The largest crater on Mars’s moon Phobos is so big that the impact which dug it nearly destroyed the moon itself.
NASA marked October 9 with an Astronomy Picture of the Day devoted to Stickney Crater, a scar more than 9 kilometres across on a moon barely twice that wide. The crater is named for Chloe Angeline Stickney Hall, a mathematician whose husband, astronomer Asaph Hall, discovered both of Mars’s moons in 1877.
The featured image, recorded by the HiRISE camera aboard the Mars Reconnaissance Orbiter as it passed within some 6,000 kilometres of Phobos in March 2008, shows a battered, grooved world. Streaks running down the inside of the crater walls suggest loose material has slid downhill over the ages — a slow-motion landslide landscape on a body whose surface gravity is less than a thousandth of Earth’s, where an astronaut could nearly throw a baseball into orbit.
Light bluish regions near the rim may expose relatively fresh surface, while the curious grooves scoring Phobos remain debated: they may trace tidal stresses on a moon slowly spiralling toward Mars, or fractures radiating from the Stickney impact itself.
Phobos is doomed either way; orbital decay will eventually shred it into a ring around Mars. Images like this one are the record of a moon living on borrowed time — and a reminder that the solar system’s most violent history is written in craters, for anyone equipped to read it.
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