Martian Minerals Found By Curiosity Hint They Formed In An Icy Lake

This illustration depicts a lake of water partially filling Mars' Gale Crater. It would have been filled by runoff from snow melting on the crater's northern rim. Evidence of ancient streams, deltas, and lakes that NASA's Curiosity rover has found in the patterns of sedimentary deposits in Gale suggests the crater held a lake like this one more than 3 billion years ago, filling and drying in multiple cycles over tens of millions of years. NASA/JPL-Caltech/ESA/DLR/FU Berlin/MSSS

Over the last eight years, NASA’s Curiosity has been exploring Gale Crater, which was once a vast lake on the surface of Mars. The rover’s work has been key to expanding our understanding of what happened to this lake when the climate changed billions of years ago. Now, results from one of the rover's experiments has provided intriguing insight.

As reported in Nature Astronomy, Curiosity has been running a multi-year experiment with its Sample Analysis at Mars (SAM), a chemistry lab used to analyze samples collected by the rover. The composition of those samples suggests the lake either froze over just before disappearing completely or between two warming periods.

“At some point, Mars’ surface environment must have experienced a transition from being warm and humid to being cold and dry, as it is now, but exactly when and how that occurred is still a mystery,” lead author Dr Heather Franz, a NASA geochemist based at NASA’s Goddard Space Flight Center, said in a statement.

SAM was used to bake 13 soil and rock samples to a temperature of 900°C (1,650°F). This method allowed Curiosity to break carbon dioxide from the minerals and measure its weight. Elements come in what we call isotopes. These have the same chemical properties but a different number of neutrons in the nucleus, which make them a bit more or a bit less heavy. 

Most carbon has 6 protons and 6 neutrons in its nucleus, known as Carbon-12. The most common isotope of oxygen has 8 protons and 8 neutrons (Oxygen-16). Stable forms of these elements are Carbon-13 and Oxygen-18, but they are much rarer. These were the ones of particular focus for the scientists.

This graphic depicts paths by which carbon has been exchanged among Martian interior, surface rocks, polar caps, waters, and atmosphere, and it also depicts a mechanism by which it is lost from the atmosphere. Lance Hayashida/Caltech

Minerals in the lake formed from gases that originated in the Martian atmosphere, which at the time was mostly CO2 and denser than it is today. The gas was converted into carbonates, and by heating it up, SAM was able to release the gas back again. Surprisingly, there were a lot more light oxygen isotopes in the minerals compared to the atmosphere.

This puzzling result could be explained if the carbonates formed in a freezing lake. Under those conditions, the ice would have sucked up the heavier oxygen atoms, leaving the lighter ones to form the minerals studied by Curiosity. The work also suggests that the ancient Martian atmosphere might not have been as thick as previously thought at roughly half of Earth’s air pressure at sea level today.

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