Dating Howardite Melt Clasts: Evidence for an Extended Vestan Bombardment?

Howardites are polymict breccias that, together with eucrites and diogenites (HED), likely originate from the vestan surface (regolith/ megaregolith), and display a heterogeneous distribution of eucritic and diogenitic material. Melt clasts are also present alongside other regolithic features within howardites, and are noteworthy for their compositional variability and appearance. Melt clasts formed by impact events provide a snapshot of the timings and conditions of surface gardening and bombardment on the vestan surface. By dating such clasts, we aim to better constrain the timings of impact events on Vesta, and to establish whether the impact flux in the asteroid belt was similar to that on the Moon. As the Moon is used as the basis for characterising impact models of the inner solar system, it is necessary to verify that apparent wide-scale events are seen in other planetary bodies. In particular, the observed clustering of Apollo melt clast ages between 3.8-4.0 Ga has led to two hypotheses: 1) The Moon was subjected to a sudden event - 'Lunar Cataclysm' or period of 'Late Heavy Bombardment' (LHB), 2) The age cluster represents the end of an epoch of declining bombardment or 'Heavy Bombardment. No consensus has emerged regarding one or other hypothesis. We are testing these hypotheses by seeking evidence for such events in materials other than those derived from the Moon.