Swiss give best estimate of Moon age at 62M years after solar system formed

William Atkins
Saturday, 05 January 2008 07:41

Science - Space

They state in their Nature article: “The Moon is thought to have formed from debris ejected by a giant impact with the early 'proto'-Earth and, as a result of the high energies involved, the Moon would have melted to form a magma ocean. The timescales for formation and solidification of the Moon can be quantified by using ¹⁸²Hf–¹⁸²W and ¹⁴⁶Sm–¹⁴²Nd chronometry, but these methods have yielded contradicting results. In earlier studies, ¹⁸²W anomalies in lunar rocks were attributed to decay of ¹⁸²Hf within the lunar mantle and were used to infer that the Moon solidified within the first ~60 million years of the Solar System. However, the dominant ¹⁸²W component in most lunar rocks reflects cosmogenic production mainly by neutron capture of ¹⁸¹Ta during cosmic-ray exposure of the lunar surface, compromising a reliable interpretation in terms of ¹⁸²Hf–¹⁸²W chronometry.”

Further, they state, “Here we present tungsten isotope data for lunar metals that do not contain any measurable Ta-derived ¹⁸²W. All metals have identical ¹⁸²W/¹⁸⁴W ratios, indicating that the lunar magma ocean did not crystallize within the first ~60 Myr of the Solar System, which is no longer inconsistent with Sm–Nd chronometry. Our new data reveal that the lunar and terrestrial mantles have identical ¹⁸²W/¹⁸⁴W. This, in conjunction with ¹⁴⁷Sm–¹⁴³Nd ages for the oldest lunar rocks, constrains the age of the Moon and Earth to 62 (+90/-10) Myr after formation of the Solar System. The identical ¹⁸²W/¹⁸⁴W ratios of the lunar and terrestrial mantles require either that the Moon is derived mainly from terrestrial material or that tungsten isotopes in the Moon and Earth's mantle equilibrated in the aftermath of the giant impact, as has been proposed to account for identical oxygen isotope compositions of the Earth and Moon."