Phobos
Image: NASA/JPL-Caltech/University of Arizona

Mars boasts two of the ugliest moons in the Solar System, including Phobos—an oddly shaped, pockmarked moon featuring a distinctive set of stripes. Astronomers have debated the origin of these grooves for decades, but a new computer simulation suggests Phobos’ stripes were made by rolling and bouncing boulders dislodged by a cataclysmic asteroid strike.

This moon’s most distinctive feature, aside from its grooves, is a gigantic impact crater known as the Stickley Crater. Phobos measures a mere 27 kilometers (18 miles) at its widest point, yet this crater extends for nine kilometers (5.5 miles).

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New research published this week in the science journal Planetary and Space Science is providing evidence linking the impact event that created the Stickley Crater to Phobos’s conspicuous grooves. A new computer model developed by Brown University scientists suggests the ancient impact sent boulders careening across the moon’s landscape, which bounced, rolled, and slid, forming the stripes we see today.

The theory certainly feels intuitive, but there’s more to these grooves than meets the eye.

For example, the grooves aren’t all diverging from the Stickney Crater, as might be expected if they were created by the crater-forming event. Also, some grooves are superimposed on top of others, which suggests they formed at different times. Some stripes run through the Stickney Crater itself, which seems to imply that the crater was already in place when the grooves formed. And finally, there’s Phobos’ Dead Spot—an area on the moon in which no grooves exist; if the grooves were caused by bouncing boulders, it seems strange they all managed to avoid this one particular area.

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The bouncing boulder theory was first proposed in 1989, but owing to these anomalous observations, other theories have managed to stick around. Some have argued that massive asteroid strikes on Mars showered Phobos with groove-carving debris, while others have speculated that gravity from Mars is ripping Phobos apart, with the grooves displaying signs of structural failure.

Computer models showing the possible paths of rolling boulders on Phobos following the Stickney impact.
Image: K. Ramsey & J. W. Head

With the true cause of these stripes still in doubt, Brown University planetary scientists Kenneth Ramsley and James Head decided to put the bouncing boulder theory to the test, which they did by running a computer simulation of the ancient asteroid strike. Their model took Phobos’ paltry gravity into account, along with its twisted topography, spin, and orbital relation to Mars. Ramsley and Head had no preconceived notions of what the simulation might show.

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“The model is really just an experiment we run on a laptop,” said Ramsley in a statement. “We put all the basic ingredients in, then we press the button and we see what happens.”

Watching the simulation unfold, the researchers saw how the debris moved outwards from the impact site, with the boulders aligning themselves in sets of parallel paths—an observation consistent with the sets of parallel grooves observed on Phobos.

Owing to the moon’s weak gravity, however, some of the boulders just kept on rolling and bouncing. In fact, some boulders rolled for so long they actually traveled all the way around Phobos—and then still kept on going. The stunning observation of circumnavigating boulders could explain why some grooves aren’t radially aligned to the crater, and why some are superimposed on top of others. The simulations also showed some boulders returning to their point of origin, which could explain why grooves can be seen inside of Stickney Crater.

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Simulation showing how boulders take a flying leap over one particular area of Phobos.
Image: K. Ramsey & J. W. Head

The models provided an explanation for the Dead Spot, too. This region of low elevation is surrounded by areas of higher elevation. Watching the simulation, the researchers saw the boulders hitting the lip and literally taking a flying leap over the Dead Spot, becoming airborne for an extended period of time, and finally coming to a landing on the other side.

“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.”

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Ramsley and Head say their new model “makes a pretty strong case” in explaining the origin of most, if not all, the grooves on Phobos. Of course, it’s just a computer model, so it would be good to corroborate these findings with other forms of data, such a geological analysis. It would also be good to see other researchers replicate these findings with their own computer simulations, as some of the variables used in the simulations may have been biased or somehow inaccurate.

Regardless, the bouncing boulder theory is emerging as the most plausible explanation for Phobos’ distinctive grooves. It’s a cool theory, but Phobos, along with its companion Deimos, are still butt ugly.

[Planetary and Space Science]

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