Mars is slowly tearing apart one of its two moons, and the circling body, dubbed Phobos, could eventually completely disappear, astronomers say.
No need to mark your calendars, they say, since we're talking 30 to 50 million years from now.
Parallel grooves on the surface of Phobos, once believed fractures left behind by a long-ago cosmic collision with another body, are now thought to be early signs of a structural failure that will lead to eventual destruction — like slowly widening cracks in a sidewalk.
That long ago impact came close to shattering Phobos and left it an agglomeration of rubble held together by a thin surface crust, scientists suggest, essentially just a big orbiting sack of rocks.
The gravitational pull of Mars on Phobos may be weakening that outer layer, they say; the moon is closer to Mars that any other moon in the solar system is to its planet, orbiting at just 3,700 miles above the Red Planet.
"We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves," says Terry Hurford of NASA's Goddard Space Flight Center in Greenbelt, Md.
Scientists have characterized the grooves seen on the surface of Phobas as "stretch marks," strong evidence the outer surface of the moon, which may be no more than 100 meters thick, is slowly being deformed by the tidal forces of Mars' gravity.
"The funny thing about the result is that it shows Phobos has a kind of mildly cohesive outer fabric," says Erik Asphaug of the School of Earth and Space Exploration at Arizona State University in Tempe.
That outer layer can behave elastically and readjust to distortions in the moon's interior caused by Mars' gravity, but it's thin and weak enough that it could, if stressed too far, fail, the researchers say.
Modeling of such stresses creates hypothetical fractures that line up very well with the grooves observed on the moon's surface, they say.
That leads them to suggest tiny Phobos, just 13.5 miles in diameter, could ultimately be destroyed in around 30 to 50 million years, as Mars pulls the moon closer to it by about 6.6 feet every hundred years.
The phenomenon isn't unique, scientists say; a similar fate may await Neptune's moon Triton, which is also circling ever closer to its parent planet and displays a surface that is also fractured.
