The discovery of gypsum crystals by China's Zhurong rover on Mars has sparked excitement and debate among scientists. These crystals, found in a thin layer of flat, hydrated rocks in southern Utopia Planitia, may still contain microscopic pockets of the brine they grew from, offering a glimpse into Mars' recent past. The crystals' branching internal patterns and chemistry suggest they are selenite, a clear, well-formed variety of gypsum that grows directly from concentrated water.
This interpretation has significant implications for our understanding of Mars' history. If correct, it suggests that Mars maintained an active water system far later than previously thought, with evidence of liquid water dating back to around 760 million years ago. This challenges the notion that Mars was once a warm, wet planet and instead points to local, episodic liquid water in an otherwise cold world.
One of the most intriguing aspects of this discovery is the potential for fluid inclusions within the crystals. These microscopic droplets, sealed during the crystal's growth, could preserve the chemistry of liquid water from a surprisingly recent chapter of Mars' history. However, the absence of such inclusions in the rover's data and the difficulty of detecting them without specialized equipment mean that this remains a prediction rather than a confirmed finding.
The study's authors propose a model where magma intruded into a volatile-rich cryosphere, melting buried ice and driving ion-rich groundwater toward the surface. This process, combined with freezing and cryoconcentration, could have resulted in the formation of the gypsum crystals. The timing of this event is also significant, falling late in the Martian Amazonian period, a time when Mars had already lost its warm, persistently wet conditions.
The potential for fluid inclusions within the crystals has sparked further excitement and speculation. If confirmed, these inclusions could provide a wealth of information about the water's salinity, acidity, dissolved elements, and gases. Isotope ratios could help distinguish between shallow melted ice and deeper groundwater, and multiple inclusions along growth zones might record changes during successive episodes.
However, the challenges of testing this prediction are significant. Researchers would need to establish that an inclusion formed with the crystal rather than in a later fracture, that it remained sealed, and that any organics were Martian rather than contamination from spacecraft or laboratory handling. Additionally, the exposure of the crystals to oxidants and ionizing radiation at the surface poses risks to the preservation of any inclusions.
In conclusion, the discovery of gypsum crystals on Mars is a fascinating development that challenges our understanding of the planet's history. While the potential for fluid inclusions within the crystals is exciting, it remains a prediction that requires further testing and confirmation. The study's authors have laid out a chain of increasingly ambitious inferences, and it is up to future missions to determine whether these inferences are correct and to what extent they can reveal the secrets of Mars' recent past.
