Spinning in orbit just 3,700 miles above the Mars' surface, the planet's largest moon, Phobos, seems to be undergoing a "structural failure," NASA says, providing a new explanation about a moon whose odd features have sparked many theories — including the idea that Phobos is hollow.

"Mars' gravity is drawing in Phobos, the larger of its two moons, by about 6.6 feet (2 meters) every hundred years," NASA says. "Scientists expect the moon to be pulled apart in 30 to 50 million years."

The story of Phobos' doom is written on its surface, where grooves that resemble stretch marks signal stress fractures, according to NASA scientists.

"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 Maryland.

The new theory, presented at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society, is an update on ideas that held that the striations on Mars were impact fractures, caused by either the same impact that made the large Stickney crater or by a series of smaller impacts.

Discussing the moon's interior, NASA says, "the interior of Phobos could be a rubble pile, barely holding together, surrounded by a layer of powdery regolith about 330 feet (100 meters) thick."

That weak structure seems to be held together by "a kind of mildly cohesive outer fabric," according to Erik Asphaug of Arizona State University's School of Earth and Space Exploration. He's a co-investigator on the Phobos study that was presented Tuesday.

Measuring less than 17 miles across at its major axis, Phobos' weak interior structure allows stress from tidal forces to alter its surface, the researchers say.

The new analysis of Phobos comes years after the idea that the moon is hollow was put forth by Russian scientist Iosif Shklovsky (for whom a crater on the moon is named), who collaborated with Carl Sagan on the book Intelligent Life in the Universe.

In part, the Phobos-as-hollow-satellite theory was based on the speed and acceleration of Phobos' orbit. Unusually, the moon rotates around Mars much faster than the planet itself spins. But Shklovsky and others were also trying to explain the moon's lack of density.

Those qualities — and the tendency for space probes to go missing once they reach Phobos — have helped make the moon an enduring mystery.

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