Phobos, the largest — yet at just 16 miles wide still quite tiny — moon of Mars is getting ripped apart by the gravitational pull of its parent planet… and it bears the scars to show it, scientists have determined.
Long parallel grooves that wrap around the surface of Phobos are thought to be stress fractures — surface evidence of the tidal forces that will one day cause the moon to break apart entirely. This fate is not surprising to scientists, but that we’re seeing it in action is fascinating.
“We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves,” said Terry Hurford of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
As a planetary satellite Phobos really is an oddity. Besides its small size and irregular shape, it also orbits Mars at an extremely low altitude — only 3,721 miles (compare that to our own Moon’s 248,000-mile distance.) It thus needs to travel at a very high speed just to stay in orbit. It is actually traveling around Mars three times faster than Mars rotates (about 2,237 mph) and so Phobos rises in Mars’ western sky (but isn’t even visible from high-latitude polar regions.)
Even at that speed, though, Phobos can’t maintain its altitude. Every hundred years or so it falls 6.6 feet closer to Mars. After another 30 to 50 million years it will reach its Roche limit and, unable to support its structure under an uneven pull of gravity across its form, will completely crumble apart over Mars.
This scenario was actually first hypothesized after the Viking 1 orbiter imaged Phobos in June 1977, but at the time it was thought that the interior of the moon was fairly solid. Now, with more data in hand about Phobos’ composition and density, it’s thought to be more of a loose rubble pile with an elastic outer “skin” of powdery material…which allows Phobos to stretch but will ultimately lead to its structural collapse.