Krypton in the Chassigny meteorite shows Mars accreted chondritic volatiles before nebular gases.

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.