Argon isotopes as tracers for martian atmospheric loss

Abstract

Recent measurements of the present-day Ar abundance and isotopic ratios in the martian atmosphere by the SAM instrument suite onboard the Curiosity rover can be used to constrain the atmospheric and volatile evolution. We have examined the role of volcanic outgassing, escape to space via sputtering, crustal erosion, impact delivery, and impact erosion in reproducing the Ar isotope ratios from an initial state 4.4 billion years ago. To investigate the effects of each of these processes, their timing, and their intensity we have modeled exchanges of Ar isotopes between various reservoirs (mantle, crust, atmosphere, etc.) throughout Mars’ history. Furthermore, we use present-day atmospheric measurements to determine the parameter space consistent with observations. We find that significant loss to space (at least 48% of atmospheric 36Ar) is required to match the observed 36Ar/38Ar ratio. Our estimates of volcanic outgassing do not supply sufficient 40Ar to the atmosphere to match observations, so in our model at least 31% of 40Ar produced in the crust must have also been released to the atmosphere. Of the total 40Ar introduced into the atmosphere about 25% must have been lost to space. By adding the present-day isotopic abundances with our results of total integrated Ar loss we find a “restored” value of atmospheric 40Ar/36Ar, which represents what that ratio would be if the total integrated Ar loss had remained in the atmosphere. We determine the restored value to be ∼900–1500. This is below the present martian atmospheric value (1900 ± 300), but 3–5 times greater than the terrestrial value.