Published on November 21, 2018 "It's like a jump," said Ken Ramsley, a planet science researcher at Brown University. "Boulders…
Published on November 21, 2018
“It’s like a jump,” said Ken Ramsley, a planet science researcher at Brown University. “Boulders continue but suddenly there is no land below them. They stop making this suborbital flight over this zone.”
“These traces are a distinctive feature of Phobos, and how they were formed have been discussed by planet scientists for 40 years,” says Ramsley who led the research. “We think this study is another step towards nullifying an explanation.”
A new Brown University study reinforces the idea that strange traces crossing the surface of Marsh Moon Phobos were made of rolling stone blocks blasted free from an old asteroid effect.
The research, published in planetary and space science, uses computer models to simulate the movement of debris from Stickney crater, a huge gash at one end of Phobo’s oblong body. The models show that rock blocks rolling over the surface in the aftermath of the Stickney support could have created the puzzling patterns of tracks seen on Phobos today.
Fobo trails, visible over most of the moon’s surface, were first gleamed in the 1
970s by NASA’s Mariner and Viking mission. Over the years, there have been no shortages of explanations as to how they were formed. Some researchers have suggested that major effects on Mars have showered the nearby moon with grout cutting products. Others believe that Mars gravity slowly breaks Phobos apart, and the traces are indicative of structural defects.
Additional researchers have made the case that there is a link between the tracks and the Stickney support. Planetaric scientists Lionel Wilson and Jim Head suggested in the late 1970s the idea that ejecta-bouncing, sliding and rolling stone blocks – from Stickney may have slipped the tracks. Head, a professor at the Browns Department of Earth, Environment and Planet Sciences, was also co-author of this new paper.
For a moon, the size of the diminutive Phobos (27 kilometers above its widest point) is a huge crater of 9 miles. Impact that formed it would have blown out lots of giant rocks, making the rolling watch tank completely credible, says Ramsley. But there are also some issues with the idea.
For example, not all of the tracks are radially aligned from Stickney, which one can intuitively expect if Stickney ejecta did the cuts. And some tracks are superimposed on top of each other, indicating that some must have been there when the superiors were created. How could there be tracks created at two different times from a single event? In addition, some traces pass through Stickney itself, which indicates that the crater has already had to be there when the tracks were formed. There is also a visible deadlock on Phobos where there are no tracks at all. Why would all the rolling rocks just jump over a certain area?
To investigate these issues, Ramsley designed computer models to see if there was any chance that “Rolling Stone Model” could recreate these confrontational patterns. The models simulate the stones thrown out of Stickney, taking into account Phobo’s form and topography, as well as its gravitational environment, rotation and orbit around Mars.
Ramsley said he had no expectations as to what the models could show. He ended up being amazed at how well the model recreated the track patterns seen on Phobos.
“The model is really just an experiment we run on a laptop,” said Ramsley. “We put all the basic ingredients in, then we press the button and we see what’s happening.”
The models showed that the stone blocks tended to align in sets of parallel roads, which jibes with the sets of parallel tracks seen on Phobos. The models also provide a potential explanation for some of the other more confusing track patterns.
The simulations show that because of the small size of Phobos and relatively weak gravity, the Stickney stones only continue to roll, rather than stopping after a mile or so as they might on a larger body. In fact, some stone blocks would have rolled and bordered all the way around the little moon. The circumvention can explain why some traces are not radically aligned with the crater. Boulders that begin rolling over Phobo’s east hemisphere produce traces that appear to be inaccurate from the crater when they reach the western hemisphere.
The round roll that rolls also explains how some tracks are laid over others. The models show that the tracks lie shortly after the support was crossed minutes to hours later by rocks that completed their global travels. In some cases, the globe-rocks rolled their entire back to where they started – Stickney Crater. That explains why Stickney himself has a track.
Then there is the dead place where there are no tracks at all. That area turns out to be a fairly low surface on Phobos surrounded by a higher altitude lip, “says Ramsley. The simulations showed that the stone blocks hit that lip and took a flying jump over the dead place before coming back on the other side.
Everyone said, Ramsley says the models answer some important questions about how Stickney’s ejecta could have been responsible for Phobo’s complex trace patterns.
“We think this is a pretty strong case that this is where this rolling rock model stands for most if not all the tracks on Phobos,” says Ramsley.
Image Credit: NASA / JPL-Caltech / University of Arizona
The Daily Galaxy via Brown University