Phobos' grooves were long thought to be fractures caused by the impact that formed Stickney crater. The long, shallow grooves lining the surface of Phobos are likely early signs of the structural failure that will ultimately destroy this moon of Mars. "We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves," said Terry Hurford of NASA's Goddard Space Flight Center in Greenbelt, Maryland. All of this means the tidal forces acting on Phobos can produce more than enough stress to fracture the surface. Orbiting a mere 3,700 miles (6,000 kilometers) above the surface of Mars, Phobos is closer to its planet than any other moon in the solar system. Scientists estimate the ultimate destruction of this tiny moon (22 kilometers/13.5-miles in diameter) might take place in about 30 to 50 million years. At the time, however, Phobos was thought to be more-or-less solid all the way through. Mars' gravity is drawing in Phobos, the larger of its two moons, by about 6.6 feet (2 meters) every hundred years. Hurford and his colleagues, who presented their latest findings at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society this week, also delivered other bad news about the interior of Phobos – which could ultimately speed up the demise of the moon. The recent thinking, however, is that the interior of Phobos could be a rubble pile, barely holding together, surrounded by a layer of powdery regolith about 330 feet (100 meters) thick. “The funny thing about the result is that it shows Phobos has a kind of mildly cohesive outer fabric,” said Erik Asphaug of the School of Earth and Space Exploration at Arizona State University in Tempe and a co-investigator on the study. “This makes sense when you think about powdery materials in microgravity, but it’s quite non-intuitive.” 
 
 Phobos’ grooves have long been an issue up for debate. However, scientists eventually determined that the grooves don’t radiate outward from the crater itself but from a focal point nearby. But new modeling by Hurford and colleagues supports the view that the grooves are more like "stretch marks" that occur when Phobos gets deformed by tidal forces. The team said that stress fractures predicted by their model coincide with the grooves seen in images of Phobos. This explanation also fits with the observation that some grooves are younger than others, which would be the case if the process that creates them is ongoing. Huford also said the same fate may await Neptune’s moon Triton, which is also slowly falling inward and has a similarly fractured surface. The work also has implications for extrasolar planets, according to researchers. “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,” said Hurford.
– The days are numbered for one of Mars' two moons, and the culprit turns out to be Mars itself. It seems that the two are engaged in what Discovery describes as a gravitational tug-of-war, and Mars is going to win. It will take a while, about 30 million to 50 million years, but eventually Phobos will literally fall apart. For now, Mars is drawing the moon toward it at a rate of about 6 feet per century, though the two won't actually collide. Someday, Phobos will drop low enough to reach what's known as the Roche Limit, explains Universe Today, and it will be "torn apart" by the planet's tidal forces. Researchers came to the conclusion by studying the moon's distinctive grooves, marks that were once thought to be created by asteroid strikes. Now, however, they liken them to "stretch marks" formed by this tug-of-war, the scientists say in a post at Science Daily. "We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves," says Terry Hurford of NASA's Goddard Space Flight Center. "Eventually, Phobos will be ripped apart before it reaches Mars’ surface." Phobos already holds the distinction of being the moon closest to its planet in our solar system, orbiting at a relatively scant distance of 3,700 miles. While the idea that the moon's grooves were formed by tidal forces, not impacts, was first floated years ago, it was generally rejected because Phobos was thought to be too solid to succumb to such forces. The new thinking is that Phobos' interior is more like a "rubble pile, barely holding together," according to NASA. (Mars may be doing a number on Phobos, but the sun is doing a number on Mars.)