Why is Mars dried out? New study points to unusual answers – Verve times

Billions of years ago, a river flowed across this scene in a Martian valley called Mawrth Vallis. A new study examines the traces of Martian rivers to see what they can reveal about the history of the planet’s water and atmosphere. Credit: NASA/JPL Caltech/University of Arizona

Mars once ran red with rivers. The telltale traces of rivers, streams and lakes of the past are visible all over the planet today. But about three billion years ago, they all dried up — and no one knows why.

“People have put forward different ideas, but we’re not sure why the climate has changed so drastically,” said University of Chicago geophysical scientist Edwin Kite. “We’d really like to understand it, especially since it’s the only planet that we know for sure has changed from habitable to uninhabitable.”

Kite is the lead author of a new study examining the traces of Martian rivers to see what they can reveal about the history of the planet’s water and atmosphere.

Previously, many scientists had assumed that the loss of carbon dioxide from the atmosphere, which helped keep Mars warm, was causing the problems. But the new findings, published May 25 in scientific progresssuggest the change was caused by the loss of another key ingredient that kept the planet warm enough for running water.

But we still don’t know what it is.

Water, water everywhere and not a drop to drink

In 1972, scientists were amazed to see photos of NASA’s Mariner 9 mission as it orbited Mars. The photos revealed a landscape full of riverbeds — proof that the planet once had plenty of liquid water, even though it’s as dry as a bone today.

Because Mars has no tectonic plates to shift and bury the rock over time, ancient river tracks still lie on the surface as evidence quickly left behind.

This allowed Kite and his collaborators, including University of Chicago graduate student Bowen Fan and scientists from the Smithsonian Institution, Planetary Science Institute, California Institute of Technology Jet Propulsion Laboratory and Aeolis Research, to analyze maps based on thousands of photos taken from orbit. by satellites. Based on which tracks overlap and how weathered they are, the team put together a timeline of how river activity changed in altitude and latitude over billions of years.

Then they could combine that with simulations of different climate conditions, and see which ones fit together best.

For years, researchers have debated whether Mars ever had enough water to form an ocean, as shown in this concept illustration. Credit: NASA/GSFC

Planetary climates are hugely complex, with many, many variables to take into account – especially if you want to keep your planet in the “Goldilocks” zone, where it’s just warm enough to liquefy water, but not so hot that it cooks. Heat can come from a planet’s sun, but it must be close enough to receive radiation, but not so close that the radiation takes away from the atmosphere. Greenhouse gases, such as carbon dioxide and methane, can trap heat near a planet’s surface. Water itself also plays a role; it can exist as clouds in the atmosphere or as snow and ice on the surface. Snowcaps tend to act as a mirror to reflect sunlight back into space, but clouds can either trap or reflect light away, depending on their height and composition.

Kite and his collaborators used many different combinations of these factors in their simulations, looking for conditions that could make the planet warm enough to allow at least some liquid water to exist in rivers for over billion years, but then abruptly turn it off. losses.

But when they compared different simulations, they saw something surprising. Changing the amount of carbon dioxide in the atmosphere did not change the outcome. That is, the driving force behind the change did not appear to be carbon dioxide.

“Carbon dioxide is a potent greenhouse gas, so it was really the leading candidate to explain the drying out of Mars,” said Kite, an expert on other worlds’ climates. “But these results suggest it’s not that simple.”

There are several alternative options. The new evidence fits nicely with a scenario suggested in a 2021 study by Kite, where a layer of thin, icy clouds high in Mars’ atmosphere acts like translucent greenhouse glass, trapping heat. Other scientists have suggested that 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 this factor is, but we must have had a lot of it to explain the results,” Kite said.

There are a number of ways to try and narrow down the possible factors; the team suggests several possible tests NASA’s Perseverance rover could conduct that could reveal clues.

Kite and colleague Sasha Warren are also part of the scientific team that will lead NASA’s Curiosity Mars rover to look for clues as to why Mars dried up. They hope these efforts, as well as measurements of Perseverance, could provide additional clues to the puzzle.

On Earth, many forces have united to keep conditions remarkably stable for millions of years. But other planets may not be so lucky. One of the many questions scientists have about other planets is just how lucky we are — that is, how often this confluence occurs in the universe. They hope that studying what happened to other planets, such as Mars, can provide clues about planetary climates and how many other planets are habitable there.

“It’s really striking that we have this puzzle next door, and yet we still don’t know how to explain it,” Kite said.

Icy clouds could have kept early Mars warm enough for rivers and lakes, study finds

More information:

Edwin S. Kite et al, Changing Spatial Distribution of Water Flow Diagrams Major Change in the Mars Greenhouse Effect, scientific progress (2022). DOI: 10.1126/sciaadv.abo5894

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Why is Mars dried out? New study points to unusual answers (2022, May 26)
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