Ancient Mars had widespread liquid water on its surface, making its climate suitable for life. But today, Mars’s surface is frigid and hostile to life.
How did the ancient Martian climate go from habitable to a surface that is inhospitable to terrestrial life?
NASA’s Curiosity rover, currently exploring Gale Crater on Mars, is providing new details about it. Using the rover’s instruments- the Sample Analysis at Mars (SAM) and Tunable Laser Spectrometer (TLS)-scientists measured the isotopic composition of carbon-rich minerals (carbonates) found in the Gale Crater. They found new clues on how the Red Planet’s ancient climate transformed.
David Burtt of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of a paper said, “The isotope values of these carbonates point toward extreme amounts of evaporation, suggesting that these carbonates likely formed in a climate that could only support transient liquid water.”
“Our samples are inconsistent with an ancient environment with life (biosphere) on the surface of Mars, although this does not rule out the possibility of an underground biosphere or a surface biosphere that began and ended before these carbonates formed.”
There may be less liquid water on Mars than previously thought
Isotopes are different versions of the same element that have different weights. When water evaporates, lighter carbon and oxygen are more likely to escape into the air, while heavier ones stay behind and build up, eventually forming carbonate rocks. Scientists study these rocks because they can provide important information about past climates, like the temperature and acidity of the water they came from.
The paper discusses two ways these carbonates might have formed in Gale Crater. The first idea is that they formed during cycles of wet and dry conditions. The second idea is that they formed in very salty water cold enough to freeze.
Jennifer Stern of NASA Goddard, a paper co-author, said, “These formation mechanisms represent two different climate regimes that may present different habitability scenarios. Wet-dry cycling would indicate an alternation between more-habitable and less-habitable environments. In contrast, cryogenic temperatures in the mid-latitudes of Mars would indicate a less-habitable environment where most water is locked up in ice and unavailable for chemistry or biology, and what is there is extremely salty and unpleasant for life.”
Scientists have suggested climate scenarios for ancient Mars based on minerals, models, and rock formations. This new finding adds evidence from isotopes found in rock samples to support those scenarios.
The heavy isotopes in Martian carbonates are much higher than those in Earth’s carbonates, making them the heaviest carbon and oxygen isotopes on Mars. The research indicates that wet-dry cycles and cold, salty conditions are needed to create these highly enriched carbonates.
Burtt said, “The fact that these carbon and oxygen isotope values are higher than anything else measured on Earth or Mars points towards a process (or processes) being taken to an extreme. While evaporation can cause significant oxygen isotope 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 driving these isotope values to be so heavy, and 2) these heavier values were preserved, so any processes that would create lighter isotope values must have been significantly smaller in magnitude.”