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Most of Phobos' surface is covered with strange linear grooves. New research reinforces the idea that the sandy rocks of Stickney Crater (the Great Depression on the right) have carved these iconic grooves. NASA / JPL-Caltech / University of Arizona
A new study supports the idea that strange furrows crisscrossing the surface of the Phobos Martian moon were created by rolled rocks, rid of an ancient asteroid impact.
The research, published in Planetary and Space Science, uses computer models to simulate the movement of Stickney Crater debris, a huge gash on one end of Phobos' oblong body. The models show that the blocks rolled on the surface after the impact of Stickney could have created the curious patterns of furrows observed on Phobos today.
"These grooves are a distinguishing feature of Phobos and planetary scientists have been discussing them for 40 years," said Ken Ramsley, Global Science Researcher at Brown University, who led the work. "We think this study is another step towards an explanation."
Phobos' grooves, visible on almost the entire surface of the moon, were first sighted in the 1970s by NASA's Mariner and Viking missions. Over the years, explanations provided to explain their training are not lacking. Some scientists have postulated that significant impacts on Mars had submerged the nearby moon with debris sculpting the furrows. Others believe that Mars' gravity is slowly tearing Phobos apart and that the grooves are a sign of structural failure.
Other researchers have already said that there is a link between the grooves and the impact of Stickney. In the late 1970s, planetary scientists Lionel Wilson and Jim Head proposed that Stickney's ejectas – bouncy, slippery and rolling blocks – would have carved the grooves. Head, a professor at Brown's Department of Earth, Environmental and Planetary Sciences, was also a co-author of this new paper.
For a moon the size of the tiny Phobos (27 km wide at its widest point), Stickney is a huge crater 9 km wide. According to Ramsley, the resulting impact would have exploded tons of giant rocks, making the idea of moving blocks quite plausible. But there are also some problems with the idea.
Computer models have traced the possible paths of Stickney's ejecta.
For example, not all grooves are radially aligned with Stickney, contrary to what one might think intuitively if Stickney ejecta did the engraving. And some grooves overlap, suggesting that some of them must already be present when creating them. How could there be grooves created at two different times from a single event? In addition, a few grooves pass through Stickney itself, suggesting that the crater must already be present during the formation of the grooves. There is also a clearly visible dead spot on Phobos where there are no furrows at all. Why were all these rolling rocks jumping from one area in particular?
To explore these questions, Ramsley designed computer models to see if it was possible for the "rolling block model" to recreate these patterns of confusion. The models simulate the trajectories of Stickney ejected blocks, taking into account the shape and topography of Phobos, as well as its gravitational environment, rotation, and orbit around Mars.
Ramsley said that he had no expectations for what the models could show. He ended up being surprised at how much the model reproduced the groove patterns observed on Phobos.
"The model is really just an experience we run on a laptop," said Ramsley. "We put all the basic ingredients, then we pressed the button and we saw what was happening."
The models showed that the rocks tended to align in sets of parallel paths, which overlap with the sets of parallel grooves observed on Phobos. The models also provide a potential explanation for some of the other more disconcerting groove patterns.
The simulations show that due to the small size of Phobos and its relatively low gravity, Stickney stones continue to roll instead of stopping after about one kilometer, as if they were placed on a larger body . In fact, some rocks would have rolled and circled all around the tiny moon. This circumnavigation could explain why some grooves are not aligned radially with the crater. Blocks that begin rolling in the eastern hemisphere of Phobos produce furrows that seem out of alignment with the crater when they reach the western hemisphere.
The simulations show how rocks make jumps on a particular area of Phobos, explaining why it is devoid of grooves.
This rolling around the globe also explains how some grooves overlap. The models show that the grooves formed just after the impact were crossed a few minutes to a few hours later by blocks completing their overall course. In some cases, these globular blocks have returned to their starting point: the Stickney Crater. That explains why Stickney himself has grooves.
Then there is the dead point where there are no grooves at all. According to Ramsley, this region turns out to be a fairly low altitude area on Phobos, surrounded by a higher lip. The simulations showed that rocks clashed with the lip and jumped over the dead center before descending to the other side.
"It's like a ski jump," Ramsley said. "The rocks continue, but suddenly there is no more land under them. They end up doing this suborbital flight over this area. "
All in all, says Ramsley, the models answer key questions about how Stickney's ejecta could be responsible for the complicated groove patterns of Phobos.
"We think this clearly shows that it is this model of moving blocks that accounts for most, if not all, furrows on Phobos," Ramsley said.
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