Curiosity Finds Evidence of Mars’ Ancient Ice-Covered Lakes
Using data from the Sample Analysis at Mars (SAM) instrument on NASA’s rover Curiosity, researchers have found that certain minerals in rocks at Gale Crater, the 150-km-wide ancient basin that Curiosity is exploring, may have formed in an ice-covered lake during a cold stage sandwiched between warmer periods, or after Mars lost most of its atmosphere and began to turn permanently cold. Their findings appear in the journal Nature Astronomy.
Gale Crater was selected as Curiosity’s 2012 landing site because it had signs of past water, including clay minerals. Indeed, while exploring the base of Mount Sharp, a mountain in the center of the crater, the rover found a 304-m-thick layer of sediments that was deposited as mud in ancient lakes.
To form that much sediment an incredible amount of water would have flowed down into those lakes for millions to tens of millions of warm and humid years. But some geological features in the crater also hint at a past that included cold, icy conditions.
“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,” said Dr. Heather Franz, a geochemist at NASA’s Goddard Space Flight Center.
‘Factors such as changes in Mars’ obliquity and the amount of volcanic activity could have caused the Martian climate to alternate between warm and cold over time. This idea is supported by chemical and mineralogical changes in Martian rocks showing that some layers formed in colder environments and others formed in warmer ones.”
Dr. Franz and colleagues found evidence for a cold ancient environment after the SAM extracted carbon dioxide and oxygen from 13 dust and rock samples.
After Curiosity fed rock and dust samples into SAM, the lab heated each one to 900 degrees Celsius to liberate the gases inside. By looking at the oven temperatures that released carbon dioxide and oxygen, scientists could tell what kind of minerals the gases were coming from. This type of information helps them understand how carbon is cycling on Mars.
Various studies have suggested that Mars’ ancient atmosphere, containing mostly carbon dioxide, may have been thicker than Earth’s is today. Most of it has been lost to space, but some may be stored in rocks at the planet’s surface, particularly in the form of carbonates, which are minerals made of carbon and oxygen.
On Earth, carbonates are produced when carbon dioxide from the air is absorbed in the oceans and other bodies of water and then mineralized into rocks. Researchers think the same process happened on Mars and that it could help explain what happened to some of the Martian atmosphere. Yet, missions to Mars haven’t found enough carbonates in the surface to support a thick atmosphere.
Nonetheless, the few carbonates that SAM did detect revealed something interesting about the Martian climate through the isotopes of carbon and oxygen stored in them. Because different chemical processes, from rock formation to biological activity, use these isotopes in different proportions, the ratios of heavy to light isotopes in a rock provide scientists with clues to how the rock formed.
In some of the carbonates SAM found, scientists noticed that the oxygen isotopes were lighter than those in the Martian atmosphere. This suggests that the carbonates did not form long ago simply from atmospheric CO2 absorbed into a lake. If they had, the oxygen isotopes in the rocks would have been slightly heavier than the ones in the air.
While it’s possible that the carbonates formed very early in Mars’ history, when the atmospheric composition was a bit different than it is today, Dr. Franz and co-authors suggest that the carbonates more likely formed in a freezing lake.
In this scenario, the ice could have sucked up heavy oxygen isotopes and left the lightest ones to form carbonates later.
“The low abundance of carbonates on Mars is puzzling,” the authors said.
“If there aren’t many of these minerals at Gale Crater, perhaps the early atmosphere was thinner than predicted. Or maybe something else is storing the missing atmospheric carbon.”
Based on their analysis, the researchers suggest that some carbon could be sequestered in other minerals, such as oxalates, which store carbon and oxygen in a different structure than carbonates.
Their hypothesis is based on the temperatures at which carbon dioxide was released from some samples inside SAM — too low for carbonates, but just right for oxalates — and on the different carbon and oxygen isotope ratios than the scientists saw in the carbonates.
H.B. Franz et al. 2020. Indigenous and exogenous organics and surface-atmosphere cycling inferred from carbon and oxygen isotopes at Gale crater. Nat Astron 4, 526-532; doi: 10.1038/s41550-019-0990-x
This article is based on text provided by the National Aeronautics and Space Administration.