Nasa probe pictures Phobos moon

The 9km-wide Stickney impact crater is the most prominent feature on Phobos

Nasa's Mars Reconaissance Orbiter (MRO) has captured two stunning images of the Red Planet's major moon, Phobos.

Stickney Crater, a 9km (5.5 mile) -wide depression that is the largest feature on Phobos dominates the pictures.

The images also show a series of grooves and crater chains; the formation of these features is the subject of debate among scientists.

MRO was launched from Florida in August 2005 and entered orbit around the Mars in March 2006.

It is mapping the Martian surface with high-resolution cameras with a view to choosing landing sites for future missions.

It will also study Mars' weather, climate, geology and atmosphere.

MRO's High Resolution Imaging Science Experiment (HiRISE) camera acquired two dramatic views of Phobos on 23 March 2008, one at a distance of 6,800km (4,225 miles) from the Martian moon and another at 5,800km (3,603 miles).

The two images were taken within 10 minutes of each other and show roughly the same features, but from a different angle so they can be combined to yield a stereo view.

Get into the groove

Some scientists believe the grooves and crater chains that can be seen in the pictures are related to the formation of the Stickney impact crater.

However, others think they may have formed from ejecta from impacts on Mars that later collided with Phobos.

In the MRO images, surface rocks near the rim of Stickney appear bluer than the rest of Phobos. Based on analogy with rocks on our own Moon, this could mean this surface is fresher, and therefore younger, than other parts of Phobos.

Phobos was discovered in 1877 by the American astronomer Asaph Hall. Both Phobos and Mars' other moon, Deimos, are thought to be captured asteroids.

Phobos' orbit around Mars is dropping by about 1.8m (5.9ft) every 100 years. This means that in 50 million years it will either crash into Mars or break up into a ring.

Russia has been working on an unmanned spacecraft that will return samples of soil and rock from the surface of Phobos. The mission, called Phobos-Grunt, has a provisional launch date of October 2009.

The unique, fist-sized Kaidun meteorite, which fell to Earth at a Russian military base in Yemen in 1980, is claimed by some researchers to be a piece of Phobos.