Evaporation and recondensation of volatiles in planetesimals

It has long been observed that the inner solar system, which includes the terrestrial planets as well as the Moon and the asteroid belt, has a chemical composition that is depleted in volatile elements compared with the average composition of the solar system, as measured by the analysis of primitive samples such as Ivuna-type carbonaceous chondrites (CIs) or the Ryugu sample brought back by the Hayabusa 2 mission.

Recent developments in isotopic tracers suggest that this depletion is linked to the loss of these elements through evaporation during the formation of the first planetary bodies, the planetesimals, which then assembled to form planets like the Earth.

To better understand this process, scientists are focusing on a very specific type of meteorite: angrites. These meteorites, formed more than 4.5 billion years ago, are derived from the most volatile-depleted planetesimals in the Solar System and are therefore prime samples for studying the formation of volatile-poor rocky planets. But their exceptionally low volatile content represents a major analytical challenge that instrumental and analytical developments are just beginning to address.

To trace the history and mechanisms of this depletion, a team of cosmochemists from the IPGP has developed a new method for analysing the isotopic signatures of two volatile alkaline elements, potassium and rubidium, with very high precision, using a plasma source mass spectrometer equipped with a collision cell. Their results are published in two studies, the second of which has just been published in the journal Proceedings of the National Academy of Science (PNAS).

The samples analysed show that the angrites are enriched in light isotopes of rubidium (Rb) and potassium (K). Although this result is the opposite of what was expected and predicted by an evaporation loss model, the researchers propose that this enrichment in light isotopes reflects a phenomenon of recondensation of a fraction of the volatiles following total evaporation; the kinetics of the reaction favouring the lightest isotopes.

These data therefore indicate that these primordial planetesimals may well have lost all their volatiles by evaporation during their formation, a priori during a magma ocean phase, but a tiny fraction of this vapour would then have recondensed, opening the way to new interpretations of the distribution of volatile elements during planetary formation processes.