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A “crazy-looking” crater on Mars caught the attention of scientists. A simple calculation cleared up how it got its strange shape, but also raised a few questions about past weather on Mars.

“Craters should be bowl shaped, but this one had terraces in the wall,” says Ali Bramson, a graduate student in the Lunar and Planetary Laboratory (LPL) at the University of Arizona. Terraces can form when there are layers of different materials in the planet’s subsurface, such as dirt, ice, or rock.

“When the crater is forming, the shock wave from an object hitting a planet’s surface propagates differently depending on what substrates are beneath the area of impact,” Bramson says. “If you have a weaker material in one layer, the shock wave can push out that material more easily, and the result is terracing at the interface between the weaker and stronger materials.”

‘Something weird is going on’

“It’s worth mentioning that terraced craters of this size are quite rare,” says Shane Byrne, associate professor in LPL. “But in this area of Mars (Arcadia Planitia), there are a lot of terraced craters. The craters may have formed at different times, but they all have terraces, which indicates something weird is going on in the subsurface.”

Thanks to Mars’ unstable obliquity—the degree the planet tilts on its axis—its climate changes often. Unlike Earth, Mars doesn’t have a large moon to keep it stable. Instead, it wobbles on its axis, and that wobbling leads to dramatic climatic shifts. Those shifts, in turn, have led to intermittent ice ages.

Using a camera on NASA’s Mars Reconnaissance Orbiter, researchers created 3D models of the area’s craters, which allowed them to measure the depth of their terraces. The researchers then beamed radar pulses to Mars to measure the time it took for the radar signals to penetrate the surface’s buried layers and bounce back.

They then combined the two data sets to measure the radar waves’ speed, a pivotal clue to the layers’ composition. Their findings are published in the journal Geophysical Research Letters.

In this crater’s case, the layers turned out to be ice, and lots of it. Just beneath Mars’ dirt surface, or regolith, the researchers found an enormous slab of water ice, measuring 130 feet thick and covering an area equivalent to that of California and Texas combined.

While the presence of ice wasn’t much of a surprise, its age, amount, and location were.

Snow and ice

Although scientists have known for some time about Mars’ icy deposits at its poles and have used them to look at its climatic history, knowledge of icy layers at the planet’s midlatitudes, analogous to earthly latitudes falling between the Canadian-US border and Kansas, is something new.

“Knowing where the ice is and how thick it is can tell you about Mars’ past climates,” Byrne says. “The fact that the ice is so thick and widespread leads us to think it came into place during one of Mars’ past climates when it snowed a bunch, ice accumulated, was buried, and then preserved.

“There have been a lot of climate changes between now and the tens of millions of years ago when we suspect the ice was put there. But it shouldn’t be stable today, and other past climates of ice instability in this region mean the ice should’ve sublimated away into the dry Martian atmosphere by now. So, that’s what we need to investigate.

“What kept the ice around all this time? There’s no climate model that we have now that explains this.”

Source: University of Arizona