LAUSR

Volatile elements are thought to have been delivered to Solar System terrestrial planets late in their formation through accretion of chondritic meteorites. Mars can provide information on inner Solar System volatile delivery during the earliest planet formation stages. We measured krypton isotopes in the martian meteorite Chassigny, representative of the planet’s interior. We found chondritic krypton isotope ratios, which imply early incorporation of chondritic volatiles. The atmosphere of Mars has different (solar-type) krypton isotope ratios, indicating that it is not a product of magma ocean outgassing or fractionation of interior volatiles. Atmospheric krypton instead originates from accretion of solar nebula gas after formation of the mantle but before nebular dissipation. Our observations contradict the common hypothesis that during planet formation, chondritic volatile delivery occurred after solar gas acquisition. Description Mars accreted in an unexpected order The Solar System formed from a nebula of gas and dust, from which the Sun and planets accreted. Their assembly sequence can potentially be reconstructed using the abundances of chemical elements and their isotope ratios. Péron and Mukhopadhyay have measured krypton and xenon isotopes in the Martian meteorite Chassigny, already known to reflect the planet’s interior composition. They found isotope ratios similar to meteorites, indicating that much of Mars’ mass was sourced from solids. However Mars’ atmosphere has isotopes indicating an origin from the nebular gas and thus must have been added later than the interior. Standard cosmochemical models predict the opposite sequence, so the results challenge current understanding of Solar System assembly. —KTS Krypton isotopes in the Martian meteorite Chassigny challenge standard models of planet formation.