Jonathan Amos, reporting for BBC News:
Scientists think that perhaps as much as 95% of the gaseous shroud Mars started out with billions of years ago has gone. All of the key components in the present-day air show a leaning towards heavier isotopes. Curiosity itself has already demonstrated this to be the case with its measurements of oxygen, carbon dioxide, and also of water vapour. But the new analysis of the ratio of argon-36 to argon-38 is particularly incisive because the element is so unreactive. There is no significant way for the ratio between the two to change other than through the preferential loss of the lightest isotope to space.