Early Mars had rivers, but the cause of the transition from a wet planet to dry remains unknown. Past Martian climate can be probed using the distribution of climate-sensitive landforms. In new research, University of Chicago scientist Edwin Kite and colleagues analyzed global databases of Martian water-worked landforms to see what they can reveal about the history of the planet’s water and atmosphere.
Billions of years ago, a river flowed across the Mawrth Vallis valley on Mars. Image credit: NASA / JPL Caltech / University of Arizona.
In 1972, planetary scientists were astonished to see pictures from NASA’s Mariner 9 mission as it circled Mars from orbit.
The photos revealed a landscape full of riverbeds — evidence that the planet once had plenty of liquid water, even though it’s dry as a bone today.
Since Mars doesn’t have tectonic plates to shift and bury the rock over time, ancient river tracks still lie on the surface like evidence abandoned in a hurry.
This allowed Dr. Kite and co-authors to analyze maps based on thousands of pictures taken from orbit by satellites.
Based on which tracks overlap which, and how weathered they are, the researchers pieced together a timeline of how river activity changed in elevation and latitude over billions of years.
Then they could combine that with simulations of different climate conditions, and see which matched best.
Planetary climates are enormously complex, with many, many variables to account for — especially if you want to keep your planet in the habitable zone.
Heat can come from a planet’s parent star, but it has to be near enough to receive radiation but not so near that the radiation strips away the atmosphere.
Greenhouse gases, such as carbon dioxide and methane, can trap heat near a planet’s surface.
Water itself plays a role, too; 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 away sunlight back into space, but clouds can either trap or reflect away light, depending on their height and composition.
The study authors ran many different combinations of these factors in their simulations, looking for conditions that could cause the planet to be warm enough for at least some liquid water to exist in rivers for more than billion years — but then abruptly lose it.
But as they compared different simulations, they saw something surprising: changing the amount of carbon dioxide in the atmosphere didn’t change the outcome; that is, the driving force of the change didn’t seem to be carbon dioxide.
“Carbon dioxide is a strong greenhouse gas, so it really was the leading candidate to explain the drying out of Mars. But these results suggest it’s not so simple,” Dr. Kite said.
The new evidence fits nicely with a scenario, suggested in a 2021 study, 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 was released from the planet’s interior, it could have interacted with carbon dioxide in the atmosphere to absorb infrared light and warm the planet.
“We don’t know what this factor is, but we need a lot of it to have existed to explain the results,” Dr. Kite said.