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According to new data obtained from the radar instruments on Mars Reconnaissance Orbiter (MRO) and Mars Express, scientists now think that the Medusae Fossae Formation, Mars’ biggest volcanic ash field and thought by some to be the source of most of the planet’s dust, might have an underground layer of ash that is also ice-rich. From their abstract:

The Medusae Fossae Formation (MFF) on Mars covers a vast area along the boundary between the rugged southern highlands and the smooth northern plains. While the MFF appears to be thick sediments or volcanic ash slowly eroding in the martian winds, how this material was emplaced remains mysterious. Most intriguing is evidence suggesting that some areas of the MFF may contain water ice. In this work we use sounding radar data from the SHARAD instrument on the Mars Reconnaissance Orbiter to probe up to 600 m below the surface and measure the electrical properties of the MFF material. The results suggest that the shallow parts of the MFF deposits are very porous and compress readily under their own weight. To match deeper probing by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on Mars Express requires a second layer of either vast porous deposits or ice‐rich material protected from sublimation by the dry sediments.

The MRO image above, originally posted here in November 2020, shows one example of the typical wind erosion found in the Medusae ash field. Apparently the ground-penetrating radar from orbit now suggests the possibility that there is an ash layer rich in ice, at depths beginning somewhere between 1,000 to 2,000 feet below the surface.

What makes this possibility important is that the Medusae ash field is located along the equator no more than 10 degrees latitude to the north and south. If a water-rich ash layer exists deep underground here this strongly suggests that an underground ice table likely exists everywhere on Mars, even in its most arid equatorial regions.

This conclusion has of course many uncertainties. First, the data could be explained not by an ice-rich layer but by a layer of porous layer made up of coarse sand. The scientists think this is unlikely because they do not see such sand at the margins of the Medusae ash field, where the lower layer might be exposed.

Second, the data is based on what the radar reveals, which includes many assumptions that could be found wrong when we finally dig down and look at actual samples.

The data also suggests that the dry upper layer of fine-grained ash could not have formed from the sublimation of its own ice. Instead, for there to be a lower layer of ice-rich material, the scientists theorize the following:

One possibility is that the Medusae Fossae Formation is a hybrid deposit, where parts of an extensive, thick, ice-rich unit was capped by a later “dry” component, perhaps through pyroclastic volcanism. The currently existing ice at depth would thus reflect the interplay of large-scale [relatively recent glacial] deposition with favorably timed volcanic eruptions that buried and preserved the ice through subsequent obliquity cycles.

If this explanation holds, it once again suggests that there is ample water on Mars. All you will need to do is dig down to find it, though at the equator it will be very deep.