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 and well-formed variety of gypsum that forms directly from concentrated water. This interpretation implies that Mars maintained an active water system far later than previously thought, challenging our understanding of the planet's history.
The study's authors propose that the crystals may also contain fluid inclusions, tiny sealed samples that could preserve the chemistry of liquid water from a surprisingly recent chapter of Mars' history. However, no such inclusions have been observed on Mars, and no droplets were measured by the rover. This raises questions about the validity of the interpretation and the potential for further investigation.
The paper's authors argue that the layer of rocks is a primary evaporite, formed at or near the bottom of a briny water body. This interpretation is supported by the large euhedral crystals, uniform composition, and thin, continuous geometry of the layer. However, it is important to note that this interpretation strengthens but does not replace earlier reports, and the study remains a testable prediction that would require microscopy, in-place chemical analysis, or sample return.
The timing of the crystal growth is also significant. The gypsum-bearing layer is estimated to be around 760 million years old, which is ancient by terrestrial standards but falls late in the Martian Amazonian period. This suggests that the water could have arrived many times, with the crystals forming during a period of local, episodic liquid water in an otherwise cold world.
The microscopic brine remains a prediction, and the study's authors caution that the crystals may contain fluid inclusions that could preserve a sample of water. However, the lack of optical or sampling equipment on the rover limits the ability to confirm this. The study also highlights the challenges of testing the archive, including the need to establish that an inclusion formed with the crystal and remained sealed, and the potential for contamination or disturbance during the process.
In conclusion, the discovery of gypsum crystals on Mars is a fascinating development that challenges our understanding of the planet's history and the potential for life. However, the study remains a testable prediction, and further investigation is needed to confirm the interpretation and explore the potential for fluid inclusions and biological activity.
