Isotopes shed light on early Martian climate

A new analysis of chemical signatures measured by NASA’s Curiosity rover provides insight into Mars’ past about 3.7 billion years ago, when it was warmer and wetter.

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

The process of evaporation, although common on Earth, provides important clues about the ancient Martian climate. The presence of evaporation signatures in the isotopic compositions of water extracted from clay minerals in Martian rocks indicates that the Martian atmosphere was warm but also dry, favoring the evaporation of stagnant water.

“‘Hot’ is relative,” says Amy Hofmann, Ph.D., a visiting associate at Caltech and a research scientist at 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.

“It was a dynamic time in Mars’ history. The planet was in the midst of a global climate transition, but we know from the Gale rocks that the surface of Mars was still undergoing chemical weathering and that the lake waters had a roughly neutral pH and were not particularly salty. So add to this mix the simple organic compounds previously discovered in these same rocks, and you have an incredibly habitable local environment.”

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

The study focuses on oxygen isotopes rather than the more commonly studied hydrogen isotopes. The project is the first to discover strong oxygen-18 enrichments 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 the presence of two extra neutrons. When the water evaporates, the H₂O molecules containing a lighter oxygen atom tend to be the first to disappear, 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 on Mars. This deep depression on Mars shows signs that it once contained a large lake. The rover sampled clay minerals, known to more precisely preserve the isotopic signatures of oxygen and hydrogen transmitted since their formation.

Although the oxygen isotope ratios in Mars’ atmosphere are quite similar to those on Earth, water extracted from clay minerals shows strong enrichments of heavier oxygen. This finding indicates that evaporation was indeed occurring in Gale Crater at the time these sediments were deposited.

“This discovery by the Curiosity rover team is an important step in our long struggle to understand how water shaped the surface of Mars in a way that reminds us of Earth but is so different in its details and results,” said co-author John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry and the Ted and Ginger Jenkins Leadership Chair in the Division of Geological and Planetary Sciences.

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