Mars's Moon Phobos Is Likely Crumbling Away, Says NASA

Stretch marks across the surface of Phobos could indicate the Mars moon is crumbling away. The shallow grooves that run across Phobos have baffled scientists for years, and on Tuesday researchers with NASA and several other institutions said they believe the grooves are the result of gravitational forces and indicate "the structural failure that will ultimately destroy this moon," according to NASA.

One of the researchers, Terry Hurford of NASA's Goddard Space Flight Center, says scientists began studying the grooves when images of Phobos came out in the 1970s. "The earlier models tended to treat Phobos as one density and one strength, which led to stresses that weren't high enough to cause fractures," Hurford says. Instead, Hurford and his colleagues tried a model of Phobos with a weak inner layer that when stretched by gravity causes fractures in the outer layer.

According to their model, Phobos is "very weak inside and has kind of an elastic outer shell that's breaking continuously," says Erik Asphaug of the School of Earth and Space Exploration at Arizona State University, who was also a researcher on the project.

Because Phobos is so close to Mars—3,700 miles, compared with the average 237,675 miles between Earth and its moon—there's significant pull between the two bodies, Hurford says. Mars "steals a little bit of its orbital energy. And as it steals the energy, it causes the orbit to become smaller to smaller." The researchers say Mars will continue pulling in Phobos at an accelerated rate. Right now, that rate is a few inches per year, they say; later, it could be 100 meters per year. Then Phobos could eventually break apart and crash into the planet.

The collapse won't happen for another 30 to 50 million years, the researchers say. Asphaug says there has been talk of humans exploring Phobos; given the new research, this seems unwise. "It's looking like Phobos might be kind of a wild carnival ride from a geology point of view. We might want to be safer and hang out a little farther back at Deimos," he says, referring to the other moon of Mars.

To study the grooves, the researchers used images from the Viking missions, which launched in the 1970s, the Mars Express mission, which has been photographing Mars since 2003 and the Mars Reconnaissance Orbiter, which launched in 2005. "There's never been a dedicated Phobos mission that's succeeded," says Asphaug. "Almost everything we know about Phobos is sort of obtained secondhand by these Mars missions, whenever they get time to point the camera at Phobos. We really haven't had the unified focus on Phobos that we've had on other satellites."

Earlier research blamed the grooves on impacts from material that flew off Mars and struck the moon—ejecta, in astrophysics parlance. Last year, a study published in the journal Planetary and Space Science said those impacts had formed the grooves 3 billion years ago.

"It's true some Mars ejecta does hit Phobos," Asphaug says, "but for that ejecta to maintain these kind of razor sharp stripes all the way around from Mars out to the distance of Phobos, that's turned out to be impossible."

Ken Ramsley, who lectures on planetary geoscience at Brown University and was not involved in the research, says the modeling is well done, but "we really don't know if these things are true, so it's sort of like you're stuck with the results you get based on your assumptions."

If the model does turn out to be accurate, there will be some visually striking phenomena: The material Phobos sheds after it breaks apart could form a debris ring around the planet. "Mars will for some brief moment in geological time look a lot like Saturn," Asphaug says. "It will have this beautiful ring system." That ring material would eventually crash down to the surface of the planet.