Curiosity rover provides new information on how Mars became uninhabitable

NASA’s Curiosity rover, currently exploring Gale Crater on Mars, is providing new details about how the ancient Martian climate changed from one potentially suitable for life (with evidence of water liquid spread on the surface) to a surface inhospitable to terrestrial life as we know it. he.

Although the surface of Mars today is frigid and hostile to life, NASA’s robotic explorers on Mars are searching for clues as to whether it might have supported life in the distant past. Researchers used instruments aboard Curiosity to measure the isotopic composition of carbon-rich minerals (carbonates) found in Gale Crater and discovered new insights into the transformation of the Red Planet’s ancient climate.

“The isotopic values ​​of these carbonates indicate extreme levels of evaporation, suggesting that these carbonates likely formed in a climate that could only support transient liquid water,” said David Burtt of the Goddard Space Flight Center of NASA in Greenbelt, Maryland, and lead author of the work. an article describing this research published in the Proceedings of the National Academy of Sciences.

“Our samples do not correspond to an ancient environment with life (biosphere) on the surface of Mars, although this does not exclude the possibility of a subsurface biosphere or a surface biosphere that began and grew. is completed before the formation of these carbonates.”

Isotopes are versions of an element with different masses. As water evaporated, the lighter versions of carbon and oxygen were more likely to escape into the atmosphere, while the heavier versions more often stayed behind, accumulating in greater abundance and, in this case, ultimately being incorporated into carbonate rocks.

Scientists are interested in carbonates because of their proven ability to serve as a climate record. These minerals can retain signatures of the environments in which they formed, including water temperature and acidity, as well as the composition of the water and atmosphere.

The paper proposes two formation mechanisms for the carbonates found at Gale. In the first scenario, carbonates form during a series of wet-dry cycles in Gale Crater. In the second case, carbonates form in very salty water under cold ice-forming (cryogenic) conditions in Gale Crater.

“These formation mechanisms represent two different climate regimes that may present different habitability scenarios,” said Jennifer Stern of NASA Goddard, co-author of the paper. “A wet-dry cycle would indicate alternation between more and less habitable environments, while cryogenic temperatures at mid-latitudes of Mars would indicate a less habitable environment where most of the water is locked in ice and is not available for chemistry or biology, and what is there is extremely salty and unpleasant to experience.

These climate scenarios for ancient Mars have already been proposed, based on the presence of certain minerals, global-scale modeling and the identification of rock formations. This result is the first to add isotopic evidence from rock samples to support the scenarios.

Heavy isotope values ​​in Martian carbonates are significantly higher than those observed on Earth for carbonate minerals and constitute the heaviest carbon and oxygen isotope values ​​recorded for any Martian material. In fact, the team says, wet-dry and cold-salty climates are necessary for the formation of carbonates so rich in heavy carbon and oxygen.

“The fact that these carbon and oxygen isotopic values ​​are higher than anything measured on Earth or Mars indicates that one or more processes are being pushed to the extreme,” Burtt said.

“Although evaporation can cause significant isotopic changes on Earth, the changes measured in this study were two to three times larger. This means two things: 1) there was an extreme degree of evaporation that made these isotopic values so heavy, and 2), these heavier values ​​were preserved, so any processes that could create lighter isotopic values ​​would have had to be of significantly lower magnitude,” he continued.

This discovery was made using the Sample Analysis at Mars (SAM) and Tunable Laser Spectrometer (TLS) instruments aboard the Curiosity rover. The SAM heats the samples to nearly 1,652 degrees Fahrenheit (nearly 900°C), then the TLS is used to analyze the gases produced during this heating phase.