Mars is tantalizingly similar to Earth in many ways, but especially in its surface features, which often resemble Earth deserts to an eerie degree. Both Earth and Mars share features such as valleys; canyons; fanlike washes of sand and rock; and long, winding gravel ridges called eskers. All are formed by flowing water, marking the surface over millennia and remaining long after the water disappears.
The puzzle of Mars is not how these features came to be — scientists know it was moving water. But figuring out how and when Mars could hold such large amounts of liquid water is the question that has stumped them for decades.
Peter Buhler, a research scientist at the Planetary Science Institute, has modeled a new suggestion: that carbon dioxide condensed out of Mars’ atmosphere in a glacier nearly 0.4 mile (0.6 kilometer) high, smothering the even-larger water ice glaciers on its surface and causing them to melt out in rivers thousands of miles long. He published his research Nov. 1 in the Journal of Geophysical Research: Planets.
Geologic seasons
Buhler’s model attempts to solve a hole in martian history: What caused the planet to warm sufficiently to melt enough water to form the many large and varied river features that cross its surface to this day?
“The current best hypothesis is that there was some unspecified global warming event,” Buhler said in a press release, “but that was an unsatisfying answer to me, because we don’t know what would have caused that warming.”
Instead of climate warming, Buhler’s model relies on a cycle scientists believe is still happening on Mars today, caused by the slow drift of Mars’ rotational tilt. Like Earth, Mars is tilted on its axis. Currently, Mars’ tilt is 25°, similar to Earth’s 23°. But over hundreds of thousands or millions of years, Mars wobbles to a far greater extent than Earth: Some studies have shown it may swing all the way from upright (0°) to 80°, with its poles nearly pointed at the Sun.
This wobble drives a carbon dioxide cycle, like seasonal changes but on a geologically long time scale. When the equatorial regolith is baked by the Sun, carbon dioxide evaporates into the atmosphere, where it cools and then condenses as ice near the poles, on top of the water-ice caps there.
When the poles are more directly warmed by the Sun, the carbon dioxide ice there sublimates back into the atmosphere. The regolith near the equator then absorbs the carbon dioxide back until the next cycle. Currently, much of Mars’ carbon dioxide is stored in the regolith, sheathing each grain of rock in a layer just one molecule thick.
Buhler looked at how this cycle would have operated earlier in Mars’ history, some 3.6 billion years ago. At that time, the planet had a thicker atmosphere containing even more carbon dioxide — and it’s also when scientists think many of Mars’ river features appeared.
Glaciers melting glaciers
The model shows that in this ancient setting, warming the equatorial regions causes carbon dioxide to condense into a sheet 0.4 mile (0.6 km) thick at the poles, on top of a water-ice cap 2.5 miles (4 km) thick — about the same as the current martian south pole ice cap.
The thick carbon dioxide puts pressure on the water ice cap and also insulates it, trapping heat from below and causing the water ice underneath to melt. Once the ground directly beneath the ice cap is saturated, the water must escape, and so it does — in rivers that stretched thousands of miles, which filled and eventually overflowed a feature called the Argyre Basin (roughly the size of the Mediterranean Sea), and finally washed out some 5,000 miles (8,000 km) away.
Buhler estimates this process repeated several times over about 100 million years, to form the varied river terrain seen across Mars’ surface today.
“This is the first model that produces enough water to overtop Argyre, consistent with decades-old geologic observations,” Buhler said, adding that it also “demonstrates that large amounts of water can mobilize in a cold climate without invoking the fraught paradigm of late-stage climatic warming.”
While only a time machine could tell us for certain what caused Mars’ ancient rivers, Buhler’s model provides a new explanation using cycles still present on Mars today, offering a fresh way to think about our nearest neighbor’s history, so similar and so different from our own.
