Oded Aharonson

Ice deposits at the lunar poles are concentrated within permanently shadowed regions where water molecules are thermally stable. The geographic distribution of the deposits has been predicted and observed. However, the timing of their formation remains poorly constrained. Owing to the gradual decrease in the lunar obliquity with time, the cold-trapping regions have expanded monotonically since the passage of the Moon through the Cassini state transition ~4 Gyr ago. Here, using reflected ultraviolet starlight, we show with observations from the Lyman-Alpha Mapping Project that there is a strong correlation between the exposed-ice fraction and the age of the permanent shadow within which it resides. A model of water delivery, burial and loss predicts such a correlation if these processes operate on timescales that are comparable with the ages of the permanently shadowed regions. The exposed-ice area ratio of ~3.4% in the youngest permanently shadowed regions aged ~100 Myr favours models with high proportional loss rates. The results indicate that polar ice has accumulated quasi-continuously at least over the last ~1.5 Gyr rather than from a discrete event.

The Jupiter and Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA) will investigate the Jovian system with multiple instruments over several years, beginning in early 2031. This paper describes the historical context and state of knowledge, as well as JUICEs scientific goals and measurement techniques of the satellites that will not be encountered in close flybys. These include the large volcanically active moon Io, the four small inner moons Metis, Adrastea, Amalthea, and Thebe, and the numerous small Irregular (outer) moons. JUICE will provide multiple opportunities to observe Io from relatively remote distances of hundreds of thousands of kilometers. These observations will enable monitoring of Ios surface for changes, and for the study of its neutral clouds and plasma torus. Io observations will be performed with the four optical remote sensing instruments and with the Particle Environment Package. For the small inner moons it is planned to obtain complete geographic longitude (scales up to 8 km/px), solar-phase and multi-color coverage, oblique polar views, and UV to near-IR spectra. Astrometric measurements will also be performed. The Irregular moons will mostly appear unresolved to the JUICE instruments. Nonetheless, long-duration disk-integrated lightcurves will be acquired to derive rotation periods, object dimensions, pole-axis orientations, and colors for most objects for the first time. From these data, convex-shape models will be generated and phase curves determined. Furthermore, the precision of the orbital elements will be improved via accurate astrometry. UV and near-IR measurements will be attempted for the largest of these objects.

Exchangeable ice deposits are present today on the surface of Mars in polar caps and in the shallow subsurface in mid-latitudes. Geologic observations indicate these deposits waxed and waned in the past, at times, along with the emplacement and loss of equatorial glaciers. Here, we couple a climate model with an ice stability criterion, to self-consistently determine the distribution of the mid-latitude ground-ice deposits in diffusive equilibrium with the atmosphere, at present and under past orbital configurations. This new coupling and iteration between the short-term and long-term models improves upon past calculations that do not allow the ice table to evolve over timescales much longer than the annual climate dynamics. The model predictions for the extent of the equilibrium ice-table in the past match the latitudinal distribution of terrain softening geologic features previously mapped. At past times, thermally stable shallow ground ice is expected even in equatorial regions, in parts of Tharsis and Arabia Terra.