Zhurong hints at a late Martian brine archive in gypsum crystals
Zhurong’s analysis of a 5–10 cm thick hydrated rock in Utopia Planitia suggests the stone hosts euhedral gypsum (selenite) crystals formed as a primary evaporite from a concentrated brine, implying localized liquid water on Mars around 760 million years ago. The interpretation hinges on crystal habit and chemistry, with the possibility of microscopic fluid inclusions that could preserve past water chemistry, but no inclusions have been observed yet. The date is based on crater dating (not a direct radiometric measurement), and testing for inclusions or sampling the crystal would require in-situ microscopy or sample return in the future.