Constraints on the Source of Lunar Cataclysm Impactors

TL;DR

This paper investigates the origin of impactors responsible for the lunar cataclysm, showing that their size distribution differs from main-belt asteroids and suggesting a different source for these impactors.

Contribution

It provides evidence that lunar impactors during the cataclysm were not primarily main-belt asteroids ejected by gravitational means, challenging current impact source models.

Findings

Fresh craters formed during the tail of the lunar cataclysm

Size-frequency distribution of impactors is steeper than that of main-belt asteroids

Impactors likely originated from a different source than main-belt asteroids

Abstract

Multiple impact basins formed on the Moon about 3.8 Gyr ago in what is known as the lunar cataclysm or late heavy bombardment. Many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main-belt asteroids destabilized by delayed planetary migration. We show that morphologically fresh (class 1) craters on the lunar highlands were mostly formed during the brief tail of the cataclysm, as they have absolute crater number density similar to that of the Orientale basin and ejecta blanket. The connection between class 1 craters and the cataclysm is supported by the similarity of their size-frequency distribution to that of stratigraphically-identified Imbrian craters. Majority of lunar craters younger than the Imbrium basin (including class 1 craters) thus record the size-frequency distribution of the lunar cataclysm impactors. This distribution is much steeper than that of main-belt asteroids. We argue that the projectiles bombarding the Moon at the time of the cataclysm could not have been main-belt asteroids ejected by purely gravitational means.