Inconsistency between the Ancient Mars and Moon Impact Records of Megameter-scale Craters

Abstract

Impact crater populations provide a record of the bombardment history of a planetary body. These craters reveal an intense bombardment history on Earth's Moon that included significant, large impacts, at least four of which created craters larger than ∼1000 km across more than 3.7 Ga. Scaling the lunar impact crater history to Mars using generally well-established scaling rules indicates Mars should have ∼40–80 similarly sized, ancient craters. However, Mars has roughly seven. This is a nontrivial mismatch between observation and expectation. Possible methods to resolve the mismatch are discussed in this work: small number statistics, incorrect scaling laws, heat flow produced larger-than-expected craters, the ratio of Mars:Moon impactors was different in early solar system history, Mars’s ancient crater chronology is wrong, and/or Mars has hidden a large fraction of its large impacts. None of these scenarios are mutually exclusive. This work details how the different scenarios could work to bring observations more in-line with the scaling expectations, or vice versa. It is posited that the most likely sources of the mismatch are that the initial bolide rate was different (this is a noncontroversial supposition), that lunar heat flow produced larger craters than expected, and the formation of the Martian Borealis basin could have kept the surface warm enough for long enough to prevent large features from forming for an extended period of time. The primary purpose of this work is to present the issue with possible ways to solve it through future efforts.