How water flowed on ancient Mars

New research describes how sheets of carbon dioxide ice protected rivers of water flowing into a lake the size of the Mediterranean Sea on ancient Mars.

The study, published in the Journal of Geophysical Research: Planets explains how an icy, cold Mars could have had flowing water more than 3 billion years ago.

“This model describes the origins of major landscape features on Mars – like the biggest lake, the biggest valleys and the biggest esker system (remnants of rivers that once flowed beneath an ice sheet) – in a self-consistent way,” says author Peter Buhler from the US Planetary Science Institute (PSI).

“And it’s only relying on a process that we see already today, which is just carbon dioxide collapsing from the atmosphere.”

Buhler’s model shows that carbon dioxide froze out of the Martian atmosphere 3.6 billion years ago. This carbon dioxide ice formed a layer on top of water ice sheet at the poles.

Heat emanating from Mars’s still active interior was insulated by the carbon dioxide ice, warming up the water ice causing about half of it to melt and flow across the Red Planet’s ancient surface.

This explanation for Mars’s ancient water flows removes the need for a yet undiscovered process of climatic warming.

Buhler’s paper emerged out of past work modelling the carbon dioxide cycle on Mars today.

Mars’s atmosphere is 95% carbon dioxide. But in the 1970s scientists worked out that most of the carbon dioxide on the planet was actually stored in the regolith – rock – on the Martian surface. The carbon dioxide forms a single-molecule thick coating around grains of sand and dust.

Buhler’s model shows that “the atmosphere is mostly just along for the ride”.

“It acts as a conduit for the real action, which is the exchange between the regolith and the southern polar ice cap, even today,” he says.

His model shows that a 650-metre-thick layer of carbon dioxide ice sat atop a 4-kilometre-thick layer of water ice 3.6 billion years ago. When the insulated water ice melted, it saturated the Martian crust under the carbon dioxide cap. It froze as permafrost.

“You now have the cap on top, a saturated water table underneath and permafrost on the sides,” Buhler explains. “The only way left for the water to go is through the interface between the ice sheet and the rock underneath it. That’s why on Earth you see rivers come out from underneath glaciers instead of just draining into the ground.”

This meltwater forms rivers at the base of the ice sheet and leave behind ridges called eskers. There are many eskers near Mars’s south pole. Their sizes are consistent with the subglacial rivers predicted by Buhler’s model.

“Eskers are evidence that at some point there was subglacial melt on Mars, and that’s a big mystery,” Buhler says. “People have been trying to discover processes that could make that happen, but nothing really worked. The current best hypothesis is that there was some unspecified global warming event, but that was an unsatisfying answer to me, because we don’t know what would have caused that warming. This model explains eskers without invoking climatic warming.”

The meltwater predicted by Buhler’s model then filled the Argyre Basin – a region the size of the Mediterranean Sea – before overflowing and emptying into the Martian northern plains.

“This is the first model that produces enough water to overtop Argyre, consistent with decades-old geologic observations,” Buhler says.

“It’s also likely that the meltwater, once downstream, sublimated (turned from solid to gas) back into the atmosphere before being returned to the south polar cap, perpetuating a pole-to-equator hydrologic cycle that may have played an important role in Mars’ enigmatic pulse of late-stage hydrologic activity. What’s more, it does not require late-stage warming to explain it.”