Large impact cratering during lunar magma ocean solidification

Tidally driven remelting around 4.35 billion years ago indicates the Moon is old

The last giant impact on Earth is thought to have formed the Moon1. The timing of this event can be determined by dating the different rocks assumed to have crystallized from the lunar magma ocean (LMO). This has led to a wide range of estimates for the age of the Moon between 4.35 and 4.51 billion years ago (Ga), depending on whether ages for lunar whole-rock samples2-4 or individual zircon grains5-7 are used. Here we argue that the frequent occurrence of approximately 4.35-Ga ages among lunar rocks and a spike in zircon ages at about the same time8 is indicative of a remelting event driven by the Moon's orbital evolution rather than the original crystallization of the LMO. We show that during passage through the Laplace plane transition9, the Moon experienced sufficient tidal heating and melting to reset the formation ages of most lunar samples, while retaining an earlier frozen-in shape10 and rare, earlier-formed zircons. This paradigm reconciles existing discrepancies in estimates for the crystallization time of the LMO, and permits formation of the Moon within a few tens of million years of Solar System formation, consistent with dynamical models of terrestrial planet formation11. Remelting of the Moon also explains the lower number of lunar impact basins than expected12,13, and allows metal from planetesimals accreted to the Moon after its formation to be removed to the lunar core, explaining the apparent deficit of such materials in the Moon compared with Earth14.

Evidence of a 4.33 billion year age for the Moon's South Pole-Aitken basin

The Moon's farside South Pole-Aitken (SPA) basin is the largest and oldest visible impact basin in the inner Solar System. Determining the timing of this catastrophic event is key to understanding the onset of the lunar basin-forming epoch, with implications for understanding the impact bombardment history of the inner Solar System. Despite this, the formation age of the SPA basin remains poorly constrained. Here we show that the chemical composition of the lunar meteorite Northwest Africa 2995 is in good agreement with lithologies exposed within the southern region of the SPA basin. Radiometric dating of a range of mineral and rock components in Northwest Africa 2995 yielded consistent dates of ~4.32-4.33 billion years old. We interpret these dates as the age of SPA basin formation, inferring that this event occurred ~120 million years before the formation of the main cluster of lunar impact basins between ~4.2 Ga and 3.8 Ga. This weakens support for a narrow period of lunar late heavy impact bombardment and also implies that the earliest formed impact basins on the Moon (that is, >4.33-4.5 Ga old) were erased either by the SPA impact itself when its formation caused massive resurfacing of the lunar surface or through other geological processes.