Since the return and analysis of the first Apollo samples, the age of the Moon has been a contentious issue. The first studies to date lunar rocks utilized the decay of ⁸⁷Rb to ⁸⁷Sr and measurements yielded ages that are as old as the Solar System (~4.56 Ga). As analytical techniques matured it became possible to apply multiple radiometric chronometers (Sm-Nd and Pb-Pb) on the same rock. Applying these techniques to what is supposed to be the oldest lunar crustal rock, a ferroan anorthosite, revealed a much younger age of 4.360 ± 0.003 Ga. This younger age is contentious because it implies that the Moon-forming impact (a collision between the proto-Earth and Mars-sized body named Theia) occurred later that suggested by dynamical models.

My postdoctoral work at LLNL sought to confirm the veracity of this young age by building upon the sparse dataset for ferroan anorthosites. This was an analytically challenging task because the Rb-Sr isotopic systematics had been disturbed, and Sm-Nd concentrations are extremely low. Nonetheless, my data, collected using thermal ionization mass spectrometry (TIMS), show a crystallization age of 4.35 ± 0.073 Ga, in agreement with the young age previously reported. This, however, is not the end of the story. Both of these ‘young’ ferroan anorthosites were collected from the near side of the Moon. There have been suggestions that crustal rocks on the lunar farside could be older. Until we can return and analyze samples from this region of the lunar crust, our picture of the Moon’s formation will remain partially eclipsed.

References:

1. Sio, C. K., Borg, L. E., & Cassata, W. S. (2020). The timing of lunar solidification and mantle overturn recorded in ferroan anorthosite 62237. Earth and Planetary Science Letters, 538, 116219.
2. Borg, L. E., Connelly, J. N., Boyet, M., & Carlson, R. W. (2011). Chronological evidence that the Moon is either young or did not have a global magma ocean. Nature, 477(7362), 70-72.
3. Papanastassiou, D.A., & Wasserburg, G.J. (1976) Rb-Sr age of troctolite 76535. In Lunar and Planetary Science Conference Proceedings (vol. 7, pp. 2035-2054).