Martian moon's secrets to be revealed during fly-bys

The deepest secrets of Mars's moon Phobos are set to be revealed, following a series of 12 fly-bys by Europe's Mars Express spacecraft. Six have been completed, including the closest ever pass of the moon, at 67 km, last week.

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, manoeuvre 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," says ESA's project scientist Olivier Witasse. "This week we switch to flying by the daylight side, allowing the camera and spectrometers to begin working." That will give the moon's composition, testing the idea that Phobos formed from rocks that somehow found themselves orbiting the planetMovie Camera.

Unfortunately, a 90-metre-high rocky outcrop called the 'monolith' is not visible to Mars Express during this series of fly-bys. The monolith could be a piece of Phobos's interior thrown to the surface during the formation of a crater. It was first spotted in 1999, on images taken by NASA's Mars Global Surveyor probe.