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For those who fantasize that Mars might still have vast underground lakes with fish, the recent data is unfortunately not encouraging. A paper published today by the American Geophysical Union suggests that if groundwater exists on Mars, it is going to be increasingly difficult to find, possibly only in the low latitudes at low elevations. These paragraphs from the conclusion of the paper say it all:
Various lines of evidence suggest that at the time of the Late Hesperian, Mars possessed a planetary inventory of water equal to a global ocean ~0.5 km deep, much of which is believed to have been stored as ground ice and groundwater in the subsurface. The potential survival of groundwater to the present-day has important implications for understanding the geological, hydrological and mineralogical evolution of the planet, as well as the potential survival of native Martian life. The two most important factors affecting the persistence of groundwater on Mars are the depth and pore volume of the cryosphere.
To date, the orbital radar sounding data from MARSIS [an instrument on Mars Orbital Express] has provided little evidence of any deep reflectors potentially indicative of subpermafrost groundwater. Here we have examined two (of several) possible explanations for this lack of evidence: (1) that subpermafrost groundwater no longer survives on Mars or (2) that groundwater is present, but that a thicker than expected cryosphere has restricted its occurrence to depths that exceed the estimated ~3 km maximum sounding depth of MARSIS.
Which one (or combination) of explanations discussed here is responsible for the lack of deep reflectors on Mars is unknown. But our revised estimates of cryosphere depth suggest that a successful detection of subpermafrost groundwater, outside of those areas on Mars that combine low latitude and low elevation, is unlikely. In an effort to better constrain this problem, a more comprehensive investigation of the MARSIS sounding data obtained over Athabasca Valles, and four other low-elevation, near-equatorial sites, is currently underway.