Red Planet Report

The inner martian moon Phobos is doomed. Scientists have long known that it is spiraling in toward Mars, and will break up from tidal forces before it falls into the planet sometime in the next few tens of millions of years. But a newly published paper says that the tidal forces have already-visible effects.

Phobos is shaped like a lumpy football, measuring 26 × 23 × 18 km (16 x 14 x 11 miles). It has a large crater, Stickney, on one end and its surface is scarred with a global system of grooves. These are largely symmetric around the side of Phobos that points directly toward Mars.

A new explanation for the grooves says that they are the result of tidal forces at work on Phobos now, and that they are the initial signs of Phobos eventually breaking up. The finding, which is the work of a team led by Terry Hurford (NASA Goddard Space Flight Center), appears in the Journal of Geophysical Research.

Previous work has attributed the grooves to ejecta or seismic shaking from the Stickney impact or to debris swept up by Phobos in its orbit. However, in the new work the team calculated the stress field of Phobos and found that the grooves “show that the first signs of tidal disruption are already present on its surface.”

The researchers note that their computer model requires a weak interior with very low rigidity on the tidal evolution time scale, overlain by an exterior shell some 10 to 100 meters thick. The shell has elastic properties similar to lunar regolith. Most of Phobos’ prominent grooves, they say, “have an excellent correlation with computed stress orientations.”  [More at link]