Phobos starting to crack under pressure

OXON HILL, Md. — Mars’ moon Phobos is stressed and starting to crack under pressure. A network of grooves encircling the moon are early signs that the Red Planet’s gravity is splintering Phobos, Terry Hurford reported November 10 at a meeting of the American Astronomical Society’s Division for Planetary Sciences.

Phobos, unlike its sibling satellite Deimos, is slowly spiraling toward Mars. “As [Phobos] gets closer… it gets pulled out into a football shape,” says Hurford, a planetary scientist at the NASA Goddard Space Flight Center in Greenbelt, Md. “The changing shape causes stresses that we think form the grooves.” Sometime in the next 50 million years, the stress will become too much and the moon will break apart.

The grooves were first seen by the Viking landers in 1976, and some researchers proposed stress fractures as a cause. But scientists also thought Phobos was a monolithic chunk of rock, making it hard to crack. If Phobos is instead a loose conglomeration of rubble slathered with a rocky crust (SN: 6/5/10, p. 11), as Hurford assumes in his computer simulations, Mars’ gravity could deform the moon and form the cracks seen today.

“It all fits together pretty well,” says Alan Harris, a planetary scientist at the Space Science Institute in Boulder, Colo. Harris is one of the scientists who proposed in 1977 that the grooves were Mars’ fault. Stress can’t explain every fissure, however; some grooves aren’t lined up the way stretch marks from Mars’ gravity should be, he notes. “The sticky wicket continues to be these other grooves.”

A rain of debris from meteorites probably created those misfit grooves, suggested Michael Nayak, a graduate student at the University of California, Santa Cruz, in a separate presentation at the meeting. Nayak ran computer simulations that showed that some rocks blown off the moon’s surface stay in orbit around Phobos for a while. The debris eventually falls back on Phobos and can form grooves that resemble the anomalous trenches.

Harris agrees that the two hypotheses together offer a tidy explanation for the Phobos fissures. While scientists generally like to keep explanations simple, he says, “every now and then, nature doesn’t have a simple cause.”