Ancient river channels on Mars may have formed beneath thick ice sheets, scientists say.
It’s an important find, because many have long wondered why Mars is covered with tens of thousands of ancient river valleys, even though climate models have a hard time explaining how it could have been warm enough for flowing water.
“It’s farther from the Sun than the Earth,” says Anna Grau Galofre, a physicist at Arizona State University and lead author of a paper in the journal Nature Geoscience. And, in the early history of Mars, when these valleys formed, the young Sun was less intense. “It’s really hard to understand how Mars could not have been frozen,” she says.
The result has been a 40-year debate about what Mars was like 3.8 billion or so years ago, when life was first emerging on Earth. “Are we thinking about New York, or Antarctica?” Grau Galofre asks. “Was Mars warm and wet – meaning rainfall, oceans, and seas – or a frozen planet which from time to time got a hot summer day, melted, and released a lot of water?”
“The two arguments are really strong, and really valid,” she says, adding that it’s very hard to “marry” the clear evidence from numerous streambeds with the climate models.
Victor Baker, a planetary scientist at the University of Arizona who has been studying Martian geology for more than 40 years (but was not part of the study team), notes that the controversy has been described as “Mars at War”.
“As one of the leaders in the modelling community, Robin Wordsworth of Harvard University stated the problem: either something is wrong with the modelling, or Mars was never warm and wet,” he says.
For Grau Galofre, the key moment was when a co-author, Gordon Osinski of the University of Western Ontario, Canada, told her that his studies on Devon Island in the Canadian Archipelago had been turning up river channels that looked a lot like the ones on Mars.
What was interesting, she says, is that Devon Island was until recently covered in an ice cap. Now, the ice has retreated from the western half of the island, exposing the landscape once beneath it. “That’s something very rare. We realised that those channels formed under the ice.”
That, she says, was the missing piece of the Mars puzzle. Nobody had ever before considered that its stream channels could have formed beneath an ice sheet.
Armed with their studies of Devon Island and computer models of how water flows and erodes the land under various conditions, her team used high-resolution images of thousands of Martian valleys, falling into 66 large clusters. They then broke them into groups according to a number of parameters, including the complexity of the drainage network, the length of its channels, and the width of each valley’s headwaters streams.
Of particular importance was that some of the streams appear to have been flowing uphill.
Normally rivers flow downhill. But if the water is flowing beneath a glacier, it responds not only to gravity but also the pressure of the ice sheet. “Just like you squeeze toothpaste out of the tube, the ice lying on top of a subglacial channel is squeezing the water, so a river can flow uphill,” Grau Galofre says.
Based on this, her team found that the valley networks on Mars didn’t all form the same way.
Some, particularly at warmer, lower elevations, do appear to have been formed by flowing water.
But many, especially in the colder highlands, appear to have formed beneath thick ice sheets. “We have evidence that a good fraction – more than half – of the valley networks on Mars could be explained very well if you bring up the possibility that they formed under ice sheets,” Grau Galofre says.
It’s a finding that may have implications for the search for life on Mars, possibly even making it more likely that NASA’s Perseverance rover, now heading for Mars, will collect samples that when eventually returned to Earth, will show the biosignatures astrobiologists are dreaming of.
That, she says, is because the presence of subglacial channels means Mars once hosted glaciers. Large ones. And that might have been a very good for life living beneath them, she says, because they would have shielded it from intense radiation from the young Sun.
“The ice would act as a blanket.” Solar radiation and energetic particles from a young and active Sun would be blocked – even if Mars never had the type of magnetic field that provides the same protection to the Earth. “You don’t need a magnetic field.”
The thick layer of ice could also have provided a steady stream of meltwater at its base, depending on the degree to which its underside was being heated by energy escaping from the Martian interior.
And, it would have been at a fairly steady temperature, protected from the day/night, summer/winter swings surface life would have had to endure. “It could provide a very stable environment,” Grau Galofre says.
Baker applauds the new paper for using a “very impressive” statistical approach. But, he notes, an unresolved question is whether the Martian riverbeds are channels or valleys.
“Channels form when the water fills the entire trough,” he says, noting that this means they can form in short, brief floods. “Valleys, on the other hand, have a small river or stream that gradually lowers the valley floor over relatively long periods of time.”
It sounds like an arcane distinction, but it might spell the distinction between conditions that continued in a stable manner for long periods of time, or were brief and intermittent.
Meanwhile, he says, the three missions now heading for Mars, especially Perseverance, might help fill in the missing links.
The Perseverance landing will not occur in a valley network, so it will not be able to collect direct evidence about how those formed. But, he says, Perseverance is tasked with looking for other indications of whether Mars was warm and wet or cold and dry.
In other words, just wait for 18 February when the rover is scheduled to parachute to the Martian surface to begin its explorations. The only thing that’s certain is that, assuming it lands safely, it’s likely to find even more things previously undreamed of by Mars scientists.
Richard A Lovett
Richard A Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to Cosmos.
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