Observations by the Lunar Prospector and the Lunar Atmosphere and Dust Environment Explorer spacecraft suggest the existence of a near-global deposit of weakly bound water ice on the Moon, extending from a depth of a decimetre to at least three metres. The existence of such a layer is puzzling, because water ice would normally desorb at the prevailing temperatures. We here determine the conditions for long-term thermal stability of such a reservoir against solar and meteoroid-impact heating. This is done by using the highly versatile thermophysics code nimbus to model the subsurface desorption, diffusion, recondensation, and outgassing of water vapour in the porous and thermally conductive lunar interior. We find that long-term stability against solar heating requires an activation energy of similar to 1.2 eV in the top metres of lunar regolith, and a global monthly night time exospheric freeze out amounting to similar to 1 tonne. Furthermore, we find that a lower similar to 0.7 eV activation energy at depth would allow for water diffusion from large (0.1-1 km) depths to the surface, driven by the radiogenically imposed selenotherm. In combination with solar wind-produced water, such long-range diffusion could fully compensate for meteoroid-driven water losses. These results are significant because they offer quantitative solutions to several currently discussed problems in understanding the lunar water cycle, that could be further tested observationally.

Molecular dynamics simulations are used to analyse the effects after 20 MeV sulfur ion impact into an ice mixture consisting of water, carbon dioxide, ammonia, and methanol. By using a so-called REAX, i.e., reactive, potential, the chemical processes occurring after the impact can be studied. Such impacts may occur in Jupiter's magnetosphere, where energetic S ions originate from Io's surface and irradiate ice surfaces of Jupiter's moons, of comets or ice dust particles entering the magnetosphere. By segmenting the ion trajectory to smaller pieces that fit into our simulation box, we can follow the ion from its impact point at the surface down to the depth where it is stopped. Electronic stopping is modelled by a thermal track model; it is necessary to use a sufficiently small track radius R in order to be able to include the hot-chemistry reactions occurring in the track volume. We find that the number of dissociations and ensuing reactions scales approximately linearly with the deposited energy density. In consequence, the total number of molecules produced is approximately proportional to the impact energy. In addition, the most complex molecules are formed at the highest energy densities. Smaller molecules such as formaldehyde and hydrogen peroxide, in contrast, are produced all along the ion track.

Saturn's rings are rock-poor, containing 90%-95% ice by mass. As a group, Saturn's moons interior to and including Tethys are also about 90% ice. Tethys itself contains 40% rock. Here we simulate the evolution of a massive primordial ice-rich ring and the production of satellites as ring material spreads beyond the Roche limit. We describe the Rocheinterior ring with an analytic model, and use an N-body code to describe material beyond the Roche limit. We track the accretion and interactions of spawned satellites, including tidal interaction with the planet, assuming a tidal dissipation factor for Saturn of Q similar to 10(4). We find that ring torques and capture of moons into mutual resonances produce a system of ice-rich inner moons that extends outward to approximately Tethys's orbit in 109 years, even with relatively slow orbital expansion due to tides. The resulting mass and semimajor axis distribution of spawned moons resembles that of Mimas, Enceladus, and Tethys. We estimate the mass of rock delivered to the moons by external cometary impactors during a late heavy bombardment. We find that the inner moons receive a mass in rock comparable to their current total rock content, while Dione and Rhea receive an order-of-magnitude less rock than their current rock content. This suggests that external contamination may have been the primary source of rock in the inner moons, and that Dione and Rhea formed from much more rock-rich source material. Reproducing the distribution of rock among the current inner moons is challenging, and appears to require large impactors stochasticity and/or the presence of some rock in the initial ring.