Martian surface/near‐surface water inventory: Sources, sinks, and changes with time

Today, a 34 m global equivalent water layer (GEL) lies in the Martian polar‐layered deposits and shallow ground ice. During the Amazonian, 3 m was outgassed, and 31 m was lost to space and to the surface, leaving 62 m at the end of Hesperian. During the Hesperian, volcanic outgassing added 5 m, 7 m was lost, and 40 m GEL of groundwater was added to form outflow channels, leaving 24 m carryover of surface water from the Noachian into the Hesperian. The Hesperian budget is incompatible with a northern ocean during this era. These figures are for near‐surface water; substantial amounts of water may have existed as deep ground ice and groundwater. Our estimate of approximately 24 m near‐surface water in the Late Noachian is insufficient to support an ocean at that time also and favors episodic melting of an icy highlands to produce the fluvial and lacustrine features.