Phobos is falling apart. A set of enigmatic grooves on the surface of Mars’s larger moon suggests that the gravity of its parent planet is slowly tearing it to shreds.

We already knew that Phobos was doomed to destruction. It is so close to Mars that the planet’s tidal pull drags on the satellite, slowing it down and shrinking its orbit. In tens of millions of years, those forces are expected to rip Phobos apart before it can crash into Mars.

But Phobos now seems to be showing signs of wear. In the 1970s, the Mariner 9 and Viking orbiters uncovered long, often parallel grooves 100 to 200 metres wide and 10 to 30 metres long, stretching across parts of Phobos. At the time, researchers assumed that Phobos was a homogeneous lump of rock, and thought the grooves were cracks from a giant impact, or rows of small craters formed by debris blasted into space by impacts on the Martian surface.


But in 2008, the Mars Express spacecraft showed that Phobos is actually a pile of rubble held together by a stronger outer layer of dust 50 to 100 metres thick. That means Phobos looks a bit like a beanbag: easily deformed, but held together by a covering.

Using that model, Terry Hurford of the NASA Goddard Space Flight Center in Maryland and colleagues calculated what stress tidal forces would cause on Phobos – and found that most grooves are perfectly aligned with the regions of greatest stress.

“The grooves are the first sign of tearing it apart,” Hurford says. He will present the results on 4 November at a meeting of the Geological Society of America.

“I find the results very interesting,” says Alexander Basilevsky of the Vernadsky Institute of Geochemistry and Analytical Chemistry in Moscow, who has written a review of the surface features of Phobos. He agrees that the grooves could be faults, and thinks this could explain why some of them appear to criss-cross each other.

The moon is in no immediate danger, Hurford says – it could still survive for millions of years. “We have not looked how far we can go before it completely fails,” he says, and no one knows the strength of the beanbag’s shell or how well it can hold Phobos together. Finding out may mean landing there – gingerly.

(Image: ESA/DLR/FU Berlin, CC BY-SA IGO 3.0)