On Earth, many forces have combined to keep the conditions remarkably stable for millions of years. But other planets may not be so lucky. One of the scientists’ many questions about other planets is exactly how lucky we are—that is, how often this confluence exists in the universe.
Studying what happened to other planets, such as Mars, can yield clues about planetary climates and how many other planets out there might be habitable.
Mars is the only world whose surface is known to have become uninhabitable. Mars’s climate 3.6 to 3.0 billion years ago was at least occasionally warm enough for rivers and lakes. We may infer it as a habitable climate. But the surface today is a cold desert. Few constraints exist on Mars’s atmospheric greenhouse effect during the wet-to-dry transition.
Was early Mars temperate or icy, was the environmental catastrophe abrupt or gradual, and what caused the change?
The prevailing view is that the cause of Mars’s drying-out was loss of atmospheric CO2. Mars’s atmosphere today is so thin that it is close to the triple point of water, so lakes on early Mars probably formed under a thicker atmosphere. CO2 is, in the modern inner solar system, a key greenhouse gas for regulating climate change.
But the new research findings, published on May 25, 2022, in the journal Science Advances, suggest that the change was caused by the loss of some other important ingredient that maintained the planet warm enough for running water.
“People have put forward different ideas, but we’re not sure what caused the climate to change so dramatically,” said University of Chicago geophysical scientist Edwin Kite. “We’d really like to understand, especially because it’s the only planet we definitely know changed from habitable to uninhabitable.”
Kite is the first author of a new research study that examines the tracks of Martian rivers to see what they can reveal about the history of the planet’s water and atmosphere.
“However, even when H2O vapor feedback is considered, additional non-CO2 warming is needed to warm early Mars enough for rivers. Therefore, changes in non-CO2 radiative forcing are an alternative explanation for Mars’s wet-to-dry transition. The relative importance of these mechanisms has not been investigated, so the prevailing explanation of the wet-to-dry transition remains untested.” Study mentions.
From being water everywhere to not a single drop of water: What made researchers analyze?
In 1972, 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 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 maps based on thousands of pictures taken from orbit by satellites. Based on which tracks overlap which, and how weathered they are, the team 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 variables to account for.
Heat can come from a planet’s sun, 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.
Kite and his collaborators 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 a billion year and then abruptly lose it.
After comparing 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,” said Kite, an expert on the climates of other worlds. “But these results suggest it’s not so simple.”
There could be various possibilities that study summarizes.
“Our analysis suggests that the shifts in river distribution were driven by loss of non-CO2 radiative forcing.The subsequent cessation of river-forming climates on Mars could have been caused by further reduction in non-CO2 greenhouse warming, by H2O loss, or by C loss. The present-day rate of C escape to space is small, and isotopic evidence indicates that most of Mars’s atmosphere was lost >3.5 Ga ago. Candidate carbon sinks include escape to space, carbonate formation, and basal melting of CO2 ice. Alternatively, CO2 could have been reversibly sequestered as CO2 ice.” Study mentions.
“We don’t know what this factor is, but we need a lot of it to have existed to explain the results,” Kite said.
There are a number of ways to try to narrow down the possible factors; the team suggests several possible tests for NASA’s Perseverance rover to perform that could reveal clues.
Kite and colleague Sasha Warren are also part of the science team that will be directing NASA’s Curiosity Mars rover to search for clues about why Mars dried out. They hope that these efforts, as well as measurements from Perseverance, can provide additional clues to solve the mystery.
“It’s really striking that we have this puzzle right next door, and yet we’re still not sure how to explain it,” said Kite.