Phobos, the larger and inner of the two natural satellites of Mars, is slowly being pulled apart by tidal forces and is expected to break up within the next 50 million years, says a team of planetary researchers led by Dr Terry Hurford of NASA’s Goddard Space Flight Center.
The grooves on Phobos, the larger of the two Martian moons, could be produced by tidal forces – the mutual gravitational pull of Mars and the moon. Initially, planetary scientists had thought the grooves were created by the massive impact that made Stickney crater (lower right). Image credit: NASA / JPL-Caltech / University of Arizona.
Phobos was discovered along with its smaller companion, Deimos, by the American astronomer Asaph Hall at the Naval Observatory in Washington, D.C., on August 18, 1877. It is named after the Greek god Phobos, a son of Ares and Aphrodite.
The moon orbits 3,721 miles (5,989 km) from the Martian surface and completes an orbit in just 7 hours and 39 minutes.
It orbits so close to the Martian surface that the curvature of the planet would obscure its view from an observer standing in Mars’ polar regions. Its orbital period is about 3 times faster than the rotation period of the Red Planet, with the unusual result among natural satellites that Phobos rises in the west and sets in the east as seen from Mars.
The origin of this strange moon is being studied in detail but remains unclear. One scenario is that Phobos is a captured asteroid. It is also possible that the moon formed in situ at Mars, from ejecta from impacts on the planet’s surface, or from the remnants of a previous moon which had formed from the Martian accretion disc and subsequently collided with a body from the asteroid belt.
Phobos is too light for gravity to make it spherical. It has a very lumpy appearance, and is heavily cratered. Its dominant feature is a huge impact crater, called Stickney, which was the maiden name of Asaph Hall’s wife.
Phobos also has very deep (100 feet or 30 m) grooves in the vicinity of Stickney crater. These grooves were long thought to be fractures caused by the impact that formed the crater. That collision was so powerful, it came close to shattering Phobos.
However, planetary researchers eventually determined that the grooves don’t radiate outward from the crater itself but from a focal point nearby. More recently, they have proposed that the grooves may instead be produced by many smaller impacts of material ejected from Mars.
But a new model developed by Dr Hurford and co-authors supports the view that the grooves are more like stretch marks that occur when the moon gets deformed by tidal forces.
According to the scientists, Mars’ gravity is drawing in Phobos by about 6.6 feet (2 m) every hundred years. They expect the moon to be pulled apart in 30 to 50 million years.
“We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves,” Dr Hurford said.
The tidal forces acting on Phobos can produce more than enough stress to fracture the surface.
“Our model results applied to surface observations imply that Phobos has a rubble pile interior that is nearly strengthless,” the scientists said. “A lunar-like cohesive regolith outer layer overlays the rubble pile interior. This outer layer behaves elastically and can experience significant tidal stress at levels able to drive tensile failure. Fissures can develop as the global body deforms due to increasing tides related to orbital decay.”
Stress fractures predicted by the new model line up very well with the grooves seen in images of this small moon. This explanation also fits with the observation that some grooves are younger than others, which would be the case if the process that creates them is ongoing.
According to the team, the same fate may await Neptune’s moon Triton, which is also slowly falling inward and has a similarly fractured surface. The study also has implications for exoplanets.
Dr Hurford and co-authors reported their results yesterday at the 47th Annual Division for Planetary Sciences Meeting in National Harbor, Maryland.
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Terry Hurford et al. 2015. Surface Evolution from Orbital Decay on Phobos. 47th Annual Division for Planetary Sciences Meeting. Abstract # 201.09
