One of the best close-ups ever taken of the Martian moon Phobos reveals fresh details of the strange object.
The impact crater named Stickney is the largest feature on Phobos with a diameter of almost 6 miles (9 km). The crater wall textures come from landslides that formed as materials fell in the weak gravity of the moon.
A series of grooves appear to radiate outwards from the crater, although studies have shown the cracks did not come from the crater. Some scientists believe the grooves are still related to the origin of Stickney, but others speculate that they originated with leftover space debris from Martian impacts that later pelted Phobos.
Just 13.5 miles (22 km) wide, Phobos appears as a lumpy, imperfect moon because its weak gravity could not compact it into a sphere. Both Phobos and its sister moon Deimos seem very similar to some asteroids, which could suggest their origin as space rocks captured by Martian gravity.
Other origin theories for the Martian moons include coming from a larger moon that broke up, forming with Mars in the early solar system, or being composed of material blasted from the Martian surface by impacts.
The Mars Reconnaissance Orbiter (MRO) angled in as close as 3,600 miles (5,800 kilometers) to eye Phobos in detail. Two similar images taken from different distances were combined to give the perception of a 3-D view.
Spacecraft such as Mars Global Surveyor have taken closer images of Phobos, but MRO's HiRISE camera provides some of the best quality data to date for the Martian moon.
Although no spacecraft have landed on Phobos, the Russians announced plans in 2005 to develop a Phobos-Grunt mission to collect soil samples.
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