Mars may one day get a ring of its own

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With Mars’ largest moon Phobos expected to eventually breakup, scientists have wondered whether it would crash into the Red Planet or do something else.

Now, a team from UC Berkeley is predicting that Phobos would be shredded into pieces before it reaches Mars, a process that would result in a ring of debris encircling the planet as is the case with Saturn, Jupiter, Uranus and Neptune.

“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,” Tushar Mittal, who co-authored a paper in Nature Geoscience on the moon with Benjamin Black, 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.”

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 and the rings might look like shadows when viewed through a telescope from Earth.

“Standing on the surface of Mars a few tens of millions of years from now, it would be pretty spectacular to watch,” Black said.

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The process - which won’t happen for some 20 to 40 million years - is driven by tidal forces that will eventually pull Phobos apart as it approaches Mars.

Just as Earth’s moon pulls on our planet in different directions, rising 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 actually pull the moon apart, the scientists say. This is because Phobos is highly fractured, with lots of pores and rubble.

“As you move further in, the tidal forces would keep increasing and ultimately they exceed the strength of Phobos and that is when you would have a breakup,” Mittal told “We are reasonably confident that it will breakup with the caveat that we still don’t know the interior structure of Phobos.  Future missions to Phobos would sort of help us improve that information and improve the model.”

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. Eventually, they would drop onto the planet in ‘moon’ showers, like meteor showers.

Only Mars’ other moon, Deimos, would remain.

The researchers were drawn to the question of Phobos because its fate is expected to be much different than other moons in the solar system.

Related: Mars' moon Phobos could be headed for destruction

“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.

Along with understanding the future of Phobos, the scientists also it can shed light on Mars formation and whether it had any other moons that suffered a similar fate. Based on the process that led to Mars forming, previous models have suggested that there could have been other moons that have gone undiscovered.

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“In terms of past moons, one thing we talked about in the paper was if we modeled Phobos not to breakup and then impact Mars, what would those impact craters look like?” Mittal said.

“If we look on Mars currently, there are thousands of impact craters which have similar shape to what you expect for an inward evolving moon,” he said. “Another thing that we plan to look at in the future is to look at those impact craters and see if we can figure out whether some of them were produced by inward evolving moons as evidence for moons that Mars might have had in the past.”