Mars volatile inventory and outgassing history

The timing of formation of the Mars atmosphere controlled the early climate and its subsequent evolution. We examine this timing through the role played in atmospheric formation by outgassing from the mantle resulting from volcanism through time. To do this, we compare the amount of gas released by volcanism, derived based on measurements of the Martian meteorites, with the amounts present at one time in the atmosphere. Results indicate that volcanic outgassing of H2O and 36Ar are insufficient to explain their current atmospheric abun-dances; 40Ar data also require a very low outgassing efficiency. The bulk of these components of the atmosphere must have been degassed earlier, during accretion/core formation, an early magma-ocean stage, and/or early-Noachian crust formation. These components of the Mars atmosphere appear to have been essentially fully formed at the time of onset of the observable geologic record in the middle and late Noachian, with subsequent evolution dominated by a draw-down of gas via loss to the crust and to space.