NASA announced on Tuesday that the days of Phobos, one of the moons of Mars, are numbered. Recent images of the moon show long, shallow grooves in its surface that show the early signs of structural failure. In about 30 million to 50 million years, Phobos will be pulled apart by Mars’ gravity.
Currently, Phobos orbits Mars at a distance of 3,700 miles, closer to its planet than any other moon in the solar system. Mars is slowly pulling Phobos closer to it at a rate of 6.6 feet per second. The tidal forces of Mars are already acting on the moon as it hurtles slowly but steadily closer, suggests Terry Hurford of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“Phobos’ grooves were long thought to be fractures caused by the impact that formed Stickney crater. That collision was so powerful, it came close to shattering Phobos. However, scientists eventually determined that the grooves don’t radiate outward from the crater itself but from a focal point nearby.
“More recently, researchers have proposed that the grooves may instead be produced by many smaller impacts of material ejected from Mars. But new modeling by Hurford and colleagues supports the view that the grooves are more like “stretch marks” that occur when Phobos gets deformed by tidal forces.
“The gravitational pull between Mars and Phobos produces these tidal forces. Earth and our moon pull on each other in the same way, producing tides in the oceans and making both planet and moon slightly egg-shaped rather than perfectly round.
“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.”
Phobos is an irregularly shaped object that is about 27 by 22 by 18 kilometers. It is too small to be rounded by its own gravity. It has the same characteristics as a carbonaceous chondrite asteroid. Scientists argue whether Phobos is a captured main belt asteroid or whether it formed concurrently with Mars.
Current NASA planning suggests that an expedition to Phobos would be a good precursor mission to the eventual crewed landing on the lunar surface. The concept would be for a crew of astronauts to land on the moon and secure a habitat on its surface. Then they would teleoperate robots on the Martian surface remotely. The mission would fulfill all of the objectives of a Mars expedition with the exception of landing on the Martian surface, just as Apollo 8 paved the way for the eventual moon landings.