Geophysical and cosmochemical evidence for a volatile-rich Mars

Constraints on the composition of Mars principally derive from chemical analyses of a set of Martian meteorites that rely either on determinations of their refractory element abundances or isotopic compositions. Both approaches, however, lead to models of Mars that are unable to self-consistently explain major element chemistry and match its observed geophysical properties, unless ad hoc adjustments to key parameters, namely, bulk Fe/Si ratio, core composition, and/or core size are made. Here, we combine geophysical observations, including high-quality seismic data acquired with the InSight mission, with a cosmochemical model to constrain the composition of Mars. We find that the FeO content of Mars' mantle is 13.7±0.4 wt%, corresponding to a Mg# of 0.81±0.01. Because of the lower FeO content of the mantle, compared with previous estimates, we obtain a higher mean core density of 6150±46 kg/m3 than predicted by recent seismic observations, yet our estimate for the core radius remains consistent around 1840±10 km, corresponding to a core mass fraction of 0.250±0.005. Relying on cosmochemical constraints, volatile element behaviour, and planetary building blocks that match geophysical and isotopic signatures of Martian meteorites, we find that the liquid core is made up of 88.4±3.9 wt% Fe-Ni-Co with light elements making up the rest. To match the mean core density constraint, we predict, based on experimentally-determined thermodynamic solution models, a light element abundance in the range of ≈9 wt% S, ⩾3 wt% C, ⩽2.5 wt% O, and ⩽0.5 wt% H, supporting the notion of a volatile-rich Mars. To accumulate sufficient amounts of these volatile elements, Mars must have formed before the nebular gas dispersed and/or, relative to Earth, accreted a higher proportion of planetesimals from the outer protoplanetary disk where volatiles condensed more readily.