Martian moon Phobos had his strange grooves from Rolling Boulders | Planetary sciences, exploration of space

A new study from Brown University reinforces the idea that the grooves that crisscross the surface of Phobos, the largest of the two Martian moons, were created by rolling sandstone rocks of enormous impact. 39; asteroid.

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 planetary researchers have postulated that significant impacts on Mars had submerged the nearby moon with debris carving grooves. Others believe that Mars' gravity is slowly tearing Phobos apart and that the grooves are a sign of structural failure.

Other scientists have argued that there is a connection between the grooves and the impact that created a large crater called Stickney.

In the 1970s, professors Lionel Wilson of Lancaster University and Jim Head of Brown University proposed the idea that sticky ejectas – bouncy, slippery and rolling blocks – would have carved the grooves .

For a moon the size of the tiny Phobos (17 miles), Stickney is a huge 9 km crater.

"The resulting impact would have exploded tons of giant rocks, making the idea of ​​boulder quite plausible," said Ken Ramsley, a researcher at the Department of Earth Sciences, of Environment and planets and at the School of Engineering at Brown University.

"But there are also some problems with the idea. For example, not all grooves are radially aligned with Stickney, as one would expect intuitively if Stickney ejecta did the cutting. Some grooves overlap, suggesting that some of them should already be present when creating overlays.

"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 have been there when the grooves were formed."

"There is also an area on Phobos where there are no grooves. Why would all these rolling rocks skip a particular area? "

To explore these questions, Ramsley and Professor Head designed computer models to determine whether it was possible for the 'rolling block model' to recreate these patterns of confusion.

These 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.

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. Their 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.

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 an area where there are no grooves at all. This area turns out to be a fairly low area on Phobos, surrounded by a higher lip. The simulations showed that rocks clashed with this lip and leaped forward over the area before descending to the other side.

"The models answer key questions about how Stickney's ejecta could be responsible for the complicated groove patterns of Phobos," said Ramsley.

"We think this proves quite clearly that it is this model of moving blocks that represents most, if not all, Phobos grooves."

The research is published in the journal Planetary and space sciences.

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Kenneth R. Ramsley and James W. Head. Origin of Phobos Gorge: Test of the ejecta model of Stickney Crater. Planetary and space sciences, published online November 16, 2018; doi: 10.1016 / j.pss.2018.11.004