Gypsum crystals discovered by Zurong on the roughly 760-million-year-old Martian terrain may still contain microscopic pockets of the brine from which they originated — tiny, sealed samples that could preserve the chemistry of liquid water from a surprisingly recent chapter in Martian history.
China’s Zhurong spacecraft crossed a thin layer of wet, flat rock in southern Utopia Planitia. The new analysis suggests that their internal patterns and chemistry are best explained by large selenite crystals, a clear, well-formed aggregate of gypsum that grows directly from concentrated water.
If this interpretation is correct, Mars maintained an active water system much later than its famous rivers and lakes. Crystals may also contain liquid inclusions, microscopic droplets sealed during growth. However, no such inclusion has been observed on Mars, and no droplet has been measured by the rover. This is one study, not a settled consensus.
The distinction is important. Zhurong found evidence of the mineral and its crystal habits on the rover’s scale. The small archive conceived within it remains a testable prediction, requiring microscopic examination, in-situ chemical analysis, or re-sampling.
Zhurong combines crystal form and chemistry
the Nature astronomy paper Reports indicate decimeter-scale crystals forming within an extensive layer of flat rock only 5 to 10 centimeters thick. Previous observations have proven that these rocks contain hydrated materials, but the exact stage is uncertain.
Zhurong’s short-wave infrared spectrometer recorded absorption bands near 1.45 and 1.95 micrometers, associated with water and hydroxyl, as well as a host of additional features that matched laboratory gypsum spectra. Laser-induced collapse spectroscopy detected the presence of sulfur, calcium and hydrogen in each flat rock target. The sulfur and calcium peaks closely resemble the on-board gypsum reference, while their correlated abundances support calcium sulphate dihydrate, CaSO.4·2H2Hey.
The photos provided another line of evidence. Some of the rocky interiors displayed radiant crystal clusters. Others showed branching “Christmas tree” patterns resembling fishtail twining of terrestrial selenite. The material appeared internally uniform and lacked the rich, sandy pink structure expected from desert rose gypsum. Detailed 2026 conference summary It shows how crystal habit, texture, infrared signatures, and elemental chemistry converge in the interpretation of selenite.
Primary vaporizer is more than just a sulfate dye
Gypsum has been identified elsewhere on Mars, but sulfates can appear in several geological roles. Mineral-rich groundwater can solidify pre-existing sediments, fill fractures, or leave fine-grained alteration products. None of these settings necessarily require a pond to be present on the surface when the metal is formed.
The new paper interprets the Zhurong layer as a primary evaporite. Large monohedral crystals, uniform composition and continuous fine geometry, are more consistent with growth at or near the bottom of salt water bodies than with subsequent growth of cement or fracture filling. “Euhedral” means that the crystals have developed recognizable outer faces rather than simply occupying any available pore space.
This interpretation reinforces previous reports but does not replace them. A 2022 Science Advances Study He used Zhurong’s images and spectra to identify sulfate or hydrated silica materials in crustacean shells. The work was found published the following year Potential brine activity in the Amazon and current water vapor cycle In data from the rover’s first 110 Martian days. SpaceDaily also reported Zhurong’s evidence of this Effects of younger salt water on nearby sand dunes. The selenite claim relates to a different layer and a different ring.
The water could have hit several times
The mass balance calculation relates the mineral layer to a large amount of water. Depending on the concentration of the initial brine, depositing 5 to 10 centimeters of gypsum would require a cumulative water equivalent column of at least 6.25 to 25 metres. “Cumulative” is essential: the site did not need a 25-meter-deep lake at once.
The authors suggest that continuous or episodic uplift occurs in a shallow basin. At any given time, the liquid layer may have been a meter or less deep, and most of it was covered by ice. Repeated delivery and freezing can result in a much larger total volume being processed through such a small body.
The southern Utopia Planitia area contains excavated cones, basins, polygonal terranes, and structures that have been interpreted as mud volcanoes or dike systems. In the proposed sequence, magma infiltrated the volatile-rich cryosphere, melted buried ice, and helped push ion-rich groundwater to the surface. This model is plausible within that geological setting, but Zurong did not directly observe the ancient pond or its water pipes.
Freezing provides the focusing mechanism. When relatively pure ice forms, most of the dissolved ions remain in the liquid. The remaining water becomes progressively saltier, a process called cryoconcentration. The magnesium in the brine can inhibit calcium sulfate nucleation, allowing fewer crystals to grow larger before the last of the liquid is gone or drained.
About 757 million years ago is only relatively recent for Mars
Demanding time requires care. Crater numbers assign the underlying Vastitas Borealis Formation to a typical age of approximately 3.2 billion years. Counting smaller craters puts the apparition at around 757 million years old, and conference analysis suggests an uncertainty of around 66 million years.
The gypsum-bearing layer is located directly under a thin sand cover. The researchers did not see a thick covering that would indicate that the ancient materials had been buried deeply and then excavated. Therefore, they associate the near-surface evaporite with a younger resurfacing event rather than a much older substrate.
This is not a radiometric age measured from an individual crystal. Crater count dating estimates how long impacts have accumulated at the surface and relies on models that translate crater density into time. The relationship between resurfacing and crystal growth is a geological explanation. The phrase “approximately 760 million years ago” is a defensible abbreviation of the host unit, not a direct laboratory dating of the gypsum.
Even with that qualification, the timing is amazing. Seven hundred and sixty million years is an old year by Earthly standards, but it falls late in the Martian Amazonian period, long after Mars had lost the persistent warm and humid conditions usually associated with its early history. The result suggests an occasional local liquid water presence in a cold world, not a return to a planet-wide temperate climate.
Microscopic saline remains a predictor
Large gypsum crystals on the ground usually trap small portions of the original solution as they grow. These liquid impurities can line growth domains or occupy small cavities within the crystal. If they remain closed, they preserve a sample of the water rather than just the solid minerals remaining after the water is gone.
Review for Fluid inclusions in Martian chemical deposits Explains why they attract astrobiologists. Terrestrial inclusions of gypsum and halite can retain dissolved salts, gases, organic compounds, and, in some environments, cellular materials. They are compact records of the hydrosphere and sometimes the biosphere at the moment of crystallization.
Zhurong did not have the optical equipment or sampling equipment needed to see such pockets in these rocks. It did not cut a transparent section, focus through the crystal, penetrate an embedding or analyze its fluid. The paper predicts that large Martian selenite likely contains impurities, as similar terrestrial crystals typically do. “May contain” is wording of scientific interest.
The same caution applies to biology. No living or fossil organism, organic molecule or biosignature has been discovered in the proposed selenite. The brine would have been chemically habitable without ever being inhabited. Finding a sealed drop would initially be a geological and geochemical finding.
The returned crystal can experience the whole story
Proper inclusion can reveal water salinity, acidity, dissolved elements and gases. Isotope ratios can help distinguish between shallow melted ice and deep groundwater and reconstruct how freezing or evaporation modified the original solution. Multiple inclusions along growth zones may record changes during successive episodes.
Testing this archive will be difficult. Researchers will need to prove that the inclusion formed with the crystal and not in a later fracture, that it remained sealed, and that any organic material was Martian and not contaminated from spacecraft or laboratory handling. Gypsum can also dehydrate or recrystallize, processes that may disturb the ancient embedding process.
A shallow setting cuts both ways. Large crystals and a 5- to 10-centimeter layer provide some protection, but materials near the surface are exposed to oxidants and ionizing radiation. The gypsum top is also relatively transparent to UV rays. Deep pockets within the crystal or layer would provide a better target for preservation.
A View of the Mars Reconnaissance Orbiter of Utopia Planitia It shows how diverse this vast northern basin is; The image is of regional context, not the Zurong outcrop itself. Locating, approaching, and sampling the selenite layer at rover scale will require a future mission designed for fine mineralogy and clean collection.
For now, the result is a series of increasingly ambitious conclusions: spectra and chemistry point to hydrated calcium sulfate; Morphology indicates primary selenite; Elemental selenite means concentrated liquid water. Ground selenite indicates there may be liquid impurities inside. Every link is testable. Only the final step, opening or scanning the Martian crystal, can show whether a microscopic sample from that late water is still present.
