Numerous, mysterious, long, shallow grooves on Phobos – the larger and closer of Mars’ two moons – have been known since 1976, when the two Viking orbiters flew near. At first, scientists believed the grooves were associated with Phobos’ large crater Stickney, which is half the size of Phobos itself. It made sense to think that whatever struck Phobos to make Stickney created the grooves as well. More recently, scientists determined that the grooves don’t radiate outward from the large crater, but instead from a focal point nearby. And this week there was more news about the grooves on Phobos. Scientists announced that they are likely early signs of the structural failure that will ultimately destroy this moon of Mars.
In this scenario, the reason for Phobos’ ultimate destruction is the mutual gravity between it and Mars. Phobos is closer to its planet than any other moon in the solar system, orbiting just 3,700 miles (6,000 km) above the surface of Mars. That’s in contrast, for example, to our own moon’s quarter-million-mile distance from Earth. A NASA statement on November 10, 2015, said:
Mars’ gravity is drawing in Phobos, the larger of its two moons, by about 6.6 feet (2 meters) every hundred years. Scientists expect the moon to be pulled apart in 30 to 50 million years.
This result comes from Terry Hurford of NASA’s Goddard Space Flight Center. He and and his colleagues are presenting the work this week at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society at National Harbor, Maryland. Hurford said in the NASA statement:
We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves.
Hurford and colleagues compared the grooves to stretch marks that occur as Phobos is deformed by tidal forces between it and Mars. Tidal forces occur between our Earth and moon, too, creating ocean tides and making both the Earth and the moon slightly egg-shaped rather than perfectly round. The NASA statement explained:
The same explanation was proposed for the grooves decades ago, after the Viking spacecraft sent images of Phobos to Earth. At the time, however, Phobos was thought to be more-or-less solid all the way through. When the tidal forces were calculated, the stresses were too weak to fracture a solid moon of that size.
The recent thinking, however, is that 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 … An interior like this can distort easily because it has very little strength and forces the outer layer to readjust. The researchers think the outer layer of Phobos behaves elastically and builds stress, but it’s weak enough that these stresses can cause it to fail.
All of this means the tidal forces acting on Phobos can produce more than enough stress to fracture the surface…
The same fate may await Neptune’s moon Triton, which is also slowly falling inward and has a similarly fractured surface. The work also has implications for extrasolar planets, according to researchers.
Bottom line: The mysterious grooves on Phobos are likely early signs of structural failure. Scientists expect the moon to be pulled apart in 30 to 50 million years. Scientists are presenting the work this week at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society at National Harbor, Maryland.