Anomalous grooves on Phobos, the larger of the two Martian moons, are the result of debris ejected by impacts eventually falling back onto the surface to form linear chains of craters, according to a study published in the journal Nature Communications.
In this image of Phobos, red arrows indicate a chain of small craters whose origin researchers were able to trace back to a primary impact at a large crater known as Grildrig. Image credit: M. Nayak & E. Asphaug / ESA / Mars Express.
Phobos, the larger and inner of the two natural satellites of Mars, is an unusual satellite, orbiting closer to its planet than any other moon in the Solar System.
It orbits the Red Planet about 3,700 miles (6,000 km) from the surface and completes an orbit in just 7 hours and 39 minutes.
Phobos is approximately 16.2 x 13.7 x 11.2 miles (26 x 22 x 18 km) and has a very lumpy appearance. It also has craters and grooves on its surface.
One set of grooves are thought to be stress fractures resulting from the tidal pull of Mars. The new study addresses another set of grooves that do not fit that explanation.
“These grooves cut across the tidal fields, so they require another mechanism,” said lead author Michael Nayak, of the University of California at Santa Cruz and Red Sky Research, LLC.
“If we put the two together, we can explain most if not all of the grooves on Phobos.”
Nayak and his colleague, Dr. Erik Asphaug of Arizona State University and the University of California at Santa Cruz, developed computer simulations showing how these anomalous grooves could result from impacts.
Material ejected from the surface by an impact easily escapes the weak gravity of Phobos. But the debris remains in orbit around Mars, most of it moving either just slower or just faster than the orbital velocity of Phobos, and within a few orbits it gets recaptured and falls back onto the surface of the moon.
The simulations enabled the team to track in precise detail the fate of the ejected debris.
They found that recaptured debris creates distinctive linear impact patterns that match the characteristics of the anomalous grooves and chains of craters that cut across the tidal stress fractures on Phobos.
“A lot of stuff gets kicked up, floats for a couple of orbits, and then gets recollected and falls back in a linear chain before it has a chance to be pulled apart and disassociated by Mars’ gravity,” Nayak said.
“The controlling factor is where the impact occurs, and that determines where the debris falls back.”
The researchers used their model to match a linear chain of small craters on Phobos to its primary source crater.
They simulated an impact at the 1.6-mile (2.6 km) crater called Grildrig, near the moon’s north pole, and found that the pattern resulting from ejected debris falling back onto the surface in the model was a very close match to the actual crater chain observed on Phobos.
“With its low mass and close orbit around Mars, Phobos is so unusual that it may be the only place in the Solar System where this phenomenon occurs,” Nayak said.
_____
M. Nayak & E. Asphaug. 2016. Sesquinary catenae on the Martian satellite Phobos from reaccretion of escaping ejecta. Nature Communications 7, article number: 12591; doi: 10.1038/ncomms12591
