How Mars is tearing apart its own moon

Mars and its moon Phobos are far closer together than Earth and its moon, causing a strong gravitational pull that will eventually tear the moon apart. 

The Curiosity rover observes the moon Phobos grazing the sun's disk on Martian day, or sol, 37 (September 13, 2012) in this NASA handout image.

NASA confirmed Tuesday that Mars’s moon Phobos is showing early signs of structural failure that will likely lead to the moon’s demise.

Visible grooves on Phobos are caused by gravitational tidal forces as Mars and Phobos mutually pull on one another, just as Earth and our moon pull on each other. 

“We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves,” Dr. Terry Hurford, a researcher with NASA’s Goddard Space Flight Center, said in a press release.

Phobos, one of the Red Planet's two moons, orbits only 3,700 miles above Mars, making it the closest planet-moon relationship of any planet in our solar system. By comparison, our moon orbits about 238,855 miles above Earth. And, while Earth's moon is inching away from our planet, Phobos is moving closer: Mars continues to pull in its moon at a rate of about 6.6 feet per century.

But fortunately for Phobos, scientists predict the moon has about 30 to 50 million more years before it is completely pulled apart.

Scientists were already aware of Phobos’ grooves and cracks, but until now they believed these “stretch marks” were caused by the impact from the Stickney crater. The crater is more than six miles in diameter, which is almost half of the irregularly shaped moon's overall average diameter, so until now it seemed like an obvious cause for the cracks.

Further analysis of the moon’s cracks found that they do not radiate outward from the Stickney crater, but instead radiate from another nearby point. As another source of evidence against the Stickney theory, some of the grooves are younger than others, “which would be the case if the process that creates them is ongoing” and not a single, forceful impact.    

Scientists have also revised their thinking regarding the consistency of Phobos’ interior. Previously thought to be solid, scientists have recently agreed that the interior of Mars’ moon is likely a rubble pile, barely held together by a 330-foot layer of powder. Scientists associated with the study changed their way of thinking because even the strong tidal forces between Phobos and Mars were “too weak to fracture a solid moon of that size.”

“The funny thing about the result is that it shows Phobos has a kind of mildly cohesive outer fabric,” Dr. Erik Asphaug, a professor of planetary science at the University of Arizona and co-investigator of the study said in a NASA press release. “This makes sense when you think about powdery materials in microgravity, but it’s quite non-intuitive.”

Not only is Phobos closer to its planet than Earth’s moon, but it also has a faster orbit. Phobos revolves around Mars up to three times during one Martian day, rising in the west and setting in the east. 

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