This artist’s image shows an ocean of liquid water on Mars, as the planet may have appeared about 4 billion years ago. Credit: ESO/M. Kornmesser
Mars is strikingly similar to Earth in many ways, but especially in its surface features, which often resemble Earth’s deserts uncannily. Both Earth and Mars share features such as valleys; canyon; fan washes of sand and rock; and long, twisting gravel ridges called eskers. They are all formed by the flow of water, which marks the surface over millennia and remains long after the water has disappeared.
The puzzle of Mars isn’t how these features formed: Scientists know it was moving water. But understanding how and when Mars could contain such large quantities of liquid water is the question that has baffled them for decades.
Peter Buhler, a researcher at the Planetary Science Institute, modeled a new hypothesis: that carbon dioxide condensed from Mars’ atmosphere into a glacier nearly 0.4 miles (0.6 kilometers) high, choking the ice glaciers of water even larger on its surface. and causing them to dissolve into rivers thousands of miles long. He published his research Nov. 1 in the Journal of Geophysical Research: Planets.
Geological seasons
Buhler’s model attempts to address a gaping hole in Martian history: What caused the planet to warm enough to melt enough water to form the numerous large river features that crisscross its surface to this day?
“The current best guess is that there was an unspecified global warming event,” Buhler said in a press release, “but this was an unsatisfactory answer to me, because we don’t know what would have caused that warming.”
Instead of climate warming, Buhler’s model is based on a cycle that scientists believe is still happening on Mars today, caused by the slow drift of the planet’s 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 oscillates to a much greater extent than Earth: some studies have shown that it can oscillate completely from vertical (0°) to 80°, with its poles almost pointing towards the Sun.
This oscillation drives a carbon dioxide cycle, like seasonal changes but on a geologically long time scale. When equatorial regolith is baked by the Sun, carbon dioxide evaporates into the atmosphere, where it cools and then condenses as ice near the poles, above water ice sheets.
When the poles are heated more directly by the Sun, the carbon dioxide ice sublimates back into the atmosphere. The regolith near the equator then absorbs carbon dioxide until the next cycle. Currently, much of Mars’ carbon dioxide is stored in the regolith, which coats each grain of rock in a layer just one molecule thick.
Buhler looked at how this cycle would have worked earlier in Mars’ history, around 3.6 billion years ago. At the time, the planet had a thicker atmosphere containing even more carbon dioxide – and it’s also when scientists think many of the features of Mars’ rivers appeared.
Glaciers melting glaciers
The model shows that in this ancient environment, warming of the equatorial regions causes carbon dioxide to condense into a layer 0.4 miles (0.6 km) thick at the poles, on top of a 2-inch-thick water ice sheet. .5 miles (4 km) – about the size of the current Martian ice cap at the South Pole.
The dense carbon dioxide puts pressure on the water ice sheet and insulates it, trapping heat from below and causing the water ice below to melt. Once the ground directly beneath the ice sheet is saturated, water must escape, and so it does – in rivers that stretched for thousands of kilometers, which filled and eventually overflowed a feature called the Argyre Basin (roughly the size of the Mediterranean Sea), and was eventually swept away about 5,000 miles (8,000 km) away.
Buhler estimates that this process repeated itself several times over about 100 million years, to form the varied riverine terrain seen on the surface of Mars today.
“This is the first model to produce enough water to overrun 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 last times.” climate warming in a single phase”.
While only a time machine could tell us for sure what caused Mars’ ancient rivers, Buhler’s model provides a new explanation using cycles still present on Mars today, offering a new way to think about the history of our nearest neighbor, so similar and so different from ours. Own.
