Determining the age of the Moon

With more than 30 missions planned to take place by 2030, culminating with the return of humans to the Moon through the NASA Artemis programme, we are currently witnessing an incredibly exciting lunar exploration renaissance more than half a century after the end of the Apollo programme. Studies of samples returned by the Apollo astronauts between 1969 and 1972 have transformed our understanding of how planets form and evolve. And 50 years after these samples were returned to the Earth, we are still making crucial discoveries about the Moon, for example that volcanic rocks from the Moon contain water trapped inside tiny crystals. Yet, many fundamental questions have remained unanswered, such as how and when did the Moon form, and how did the Moon's primordial crust form. Canonical models postulate that the Moon formed as a result of a giant impact between the proto-Earth and a body called Theia. In these models, the Moon started as partially to wholly molten, a stage known as the lunar magma ocean. During cooling, dense mafic minerals crystallised first and sank to the bottom of the lunar magma ocean; Ca-rich plagioclase then formed and floated atop the lunar magma ocean after 70-80% solidification, forming a global anorthosite crust. Chronological investigations of Apollo samples from this primordial lunar crust have yielded dates clustering around 4.35 billion years - some have argued that this indicates protracted crystallisation of the lunar magma ocean over ~150 million years with a Moon formation 4.5 billion years ago, though others have suggested that the Moon was much younger and only formed shortly before 4.35 billion years ago. Thanks to detailed remote sensing investigations of the composition of the lunar surface over the past couple of decades, we now know that samples returned by the Apollo missions all originate from an anomalous region of the lunar nearside characterised by elevated abundances of some geochemical elements such as potassium (K), rare earth elements (REE), and phosphorus (P) - an area called the Procellarum KREEP Terrane. These features may have been caused by a large impact soon after the Moon’s formation, forming the putative ~3000 km diameter Procellarum basin. This Procellarum impact event could have triggered large-scale melting of the lunar nearside and resetting of chronological systems at 4.35 Ga, explaining why Apollo sample-based models could be biased towards younger Moon formation ages. To decipher when the Moon formed, we will investigate samples from outside the Procellarum area, and notably newly discovered anorthosite meteorites that likely originated from the Moon's farside. Our work will involve two complementary approaches, including (1) precisely dating the formation of these farside meteorites using the Sm-Nd and Pb isotope chronological systems, and (2) revisiting the model formation age of the Moon using the iodine-plutonium-xenon system. Our team has a strong scientific track record of tackling important lunar and planetary science questions through sample analyses, and we have pioneered the development of most of the techniques that are central to this project. We are, therefore, ideally placed to finally unravel when the Moon formed.