Rolling stones leave Mars moon feeling groovy

21 Nov 2018

New research bolsters the idea that boulders blasted free from Stickney crater (the large depression on the right) carved those iconic grooves. Image: NASA/JPL-Caltech/University of Arizona

Researchers looking at strange features on the surface of Martian moon Phobos may have found an answer in rolling stones.

One of the most puzzling sights on the Martian moon of Phobos is a series of huge criss-crossing grooves across its surface that appear to have no clear origin.

Back on Earth, researchers from Brown University in the US have revealed new findings that suggest the grooves could have been formed by rolling boulders moving across the moon’s surface. Even more fascinating is the fact that they likely blasted free from an ancient asteroid impact.

Publishing its findings to Planetary and Space Science, the research team used computer models to simulate the movement of debris from a huge gash on one end of the moon’s oblong body known as Stickney crater.

“These grooves are a distinctive feature of Phobos, and how they formed has been debated by planetary scientists for 40 years,” said Ken Ramsley, a planetary science researcher at Brown University who led the work. “We think this study is another step toward zeroing in on an explanation.”

The grooves were first discovered in the 1970s by NASA’s Mariner and Viking spacecraft. Not long after, planetary scientists Lionel Wilson and Jim Head proposed the idea that rolling boulders from Stickney crater caused the formations. With the crater measuring 9km across, the impact that formed it would have seen giant rocks blown free, making the idea plausible.

Computer simulations of how boulders would move on the surface of Phobos.

The simulations showed that boulders could have carved the iconic grooves seen on Phobos’s surface. Image: Ken Ramsley/Brown University

‘It’s like a ski jump’

However, at the time, this theory was not solid as there are a number of grooves crossing over one another, and even some found in the crater itself.

Adding even further mystery, there is a noticeable dead spot on the moon where no rolling boulders have ever crossed.

Ramsley wanted to test the theory anyway using computer simulations, admitting that he and his team put all the basic ingredients in and then it was just a case of pressing a button and seeing what happened.

The results showed that the boulders tended to align themselves in sets of parallel paths. Because Phobos is small and has a weak gravity, they just keep rolling, rather than stopping after a kilometre or so like they might on a larger body. In fact, some boulders would have rolled and bounded their way all the way around the tiny moon, explaining why some are not radially aligned to the centre.

As for the dead spot, it is a fairly low-elevation area on Phobos surrounded by a higher elevation. “It’s like a ski jump,” Ramsley said. “The boulders keep going but suddenly there’s no ground under them. They end up doing this suborbital flight over this zone.”

Colm Gorey was a senior journalist with Silicon Republic