Mars’ Gravity Is Slowly Shredding Its Moon Phobos

Telltale grooves on the moon’s surface are an early sign of Phobos’ grim fate

NASA/Science Faction/Corbis

Ever since scientists got their first glimpse at Mars’ moon Phobos, they have wondered what could have given the misshapen moon its mysterious grooves. Now, NASA scientists believe these marks are signs that Mars’ gravity is slowly ripping Phobos apart.

Scientists have known for years that the lumpy little moon is bound to be destroyed sometime in the next 50 million years or so. Phobos has the closest orbit of any moon in the solar system and is only about 3,700 miles away from Mars (in comparison, our moon is about 238,900 miles away). That means that compared to our Earth-moon system, Mars' gravity affects its moon much more—Phobos actually draws closer to Mars at a rate of roughly 6.6 feet every 100 years.

According to recent research, Phobos' "stretch marks" indicate that the moon is feeling the pull in another way—it is starting to collapse. "We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves," NASA scientist Terry Hurford says explains in a statement.

This theory was first proposed in the 1970s after Phobos’ grooves were discovered in photos taken by the Mariner 9 and Viking orbiters. At the time, NASA scientists believed that Phobos was completely solid, just like our own moon. Because the grooves appear to stretch from Phobos’ massive Stickney Crater (which is almost half as big as the moon itself), scientists thought they might have been the result of whatever impact created the crater, Deborah Byrd writes for

But new evidence suggests that Phobos is actually a big pile of rubble held together by a few hundred feet of space dust, meaning that Mars’ gravity and tidal forces could easily tear the moon to shreds.

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

While our moon is a solid, homogenous hunk of rock, Phobos is more like a beanbag, Jeff Hecht writes for New Scientist. Although the moon’s surface holds it together for now, its insides can shift around easily, giving Phobos its lumpy, uneven shape.

When Hurford and Asphaug applied that model to a simulation recreating Mars’ tidal forces, they discovered that the areas of highest stress on the moon’s structure neatly lined up with Phobos’ grooves. However, it’s still unclear how long Phobos’ thin surface might hold the moon together.

“We have not looked how far we can go before it completely fails,” Hurford tells Hecht. On the bright side, there’s plenty of time before that might happen, as Hurford estimates that Mars’ tides could tear the moon to pieces sometime in the next 30 to 50 million years.

In the meantime, these findings could help NASA scientists prepare to build a possible moonbase on Phobos as well as understand how distant stars might tear their own planets apart the same way.

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