China’s Zhurong rover crossed a thin layer of flat, hydrated rocks in southern Utopia Planitia. A new analysis argues that its branching internal patterns and chemistry are best explained by large crystals of selenite, a clear, well-formed variety of gypsum that grows directly from concentrated water. If that interpretation is correct, Mars maintained an active water system much later than the era of its famous rivers and lakes. Crystals may also contain fluid inclusions, microscopic droplets sealed during growth. However, no such inclusion has been observed on Mars and the rover did not measure any droplets. This is a study, not an established consensus. The distinction matters. Zhurong found evidence of the mineral and its crystalline habit on a rover scale. The small file imagined inside remains a testable prediction, which would require microscopy, in situ chemical analysis, or sample return. Zhurong combined crystal shape with chemistry. The Nature Astronomy article reports that decimeter-scale crystals form within an extensive lateral layer of platy rock only 5 to 10 centimeters thick. Previous observations had established that these rocks contained hydrated material, but the exact phase was uncertain. Zhurong’s shortwave infrared spectrometer recorded absorption bands near 1.45 and 1.95 micrometers, associated with water and hydroxyl, plus a set of additional features that matched laboratory gypsum spectra. Their laser-induced decomposition spectrometer detected sulfur, calcium and hydrogen in each sheet rock target. The sulfur and calcium peaks closely resembled an incorporated gypsum reference, while their correlated abundances supported the calcium sulfate dihydrate, CaSO4·2H2O. The images provided another line of evidence. Some rock interiors showed clusters of radiant crystals. Others showed branching “Christmas tree” patterns that resembled the fishtail twinning of terrestrial selenite. The material appeared internally uniform and lacked the sand-rich rosette structure expected of gypsum desert roses. A detailed summary of the 2026 conference establishes how crystal habit, texture, infrared signatures, and elemental chemistry converge in the interpretation of selenite. A primary evaporite is more than a sulfate stain. Gypsum has been identified elsewhere on Mars, but sulfate can appear in various geological features. Mineral-rich groundwater can cement preexisting sediments, fill fractures, or leave behind fine-grained alteration products. Neither of those environments necessarily required a pond at the surface when the mineral formed. The new paper interprets the Zhurong layer as a primary evaporite. The large euhedral crystals, uniform composition, and thin, continuous geometry are more consistent with growth at or near the bottom of a brackish water body than with later cementing or fracture filling. “Euhedral” means that the crystals developed recognizable external faces rather than simply occupying any available pore space. That interpretation reinforces, but does not replace, previous reports. A 2022 Science Advances study used Zhurong imaging and spectra to identify hydrated sulfate or silica materials in duricrusts. Work published the following year found potential Amazonian brine activity and current water vapor cycles in data from the rover’s first 110 sols. SpaceDaily also reported on Zhurong’s evidence of much more recent saline water effects on nearby dunes. The Selenite statement refers to a different layer and a different episode. The water could have come many times. A mass balance calculation links the mineral layer to a substantial amount of water. Depending on the initial brine concentration, precipitating 5 to 10 centimeters of gypsum would require a cumulative water equivalent column of at least 6.25 to 25 meters. The “cumulative” is fundamental: the place did not suddenly need a lake 25 meters deep. The authors propose sustained or episodic upwelling in a shallow basin. At any time, the liquid layer could have been a meter deep or less and mostly covered by ice. Repeated delivery and freezing could process a much larger total volume through that small body. Southern Utopia Planitia contains pitted cones, depressions, polygonal soil, and structures interpreted as mud volcanism or dike systems. In the proposed sequence, magma burst into a volatile-rich cryosphere, melting buried ice and helping to propel ion-rich groundwater to the surface. The model is plausible within that geological setting, but Zhurong did not directly observe the ancient pond or its pipes. Freezing provides the concentration mechanism. As relatively pure ice forms, most of the dissolved ions remain in the liquid. The wastewater becomes progressively saltier, a process called cryoconcentration. The magnesium in brine can suppress the nucleation of calcium sulfate, allowing fewer crystals to grow before the last liquid disappears or drains away. Approximately 757 million years are recent for Mars alone. Time affirmation requires care. Crater counts assign the underlying Vastitas Borealis Formation a model age of around 3.2 billion years. Counting the smaller craters gives a reappearance age of about 757 million years, and the conference analysis reports an uncertainty of about 66 million years. The gypsum-containing layer lies immediately beneath a thin mantle of sand. The researchers did not see a thick layer that suggested ancient material had been deeply buried and then excavated. Therefore, they associate the near-surface evaporite with the younger resurfacing event rather than the much older substrate. This is not a radiometric age measured from a single crystal. Crater count dating estimates how long a surface has accumulated impacts and relies on models that translate crater density into time. The link between resurfacing and crystal growth is a geological interpretation. “About 760 million years old” is a defensible abbreviation for the host unit, not a direct laboratory date for the gypsum. Even with that caveat, the moment is surprising. Seven hundred and sixty million years is old by Earth standards, but it falls late in the Martian Amazonian period, long after Mars lost the warm, persistently humid conditions usually associated with its early history. The result points to local, episodic liquid water in an otherwise cold world, not a return to a planet-wide temperate climate. Microscopic brine remains a prediction Large gypsum crystals on Earth often trap small portions of their original solution as they grow. These fluid inclusions may line growth bands or occupy small cavities within the crystal. If they remain closed, they preserve a sample of water rather than simply the solid minerals that remain after the water disappears. A review of fluid inclusions in Mars-analog chemical sediments explains why they appeal to astrobiologists. Terrestrial inclusions of gypsum and halite can retain dissolved salts, gases, organic compounds, and, in some environments, cellular material. They are compact records of the hydrosphere and sometimes the biosphere at the time of crystallization. Zhurong did not have the optical or sampling equipment necessary to see such pockets in these rocks. It didn’t cut a transparent section, center through a glass, drill an inclusion or analyze its liquid. The paper predicts that large Martian selenite is likely to contain inclusions because comparable terrestrial crystals commonly contain them. “May contain” is the scientifically important phrase. The same caution applies to biology. No organism, fossil, organic molecule or biological signature was detected in the proposed selenite. A brine could have been chemically habitable without ever having been inhabited. Finding a sealed drop would be, first of all, a geological and geochemical result. A returned crystal could prove the whole story. An intact inclusion could reveal the salinity, acidity, dissolved elements and gases of the water. Isotope ratios could help distinguish shallow melted ice from deeper groundwater and reconstruct how freezing or evaporation modified the original solution. Multiple inclusions across growth zones could record changes during successive episodes. Testing that file would be difficult. Researchers would need to establish that an inclusion formed with the crystal and not in a subsequent fracture, that it remained sealed, and that any organic matter was Martian rather than contamination from spacecraft or laboratory manipulation. Gypsum can also dehydrate or recrystallize, processes that can alter an old inclusion. The shallow configuration cuts both ways. Large crystals and a 5 to 10 centimeter layer provide some protection, but material near the surface is exposed to oxidants and ionizing radiation. The top plaster is also relatively transparent to ultraviolet light. Deeper pockets within a crystal or layer would offer a better preservation target. A view of Utopia Planitia from the Mars Reconnaissance Orbiter shows how varied this vast northern basin is; the image is the regional context, not the Zhurong outcrop itself. Locating, approaching and sampling the selenite layer at rover scale would require a future mission designed for fine mineralogy and clean collection. For now, the result is a chain of increasingly ambitious inferences: the spectra and chemistry indicate hydrated calcium sulfate; the morphology points to primary selenite; Primary selenite involves concentrated liquid water; Terrestrial selenite suggests that fluid inclusions may survive within it. Every link is verifiable. Only the final step, opening or scanning a Martian crystal, can show whether a microscopic sample of that last water is still there.
