In 2003, strange features on Mars’s surface got scientists’ “spidey senses” tingling when they saw them. That’s when unusual “anareiform terrain” landforms appeared in images from the Mars Reconnaissance Orbiter. They’ve returned each year, spreading across the southern hemisphere surface.
At first, nobody knew what caused these weird wrinkly spider-like formations. Now, NASA researchers have duplicated them in the lab to explain their existence. No doubt about it, though, these Mars spiders look weird. Some of them stretch across a kilometer and generally appear in clusters.
Since carbon dioxide is common on Mars, scientists figured it had something to do with creating these weird formations. They used the “Kieffer model” to delve into the history of Mars spiders. That model explains how carbon dioxide ice slabs under the surface trap gas as it sublimates (turns to gas), usually during southern hemisphere spring.
Sunlight heats the surface and shines through transparent slabs of carbon dioxide. Those ice layers build up each winter. The soil beneath the ice absorbs heat from the Sun and causes the ice closest to it to sublimate. Gas pressure builds up, which cracks the ice and allows gas to escape. As it seeps upward, the gas takes with it a stream of dark dust and sand from the soil that lands on the surface of the ice. Those deposits take the form of spidery landforms.
Confirming Mars Spiders
To see if that process is what’s creating Mars spiders, NASA JPL scientists, led by Lauren McKeown, decided to simulate Mars conditions in their lab. “The spiders are strange, beautiful geologic features in their own right,” said McKeown. “These experiments will help tune our models for how they form.”
Not that it’s easy to replicate Mars on Earth, even in strict laboratory conditions. For Mc Keown and her team, the hardest part was re-creating conditions found on the Martian polar surface. That region experiences extremely low air pressure. Seasonal changes bring the air and surface temperatures down to a chilly -301 degrees Fahrenheit (minus 185 degrees Celsius). To make it work, the team 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,” Mc Keown said, noting that the wine barrel-size chamber was used to test a prototype of a rasping tool designed for NASA’s Mars Phoenix lander. For their experiment, the team chilled Martian soil simulant in a container dipped into a nitrogen bath. Then they put the whole thing into DUSTIE and replaced Earth-normal pressure with Mars air pressure. Carbon dioxide gas flowed in and condensed to ice. The next step was to put a heater inside to simulate Martian conditions in early spring. The team did this several times before the experiment created simulated “spiders” similar to those on Mars.
The Next Steps
That simulation created plumes of carbon dioxide gas escaping from the soil simulant. It’s close to what happens on Mars, but not quite. So, the next step is to do the same experiment and use a simulated Sun to heat the surface materials. If that produces the same results, then the team has a good chance of proving this is what happens on Mars.
However, Mars being what it is—there are still a lot of questions about why the spiders only form in the southern hemisphere at spring. Since subsurface carbon dioxide ice isn’t limited to that region of the planet, why don’t spiders form in other places? One possibility is that these aren’t recent features. They could be left over from a more active time in the planet’s past. Maybe the climate was very different when they formed. Or something catastrophic happened to enable the formation and growth of spiders in the southern hemisphere.
The study at JPL is a good step forward in understanding the Martian terrain. It confirms several formation processes described by the Kieffer model. Of course, it would be really cool to visit those spiders someday. For now, however, lab work is as close as it gets to explaining them. Future rovers and landers could be used to study those landforms up close and personal. However, there aren’t any planned in the near future, and no other spacecraft has landed in the spider-rich southern hemisphere region. For now, scientists will continue testing the lab to understand the conditions that make these strange-looking features.
For More Information
NASA Scientists Re-Create Mars ‘Spiders’ in a Lab for the First Time
A Lab-scale Investigation of the Mars Kieffer Model