Zhurong selenite points to late liquid water on Mars while a salt archaeon grew in low-pressure brine
Peer-reviewed Zhurong data place selenite on terrain dated near 757 million years, while a bioRxiv lab study reports Haloferax volcanii growing 160 days at 24 millibar in perchlorate brine.
Science··Midday
Gypsum crystals on mid-Amazonian terrain
Writing in Nature Astronomy, Jiacheng Liu and colleagues report that the laser and infrared spectrometers on China's Zhurong rover identified large gypsum crystals in southern Utopia Planitia. Selenite of that kind grows only by crystallising out of concentrated salty water, which would put liquid water on Mars far later than the planet's wet early era. The team estimates that the mineral layer would have needed between 6.25 and 25 metres of water to accumulate, and dates the terrain by counting craters, so the age of about 757 million years carries the uncertainty that method carries. Their proposed source is underground brine melted and pushed upward by volcanic heat, an interpretation the mineral evidence permits without confirming. Zhurong worked on Mars from May 2021 until it fell silent in 2022, and the peer-reviewed paper reanalyses what it recorded. The claim is mineralogical: a late liquid-water setting is allowed by the crystals and the crater-count age, not proven as a standing surface ocean or as present-day habitability.[1]
A salt-loving archaeon under near-Mars lab pressure
Adam Robinson at the University of Florida and colleagues kept Haloferax volcanii in water holding 225 grams of salt per litre at 21 degrees Celsius and 24 millibar, without oxygen and with added nitrates and chlorates, for 160 days. Growth was slower than in the controls at 1,013 millibar but did happen, shown by clouding of the medium, biological reduction of nitrate and perchlorate, and extensive biofilm under an electron microscope. Robinson says the results point past survival to active growth, which few earlier studies have shown for salt-loving extremophiles. The work is posted on bioRxiv and has not been peer-reviewed. The setup departs from Mars in two ways the team names: the medium contained yeast extract as a carbon source, which Mars does not offer, and the pressure and temperature pairing may be more generous than the planet allows. Sean McMahon at the University of Edinburgh points out that surface pressure on Mars is around 6 to 12 millibar, where water cannot stay liquid, and that below the surface, where 24 millibar is plausible, temperatures sit well below freezing. Whether pockets of briny water exist under the Martian surface is unresolved.[2]
Past water chemistry and present lab limits
The two items sit on one habitability question without answering it the same way. Zhurong's selenite reopens late liquid water as a mineral possibility on terrain dated near 757 million years, with a proposed volcanic brine source that the crystals allow rather than prove. The Haloferax experiment tests whether a salt-loving archaeon can grow under low pressure, high salt and perchlorate chemistry for 160 days, and the team reports active growth under those controlled conditions while naming carbon supply and temperature-pressure pairing as gaps relative to Mars. One line is geological and peer-reviewed; the other is a laboratory preprint about an extremophile already used as a model organism. Neither shows living cells on Mars today, and neither closes whether briny pockets exist under the surface. Read together they separate a possible late watery past from a present experimental envelope for one extremophile, which is useful context for habitability talk that often slides from mineral evidence to life claims in a single step.[1], [2]
