Secondary craters on Europa and implications for cratered surfaces

For several decades, most planetary researchers have regarded the impact crater populations on solid-surfaced planets and smaller bodies as predominantly reflecting the direct ('primary') impacts of asteroids and comets. Estimates of the relative and absolute ages of geological units on these objects have been based on this assumption. Here we present an analysis of the comparatively sparse crater population on Jupiter's icy moon Europa and suggest that this assumption is incorrect for small craters. We find that 'secondaries' (craters formed by material ejected from large primary impact craters) comprise about 95 per cent of the small craters (diameters less than 1 km) on Europa. We therefore conclude that large primary impacts into a solid surface (for example, ice or rock) produce far more secondaries than previously believed, implying that the small crater populations on the Moon, Mars and other large bodies must be dominated by secondaries. Moreover, our results indicate that there have been few small comets (less than 100 m diameter) passing through the jovian system in recent times, consistent with dynamical simulations.     

An integrated view of the chemistry and mineralogy of martian soils

The mineralogical and elemental compositions of the martian soil are indicators of chemical and physical weathering processes. Using data from the Mars Exploration Rovers, we show that bright dust deposits on opposite sides of the planet are part of a global unit and not dominated by the composition of local rocks. Dark soil deposits at both sites have similar basaltic mineralogies, and could reflect either a global component or the general similarity in the compositions of the rocks from which they were derived. Increased levels of bromine are consistent with mobilization of soluble salts by thin films of liquid water, but the presence of olivine in analysed soil samples indicates that the extent of aqueous alteration of soils has been limited. Nickel abundances are enhanced at the immediate surface and indicate that the upper few millimetres of soil could contain up to one per cent meteoritic material.     

Water alteration of rocks and soils on Mars at the Spirit rover site in Gusev crater

Gusev crater was selected as the landing site for the Spirit rover because of the possibility that it once held a lake. Thus one of the rover's tasks was to search for evidence of lake sediments. However, the plains at the landing site were found to be covered by a regolith composed of olivine-rich basaltic rock and windblown 'global' dust. The analyses of three rock interiors exposed by the rock abrasion tool showed that they are similar to one another, consistent with having originated from a common lava flow. Here we report the investigation of soils, rock coatings and rock interiors by the Spirit rover from sol (martian day) 1 to sol 156, from its landing site to the base of the Columbia hills. The physical and chemical characteristics of the materials analysed provide evidence for limited but unequivocal interaction between water and the volcanic rocks of the Gusev plains. This evidence includes the softness of rock interiors that contain anomalously high concentrations of sulphur, chlorine and bromine relative to terrestrial basalts and martian meteorites; sulphur, chlorine and ferric iron enrichments in multilayer coatings on the light-toned rock Mazatzal; high bromine concentration in filled vugs and veins within the plains basalts; positive correlations between magnesium, sulphur and other salt components in trench soils; and decoupling of sulphur, chlorine and bromine concentrations in trench soils compared to Gusev surface soils, indicating chemical mobility and separation.