Cloudy skies may have allowed water to flow on Mars long ago
Though the Red Planet is a frigid desert today, around four billion years ago it hosted relatively long-lived lakes and river systems, as observations by NASA's Curiosity rover and other Mars robots have shown.
Mars' atmosphere was much thicker back then, which helped to keep liquid surface water from boiling away. However, it remains unclear how that water could have been liquid in the first place; the sun was about 30% dimmer during that ancient epoch than it is today, so the Martian surface should seemingly have been a permanent winter wonderland.
A new study may help to solve this puzzle. A thin layer of icy clouds high in the Martian atmosphere could have provided a greenhouse effect, warming the planet for long stretches, researchers found.
"There's been an embarrassing disconnect between our evidence and our ability to explain it in terms of physics and chemistry," lead author Edwin Kite, a planetary scientist at the University of Chicago, said in a statement. "This hypothesis goes a long way toward closing that gap."
This cloud idea was first proposed nearly a decade ago but fell out of favor because "it was argued that it would only work if the clouds had implausible properties," Kite said. For example, previous modeling work suggested that water would have had to persist for unrealistically long periods in the Martian atmosphere for the clouds to do their warming work.
However, the new modeling work by Kite and his colleagues indicates that wispy but warming clouds could indeed have formed high above the planet, trapping enough heat for liquid water to persist on the surface — but only if certain conditions were met.
For example, the Martian surface had to be covered in ice only patchily, likely on mountaintops and near the planet's poles. More extensive ice cover would have favored the formation of low-altitude clouds, which aren't great at trapping heat and may even be a net negative, reflecting enough sunlight away to cool Mars down further, the researchers said.
The team's work reinforces that Mars is an alien world, with its own special processes and properties.
"In the model, these clouds behave in a very un-Earth-like way," Kite said. "Building models on Earth-based intuition just won't work, because this is not at all similar to Earth's water cycle, which moves water quickly between the atmosphere and the surface."
Still, the new research could help us better understand our own planet, as well as the history and evolution of habitable worlds more generally, he added.
"Mars is important because it's the only planet we know of that had the ability to support life — and then lost it," Kite said. "Earth's long-term climate stability is remarkable. We want to understand all the ways in which a planet's long-term climate stability can break down — and all of the ways (not just Earth's way) that it can be maintained. This quest defines the new field of comparative planetary habitability."
The new study was published online Monday (April 26) in PNAS. You can read it for free, because PNAS is an open-access journal.
Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.
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Michael Wall is a Senior Space Writer with Space.com and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.