NASA scientists recreate Mars’ spider-like geological formations in lab for the first time

Tests on Earth appear to confirm that the Red Planet’s spider-shaped geological formations are sculpted by carbon dioxide.

Since their discovery in 2003 in images taken by orbiting probes, scientists have marveled at these spider-like features that stretch across the southern hemisphere of Mars. No one knows exactly how these geological formations are created. Each branching formation can stretch more than a kilometer from one end to the other and includes hundreds of spindly “legs.” Called spider-like terrains, these features often occur in groups, giving the surface a wrinkled appearance.

The prevailing theory is that spiders are created by processes involving carbon dioxide ice, which does not exist naturally on Earth. Thanks to experiments detailed in a new paper published in Journal of Planetary SciencesScientists have, for the first time, recreated these formation processes in simulated Martian temperatures and atmospheric pressure.

“Spiders are strange and beautiful geological phenomena in their own right,” said Lauren McKeown of NASA’s Jet Propulsion Laboratory in Southern California. “These experiments will help us refine our models of how they form.”

The study confirms several formation processes described by the so-called Kieffer model: Sunlight warms the ground when it shines through transparent patches of carbon dioxide ice that accumulate on the surface of Mars each winter.

Because the Martian soil is darker than the ice above it, it absorbs heat and converts the nearby ice directly into carbon dioxide, without first liquefying, in a process called sublimation (the same process that causes clouds of “smoke” to rise from dry ice). As the gas pressure increases, the Martian ice cracks, allowing it to escape. As the gas seeps upward, it carries with it a stream of dark dust and sand from the soil that lands on the ice surface.

When winter turns to spring and the remaining ice sublimates, the theory goes, the spider-like scars of these small eruptions are what’s left behind.

Recreating Mars in the laboratory

For McKeown and his co-authors, the most challenging part of conducting the experiments was recreating the conditions found on the polar surface of Mars: extremely low atmospheric pressure and temperatures as low as minus 185 degrees Celsius. To do this, McKeown used a liquid nitrogen-cooled test chamber at JPL, the Dirty Under-vacuum Simulation Testbed for Icy Environments, or DUSTIE.

“I love DUSTIE. It’s historic,” McKeown said, noting that the wine-barrel-sized chamber was used to test a prototype grating tool designed for NASA’s Phoenix Mars lander. The tool was used to break up water ice, which the spacecraft collected and analyzed near the planet’s north pole.

For the experiment, the researchers cooled a mockup of Martian soil in a container submerged in a bath of liquid nitrogen. They placed it in the DUSTIE chamber, where the air pressure was reduced to match that of the southern hemisphere of Mars. Carbon dioxide gas then flowed into the chamber and condensed into ice over three to five hours. It took many trials before McKeown found the ideal conditions for the ice to become thick and translucent enough for the experiments to work.

Once the ice had the right properties, they placed a heating element inside the chamber under the simulant to warm it up and break up the ice. McKeown was thrilled when she finally saw a plume of carbon dioxide gas erupt from inside the powdery simulant.

“It was a Friday night and the lab manager came in after hearing me scream,” said McKeown, who had been working on making a plume like this for five years. “She thought there had been an accident.”

The dark plumes tore holes in the simulant as they flowed, spewing simulant for 10 minutes before all the pressurized gas was expelled.

The experiments yielded a surprise that wasn’t reflected in Kieffer’s model: ice formed between the grains of the simulant and then cracked it. This alternative process could explain why the spiders have a more “cracked” appearance. Whether this happens or not seems to depend on the size of the soil grains and the depth of the water ice buried underground.

“It’s one of those details that shows that nature is a little messier than the image we get in textbooks,” said Serina Diniega of JPL, a co-author of the paper.

What’s next for plume testing?

Now that the conditions for plume formation have been determined, the next step is to try the same experiments simulating sunlight from above, rather than using a radiator below. This could help scientists narrow down the range of conditions under which plumes and soil ejection can occur.

There are still many questions to be answered in the laboratory about spiders. Why did they form in some places on Mars and not others? Since they appear to be the result of seasonal changes that are still occurring, why don’t they seem to be increasing in number or size over time? It’s possible that they are remnants of a distant time when the climate was different on Mars, and that they could therefore offer a unique window into the planet’s past.

For now, the lab experiments will be conducted as close as possible to the spiders. The Curiosity and Perseverance rovers are exploring the Red Planet far from the southern hemisphere, where these formations appear (and where no spacecraft has ever landed). The Phoenix mission, which landed in the northern hemisphere, lasted only a few months before succumbing to intense polar cold and limited sunlight.