Mars could gain a ring in 10-20 million years when its moon Phobos is torn to shreds by tidal forces due to Mars’ gravitational pull. Credit: Image by Tushar Mittal using Celestia 2001-2010, Celestia Development Team.
In 10-20 million years, the moon will get so close to Mars that it’ll be shredded into a ring, eventually be torn apart by tidal forces and distributed in a ring around the planet, a study of the cohesiveness of Phobos has concluded. This would take about 10-20 million years, and the ring will persist for up to 100 million years before the dust falls into Mars’ atmosphere and burns up as ‘moon’ showers.
The loss is due to Phobos being highly fractured, with lots of pores and rubble. Dismembering it is analogous to pulling apart a granola bar, Black said, scattering crumbs and chunks everywhere. The resulting rubble from Phobos – rocks of various sizes and a lot of dust – would continue to orbit Mars and quickly distribute themselves 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 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.
“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 moving closer to its planet.
Studying such moons is relevant to conditions in our early solar system, 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% 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.
To estimate the strength of Phobos, Black and Mittal looked data from similarly fractured rocks on Earth and from meteorites that struck Earth and have a density and composition similar to Phobos + simulations of the 10-kilometer diameter Stickney impact crater, which formed in the past when a rock rammed into Phobos without quite smashing the moon apart. That crater spans 1/6 the circumference of Phobos. Once they determined when and how they expected tidal forces to tear Phobos apart, Mittal modeled the evolution of the ring, adapting techniques developed to understand Saturn’s rings.
“If the moon broke apart at 1.2 Mars radii, about 680 kilometers above 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.
Mittal said it’s not clear whether the dust and debris rings would be visible from earth, since dust does not reflect much sunlight, whereas ice in the rings of the outer planets makes them easily visible. But Mars’ ring may reflect enough light to make Mars slightly brighter as seen from Earth, he said, and through a telescope the shadows of the rings might also be visible on the surface. http://news.berkeley.edu/2015/11/23/mars-to-lose-its-largest-moon-but-gain-a-ring/
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