天文学：月の年齢はより古い

Evidence to support an older age for the formation of the Moon, around 4.51 billion years ago, is presented in Nature. This new analysis suggests that a ‘remelting’ of the Moon’s surface around 4.35 billion years ago may have masked a far older history.

The Moon is thought to have been formed through a collision between the early Earth and a Mars-sized protoplanet, the last giant impact in our planet’s history. The timing of this event has been estimated by dating lunar rock samples presumed to have crystallized from the lunar magma ocean that existed post-impact, placing the Moon’s age at approximately 4.35 billion years old. However, this age fails to account for several discrepancies with thermal models and other pieces of evidence, such as the number of craters on the Moon and the age of some zircon minerals on the Moon’s surface, which suggest that the Moon could be up to 4.51 billion years old.

Francis Nimmo and colleagues argue that the frequent occurrence of approximately 4.35-billion-year-old rocks may be indicative of a remelting event, driven by the Moon’s orbital evolution, rather than the first solidification of the lunar magma ocean. The authors use modelling to show that the Moon may have experienced sufficient tidal heating to cause this remelting approximately 4.35 billion years ago, which could ‘reset’ the apparent formation age of these lunar samples. In addition, the remelting of the Moon would explain why there are fewer lunar impact basins from early bombardment than might be expected, as they would have been erased during a heating event. The authors posit that this explanation would suggest the formation of the Moon occurred between 4.43 and 4.53 billion years ago, at the upper limit of previous age estimates. These insights help us align our understanding of the Moon’s formation with existing knowledge of terrestrial planet formation.