Phobos has spent its life slowly spiraling toward Mars. As it cozies up to its host planet, gravity stretches the moon, which appears to be fracturing already (SN Online: 11/11/15). But Phobos’ ultimate fate depends on how strongly it is held together. The moon is probably fragile, it turns out, and will crumble and spin into a ring in the next 20 million to 40 million years, say planetary scientists Benjamin Black and Tushar Mittal, both of the University of California, Berkeley. Their findings appear online November 23 in Nature Geoscience.
By piecing together data on the moon’s composition, density and the state of its largest crater, Black and Mittal inferred that Phobos is a loose conglomeration of rubble. Computer simulations then revealed the moon’s grim future. Pieces of Phobos will survive in a new ring around Mars for anywhere between 1 million and 100 million years after breaking apart, but the moon won’t be permanently forgotten. Some tightly bound chunks could leave a lasting impact by adding a few craters to the Martian surface.