Mars Moon Phobos is Slowly Being Ripped Apart by Gravitational Forces

Posted on November 13, 2015

Grooves on Mars moon Phobos

Phobos is literally being ripped apart. The moon of Mars is being drawn in by Mars' gravity by approximately 2 meters every 100 years. The gravity is slowly pulling the moon apart: scientist say it will be completely pulled apart in 30 to 50 million years.

Terry Hurford of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues presented their findings at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society at National Harbor, Maryland, on November 10th. There are long, shallow grooves that line the surface of Phobos, which the scientists believe are the early the signs of structural failure.

Phobos orbits Mars 6,000 km above the surface. It is closer to Mars than another moon is to its planet in our solar system. There have been several theories about the mysterious grooves on Phobos. Some scientists thought the grooves were caused by fractures from the impact that caused the Stickney crater. But the pattern of the grooves are wrong for them to be caused by an impact: the grooves radiate outward from a nearby focal point, not outward from the crater.

Hurford's work used computer modeling to show that the grooves were not caused by impacts of any kind, but are more like "stretch marks" which are formed by tidal forces which are the result of the gravitational pull between Mars and Phobos. Hurford and his colleagues also believe that Phobos is not really solid, as previously thought. Instead the center is a pile of rubble which is barely held together and which is surrounded by a 100 meter thick layer of powdery regolith.

Erik Asphaug of the School of Earth and Space Exploration at Arizona State University in Tempe is a co-investigator on the study. He explained, "The funny thing about the result 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."

Phobos is not alone. Hurford noted that Neptune's moon Triton may also come to a similar end. Triton is slowly falling inward towards Neptune and its surface has similar fractures to the ones on Phobos. Huford notes that they can't get imaging on the outer planets to tell exactly what's going on but the research has important implications for planets orbiting stars. He says, "We can't image those distant planets to see what's going on, but this work can help us understand those systems, because any kind of planet falling into its host star could get torn apart in the same way."

Photo: NASA/JPL-Caltech/University of Arizona