Recent traces of liquid water discovered on Mars

While the Zhurong rover is slow to wake up from its long hibernation on the surface of Mars, Chinese scientists continue to exploit the multitude of data it collected for nearly a year before falling into a sleep which, we hopefully, is not final.

Just like Perseverance and Curiosity, the two other rovers in activity at the moment on the Red Planet, Zhurong is closely studying the composition of the Martian soil in order to better understand the geological and climatic evolution of the planet.

If we now know, without any possible doubt, that water did indeed flow on the surface several billion years ago, the drastic modification of the climate of Mars and the loss of its atmosphere led to the evaporation of masses of water, leaving an arid and desert planet. Today, the planet’s water activity seems almost non-existent and is limited to a few droplets condensing in the early morning in high latitude regions, or to the ice forming the polar caps.

Equatorial regions may not be so arid

The data reported by Zhurong, however, show that even very low water activity could well exist at much lower latitudes. Until now, no trace of liquid water had been detected in these equatorial regions of the planet. However, Zhurong discovered some in sand dunes formed only a few hundred thousand years ago.

On May 15, 2021, the rover indeed landed in the southern part of Utopia Planitia, just 25°N latitude. Thanks to its scientific measuring equipment, the little robot immediately began to scrutinize the composition of the dunes dotting the landscape of this vast plain. And surprise: the data shows that the surface of some dunes has a rather particular morphology. The surface is indeed encrusted and dotted with cracks, polygonal shapes and other traces generally formed by interactions with fluids. Spectral analysis revealed that the surface of the dune indeed presented a composition rich in hydrated sulphates, opal-CT (a water-rich mineral), but also in iron oxides (ferrihydrite) and some chlorides. A typical chemical signature that results from reactions with liquid water! Reactions which, given the relatively low age of these dunes (between 1.4 and 0.4 million years), occurred during the so-called “modern” period of Mars.

Typical minerals resulting from chemical reactions with liquid water

Liquid water would therefore have been present in the very recent past, leading to the formation of these encrusted deposits on the surface of the dunes of Utopia Planitia. Nothing to do however with the expanses of liquid water which would be at the origin of the much older sedimentary deposits studied by Perseverance in the Jezero crater, for example.

The crust composed of hydrated minerals would be the result of the melting of frost, or even a thin film of snow, on the surface of dunes containing mineral salts.

Like the salt that we disperse on our roads in winter, the presence of salts in the sand of the dunes would have allowed the frost to melt at lower temperatures than usual, leading to the formation of droplets of salty liquid water on the surface. Droplets however ephemeral under the harsh pressure conditions of the planet. As this water evaporated, it would thus have left behind precipitations of hydrated sulphates, opal, iron oxide and other minerals which gradually cemented the sand particles to form the crust observed by Zhurong.

A major result for the search for life on Mars

A story that may only date back 400,000 years. These observations, published in the journal Science Advance, could be linked to periods of low latitude cooling during episodes of strong planetary obliquity. During these periods, the transfer of water vapor from the polar caps towards the equator would have allowed the repeated humidification of these low latitude zones. An important result, because these regions are generally marked by higher surface temperatures. Add to that liquid water, even in very small quantities, and you have an environment favorable to life!