Mars has two small moons which it may once have taken from an asteroid belt. Phobos orbits just 9,376 km away from the planet, and Deimos is somewhat further way. Phobos is a very groovy place.
It has large grooves that could have been caused by massive boulders rolling across its surface after an asteroid impact. The grooves are enormous – up to 30 kilometres long and 200 metres wide.
In the 1970s researchers Lionel Wilson and Jim Head suggested that bouncing, rolling boulders freed by the Stickney-causing impact could have created the grooves. The Stickney Crater, which was formed when a rock about one kilometre across smashed into Phobos around 150 million years ago. Head along with Kenneth Ramsley recently used a simulation of the Phobos-Mars system to see if these grooves could come from rolling stones. They say that the boulders could have come from the impact on at Stickler Crater which you can see in the photo left a fairly hefty dent in good old Phobos. The crater is named after Chloe Angeline Stickney Hall, wife of Phobos’s discoverer, Asaph Hall.
They believe that gravity on Mars, as it was so nearby, would have kept the boulders on Phobos for about 12 hours – and these movements led to the lines also noticeable in the photo.