When and where a planet gets its volatile constituents are important questions in planetary science. Typical models of planet formation often start with an inventory of volatile elements (e.g., hydrogen, carbon, nitrogen, and noble gases) derived from the solar nebula. This is then modified by processes such as atmospheric escape and the addition of chondritic meteorites, either during the main accretionary phase or as a “late veneer” in the final stages of planet formation.

Because of their chemical inertness, the noble gases (e.g., neon, krypton) are excellent recorders of accretion and any processes (such as mixing/degassing) associated with it. On Earth, the mantle displays solar-like isotope ratios for light noble gases, but the heavier noble gases are more like those in chondritic meteorites. Two hypotheses may explain this: 1) simultaneous accretion of solar and chondritic components or 2) early acquisition of a solar component, with the chondritic component being mixed in later. Given that Mars was formed within the first four million years of the solar system, isotopic compositions of its noble gases can provide insight into volatile accretion during the earliest stages of planet formation.

New work by Sandrine Péron (University of California Davis, now at ETH Zürich) and Sujoy Mukhopadhyay (University of California Davis) shows that the krypton and neon isotope ratios of the martian interior (measured in the martian meteorite Chassigny) are indistinguishable from those in carbonaceous chondrites. These results contrast with the solar Kr isotopic ratio of Mars’ atmosphere. The authors suggest that the chondritic component of volatiles on Mars accreted first, and the solar component was added later — opposite to what is suggested by most models. The work also suggests that the martian atmosphere did not form by outgassing or volatile fractionation from the interior of Mars and thus raises questions about how it did originate. READ MORE