At the present time, Mars is a dry and cold planet. Surface ice is unstable for more than one season outside the polar regions, and the atmosphere is so cold or so dry that the presence of liquid water, never detected, is unlikely anywhere on the surface.
Things may have been different in the past. On the one hand, the surface of Mars is characterized by multiple geological evidences that suggest that various kind of glaciers and ice sheets formed not that long ago at low and mid-latitudes. On the other hand, the observations of the geology and mineralogy of the oldest surface on Mars (dating back to more than 3.8 billion years ago) provide evidence that the Martian climate was then completely different, with abundant liquid water on the surface.
To help understand what have happened, we have developed global climate models design to simulate the possible past Martian environment. These simulations unveil parts of Mars history, but also raise new questions.