An explanation for the scarce presence of gold and other rare elements on the Moon

An article published in the journal “Nature” offers a possible explanation of the remarkable difference in the presence of some chemical elements on the Earth and on the Moon still accepting the theory of their common origin following an impact with the primordial Earth. A team of researchers carried out a series of simulations of the impacts that could have happened on the Moon during the first phase of its history, concluding that the retention of the elements classified as highly siderophilous began 4.35 billion years ago, at the time when most of the magma that covered the lunar surface solidified.

An article published in May 2019 in the journal “Journal of Geophysical Research: Planets” offered evidence that the differences between the two faces of the Moon could be due to the impact of a dwarf planet. Some of the authors of that research continued to study the impacts that could have occurred on the primordial Moon together with other colleagues to understand another difference, that in the amount of highly siderophilic elements, meaning gold, iridium, platinum, rhodium, rhenium, osmium and ruthenium, which are 1,200 times more abundant on Earth than on the Moon.

The theories proposed over time to explain the difference in highly siderophilic elements were based on a high impact/retention ratio, which indicates the fraction of the meteorite mass that gets incorporated into the celestial body struck. The discussions continued because none of those theories could match the observations of lunar craters, the amounts of highly siderophile elements and the age of Moon rock samples.

To overcome those problems, the authors of this new research carried out a series of simulations of the possible impacts suffered by the Moon at the beginning of its history by changing speed and angle of impact and size of meteorites. The reduced size of the Moon and the angle of the meteorites trajectory are significant factors in the retention.

The image (Courtesy Nature. All rights reserved) shows the consequence of an impact with an angle of around 20° (a) in which 80% of the materials get lost while in an impact with an angle of at least 80° (b) most materials get incorporated in the crust or mantle, as shown in the inset at the right.

The results showed the importance of the penetration depth with the possibility that the materials end up in the mantle or in the crust and its close relationship with the thickness of the lunar crust and with the part of the materials ejected from the crater during the impact that later fall back to the surface. The impact/retention ratio that resulted at the end of the simulations is about one third of that generally considered in the models used for the theories proposed in the past.

Things add up if the retention of highly siderophilic elements began 4.35 billion years ago. The ones that arrived earlier penetrated into the magna and got absorbed into the Moon’s core. On the Earth and on the Moon those solidification processes occurred at different times and that adds to different impact/retention relationships.

As in the research about the possible impact of a dwarf planet on the primordial Moon, the conclusions about the highly siderophile elements are the result of simulations. The new interest regarding the Moon could allow to conduct analyzes on new Moon rocks to obtain more precise and complete data on its geology.

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