New research from Brown University reinforces the idea that the grooves crisscrossing the surface of Phobos, the big of the two Martian moons, were made by rolling boulders blasted free from a huge asteroid effect.
Phobos' grooves, which are the most of the moon's surface, were first glimpsed in the 1970s by NASA's Mariner and Viking missions.
Over the years, there is no shortage of explanations for how they formed.
Some planetary researchers have posited that large impacts on Mars have showered the nearby moon with groove-carving debris. Others think that Mars' gravity is slow, and the grooves are signs of structural failure.
Still other scientists have made the case that there is a connection between the grooves and the effect that created a large crater called Stickney.
In the 1970s, University of Lancaster's Professor Lionel Wilson and Brown University 'Professor Jim Head proposed the idea that ejecta – bouncing, sliding and rolling boulders – from Stickney may have been carved the grooves
For a moon the size of the diminutive Phobos (17 miles, or 27 km, across), Stickney is a huge crater at 5.6 miles (9 km) across.
"The impact that formed it had had blown free tons of giant rocks, making the rolling boulder idea completely plausible," said Ken Ramsley, a researcher in the Earth, Environmental and Planetary Sciences, and Brown University at the School of Engineering.
"But there are some problems with the idea. For example, not all of the grooves are aligned radially from Stickney as one might possibly have the idea if Stickney ejecta did the carving and some grooves are superposed on top of each other, which suggests some have already been there when superposed ones were created. "
"How was there grooves created at two different times from one single event?"
"What's more, a few grooves run through Stickney itself, suggesting that the crater was already done there when the grooves formed."
"There is also an area on Phobos where there are no grooves at all. Why would those all rolling boulders just skip one particular area? "
To explore those questions, Ramsley and Professor Head designed computer models to see if there was any chance that the 'rolling boulder model' could recreate these confounding patterns
These models are the paths of the boulders ejected from Stickney, taking into account Phobos' shape and topography, as well as its gravitational environment, rotation and orbit around Mars.
The models showed that the boulders tended to align themselves in sets of parallel paths, which jibes with the sets of parallel grooves seen on Phobos. They also provide a possible explanation for some other puzzling groove patterns.
The simulations show that because of Phobos' small size and relatively weak gravity, Stickney stones just keep on rolling, rather than stopping after a kilometer or so.
In fact, some boulders would have rolled and bounded their way. That circumnavigation can explain why some grooves are not radially aligned to the crater. Boulders that start out rolling across the eastern hemisphere of Phobos produce grooves that appear to be misaligned from the crater when they reach the western hemisphere.
That round-the-globe rolling also explains how some grooves are superposed on top of others.
The models show that the grooves laid down right after the impact was crossed minutes to hours later by boulders completing their global journeys. In some cases, those globetrotting boulders rolled up where they started – Stickney crater. That explains why Stickney itself has grooves
Then there is an area where there are no grooves at all. That area turns out to be a fairly low-elevation area on Phobos surrounded by a high-elevation lip. The simulations showed that the boulders hit that area and lip over the area, before coming down again on the other side.
"The models answer some key questions about how ejecta from Stickney could have been responsible for Phobos' complex groove patterns," Ramsley said.
"We think this makes a pretty strong case that is the rolling boulder model accounts for most if not all the grooves on Phobos."
The research is published in the journal Planetary and Space Science.
Kenneth R. Ramsley & James W. Head. Origin of Phobos grooves: Testing the Stickney Crater ejecta model. Planetary and Space Science, published online November 16, 2018; doi: 10.1016 / j.pss.2018.11.004