New research from Brown University reinforces the idea that grooves crisscrossing the surface of Phobos, the larger of the two Martian moons, were made by rolling boulders blasted free from a huge asteroid impact.
Martian moon Phobos. Image credit: NASA / JPL-Caltech / University of Arizona.
Phobos’ grooves, which are visible across most of the moon’s surface, were first glimpsed in the 1970s by NASA’s Mariner and Viking missions.
Over the years, there has been no shortage of explanations put forward 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 slowly tearing Phobos apart, and the grooves are signs of structural failure.
Still other scientists have made the case that there’s a connection between the grooves and the impact 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 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 would have blown free tons of giant rocks, making the rolling boulder idea entirely plausible,” said Ken Ramsley, a researcher in the Department of Earth, Environmental and Planetary Sciences and the School of Engineering at Brown University.
“But there are also some problems with the idea. For example, not all of the grooves are aligned radially from Stickney as one might intuitively expect if Stickney ejecta did the carving And some grooves are superposed on top of each other, which suggests some must have already been there when superposed ones were created.”
“How could there be grooves created at two different times from one single event?”
“What’s more, a few grooves run through Stickney itself, suggesting that the crater must already have been there when the grooves formed.”
“There’s also an area on Phobos where there are no grooves at all. Why would all those 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 simulate 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 models also provide a potential explanation for some of the other more 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 like they might on a larger body.
In fact, some boulders would have rolled and bounded their way all the way around the tiny moon. That circumnavigation could explain why some grooves aren’t 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 grooves laid down right after the impact were crossed minutes to hours later by boulders completing their global journeys. In some cases, those globetrotting boulders rolled all the back to where they started — Stickney crater. That explains why Stickney itself has grooves.
Then there’s 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 higher-elevation lip. The simulations showed that boulders hit that lip and take a flying leap 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’ complicated groove patterns,” Ramsley said.
“We think this makes a pretty strong case that it was this rolling boulder model accounts for most if not all the grooves on Phobos.”
The research is published in the journal Planetary and Space Science.
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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
