We’ve long known that Mars has water trapped in its polar ice caps, but just how wet the planet was billions of years ago, and what that means for supporting life, is still a hot topic. The predominant theory is that Mars was once a warm and wet planet, but a new study has effectively thrown cold water on that, instead positing early Mars was covered in ice sheets, not flowing rivers.
The warm and wet ancient Mars hypothesis suggests that rivers, oceans, and rainfall once existed on Mars. One of the main pieces of evidence put forward is the vast network of valleys in Mars’ southern hemisphere. These look very similar to Earth’s river drainage basins, which are carved by surface water runoff from rain, sleet, and snow forming rivers as they run downslope from nearby mountains.
Mars’s valleys formed around 3.5 to 3.9 billion years ago. If they were indeed carved by flowing rivers, this means that Mars’s climate back then was warm and wet, which increases the chance of it supporting life. However, a new study in Nature Geoscience, led by Anna Grau Galofre from the University of British Columbia, claims that on closer inspection these valleys have more in common with subglacial channels in the Canadian Arctic and were instead carved by water melting beneath glacial ice, not rivers.
"For the last 40 years, since Mars's valleys were first discovered, the assumption was that rivers once flowed on Mars, eroding and originating all of these valleys," explained Grau Galofre in a statement. "But there are hundreds of valleys on Mars, and they look very different from each other. If you look at Earth from a satellite you see a lot of valleys: some of them made by rivers, some made by glaciers, some made by other processes, and each type has a distinctive shape. Mars is similar, in that valleys look very different from each other, suggesting that many processes were at play to carve them."
It was the similarities between many of these Martian valleys and the subglacial channels on Devon Island in Nanuvet, the most northerly territory in Canada, that led Grau Galofre and colleagues to explore this possible explanation.
"Devon Island is one of the best analogs we have for Mars here on Earth – it is a cold, dry, polar desert, and the glaciation is largely cold-based," said co-author Dr Gordon Osinski, a professor in the department of earth sciences and the Institute for Earth and Space Exploration at Western University.
The team analyzed the shape of more than 10,000 Martian valleys using data gathered from the Mars Global Surveyor, which surveyed the planet between 1996 and 2006. One of its instruments, the Mars Orbiter Laser Altimeter, measured the heights of the planet’s surface features by shooting infrared laser pulses at it. By looking at the physical features of the valleys, Grau Galofre determined that though some valleys had features that suggested they were carved by running rivers, many looked like they had been carved by glaciers too, suggesting vast ice sheets covered the planet at some point.
This finding adds weight to the “cold and icy ancient Mars” theory. "Climate modeling predicts that Mars' ancient climate was much cooler during the time of valley network formation," Grau Galofre said. "We tried to put everything together and bring up a hypothesis that hadn't really been considered: that channels and valleys networks can form under ice sheets, as part of the drainage system that forms naturally under an ice sheet when there's water accumulated at the base."
More recently, the idea that Mars started off with a warm and wet climate that gradually got colder has gained traction, which would also explain the valleys formed by both rivers and glaciers on Mars, we just don't know what exactly might have changed the climate billions of years ago. However, the formation of these valleys coincides with the loss of Mars' magnetic field some 3.7 billion years ago.