Planetary scientists have discovered how Mars absorbs water.

The Chinese rover "Zhurong" recently discovered “beach deposits” in the Utopia Planitia, situated in the mid-latitudes of the northern hemisphere of the Red Planet. There, the shoreline of an ancient sea can be observed. In many other locations, winding channels of dried-up rivers are visible, and clay minerals have been found, which can only form in the presence of liquid water.

All of this indicates that Mars once had a fully developed hydrosphere. According to planetary scientists, it began to dehydrate about three billion years ago. Researchers continue to investigate why and how the planet lost its oceans.

Possible reasons include its weak gravity and almost nonexistent magnetic field, but it is crucial to understand the very process of water disappearance. It is generally accepted that water evaporated from the atmosphere into interplanetary space due to cosmic radiation, but there is also an alternative theory — it is likely that the Red Planet could have absorbed a significant amount of water.

The Martian soil can retain water in several different ways: not only as ice but also in an adsorbed state, meaning as molecular layers on the surfaces of soil particles. Recently, scientists from Tohoku University (Japan) discussed these “capabilities” of Mars in an article for the Journal of Geophysical Research: Planets. They compiled all the data from spacecraft regarding the surface properties of the Red Planet and created an unusual map based on this information.

It turns out that in two extensive regions of Mars, an interesting picture emerges: the regolith is much finer, more porous, and absorbs water better. Both areas encompass equatorial and mid-latitudes, one of which is located around the Olympus volcano, while the edge of the second area is in the Jezero crater, where the Perseverance rover is currently operating. By the way, the porosity of the soil in this former lake has been estimated at 36 percent. This means that more than a third of the total volume of “land” consists of voids.

Scientists modeled how much water should have accumulated in the soil of the Red Planet over the last 500 Martian years, which is equivalent to a millennium by Earth standards, taking these characteristics into account. It was found that near the equator, each square meter of soil should contain approximately two to four kilograms of adsorbed water at depths of up to two meters: it is firmly bound to solid particles through physical and chemical interactions and cannot move freely.

Thus, once water enters the soil from the atmosphere, it does not return to the air. It is likely that water exists even deeper than two meters, but that is the maximum depth at which instruments can "detect" hydrogen.

In the polar regions, the soil is coarse-grained, and a different mechanism for retaining water operates — in the form of ice particles in the voids. Research has shown that at a depth of four centimeters, there may be more than a kilogram of such “pore ice” in every cubic meter. Now it remains to be determined what role all this played in the history of Mars.