectra and physical appearance of Ontario Lacus, a lake-like structure near Titan's south pole, support the idea that it is filled with liquid ethane, probably in solution with methane, nitrogen and other hydrocarbons. Titan was once hypothesized to have global oceans of light hydrocarbons, but it has become clear that none exist. However there are features similar to terrestrial lakes and seas. This paper reports infrared spectroscopic data that strongly indicate that ethane, probably in liquid solution with methane, nitrogen, and other low-molecular-weight hydrocarbons, is contained within Titan's Ontario Lacus. Titan was once thought to have global oceans of light hydrocarbons on its surface 1 , 2 , 3 , 4 , 5 , but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist 6 . There are, however, features similar to terrestrial lakes and seas 7 , and widespread evidence for fluvial erosion 8 , 9 , presumably driven by precipitation of liquid methane from Titan’s dense, nitrogen-dominated atmosphere 10 . Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer 11 (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan’s Ontario Lacus.