Like a rolling stone: how the Mars moon Phobos got its grooves

Like a rolling stone: how the Mars moon Phobos got its grooves
©NASA/JPL-Caltech/University of Arizona

A new study suggests that the grooves seen on the Mars moon Phobos were caused by rolling stones from an ancient asteroid impact.

NASA‘s Mariner and Viking missions in the 1970s saw the distinctive grooves of Phobos for the first time. Since then, there have been many interpretations posited by scientists for how the grooves were formed on the Mars moon Phobos.

The distinctive grooves on Phobos

Groovy: a close-up image of Phobos from the Viking mission. ©NASA

These potential explanations include the idea that large impacts on Mars created a shower of debris which carved the grooves into it. There is also the idea that the gravity on Mars is slowly destroying Phobos, and the grooves are a visible sign of the structural failure on the Mars moon.

A new explanation

Ken Ramsley, the leader of the study and a planetary science researcher at Brown University, said: “These grooves are a distinctive feature of Phobos, and how they formed has been debated by planetary scientists for 40 years. We think this study is another step toward zeroing in on an explanation.”

Recreating the rolling stones

Ramsley designed computer models to see if a “rolling boulder model” could recreate the unsual groove patterns seen on the Mars moon. The models simulate the paths of the boulders ejected from the Stickney crater. To Ramsley surprise, the model recreated the groove patterns well. Explaining the process, he said: “The model is really just an experiment we run on a laptop…We put all the basic ingredients in, then we press the button and we see what happens.”

According to Brown university,  the model simulations show that Stickney stones keep on rolling due to the moon’s small size and weaker gravity, whereas on a larger body they might stop after a kilometre or so. Some of the boulders might even have rolled all the way around the Mars moon, which could explain the misaligned grooves.

Ramsley concluded: “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.”

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