Feeble Phobos flaking as it falls to Mars

Grooves show how moon is cracking up, says NASA

Cracks on Phoebus - NASA Goddard
Phobos: Mars's groovy satellite is falling to bits. Image: NASA/JPL-Caltech/University of Arizona

Mars' larger moon, Phobos, is already showing signs of the structural failure that will one day mean it breaks up, according to boffins from NASA Goddard.

The finding, announced here, was presented to November 10's annual Meeting of the Division of Planetary Sciences of the American Astronomical Society in Maryland.

Goddard's Terry Hurford explains that the moon is so close to Mars (its orbit is just 6,000 km away from the Red Planet) that it's subject to huge tidal forces.

In fact, NASA's release notes, the idea that the grooves on the moon are the result of tidal stresses isn't new.

When the Viking spacecraft sent back their first close-ups in the mid-1970s, the same explanation was offered for the grooves, but there was a problem: if the moon was solid, the tidal forces wouldn't be sufficient to break it up.

Now, however, space boffins are more inclined to think of the moon as having an interior of rubble surrounded by a powdery regolith around 100 metres thick. That kind of structure is elastic enough to distort, showing stress fractures on the surface.

Since Phobos is also losing altitude at the rate of 2 metres every hundred years, the stresses will get worse.

Another curiousity is that the dusty regolith hangs together well enough to show the cracks, as Hurford's co-investigator Erik Asphaug (Arizona State University's School of Earth and Space Exploration) points out:

“The funny thing about the result is that it shows Phobos has a kind of mildly cohesive outer fabric. This makes sense when you think about powdery materials in microgravity, but it's quite non-intuitive.”

Earlier hypotheses about the grooves suggested they might have been caused by the same impact as caused a feature called the Stickney crater, but the grooves emanate from a focal point slightly outside the crater; or that they are an accumulation of smaller impacts.

Phobos' future? Hurford and Asphaug reckon it will be pulled apart in 30-50 million years. ®

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