Mars to lose its largest moon, Phobos, but gain a ring

Phobos is slowly falling towards the planet and is likely to be shredded into pieces that will be strewn about the planet

Mars' largest moon, Phobos, is slowly falling towards the planet and is likely to be shredded into pieces that will be strewn about the red planet in a ring like those encircling Saturn and Jupiter, scientists, including one Indian-origin, say.

Though inevitable, the demise of Phobos is not imminent. 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 scientists at the University of California, Berkeley.

UC Berkeley postdoctoral fellow Benjamin Black and graduate student Tushar Mittal estimate the cohesiveness of Phobos and conclude that it is insufficient 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 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. 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.

Black and Mittal, both in UC Berkeley's Department of Earth and Planetary Science, were drawn to the question of what might happen to Phobos because its fate is expected to be so different from that of most other moons in our solar system.

"While our moon is moving away from Earth at a few centimetres per year, Phobos is moving towards Mars at a few centimetres per year, so it is almost inevitable that it will either crash into Mars or break apart," Black said.

Only one other moon in the solar system, Neptune's largest moon, Triton, is known to be moving closer to its planet.

Studying such moons is relevant to conditions in our early solar system, Mittal said, when it is 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.

The research appears in the journal Nature Geoscience.