Scientists have long debated the cause of the distinctive grooves on the surface of Mars' moon Phobos. One leading theory has it that the lines are signs of structural failure as the tiny body is pulled apart by the forces of gravity acting on it and its parent planet. But a competing idea suggests the marks may be superficial, caused simply by rolling stones on Phobos' surface. Now, new research addresses the long-standing objections to that theory.

First proposed in the 1970s by Lionel Wilson and Jim Head, the theory hinges on Phobos' Stickney crater, which is, relatively speaking, enormous at 9 km across (5.6 miles). Phobos is only three times that at its widest. Inevitably, the impact that caused Stickney would have thrown up large amounts of rock that would have been scattered across Phobos' surface. So far, so plausible – but at second glance there are some apparently show-stopping problems with the idea that these could have caused Phobos' grizzled complexion.

Phobos anomalies

First of said problems: the grooves don't all align radially from the Stickney crater. How could that be if the stones causing the grooves were thrown out from that crater? Second: some of the grooves actually run through the crater, which would seem to suggest that the grooves came first.

Also problematic, according to the researchers, is that some of the grooves overlap. Though it may seem plausible that one rock may follow another in quick succession, the researchers think that this is problematic. And finally, there's an area on Phobos with no lines at all – its so-called dead spot. If caused by rolling stones, why would the lines just stop for no clear reason?

To work out whether these apparent anomalies truly put paid to the rolling stones theory, the researchers built a computer model to simulate the crater impact. The model recreates the ejection of rock from the impact site, while factoring in Phobos' topography, gravitational field, rotation and orbit around the red planet.

"The model is really just an experiment we run on a laptop," lead researcher Ken Ramsley, who built the simulation, says in a press release. "We put all the basic ingredients in, then we press the button and we see what happens." What did happen? Broadly speaking, the model produced the sorts of patterns evident on Phobos itself. But what of the problematic details? It's good news so far as the simulation goes…

The stones' longevity

The simulation hints at one explanation for most of the problematic phenomena. It turns out that Phobos is so small and gravitationally weak that the ejected rocks keep rolling much farther than you might expect – all away around Phobos and beyond. Could that explain why not all the grooves appear to originate from Stickney crater? Yes. And that some might overlap? Yes again. And that some pass right through the crater? Thrice yes.

But what about the dead spot where the lines aren't evident? It turns out this dead zone is an area of low elevation, surround by a lip. "It's like a ski jump," Ramsley explains in the press release. "The boulders keep going but suddenly there's no ground under them. They end up doing this suborbital flight over this zone."

It would seem the researchers have added much-needed credence to the rolling stones theory of Phobos' lines. But though this may call into question the idea that these features are caused by gravitational stress, it doesn't mean that Phobos isn't doomed. As it's pulled ever closer to Mars, Phobos' weak structural composition means that, while it may still circle Mars, it will do so as a ring rather than a tiny moon.