Search

Europa, the innermost icy satellite of Jupiter, has a tortured young surface (1-4) and sustains a liquid water ocean (1-6) below an ice shell of highly debated thickness (1-5,7-10). 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 (1-5,7-17). 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 (1,9-15) 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., Although the settings are different, terrestrial environments can provide critical context for Europa, particularly where water and ice interact under pressure. Melting of ice occurs at subglacial volcanic craters on [...]

Abstract The Imaging Science Subsystem (ISS) on-board Cassini took a few high-resolution images of the icy Saturnian satellites Mimas and Enceladus over the last seven years of the Cassini mission during non-targeted flybys. We used the new Mimas images to improve the existing semi-controlled mosaic of Mimas. A new controlled Enceladus mosaic was published recently (Bland et al., 2015; Bland et al. in prep.). Both new mosaics are the baseline for improved atlases of Mimas in three tiles with a map scale of 1:1,000,000 and Enceladus in 15 tiles with a map scale of 1:400,000. The nomenclature for both satellites was proposed by the Cassini-ISS team and approved by the IAU. Highlights • The mosaics of Mimas and Enceladus were created using camera data from the recent Cassini orbits. • The final atlas of Mimas consisting of four tiles was created. • The final controlled atlas of Enceladus consisting of 15 tiles was created. [ABSTRACT FROM AUTHOR]

• The dielectric constant of regolith fines on crater floors ranges from 2.3 to 3.8. • Regolith fines in small equatorial craters show low metal abundance. • Small shadowed polar craters are optimal targets for detecting water-ice using SAR. Identifying polarimetric radar signatures of ice in smooth regolith fines on the floors of permanently shadowed lunar craters is hindered by uncertainties in their dielectric properties. We address this deficiency through polarimetric radar analysis of surface backscatter to derive the dielectric constant (ε r ′) of smooth, rock-free regolith fines covering brecciated crater floors observed by Mini-RF, which offer ideal locations for unambiguous retrieval of surface ε r ′ from linear polarimetric scattering models and CPR analysis for volatile identification. Specifically, we select fines covering crater fills in north polar and equatorial regions to constrain the range of variability of ε r ′ as a function of latitude and crater diameter, where we hypothesize that the latter is indicative of the excavation depth of these fines. Our observations suggest that there is measurable variability in the dielectric properties of fines on lunar crater floors as a function of crater size and potentially with impact excavation depth, suggesting that small craters <5-km in diameter have ε r ′ ranging from 2.3-to-3, and large ones >5-km have higher values of ε r ′ that range from 3-to-3.8. We find that the most plausible explanation for the observed variability of ε r ′ of regolith fines on crater floors is mineralogical differences, suggesting an increase in metal abundance in the original excavated substrate with depth, i.e., in the uppermost kilometer of the lunar crust. Finally, we suggest that regolith fines on the floors of permanently shadowed craters <5 km in diameter are optimal targets for the unambiguous detection of water-ice enrichment using S-band radar observations. [ABSTRACT FROM AUTHOR]