New insights into how Mars became uninhabitable

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

“The isotope values ​​of these carbonates indicate extreme amounts of evaporation, indicating that these carbonates likely formed in a climate that could only temporarily support liquid water,” said David Burtt of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the book and paper describing this research was published on October 7 in the Proceedings of the National Academy of Sciences. “Our samples are not consistent with an ancient environment with life (biosphere) on the surface of Mars, although this does not rule out the possibility of a subsurface biosphere or a surface biosphere that began and ended before these carbonates formed.”

Isotopes are versions of an element with different masses. As water evaporated, light versions of carbon and oxygen were more likely to escape into the atmosphere, while the heavy versions were more likely to remain, accumulate in greater quantities and, in this case, eventually be incorporated into the carbonate rocks. Scientists are interested in carbonates because of their proven ability to act as climate records. These minerals can retain characteristics of the environment in which they formed, including the temperature and acidity of the water, and the composition of the water and atmosphere.

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

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

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

The heavy isotope values ​​in Martian carbonates are significantly higher than what is seen on Earth for carbonate minerals and are the heaviest carbon and oxygen isotope values ​​recorded for any Martian material. According to the team, both wet-dry and cold-salty climates are necessary to form carbonates that are so enriched in heavy carbon and oxygen.

“The fact that these carbon and oxygen isotope values ​​are higher than anything else measured on Earth or Mars indicates that a process (or processes) is being taken to the extreme,” says Burtt. “Although evaporation can cause significant changes in the oxygen isotope on Earth, the changes measured in this study were two to three times greater. This means two things: 1) there was an extreme rate of evaporation that caused these isotope values ​​to be so high, and 2) these heavier values ​​were retained, so any processes that would create lighter isotope values ​​must have been significantly smaller in magnitude.”

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

Funding for this work came from NASA’s Mars Exploration Program through the Mars Science Laboratory project. Curiosity was built by NASA’s Jet Propulsion Laboratory (JPL), which is operated by Caltech in Pasadena, California. JPL is leading the mission on behalf of NASA’s Science Mission Directorate in Washington. NASA Goddard built the SAM instrument, a miniaturized science laboratory that includes three different instruments for analyzing chemistry, including the TLS, plus mechanisms for handling and processing samples.