The Late Heavy Bombardment on Mars: Implications for the Inner Solar System

Crater retention ages, and inferred model “absolute” ages, for the 20 largest impact basins on Mars (D>1000 km) suggest most of these formed in a relatively short time. If the conversion to a Hartmann-Neukum model chronology is correct, 75% of the population may have formed in 200 million years or less, at about 4.1 BYA. The narrow interval is consistent with a Late Heavy Bombardment due to trans-Neptunian objects scattered into the inner solar system, as described by the so-called NICE model. If the large diameter impact basins on Mars are part of a NICE-like Late Heavy Bombardment (LHB), the impacts should have formed on Mars at 3.8-4.0 BYA, as did many of the large impact basins on the Moon. The current martian chronology is likely uncertain at this level, an may need revision. The formation of the large impact basins on Mars was also coincident with the demise of the martian global magnetic field, which appears to have died in a relatively short time before the formation of the last very large (D>2500 km) basin in Utopia. Thus in addition to the environmental damage of the impacts themselves, the martian atmosphere was left unprotected from the solar wind at about the same time as the LHB ended. More interesting, the tight distribution of ages for the martian basins and the apparent absence of any large impacts prior to those now dated may suggest a lull in intense bombardment in the inner solar system prior to the “terminal lunar (and martian) cataclysm”. If true, this could support the idea of a relatively hospitable period on the early Earth before the LHB there.