This odd-shaped chunk of rock, as imaged by the Mars Express spacecraft last month, is Phobos, the larger of Mars' two moons. The moon is about 27 by 22 by 19 kilometers in size and despite its solid appearance, is about 25 to 35 percent porous (in the 1950s and 1960s, scientists speculated the that moon might be hollow). The pockmarks in the lunar surface are easily identified as impact craters (the largest of which—the one that took a chunk out of the left side of the moon in this image—is Stickney, an appellation that comes from the maiden name the wife of the man who discovered Phobos in 1877). But what are the grooves?
Researchers once thought that the grooves, about 30 meters deep and 100 to 200 meters wide, were made by the same impact that created Stickney Crater. However, by imaging the whole moon with Mars Express, they learned that the grooves do not all radiate from Stickney and can be grouped into 12 families of different age. Emily Lakdawalla explains on the Planetary Society blog:
present a different idea: that the grooves are secondary craters, but not from the Stickney impact; instead, they were made by ejecta from impacts that happened on Mars. Mars gets hit, stuff gets blasted into space, and Phobos, orbiting Mars quickly and closely about its equator, runs in to the streams of flying rocks, effectively splattering its windshield with chains of craters.
The images of Phobos from Mars Express will be used to select a landing site for the Russian Phobos-Grunt (meaning Phobos-Soil) mission. That mission will land a spacecraft on the moon's surface in 2011, collect samples of soil and return them to Earth in 2014.
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(HT: Bad Astronomy)