NASA scientists have discovered the first signs of structural failure on one of Mars’ moons, and new research presented Tuesday during the annual meeting of the Division of Planetary Sciences of the American Astronomical Society reveals that the planet is likely to blame.

Long, shallow grooves found on the surface of Phobos are evidence that the moon is being torn apart by the gravitational pull of the Red Planet, the study authors reported. Phobos is closer to its planet than any other moon in the solar system, and the gravitational pull of Mars is drawing it approximately 6.6 feet (2 meters) closer each century.

At this rate, experts believe that the moon will be pulled apart within 50 million years, and Terry Hurford of the Goddard Space Flight Center in Maryland noted, NASA believes that Phobos “has already started to fail” and that the grooves are “the first sign of this.”

Fractures not caused by Stickney crater impactor, researchers find

Previously, it was believed that the grooves on Phobos were fractures caused by the same impact that formed Stickney crater—a collision so powerful that it nearly destroyed the moon. However, scientists ultimately determined that the grooves do not actually radiate outward from the crater, but instead originate in another, nearby location.

After dismissing that hypothesis, scientists suggested that the grooves may have been caused by a series of smaller impacts of material ejected from Mars. Now, however, Hurford and his fellow researchers have come up with models supporting a different view. They believe that the grooves are like “stretch marks” caused when Phobos is deformed by tidal forces.

These forces are produced by the gravitational pull between Mars and Phobos, much the same way that the ocean tides on Earth are the result of a shared gravitational pull with the moon. The same explanation for the grooves had previously been proposed, but dismissed due to the belief that beneath its surface, Phobos was believed to be largely solid all of the way through.

A calculation of the tidal forces indicated that the gravitational pull was not strong enough to cause fractures on a solid moon of that size. However, new thinking is that Phobos may actually be far less solid than previously believed, and thus more susceptible to interior distortion.

Models produce grooves similar to those actually found on Phobos

If, as scientists now believe, the interior of Phobos is a pile of rubble that is barely being held together by a 330-foot-thick layer of powdery regolith, it could easily be distorted because it has little interior strength. They also believe that the outer layer behaves elastically and builds stress, but is not strong enough to prevent those stresses from causing surface failures.

“The funny thing about the result,” explained study co-author Erik Asphaug of the School of Earth and Space Exploration at Arizona State University, “is that it shows Phobos has a kind of mildly cohesive outer fabric. This makes sense when you think about powdery materials in microgravity, but it’s quite non-intuitive.”

In short, the research indicates that the tidal forces affecting Phobos are capable of producing enough stress to fracture the surface, and that the fractures produced in the model are very close in nature to the grooves found in images of the moon. Furthermore, this hypothesis explains why some of the grooves appear to be older than others because the process is ongoing.

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Feature Image: NASA/JPL-Caltech/University of Arizona

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