SPACE — Mars once turned red because of a river. Traces of ancient rivers, streams, and lakes are still visible today across the planet’s surface. But about three billion years ago, everything dried up, and no one knows why that happened.
“People have come up with different ideas, but we’re not sure what caused the climate to change so dramatically,” said University of Chicago geophysicist Edwin Kite. Phys.orgThursday, May 26, 2022.
“We were eager to understand it, especially since it’s the only planet we know for sure going from habitable to uninhabitable.”
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Kite is the first author of a new study examining river tracks on Mars. They want to uncover the history of the red planet’s water and atmosphere.
Previously, many scientists assumed that the cause of the water loss was because Mars lost carbon dioxide from its atmosphere. That carbon dioxide has been helping Mars warm enough to retain water. But the new findings, published Wednesday, May 25, 2022 in Science Advances, suggest the changes were caused by the loss of several other important ingredients that keep the planet warm enough to allow water to flow. But we still don’t know what it is.
In 1972, scientists were astonished to see images from NASA’s Mariner 9 mission as it circled Mars from orbit. The photos reveal a landscape full of riverbeds, evidence that the planet once had a lot of liquid water, although it is now dry as bone.
Mars does not have tectonic plates that can shift and bury rock over time. Because of that, the traces of the ancient river still lay on the surface like a hastily abandoned piece of evidence.
The trail’s existence allowed Kite and his collaborators, including University of Chicago graduate student Bowen Fan as well as scientists from the Smithsonian Institution, Planetary Science Institute, California Institute of Technology Jet Propulsion Laboratory, and Aeolis Research, to analyze the map based on thousands of images taken by satellites from orbit. The team reconstructed a timeline of how river activity changed in altitude and latitude over billions of years. Those lines are then combined with simulations of different climatic conditions, and see which one works best.
The planet’s climate is very complex, with many variables to take into account, especially if you are to keep the planet in the ‘Goldilocks’ zone warm enough for liquid water, but not too hot to boil. Heat can come from the sun, but it must be close enough to receive the radiation, but not so close that the radiation removes the atmosphere. Greenhouse gases, such as carbon dioxide and methane, can trap heat near the planet’s surface.
Water itself also plays a role, namely as clouds in the atmosphere or as snow and ice on the surface. Snow tends to act as a mirror to reflect sunlight back into space, but clouds can trap or reflect light, depending on altitude and composition.
Kite and his colleagues tried many different combinations of these factors in research simulations. They are looking for the right conditions, which could cause Mars to be warm enough for at least some of the liquid water present in the rivers to last for more than billions of years, but then suddenly disappear.
But when they compared the different simulations, they noticed something surprising: changing the amount of carbon dioxide in the atmosphere didn’t affect the results. That is, the driving force for change does not appear to be carbon dioxide, as scientists have long assumed.
“Carbon dioxide is a strong greenhouse gas, so it’s really a prime candidate to explain the drying of Mars,” Kite said. “But these results show that drying (Mars) is not that simple.”
There are several alternative options. This new evidence fits a scenario, suggested in a 2021 study from Kite, in which a thin layer of ice clouds high in the Martian atmosphere acts like a see-through greenhouse glass, trapping heat. Other scientists have suggested if hydrogen were released from the planet’s interior, it could interact with carbon dioxide in the atmosphere to absorb infrared light and warm the planet.
“We don’t know what these factors are, but we need a lot of factors to explain the results,” Kite said.
There are a number of ways to try to narrow down the possible factors. The team suggested several tests involving NASA’s Perseverance rover. Robots roaming Mars could do something that could reveal clues.
Kite and partner Sasha Warren are also part of the science team that will direct another Mars rover, NASA’s Curiosity, to look for clues to why Mars is drying up. They hope this effort, as well as measurements from Perseverance, can provide additional clues to the puzzle.
On Earth, many forces have combined to keep the planet’s conditions extremely stable for millions of years. But other planets may not be as lucky as our beloved Earth. One of the many questions scientists ask is how many planets are as lucky as Earth in the universe.
They hope that studying what happens to other planets, such as Mars, can yield clues about the climate and how many other planets out there are habitable. “It’s surprising that we have this puzzle on the other side, but we’re still not quite sure how to explain it,” Kite said.
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