New pictures have been released of the Martian moon Phobos, acquired by the European Mars Express (Mex) probe during its recent flybys.

The images reveal details down to a resolution of just 4.4m per pixel.

Mex began a series of 12 close passes in mid-February. The observation schedule continues until 26 March.

One flyby skimmed past the surface at just 67km, the nearest any manmade object has ever got to the little Martian moon.

The new images of Phobos come from 7 March approach when the spacecraft achieved an altitude just above 100km.

The pictures will help the Russians as they prepare to launch their Phobos-Grunt mission next year. The spacecraft will attempt to land on the 27km-by-22km-by-19km moon, collect a soil sample and return it to Earth for analysis.

The excellent resolution shows the Phobos-Grunt mission planners the precise conditions at their potential landing sites.

Planetary scientists are trying to explain the origin of the moon, one of two natural satellites at Mars (the other being Deimos).

Previous study had indicated that Phobos has an extremely low density, suggesting that its surface probably hides many large interior voids.

Researchers suspect the moon is simply a collection of planetary rubble that coalesced around the Red Planet sometime after its formation. Another explanation is that it is a captured asteroid.

Phobos is very slowly falling in towards Mars and tidal forces are expected to tear it apart one day.

The European Space Agency's Mars Express satellite has been in orbit since 25 December 2003.

It has made many discoveries including measurements of previously unrecognised methane in the planet's atmosphere.