AstronomyPicture of the Day—20181127

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2018 November 27

Phobos:Doomed Moon of Mars

ImageCredit: Viking Project, JPL, NASA; Mosaic Processing: Edwin V. Bell II(NSSDC/Raytheon ITSS)

Explanation:This moon is doomed. Mars, the red planet named for the Roman god of war, hastwo tiny moons, Phobos and Deimos, whose names are derived from the Greek forFear and Panic. The origin of the Martian moons is unknown, though, with aleading hypothesis holding that they are captured asteroids. The larger moon,at 25-kilometers across, is Phobos, and is indeed seen to be a cratered,asteroid-like object in this false-colored image mosaic taken by the roboticViking 1 mission in 1978. A recent analysis of the unusual long grooves seen onPhobos indicates that they may result from boulders rolling away from the giantimpact that created the crater on the upper left: Stickney Crater. Phobosorbits so close to Mars - about 5,800 kilometers above the surface compared to400,000 kilometers for our Moon - that gravitational tidal forces are draggingit down. The ultimate result will be for Phobos to break up in orbit and thencrash down onto the Martian surface in about 50 million years. Well before that-- tomorrow, in fact, if everything goes according to plan -- NASA's roboticInSight lander will touch down on Mars and begin investigating its internalstructure.