For many years, the cause of grooves on Phobos’s surface has remained a mystery. Until now. According to a study published on Aug. 19 in the journal Nature Communications, the grooves on the Martian moon are actually caused by impacts.

Previously, scientists believed that the grooves are the result of stress fractures caused by the tidal pull of the Red Planet. However, according to the study’s first author Michael Nayak of the University of California Santa Cruz, this does not explain the formation of other grooves.

Now, the new study reveals that the debris ejected by the impacts eventually falling back to the surface caused these, forming a long line of craters. To come up with this conclusion, the researchers had to develop computer simulations that showed how these grooves resulted from impacts. This also allowed them to track where the ejected objects would hit.

“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,” says Nayak. “The controlling factor is where the impact occurs, and that determines where the debris falls back.”

The team also had to simulate an impact at Grildrig, a 2.6-kilometer crater near the moon’s north pole. They observed that when the ejected debris fell back onto the surface, the pattern it followed was similar to the one observed on the crater chain of Phobos.

The research team believes that this phenomenon only occurs on Phobos due to its low mass and close distance to Mars. The distance of its orbit to Mars, which is estimated to be only 9,000 kilometers, is closer than any other moon in our solar system.

Overall, it orbits Mars within seven hours. Although it experiences tidal forces from Mars, it does not break apart, thanks to its elastic shell.