Mars' largest moon, Phobos, is slowly falling toward the planet, but rather than smash into the surface, it likely will be shredded¹ and the pieces strewn about the planet in a ring like the rings encircling Saturn², Jupiter, Uranus³ and Neptune⁴. Though inevitable⁵, the demise⁶ of Phobos is not imminent⁷. It will probably happen in 20 to 40 million years, leaving a ring that will persist for anywhere from one million to 100 million years, according to two young earth scientists at the University of California, Berkeley.

 

In a paper appearing online this week in Nature Geoscience, UC Berkeley postdoctoral fellow Benjamin Black and graduate student Tushar Mittal estimate the cohesiveness⁸ of Phobos and conclude that it is insufficient⁹ to resist the tidal forces that will pull it apart when it gets closer to Mars. 

 

Just as earth's moon pulls on our planet in different directions, raising tides in the oceans, for example, so too Mars tugs¹⁰ 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 rubble¹¹. Dismembering it is analogous¹² to pulling apart a granola bar, Black said, scattering¹³ crumbs¹⁴ and chunks¹⁵ 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 craters¹⁶, the majority of the debris¹⁷ 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.

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