Rather than flowing across its surface during a warm, wet phase early in its history, the water that carved the river channels on Mars may have come from beneath thick ice sheets that slowly melted under the influence of heat escaping from the planet’s interior, scientists say.
For nearly 50 years, planetary scientists have known that early in its history Mars had enough water to scour deep valleys, but how the Red Planet could have been warm enough for that to occur has been a mystery.
Astrophysicists know when these valleys were formed billions of years ago the Sun was 25-30% dimmer than today. And, given that Mars is millions of kilometres farther out from the Sun than the Earth, that raises a problem.
“The climate of the early Mars should have been extremely cold,” says Lujendra Ojha, a planetary scientist at Rutgers, The State University of New Jersey, US.
Previous theories for how enough liquid water to create these valleys could have been present have focused on the early Martian atmosphere, positing that it might have been full of planet-warming greenhouse gases like water vapour, methane or vastly more carbon dioxide than remains today.
But even if you assume such gases were present, Ojha says, it still doesn’t appear to have been enough to warm the temperate above freezing.
The result is a problem known as the faint young Sun paradox, in which the early Sun doesn’t seem to have been bright enough to produce temperate climates, but in which we can see the evidence that Mars once had lakes and rivers.
The answer, Ojha’s team suggests in a paper in the journal Science Advances, is that parts of the early Mars may have been covered with glaciers thick enough to trap substantial amounts of heat from radioactive decay in the Martian crust and mantle.
In support of this idea, Ojha points to Paralana Hot Springs in South Australia, where – even without anything on the surface to trap it – heat from the decay of high concentrations of uranium and other radioactive elements in the underlying rocks is intense enough to produce the springs.
Early Mars wasn’t South Australia but, Ojha says, it did have about four times as much radioactivity in its crust and mantle as it does today. And that, combined with glaciers, is enough to explain what we see.
Today, Mars doesn’t have a lot of water. But it does have enough in its polar ice caps that if it were redistributed as glaciers on the planet’s southern highlands, the region where river channels are seen, there would be enough to blanket them to a depth of 700 metres.
And in its youth, Mars would have had substantially more water – water that has been slowly escaping to space, ever since.
If, four billion years ago, there was only a little more than twice as much as is now contained in the ice caps, Ojha says, that would be sufficient to create glaciers thick enough that heat escaping from the planet’s interior would be able to slowly melt them from beneath.
The meltwater, flowing beneath the ice, could then have scoured the river valleys we see today—many of which, Ojha adds, actually look like the type of valleys known to have formed beneath vanished glaciers, here on Earth.
That’s not only important for understanding the processes that might have produced water (and possibly conditions for the emergence of life) on the early Mars, but also for our understanding of exoplanets.
Normally, Ojha says, we think in terms of life being possible in the “Goldilocks Zone”, where a planet is in the right range of distances from its star for liquid water to exist on its surface.
But maybe, he says, a planet can still be habitable substantially farther out – if, as may have been the case for the infant Mars, it has glaciers thick enough to trap enough heat escaping from its interior for them to melt from below. “Ice is a great insulator,” he says.