Phobos, the innermost and larger moon of Mars, has some peculiar grooves across its surface. Several explanations have been put forward, the most popular is that the grooves are stress marks as the planet’s gravity is slowly pulling the moon apart.
A fresh look at an old theory offers an alternative and intriguing explanation. The groves were actually produced by the boulders that were launched into space by the major impact that created the Stickney crater, the distinctive dent on Phobos' surface. As these rocks began to fall back onto the misshapen moon, they rolled on its surface, creating the peculiar marks. The idea is reported in the journal Planetary and Space Science.
“These grooves are a distinctive feature of Phobos, and how they formed has been debated by planetary scientists for 40 years,” lead author Ken Ramsey, a planetary science researcher at Brown University, said in a statement. “We think this study is another step toward zeroing in on an explanation.”
From its inception, the idea of the boulders carving the moon has a lot of supporting evidence. Phobos is 27 kilometers (16.7 miles) across at its widest point, the Stickney crater is about one-third of that length. It has obviously thrown a lot of big rocks into space and certainly, some came back down onto Phobos. But there are certain features that don’t match this hypothesis.
For example, not all the grooves spread radially from the crater. The crater itself has grooves inside, and there is a big area of Phobos with no grooves at all. The team constructed a computer model to simulate what boulders landing on Phobos would look like and were pleasantly surprised how these concerns were addressed.
Due to its small size and weak gravity, rolling boulders were able to roll for a very long time, often moving across the entire moon, crossing their path at a slightly different angle mere hours after they first hit the surface. In the simulation, some of the boulders ended up rolling back into Stickney crater, a good explanation for the grooves found there. The area with no grooves has the best explanation of all: it's surrounded by a higher elevation lip and the model has shown that boulders approaching it would end up taking a leap over it.
“It’s like a ski jump,” Ramsley added. “The boulders keep going but suddenly there’s no ground under them. They end up doing this suborbital flight over this zone.”
The team is confident that the model accounts for most, if not all, grooves of Phobos. It will be interesting to see what the proponents of other explanations will make of it.