How water flowed on an icy Mars

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On a cold, ancient Mars, rivers flowed and a lake the size of the Mediterranean Sea swelled under the protection of thick ice ceilings, according to new research published in the Journal of Geophysical Research: Planets.

The paper, led by Planetary Science Institute Research Scientist Peter Buhler, describes how 3.6 billion years ago, carbon dioxide froze out of Mars’ atmosphere and deposited on top of a water ice sheet at the poles, insulating heat emanating from Mars’ interior and increasing the pressure on the ice. This caused roughly half of Mars’ total water inventory to melt and flow across its surface without the need for climatic warming.

Buhler’s past work has focused on modeling the modern carbon dioxide cycle on Mars. Recently, he extended his model to investigate the exchange of carbon dioxide with the Martian regolith—or sand and rocks. By doing so, his model encapsulates the full carbon dioxide cycle from the regolith, to atmosphere, to frozen polar caps.

For this study, he applied the model to a pivotal era in Mars’ history.

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

Mars in motion

Scientists have known since the 1970s that much of Mars’ carbon dioxide is currently bound in the regolith in single-molecule-thick layers around each grain.

So when Buhler incorporated regolith into his model, he found that “the atmosphere is mostly just along for the ride,” he said. “It acts as a conduit for the real action, which is the exchange between the regolith and the southern polar ice cap, even today.”

The cycle is controlled by the degree of Mars’ rotational tilt, which slowly shifts back and forth every 100,000 Martian years.

When Mars spins nearly straight up and down, the poles don’t receive much direct sunlight, while the sun bakes the equator. Under these conditions, the carbon dioxide gas escapes the regolith into the atmosphere. When it reaches the frigid poles, it deposits atop the water ice cap.

Conversely, when Mars is tilted dramatically, the sun easily heats the poles. As a result, the carbon dioxide ice sublimes—or directly transforms from solid ice to gas—into the atmosphere where the cooler regolith can then “soak it back up like a sponge,” Buhler said.

This model works well for modern-day Mars, so Buhler wanted to test how it would perform during a time when the planet had a much thicker carbon dioxide atmosphere—about 3.6 billion years ago. This is when scientists think Mars’ atmosphere first began to collapse, and the current day carbon dioxide cycle Buhler described began to function.

There’s also evidence that this era also coincides with the origin of many river valley networks, but scientists still don’t agree on the climate conditions that would explain their formation.

In Buhler’s model, a 0.4-mile-thick layer of carbon dioxide deposits on top of a 2.5-mile-thick layer of water ice, a layer of water ice about as thick as that which exists on the south pole today. The carbon dioxide ice acts as a powerful insulator, trapping heat radiating from the planet’s hot interior below. It also adds weight and pressure on top of the water ice cap.

Together, these conditions melt massive amounts of water from the base of the ice cap, he found.

The meltwater then saturates the Martian crust out to the sides of the ice sheet. There, the underground water tries to escape, but instead freezes as permafrost.

“You now have the cap on top, a saturated water table underneath and permafrost on the sides,” Buhler said. “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. These subglacial rivers leave behind long gravel ridges, called eskers. Scientists have observed many eskers near the south pole, with sizes 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 said. “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.”

Once the subglacial rivers reach the edge of the ice sheet, they encounter the cold atmosphere and initially form oozing flows, like slow-moving lava covered by a frozen skin. Eventually, these ice-encrusted oozes inflate with enough water that they become proper ice-topped rivers.

Buhler predicts that the ice covering the rivers should reach tens to hundreds of feet thick and cap a few-foot-deep river with water flowing through at a few feet per second, with enough meltwater to reach lengths up to thousands of miles long.

There are several long, sinuous valleys leading downhill from the south polar, esker-rich region into Argyre Basin that have previously been identified as ancient river channels, consistent with his model’s prediction.

The ice-covered water then fills Argyre Basin, which is about the volume of the Mediterranean Sea, over tens of thousands of years before it overflows and empties nearly 5,000 miles away in the northern plains, Buhler said. The process likely happened multiple times, millions of years apart during a one-hundred-million-year era.

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

“It’s also likely that the meltwater, once downstream, sublimated 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.”

Next, Buhler plans future tests of his model. If the results continue to hold, they will dramatically change our understanding of Mars’ ancient water cycle, he said.