Reports indicate that a series of 12 flybys by Europe's Mars Express spacecraft might reveal the deepest secrets of the Martian moon Phobos.

According to a report in New Scientist, the flights will probe the moon's gravity better than ever before, revealing the distribution of material throughout its body.

The MARSIS radar will also search for underground structures in the rubbly moon, which is probably riddled with caverns.

The gravity data will help Russia's Phobos-Grunt mission, set to launch in 2011 or 2012, maneuver efficiently around the moon before coming in for a landing.

New portraits of Phobos are also on the way. "Until now, the encounters have been on the (moon's) nightside," said ESA's project scientist Olivier Witasse. "This week, we switch to flying by the daylight side, allowing the camera and spectrometers to begin working," he added.

That will give the moon's composition, testing the idea that Phobos formed from rocks that somehow found themselves orbiting the planet. Unfortunately, a 90-metre-high rocky outcrop called the 'monolith' is not visible to Mars Express during this series of flybys.

The monolith could be a piece of Phobos's interior thrown to the surface during the formation of a crater.