The Late Heavy Bombardment and Mars

Except where locally covered by lavas or sediment, Mars is as heavily cratered as the Moon, presumably as a result of the same Late Heavy Bombardment (LHB), dating from 4.0 to 3.8 Ga. The fact that this cratering record remains largely intact implies that surficial geologic agents, other than local volcanoes, ground ice, glaciers, landslides, outflow channels, minor weathering, and the wind, must have ceased being geologically important by the end of the LHB. The reason is probably the LHB itself. Given the relatively small size of Mars, catastrophic cratering should have resulted in catastrophic loss of atmosphere and hydrosphere (and given its distance from the Sun, Mars may have been icy beforehand). If the cold, dry surface of Mars is largely impact-formed, how then can one explain evidence of ancient drainage networks, crater lakes, buried clay horizons, the layered, cross-bedded sediments and spherules found by the two rovers, and the ubiquitous sulfate-rich salt assemblages (including ferric acid sulfates)? Most of these could have formed as a result of the LHB too (compare Knauth et al., 2005, Nature). Salt crystallization results from brine freezing and ice sublimation as well as from liquid water evaporation, and impacts into either sulfides or sulfates should produce ferric acid sulfates. Cross-bedded fine-grained sediments with various types of spherules and even radial troughs are observed to form as a result of volcanic or impact-related explosions on Earth, and Mars, like Earth, has an atmosphere and subsurface volatiles. At the height of the LHB, if not often afterwards, enough steam should have been generated to create a temporary greenhouse, and condensation of this steam could explain ancient features such as clays, drainage networks, and ephemeral crater lakes. If the LHB is considered, Mars is potentially simplified (and astrobiology still remains open).