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Moon formation at low temperature

As the closest and most visible celestial object to Earth, the Moon has always fascinated human beings. Yet questions remain about its origin. A team of researchers from the Institut de Physique du Globe in Paris has come up with some new answers about its formation.

Moon formation at low temperature

Publication date: 08/10/2018

General public, Press, Research

Related themes : Origins

The current dominant theory is that the Moon is composed of material ejected from the Earth (in particular from its mantle, the rocky zone that extends 2890 km below our feet) following a ‘giant’ impact with a body the size of Mars. This theory could explain why terrestrial and lunar volcanic rocks share many similarities in terms of chemical composition.

However, there are many differences between the two planets; the absence of oceans on the Moon is evidence of the extreme depletion of volatile elements such as hydrogen (which forms water – H2O – when combined with oxygen) in lunar rocks. Despite this fundamental observation, the scientific community continues to question how the Moon became so arid.

In this context, IPGP researchers have measured, in lunar rocks brought back from the Apollo missions, slight variations in the content of chromium (Cr) isotopes, an element that becomes volatile under oxidising conditions. Isotopes are different forms of the same chemical element with a slightly different mass, which can separate or split in different materials, such as liquids and gases. Chromium is unique in that it forms several chemical species depending on the level of oxygen in the atmosphere.

According to the giant impact theory and the most recent numerical models, the material that formed the Moon should have been vaporised in space following evaporation at very high temperatures (> 4000°C) due to the impact. In a reduced atmosphere, the vaporisation residue should be enriched in heavy Cr isotopes, while the light isotopes should have been lost in the gaseous phase and then in space. However, the team of researchers from the IPGP has now revealed that the chromium isotopes split in the opposite direction, testifying to an oxidising atmosphere during the formation of the Moon. This property highlights, for the first time, that temperatures had to be much lower (< 1600°C) to create such an isotopic signature.

This study therefore reveals that the Moon had already been formed and cooled when volatile substances were lost, most probably during the outgassing of an ocean of magma on its surface. The Moon’s mass, which is much lower than that of the Earth, means that its gravity (which is 1/6 that of the Earth at its surface) cannot hold back a hot atmosphere, thus losing its chromium and potentially its water.

Ref : Sossi, Moynier, Van Zuilen, Volatile loss following cooling and accretion of the Moon revealed by chromium isotopes, PNAS, doi/10.1073/pnas.1809060115

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