Why did Mars lose its water? A new study finds strange answers

Billions of years ago, a river flowed across this scene in a Mars valley called Mawrth Vallis. A new study examines the tracks 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 used to have red rivers. Today, you can see where rivers, streams, and lakes used to be all over the world. But they all stopped growing about three billion years ago, and no one knows why.

Edwin Kite, a geophysicist at the University of Chicago, said, “People have put forward different ideas, but we don’t know what caused the climate to change so drastically.” “We’d really like to know, especially since it’s the only planet we know for sure went from being able to be lived on to not being able to be lived on.”

Kite is the first author of a new study that looks at the paths of rivers on Mars to find out what they can tell us about the planet’s water and atmosphere in the past.

Before, many scientists thought that the trouble was caused by the loss of carbon dioxide from Mars’s atmosphere. Carbon dioxide helped keep Mars warm. But the new findings, which were published on May 25 in Science Advances, suggest that the change was caused by the loss of some other important ingredient that kept the planet warm enough for running water.

We don’t know what it is yet.

There was water everywhere, but no one could drink it.

In 1972, when NASA’s Mariner 9 mission took pictures of Mars from space, scientists were shocked by what they saw. The photos showed that the landscape was full of riverbeds, which is proof that the planet used to have a lot of liquid water even though it is very dry now.

Since Mars doesn’t have tectonic plates that move and bury rocks over time, old river tracks are still on the surface, looking like they were left quickly.

This made it possible for Kite and his team, which included Bowen Fan, a graduate student at the University of Chicago, as well as scientists from the Smithsonian Institution, the Planetary Science Institute, the Jet Propulsion Laboratory at the California Institute of Technology, and Aeolis Research, to look at maps made from thousands of pictures taken by satellites in orbit. The team put together a timeline of how river activity changed in elevation and latitude over billions of years by looking at which tracks overlap and how worn down they are.

Then, they could put that information together with simulations of different climates to see which one fit best.

Planetary climates are very complicated and depend on a lot of different factors. This is especially true if you want your planet to stay in the “Goldilocks zone,” where it’s warm enough for water to be liquid but not so hot that it boils. A planet’s sun can give off heat, but it has to be close enough to get radiation but not so close that the radiation destroys the atmosphere. Greenhouse gases, like carbon dioxide and methane, can hold heat close to the surface of a planet. Water plays a part, too. It can be in the form of clouds in the sky or snow and ice on the ground. Snowcaps tend to act like mirrors, reflecting sunlight back into space. Clouds, on the other hand, can either absorb or reflect light, depending on how high they are and what they are made of.

Kite and his colleagues ran many different simulations with different combinations of these factors to try to find a way for the planet to stay warm enough for liquid water to flow in rivers for more than a billion years, but then suddenly stop.

But when they looked at the different simulations side by side, they noticed something surprising. The outcome didn’t change when the amount of carbon dioxide in the air was changed. That is, carbon dioxide didn’t seem to be the cause of the change.

“Carbon dioxide is a strong greenhouse gas, so it was the most likely cause of Mars’s drying out,” Kite, an expert on the climates of other worlds, said. “But the results show that it’s not that easy.”

There are a number of other choices. The new evidence fits well with a theory from a Kite study from 2021, which says that a thin layer of icy clouds high in Mars’ atmosphere acts like clear greenhouse glass and traps heat. Other scientists have said that if hydrogen was released from the planet’s interior, it could have reacted with carbon dioxide in the atmosphere to soak up infrared light and warm the planet.

Kite said, “We don’t know what this thing is, but we need a lot of it for the results to make sense.”

There are a number of ways to try to narrow down the possible factors. The team suggests that NASA’s Perseverance rover perform a number of tests that might give clues.

Kite and his colleague Sasha Warren are also on the science team that will guide NASA’s Curiosity Mars rover as it looks for clues about why Mars dried up. They hope that these efforts and measurements from Perseverance will help them figure out what’s going on.

Many things on Earth have worked together to keep things very stable for millions of years. But this might not be the case for other planets. Scientists have a lot of questions about other planets. One of them is how lucky we are, or how often this kind of thing happens in the universe. They hope that looking at what happened on other planets, like Mars, will help them figure out how climates work and how many other planets might be able to support life.

Kite said, “It’s amazing that this puzzle is right next door, but we still don’t know how to explain it.”

Further information: Edwin S. Kite et al, Changing spatial distribution of water flow charts major change in Mars’s greenhouse effect, Science Advances (2022). DOI: 10.1126/sciadv.abo5894

Journal information: Science Advances

Source: University of Chicago