The early Martian atmosphere

Atmospheric noble gases on Mars, Earth, and Venus are severely and variably depleted with respect to solar abundances, and are characterized by isotopic signatures that differ from planet to planet and from meteoritic and inferred solar compositions. Earlier theories based on simple accretion of meteoritic veneers as planetary volatile sources are unable to replicate these distributions, and so attention is now focusing on evolutionary processes that can fractionate both elements and isotopes, operating either on the early planets themselves or in large preplanetary planetesimals.