The larger of Mars’s two moons (the other being Deimos), Phobos is not circular in appearance like most other moons in the solar system. At its largest extreme it is 26 kilometres (16 miles) across, but only 18 kilometres (11 miles) across at its shortest.
Eons of meteoroid impacts have given Phobos a rather battered appearance, with dark trails resulting from landslides marking the steep slopes of the large craters on its surface, in addition to a host of smaller craters.
The moon is tidally locked to Mars, and its close proximity to the Red Planet – an average distance of 9,378 kilometres (5,828 miles) above its surface – means that half of the moon has a temperature of -4°C (25°F), while in contrast, the far outward-facing side can drop as low as -112°C (-170°F).
The largest feature on this Martian moon is the Stickney Crater (above), a ten-kilometre (six-mile)-wide crater caused by an impact from a large meteoroid. The crater is full of ﬁne dust and debris, suggesting that boulders slide down its sloped walls and settle further down in the basin.
Phobos is moving 20m (66ft) closer to the Red Planet every 100 years and is expected to crash into the surface of Mars or burn up in its atmosphere within the next 10 million years.
Images courtesy of NASA/JPL