Very Large Impacts in the Pre-Noachian on Mars: Conditioning the Noachian Environment

There are at least 20 very large impact basins on Mars with diameters greater than 1000km. Crater retention ages for these indicate that most of them formed in a relatively short interval of time: a conversion to a Hartmann-Neukum chronology suggests 15 of 20, including 4 of the 5 largest (with diameters larger than 2500 km), may have formed in only 100 million years and 18 of the 20 may have formed in only 200 million years. The short, spike-like duration supports a Nice-like Late Heavy Bombardment (LHB) scenario which may have produced a Terminal Lunar Cataclysm on the Moon at about 3.9 BYA, and which would have produced a similar cataclysm on Mars and the other planets of the inner solar system at the same time. The Noachian period on Mars which followed was clearly conditioned by these surface-sterilizing, atmosphere- disrupting impacts which may also have contributed to the demise of the global magnetic field. It is clear that the martian global field disappeared in perhaps less than a few tens of millions of years near the end of the period of large basin formation, based on which basins were and were not remagnetized after their formation. Recent thermal modeling suggests the impacts could produce the necessary perturbation across the core- mantle boundary to kill of a subcritical dynamo. Without a magnetic field, the Noachian atmosphere was subject to solar wind erosion, further reducing the habitability of this early time. But the relatively brief duration of the large impact bombardment (less than 200 million years) may allow a longer (400 million year?) more habitable earlier time prior to the bombardment, when the magnetic field was still present and the surface relatively unaffected by such large impacts.