Rolling Stones May Have Given Phobos its Enigmatic Grooves


The impact crater Stickney (and the smaller crater Limtoc) as imaged by NASA’s Mars Reconnaissance Orbiter in 2008 [NASA/JPL-Caltech]

Earth has them. So does the Moon. As does Mars. And now we know dwarf planet Ceres has them, too. Could a Martian moon also have them? Well, according to new research, they could explain the mystery behind Phobos’ strange lines that are carved into its dusty surface.

What am I talking about? Boulders. Specifically boulders that have been on the move. Boulders that — in the presence of a gravitational field, no matter how weak — roll and bounce, leaving their grooves on some of our most beloved celestial bodies.

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

Ever since NASA’s Mariner and Viking missions spied Phobos’ lines in the 1970’s, scientists have debated what could have created them. The ancient natural satellite of Mars is only 27 kilometers wide and possesses long, etched lines that, in some cases, loop around the entirety of the moon’s circumference.

A popular hypothesis for these lines focused on the possibility that Phobos is a dying moon; the tidal forces from Mars ultimately pulling the body apart. In this scenario, the lines are a sign that the moon’s interior is crumbling, creating fault lines in the surface that our space robots have been able to image. Another idea is that the lines were created by crater chains; multiple impacts by smaller rocks that etched out long lines around Phobos’ surface.

However, according Ramsley’s study, which is published in the journal Planetary and Space Science, the real mechanism that created Phobos’ stripes is far more elegant, and more familiar to us Earthlings. What’s more, it was one of the original hypotheses that was posited when the lines were discovered over 40 years ago.

In 1998, NASA’s Mars Global Surveyor imaged Phobos’ stripes [NASA/JPL-Caltech/LLNL]

You see, Phobos has a huge, nine-kilometer-wide crater on one side, called Stickney (named after Angeline Stickney who motivated the search for Mars’ natural satellites in the late 19th Century), that was excavated by a massive impact in the moon’s ancient past. Using computer models, the researchers simulated what would happen post-impact and where the excavated material (including some hefty boulders) would have ended up. Although a huge quantity of material would have been lost to space during the Stickney impact, a few large rocks may have been kicked across the moon’s surface — these boulders would have rolled slowly, slow enough to be held in contact with Phobos, but fast enough, in some cases, to make more than one trip around the moon. 

But many of these lines intersect one another and don’t appear to be radially blasted from the crater. Also, there are regions on the surface where the lines entirely disappear. Ramsley’s simulation explains these oddities.

The simulations show that because of Phobos’ small size and relatively weak gravity, Stickney stones just keep on rolling, rather than stopping after a kilometer 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. That circumnavigation could explain why some grooves aren’t radially aligned to the crater. Boulders that start out rolling across the eastern hemisphere of Phobos produce grooves that appear to be misaligned from the crater when they reach the western hemisphere.

Brown University

This also helps to explain why many of these lines cross and superimpose themselves on one another: Grooves that were laid down by boulders rolling immediately after the impact were crossed by boulders that completed a complete traverse of the globe of the moon, some ending up where they started, minutes or hours later. This also explains why Stickney itself has grooves inside its crater basin.

The dark surface of Phobos with Mars as the backdrop, as seen by the European Mars Express [ESA]

But there’s a blank area on Phobos that appears to contain no grooves, a phenomenon that the simulation also addresses. This region is located at a comparatively low elevation part of Phobos, surrounded by a higher-elevation lip. “It’s like a ski jump,” said Ramsley. “The boulders keep going but suddenly there’s no ground under them. They end up doing this suborbital flight over this zone.

“We think this makes a pretty strong case that it was this rolling boulder model accounts for most if not all the grooves on Phobos.”

As a fan of rolling boulders on other worlds, I particularly enjoy imagining the lumbering slow roll of these massive rocks that circumnavigated Phobos. They had to keep their roll slow so not to achieve escape velocity, but fast enough to leave their indelible marks for humans to ponder their origins.