Mars is ripping its moon apart

Phobos will eventually disintegrate and form a ring around the red planet. Belinda Smith reports. 

Naming the red planet for the god of war was a prescient move. But naming it for Hercules, the god who slaughtered his own children, would have been better.

Mars, it turns out, is mercilessly crushing one of its own moons. When its death throes are finally over, the moon Phobos will have disintegrated into a orbiting disc of dust and rock, and Mars will be a ringed planet, the Solar System’s fifth. That’s the conclusion of a study in Nature Geoscience in November by Benjamin Black and Tushar Mittal from the University of California, Berkeley.

Mars has two moons, Deimos and Phobos. Phobos, the innermost, larger moon is creeping closer to Mars by a few centimetres each year, and is clearly destined for destruction. The question was, would death be by a fatal plunge into Mars’ surface, or by being torn asunder in orbit?

“[Black and Mittal have] rather neatly shown that before Phobos crashes on to Mars, it’ll be ripped apart”, says planetary scientist Helen Maynard-Casely of the Australian Nuclear Science and Technology Organisation in Sydney.

Black and Mittal used several lines of evidence to build their case. Astronomers think Phobos, like sister Deimos, started life as a ‘rubble pile’ asteroid – a conglomerate of rock pieces squished together like a clod of soil – that was captured by Mars’ gravity.

Phobos' most prominent feature is the huge impact crater, almost 10 kilometres in diameter, known as the Stickney crater. A smaller crater, created by a later impact, is visible within it.CREDIT: NASA /JPL-CALTECH / UNIVERSITY OF ARIZONA

By analysing a 10-kilometre-wide impact crater scaring Phobos’ surface, Black and Mittel worked out the clod’s strength. Had Phobos been solid rock, an impact this size would have shattered it. Had Phobos been a loose bundle of rubble, it would have disintegrated. Phobos fell in between – a pebbly, crumbly composition held together by gravity. 

Next, Black and Mittel examined the forces Mars exerts on Phobos. ‘Stretch marks’ on Phobos’s surface are graphic reminders of how the moon has already been stretched and squeezed by Mars’ gravitational pull.

As Phobos spirals closer to Mars, the researchers concluded, these forces will take it to breaking point. At the moment, Phobos orbits 9,400 kilometres from Mars’ surface. Once Phobos gets to between 4,700 and 680 kilometres, in roughly 20 to 40 million years, it will rupture, its rubble forming a ring around Mars.

Retribution for Mars won’t be swift, but it will be certain. Its new ring will eventually disintegrate, in up to 100 million years. And its other moon Deimos, orbiting at around 23,000 kilometres and so protected from the destructive gravity of Mars, will continue to slowly pull away. Millions of years down the track, Deimos will fling off into the Solar System, leaving Mars alone. Moonless. Ringless.