These MARSIS 'radargram' images show echoes obtained from an approximately 250 km diameter circular structure in the subsurface of Mars, interpreted to be a buried impact basin. MARSIS detected a series of arc-shaped reflectors that have no apparent source in the surface topography or geology. In the lower image, a linear reflector nearly parallel to the surface is seen embedded in the arcs. This reflection may be coming from the floor of the basin. The time delay to the linear reflector suggests a depth of 1.5-2.5 km. Credits: ASI/NASA/ESA/Univ. of Rome/JPL

The European Space Agency (ESA) spacecraft Mars Express has discovered a layer of water ice deep beneath the surface of Mars.

Mars Express was launched in June 2003 and arrived at the red planet in December 2005. Just before entering orbit, it released the UK’s Beagle 2 lander, which failed to send signals back and is now assumed to have crashed. Despite this setback, the main spacecraft has been working successfully for almost two years, sending back some of the sharpest-ever images of the Martian surface.

One of the key instruments on Mars Express is the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). Radio waves from MARSIS can penetrate several km below the surface of the planet before being reflected back up to the space probe. This allows scientists to study the layers that make up the Martian crust.

This topographic map, based on Mars Orbiter Laser Altimeter data, shows the MARS Express groundtracks and the arc structures detected by MARSIS that are interpreted to be part of a buried impact basin about 250 km in diameter. The topographic relief represented in the image is 1 km, from low (purple) to high (red). The projected arcs are shown in red and white. There is no obvious feature on the surface that matches the buried feature identified in the MARSIS data. Credits: ASI/NASA/ESA/Univ. of Rome/JPL/MOLA

During its first few weeks of operation MARSIS probed the highlands of Chryse Planitia in the northern hemisphere. These preliminary results indicate the presence of a 250 km-wide circular structure, buried between 1.5 and 2.5 km below the surface. ESA scientists believe this may be an impact crater containing what looks like a layer of water-ice rich material.

From late December until March 2006, astronomers will use MARSIS to search for underground liquid water in the Martian southern hemisphere. Indirect evidence for water can be seen all over the planet, from flood plains to dried up channels. If liquid water is found, it might just be a niche for simple extraterrestrial life – future human explorers will be keen to take a closer look.