Isotopes illuminate early Martian climate

A new analysis of chemical signatures measured by NASA’s Curiosity Rover gives a peek at Mars’s past to a time some 3.7 billion years ago, when it was warmer and wetter.

Through measurements of isotopic ratios of oxygen, a team of collaborators, including researchers from Caltech’s campus and NASA’s Jet Propulsion Laboratory (JPL) have discovered that the lake which once existed in Mars’s Gale Crater was undergoing significant evaporation earlier than the mineralogy and geochemistry of the lake bed sediments would suggest.

The process of evaporation, while commonplace to us on Earth, gives important clues to the ancient Martian climate. The presence of evaporation signatures in the isotopic compositions of water extracted from clay minerals in the Martian rocks indicates that the Martian atmosphere was warm but also dry, promoting evaporation of standing water.

“‘Warm’ is relative,” says Amy Hofmann, Ph.D., a visiting associate at Caltech and research scientist with JPL, which Caltech manages for NASA. “We’re talking a little above freezing, but it was warm enough to potentially support the kinds of prebiotic chemistries that astrobiologists are interested in.

“This was a dynamic time in Mars’s history. The planet was in the midst of a global climate transition, but we know from the rocks at Gale that Mars’s surface was still experiencing chemical weathering, and the lake waters had a roughly circumneutral pH and were not particularly salty. So, add to that mix the simple organic compounds previously discovered in these same rocks, and you’ve got yourself a compellingly habitable local environment.”

Hofmann is the lead author on a paper describing the study, which appears in the journal Proceedings of the National Academy of Sciences.

The study focuses on oxygen isotopes rather than more commonly studied hydrogen isotopes. The project is the first to find strong enrichments of oxygen-18 in an ancient Martian water reservoir. Oxygen-18 is a relatively rare form of oxygen that is heavier than its typical counterpart, oxygen-16, due to having two more neutrons. When water evaporates, the H₂O molecules containing a lighter oxygen atom tend to be the first to go, leaving behind liquid water containing a higher concentration of heavy oxygen.

The team studied samples collected by the Curiosity rover between 2012 and 2021 from the Gale Crater region of Mars. This deep depression on Mars shows signs of once having contained a large lake. The rover sampled clay minerals, which are known to more accurately retain the oxygen and hydrogen isotopic signatures imparted from the time they were formed.

Though the oxygen isotope ratios in Mars’s atmosphere look quite similar to the ratios on Earth, water extracted from the clay minerals showed strong enrichments of heavier oxygen. This discovery indicates the evaporation was indeed occurring in Gale Crater at the time when those sediments were deposited.

“This discovery of the Curiosity rover team is an important step forward in our long struggle to understand how water shaped the surface of Mars in ways that remind us of Earth yet are so different in their details and their outcomes,” says co-author John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry and the Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences.

“Most important to me is the new understanding we have gained of ways the drier atmosphere and wildly changing hydrosphere on Mars controlled the life cycles of its lakes—arguably our best targets for discovering evidence of life or its chemical precursors beyond Earth.”