The Chinese rover Zhurong observed on Mars structures that researchers interpret as gypsum crystals formed by evaporation. These deposits are located in southern Utopia Planitia, within a shallow rocky layer. They would indicate the past presence of liquid salty water, well after the planet's wettest periods.
The images show crystalline shapes several tens of centimeters in size. Their geometry is reminiscent of selenite, a transparent or clear variety of terrestrial gypsum. This hydrated mineral contains water in its structure.
Selfie of the Zhurong rover with the Tianwen-1 lander, taken by the deployable Tianwen-1 remote camera.
The studied layer is about 5 to 10 cm thick and rests on terrain about 760 million years old. This estimate places the deposit in the Amazonian, a geologically recent period on the Martian scale. Mars was then generally cold and dry, which makes these water-related clues particularly interesting.
According to the proposed model, salty groundwater could have risen to the surface as a result of magmatic activity. It would have fed a very shallow body of water, probably largely covered by ice. Progressive evaporation or freezing would have concentrated the salts until the crystals grew.
These deposits differ from many already known Martian sulfates, often linked to the alteration of ancient rocks. Here, the authors argue for the existence of a primary evaporite, formed directly during the concentration of a brine. This interpretation rests on the morphology of the crystals and must be confirmed by further observations or sample analyses.
Large selenite crystals can enclose tiny pockets of the liquid in which they grew. Such inclusions could preserve information about the chemistry of ancient Martian brines, which could be analyzed by a mission designed for that purpose.
Zhurong (in Chinese: 祝融, Zhùróng) is the first Chinese rover on Mars. It landed on May 14, 2021. The rover was deactivated in May 2022 for a planned period of about 7 months to protect it from the Martian winter. However, it could not be reactivated due to the accumulation of dust on its solar panels and is now considered lost. Scientific results are nevertheless still being published today, based on data the rover was able to collect during its year of operation.