Mars’ largest moon is slowly falling toward the planet—but rather than smash into it, the moon will probably disintegrate into bits that will form a ring around Mars, scientists predict.





The demise of the moon Phobos will occur in an estimated 20 million to 40 million years, leaving a ring that will last for anywhere from one million to 100 million years, according to the two earth scientists at the University of California, Berkeley.





In a paper appearing online this week in the journal Nature Geoscience, postdoctoral fellow Benjamin Black and graduate student Tushar Mittal estimate the cohesiveness of Phobos. They conclude that it can’t resist tidal forces that will pull it apart when it gets closer to the Red Planet.





“Standing on the surface of Mars a few tens of millions of years from now, it would be pretty spectacular to watch,” Black said. However, it’s not clear whether the ring would be visible from Earth, as is the case with Saturn’s impressive rings, Black and Mittal said.





Just as earth’s moon pulls on our planet in different directions, raising tides in the oceans, for example, so too Mars tugs differently on different parts of Phobos. As Phobos gets closer to the planet, the tugs are enough to pull the moon apart, scientists say. This is because Phobos is already highly fractured, with lots of pores and rubble.





Dismembering it is like pulling apart a granola bar, Black said, scattering crumbs and chunks everywhere. The resulting rubble—rocks of various sizes and a lot of dust—would, in this view, continue to orbit Mars and quickly spread around the planet in a ring.





While the largest chunks would eventually spiral into the planet and collide at a grazing angle to produce egg-shaped craters, the researchers said, the majority of the debris would circle the planet for millions of years until these pieces, too, drop onto the planet in “moon” showers, like meteor showers. Only Mars’ other moon, Deimos, would remain.





Black and Mittal were drawn to the question of what might happen to Phobos because its expected fate is so different from that of most other known moons.





“While our moon is moving away from earth at a few centimeters per year, Phobos is moving toward Mars at a few centimeters per year, so it is almost inevitable that it will either crash into Mars or break apart,” Black said. “One of our motivations for studying Phobos was as a test case to develop ideas of what processes a moon might undergo as it moves inward toward a planet.”





Only one other moon in the solar system, Neptune’s largest moon, Triton, is known to be moving closer to its planet, Mittal and Black said.





Studying such moons is relevant to conditions in our early solar system, Mittal said, when it’s likely there were many more moons around the planets that have since disintegrated into rings—the suspected origins of the rings of the outer planets.





Some studies estimate that during planet formation, 20-30 percent of planets acquire moons moving inward and destined for destruction, though they would have long since disappeared. Some of Mars’ several thousand elliptical craters may even have been formed by remnants of such moonlets crashing to the surface at a grazing angle.





The nature of rings resulting from a Phobos breakup would depend on where the breakup happens, Black and Mittal said.





If it occurred relatively low, say 680 km (420 miles) from the surface, “it would form a really narrow ring comparable in density to that of one of Saturn’s most massive rings,” Mittal said. “Over time it would spread out and get wider, reaching the top of the Martian atmosphere in a few million years, when it would start losing material because stuff would keep raining down on Mars.”





If the moon breaks up farther from Mars, the ring could persist for 100 million years before raining down on Mars, they found.



