Boiling Water Could Explain Mysterious Dark Streaks on Mars

Researchers simulate some of the Red Planet’s unique features in an Earth-bound chamber

Hale Crater
The Hale Crater on Mars, one area where slope lineae occur NASA/JPL-Caltech/Univ. of Arizona

In 2011, researchers began noticing long narrow streaks on some of the images the Mars Reconnaissance Orbiter was sending to Earth. The dark lines, which appear mainly on the edges of craters, are called recurring slope lineae, and they wax and wane throughout the Martian year, shrinking during cold weather and widening during warm periods. Last September, researchers put forward a compelling case that the lines were created by briny liquid water.

Now, a new study in Nature Geosciences shows how water could boil out of the Martian soil, adding a little more bulk to that claim. Alfred McEwen, a professor of planetary geology at the University of Arizona and his colleagues conducted their study using the Large Mars Chamber at The Open University in the United Kingdom. This chamber simulates conditions on Mars, allowing researchers to control the temperature, air pressure, and nitrogen and carbon dioxide levels.

According to Michael Greshko at National Geographic, the researchers put a sloping three- by seven-foot plank covered in fine sand in the chamber. They then melted a large ice cube at the top of the ramp, recording what the meltwater did as it rolled down the slope. Under Earth-like conditions, the water crept downhill, darkening the sand along the way, but not affecting it too much.

When the researchers simulated Mars, however, the water seeped into the sand and began to boil away in the low-pressure system, creating little piles at the leading edge of the flow. Eventually the slope was covered in a series of ridges.

video brine

“We weren’t expecting it,” Susan Conway, study co-author tells Greshko. “We all crowded around the chambers, going, ‘Aw, that’s so cool! Let’s hope it’s not a one-off.’”

The researchers contend that through this process even a relatively small amount of water could create large slope lineae. “That’s what this lab experiment shows, that even with a small amount of water boiling, you trigger larger-scale geomorphology changes,” McEwan tells Alessandra Potenza at The Verge. “That’s why I think this is so promising. It explains it with small amounts.”

The study, of course, has its limits. Because the chamber is so small, it’s difficult to know how the processes would work on a larger landscape. And the chamber isn’t able to reproduce all the variables found on Mars, like its average -80 degree temperatures.

Still, it’s a reasonable explanation for the slope lineae and illustrates some of the unique geological processes on Mars. “It’s a great laboratory study, like a science fair project,” David Stillman, a senior research scientist at Southwest Research Institute’s department of space studies tells Potenza. “It’s really neat how they were able to go into the lab...and simulate what would happen on Mars, because no one had ever predicted anything like this happening before.”

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