The mystery of Phobos’ grooves are finally solved

A recent study suggests that the strange grooves crisscrossing the surface of the Martian moon Phobos have been dug by debris resulting from an ancient asteroid impact.

The first photos of Phobos, the moon of Mars — captured in the 1970s by NASA’s Mariner and Viking missions — revealed strange grooves on the surface of the object at the time. To explain them, some have suggested that these formations could testify to a structural disintegration resulting from the gravity of the planet Mars. Others have also thought of a link with the Stickney impact crater. So, who is right ?

It seems that the second explanation is the most convincing. And to reach this conclusion, Kenneth Ramsley and James Head, of Brown University (USA), have recently relied on computer simulations. The idea here was to evaluate the movement of the debris projected as a result of the asteroid impact that led to the formation of the Stickney crater, a 9-kilometer wide structure found north of the small potato-shaped moon. (27 kilometers in diameter).

There was another problem to take into account. If some grooves evolve in a parallel way as one would expect, others overlap, even crossing the crater itself. There is also a well defined area where no groove appears. Can the simulations explain these observations? After all, if a groove pass through the crater, it is apparently absurd to think that it was the crater impact that caused it.

But it turns out that yes, the simulations can explain it. By retrieving all available data, it appears that the impact at the origin of the crater would have actually led many rocks to “roll” on the surface of the moon, creating the many grooves observed. How is it that grooves pass through the crater? Because the moon is tiny.

On a larger object, the rocks would have simply rolled before stopping their way a few kilometers further. But the diameter and shape of Phobos are such that some rocks have simply managed to circumnavigate the little moon. Once a rock has circled Phobos, its grooves are no longer aligned, and some have crossed the crater.

As for the “dead zone”, devoid of groove, the researchers argue that it could simply be a lower altitude area surrounded by ledges. While rolling, the rocks would have collided with these edges. Some even rolled so fast that they would have used the edges as a springboard to fly over the area and land on the other side.

Carl Frantz

Polyglot, humanitarian, Carl was born in Germany but raised in the USA. He writes mostly on tech, science and culture.