Fig. 1. Schematics of two possible end-member aqueous environments on Mars during the Noachian.

(Left) Warm and wet view of early Mars, where liquid water can be stable on the surface. (Right) Cold, arid view of early Mars, where liquid water is mostly generated in the subsurface by intermittent melting of snow and ice. The curly red arrows show where hydrothermal reactions could take place.