New sediment analysis from the Red Planet reinforces the theory that there was a frozen lake where today is the Gale Crater.
Three billion years ago a frozen lake existed where today is the Gale Crater on Mars. The conclusion is the result of several studies based on data sent by NASA’s Curiosity rover, which has been in that region (225 km or 140 mi wide) of the Red Planet since 2012.
According to an article published in early 2020 in the journal Nature, scientists decided to investigate the area because of the abundance of sediments present there: there are signs of water, water minerals, and even traces of organic molecules (essential for the existence of life as we know it).
As scientists explain, to form so much sediment, an enormous amount of water would have flowed through this region thousands of years ago. In addition, some geological features of the crater suggest that this debris formed in an ice-covered lake.
“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 Heather Franz, a geochemist who participated in the study, in a statement.
Curiosity has a chemical laboratory called Sample Analysis at Mars (SAM). It was from the analysis of 13 samples of dust and Martian rock collected in the last five years that experts were able to deduce the existence of this hot lake.
For study purposes, the samples were heated to almost 900ºC, releasing their internal gases. By looking at the temperatures emitted by the sediments, scientists can deduce which minerals they come from – and this type of information helps to understand how carbon is circulating on Mars.
According to the team, the studies suggest that the ancient atmosphere of Mars, containing mainly CO2, may have been thicker than that of Earth today. Although most of the substance was lost when the Red Planet lost its atmosphere, part of it was stored in rocks – mainly in the form of carbonates, minerals made of carbon and oxygen.
On Earth, carbonates are produced when CO2 from the air is absorbed into the oceans and other bodies of water and then mineralized in rocks. Scientists believe that the same process happened on Mars and that it could help explain what happened to the Martian atmosphere.
However, the few carbonates that SAM detected reveal something interesting: the oxygen atoms in the sediments were lighter than those in the Martian atmosphere. This suggests that the carbonates were not formed from the atmospheric CO2 absorbed by a lake. According to the scientists, if that had happened, the oxygen isotopes would have to be heavier than those in the air.
Taking all of this into account, the scientists suggest that the analyzed carbonates probably formed in a freezing lake, because in this way, the ice would have “sucked” heavy isotopes of oxygen. Still, the low abundance of carbonates on Mars intrigues astronomers.
They assume that if there aren’t many of these minerals in the Gale Crater, perhaps the planet’s initial atmosphere may have been thinner than expected – or maybe something else is storing the missing carbon. Based on their analysis, the team assumes that the substance has been absorbed by other minerals, such as oxalates, which store carbon and oxygen in a different structure than carbonates.
The hypothesis is based on the temperatures at which CO2 was released from some samples within the SAM: very low for carbonates, but suitable for oxalates. In addition, the different proportions of isotopes of carbon and oxygen that scientists have observed in other carbonate samples support the idea.
Oxalates are the most common type of organic mineral produced by plants on Earth – which is why it is interesting for researchers. However, these molecules can also be produced by a process known as abiotic photosynthesis, which does not involve living beings.
Studying carbon is also important because it is a simple yet versatile element, which is as critical as water in the formation of life as we know it, since it is in the organic molecules known as “life building blocks”. On Earth, for example, the substance flows continuously through the air, water, and the surface in a cycle in which living beings are protagonists.
In recent years, scientists have found several clues that the carbon cycle on Mars is very different from that on Earth. “Nevertheless, the carbon cycling is still happening and is still important because it’s not only helping reveal information about Mars’ ancient climate, it’s also showing us that Mars is a dynamic planet that’s circulating elements that are the buildings blocks of life as we know it,” said research leader Paul Mahaffy.