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Has the mystery of how Phobos got it’s grooves finally been solved

Mars', moon Phobos, is covered in grooves. Credits : NASA/JPL-Caltech/Université de l’Arizona

A recent study suggests that the strange grooves traversing the surface of Phobos, the Martian moon, were formed by debris after an asteroid impacted the surface. 

In the 1970 s NASA’s Viking and Mariner missions captured the first pictures of Phobos revealing the strange grooved surface of the Martian moon. Some have tried to explain these groves by suggesting that they represent structural disintegration due the gravity of the planet Mars.  Others have also thought they might be linked to the Stickney crater.  But what is the real reason?

Rolling stones

It seems that the second solution is the most convincing. Kenneth Ramley and James Head from Brown University, have based their conclusion on computer simulations.  Their idea consisted of evaluating the movement of projected debris after the asteroid impact which caused the Stickney crater.  This structure which is 9 kilometres wide is found in the northern area of this little potato shaped moon which is only 27 kilometres in diameter.

However there was an additional problem to take into consideration.  While some of the grooves appeared in a parallel formation as would be expected, others however overlapped one another, crossing even the crater itself.  There is also a well outlined area where no grooves appear.  Can simulations explain these observations?  After all, if the grooves cross the crater, it would be weird to think that they were formed by the crater itself.

Mars Phobos
Phobos moon (27 km in diametre, with Mars in the background. Credit : L G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA

Confirmation from the simulations

However it turns out that the simulations can explain this. By entering all the available data, it appears that the asteroid impact which created the crater would have effectively caused lots of rocks to roll on the moon’s surface creating the many grooves that are seen today.  But how do the grooves cross the crater?  The answer is simple: because the moon is tiny.

Rocks roll around the moon

On a larger object, rocks would simply roll until they stop a few kilometres further away.  However as the diametre and shape of Phobos is as small, rocks could quite simply roll all around the little moon.  Once a rock has made the tour of the moon the grooves are no longer aligned and some can cross the crater.

As for the “dead zone” which is deprived of grooves completely, researchers suggest that it could quite simply be an area which has a lower altitude surrounded by a brimmed edge.  Rolling rocks would hit against the edge and eventually continue their path by avoiding this area.  Some rocks could have rolled as fast that they could have used the edge of this area as a springboard so that they can fly over and land on the other side.


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