The Martian Hydrosphere

In his 1996 Water on Mars, Carr provided an estimate for the depth of the Martian regolith--the near surface layer fractured by impacts, and hence porous. The quantity of water underground depends on the extent of this porous zone. It was thought Mars may hold water down to about 10km, the estimated level of the deepest cracks. The upper 6km or so is the perpetually frozen cryosphere. Since the bulk of fracturing, hence porosity, is at relatively shallow depths, the hydrosphere, farther down, may not contain much liquid water. It may be just a 2-3 km zone of limited capacity.
Some evidence, however, suggests the Martian hydrosphere is far more extensive. Enormous outflow channels attest to the veritable seas that once existed underground. The channels are very old but some capacity may persist. Part of it may have resulted from an ancient cataclysm.
The crustal dichotomy was long an enigmatic feature of Martian topography. Covering a third of the Martian surface, the smooth northern plains contrast with the more cratered southern highlands. This whole northern area is now interpreted as a gigantic impact crater. Long ago, Mars was subjected to a truly stupendous impact, greater than anything which hit the moon. It is reasonable to assume an impact of this magnitude caused very extensive fracturing. Cracks may extend 50, 100, 500 or more km below the surface. The Martian hydrosphere may be orders of magnitude more voluminous than originally thought. In addition, its possible great depth may expose it to substantial heat from the core. Such heat may provide an energy source for life.
Not long ago, methane was detected in the Martian atmosphere and considered possible evidence of biological activity. Any life is almost certainly underground and dependent on planetary heat. If the methane is due to life, it may confirm a much deeper hydrosphere, and biosphere.