Today, Mars is a desiccated world, but missions to the Red Planet have suggested that it may have had liquid water early in its lifetime. Whether that water ran freely or was locked up in ice remains a subject of debate.
According to new research, the answer may be both. “If the early Martian climate was warm enough to cause some glacial ice melting, meltwater could create river systems in the downstream regions,” said Arihiro Kamada, a planetary scientist at Japan’s Tohoku University and first author on the new research, which was published in the journal Icarus.
Kamada and his team created a global atmospheric model of Mars during the Noachian period, roughly 4 billion years ago. At the time, water could have been trapped in ice sheets a few kilometers thick. As Mars warmed with the changing seasons, the base of those glaciers could have melted, their water draining away and carving intricate valley networks.
“Solving how and why surface water flowed when mean surface temperatures were most likely below freezing for the majority of the year is the key question for understanding Mars’ climate evolution and, by extension, the climates of other habitable terrestrial planets,” said Bethany Ehlmann of the California Institute of Technology, who was not involved in the research.
Melting Glaciers, Carving Valleys
Hundreds of valleys crisscross the Martian southern highlands. Only a few kilometers across on average, the narrow channels weave together, bearing a strong resemblance to features cut by flowing water on Earth. In the decades since their discovery, the valley networks have provided some of the strongest evidence that liquid water once streamed across the Martian surface. Other hints of water on the Red Planet have also emerged, including signs of oceans and suggestions of clays and silicas that required water to form.
Today, Mars has a thin atmosphere and frigid temperature that cannot support liquid surface water. Polar ice caps trap some liquid, and subsurface water may occasionally work its way through the crust, only to be immediately sublimated into gas at the surface.
Scientists think that the Martian atmosphere was thicker and possibly able to support liquid water in the past, but “once you start building ice sheets, it’s actually really difficult to turn them off and stop building them,” said Anna Grau Galofre, a researcher at French National Centre for Scientific Research who was not involved in the new study. The new model “ties in quite nicely” with Grau Galofre’s previous work analyzing the geomorphology and characteristics of the Martian valley network, she said.
The new research is not without criticism. For Ramses Ramirez, who simulates Mars atmospheres at the University of Central Florida, one of the biggest problems is highlighted by the authors themselves—the predicted valley networks don’t line up well with the actual observed valleys.
“I don’t see good agreement [between the results and observations] even in the best case,” Ramirez said.
Kamada and his colleagues note that this discrepancy may be due to other processes forming some of the Martian valleys. Glacier melting could have created a significant number of the valleys, they conclude, whereas forces like volcanic eruptions and meteoritic impacts could have played a hand in forming others.
Grau Galofre pointed out that the new study compared its results to all of the observed valley networks, rather than focusing on those thought to have formed during the Noachian period. “They’re trying to model the early climate but comparing it to the entire history,” she said. She suspects that narrowing down the time frame for the observed tributaries would make a stronger case for glacial runoff.
Ramirez also pointed out that the ice sheets should have created glacial features in the southern highlands, features he said that are not present. “It doesn’t add up with the geologic evidence that we see,” he said.
But Grau Galofre’s past research did identify potential glacial features in the valley networks, leading her team to conclude that there was evidence for ice sheets having formed some of the branches. Other researchers argue those features were made by flowing water rather than inching ice.
The new research adds more data to the ongoing schism in Martian science, where some scientists fall firmly on the side of a semiarid Red Planet with occasional rainfall and others stand strongly with a cold Mars of ice and snow. Some have referred to the disconnect as a war, though the researchers involved quickly noted that it is more a case of healthy debate than outright hostility.
Although the idea of a cool and wet Mars initially appears to bridge the gap, Ramirez isn’t convinced the new research offers conclusive evidence. “I don’t think [the debate] has really moved much at all,” he said. “I think people are pretty set in one way or the other.”
Grau Galofre is more optimistic.
“This is a really novel and intricate approach,” she said. “Probably the climate of early Mars was not one extreme or the other but somewhere in between.”
—Nola Taylor Tillman (@Nola_T_Tillman), Science Writer