The Waning Gibbous
20.8 days old
64.1 percent lit (Quarter Moon)
30 October 2015
Phobos is falling apart. (click here) A set of enigmatic grooves on the surface of Mars’s larger moon suggests that the gravity of its parent planet is slowly tearing it to shreds.
We already knew that Phobos was doomed to destruction. It is so close to Mars that the planet’s tidal pull drags on the satellite, slowing it down and shrinking its orbit. In tens of millions of years, those forces are expected to rip Phobos apart before it can crash into Mars.
But Phobos now seems to be showing signs of wear. In the 1970s, the Mariner 9 and Viking orbiters uncovered long, often parallel grooves 100 to 200 metres wide and 10 to 30 metres long, stretching across parts of Phobos. At the time, researchers assumed that Phobos was a homogeneous lump of rock, and thought the grooves were cracks from a giant impact, or rows of small craters formed by debris blasted into space by impacts on the Martian surface.
But in 2008, the Mars Express spacecraft showed that Phobos is actually a pile of rubble
held together by a stronger outer layer of dust 50 to 100 metres thick.
That means Phobos looks a bit like a beanbag: easily deformed, but held
together by a covering....The illustration at right is a composite of the two moons of Mars with Deimos (left), and Phobos (right).
