Mars, a Post-Habitable Planet?

Abstract: Mars provides a critical analog to once habitable exoplanets that have since lost their surface liquid water. The current atmospheric state of Mars retains the chemical fingerprints of that transition, including isotopic signatures of atmospheric escape and climate evolution. As the closest accessible example of a terrestrial world with definitive evidence for once supporting liquid water on its surface, Mars presents a unique opportunity to test hypotheses about planetary habitability and atmospheric evolution in a spatially and temporally resolved way.