Planetary Geoscience

Physical weathering of rocks on bodies other than the Earth occurs mostly through impact fragmentation, producing regoliths. The lunar regolith is finer-grained and contains more agglutinates than asteroidal regoliths, indicating its greater maturity. Mars exhibits both physical and chemical weathering, and its sedimentary deposits superficially resemble those on Earth. However, its basalt-derived sediments differ from those formed from felsic protoliths on Earth, and evaporation of its aqueous fluids is dominated by sulfates, distinct from terrestrial evaporites that are mostly carbonates and halides. On the surfaces of airless bodies, recondensation of vapor produced by micrometeorite impacts accounts for spectral changes, known as space weathering. In the interiors of carbonaceous chondrite asteroids, isochemical reactions of rocks with cold aqueous fluids produced by melting of ice have altered their mineralogy. Thermal metamorphism of dry chondritic asteroids has modified all but near-surface rocks. Hydrothermal metamorphism on Mars, likely associated with large impacts, has produced low-grade mineral assemblages in metabasalts and serpentinites. Conditions at Venus’ surface are severe enough to cause thermal metamorphism, and reactions with rocks may control the composition of the atmosphere. Because all bodies have gravity, some sloping topography, and some unconsolidated materials, mass wasting is among the most common processes modifying planetary surfaces.