The European Space Agency's Mars Express lander, Beagle 2, will land on Isidis Planitia, a large flat region that overlies the boundary between the ancient highlands and the northern plains. The choice of site was announced last week at a meeting of the Mars Express science working team in ESTEC, Noordwijk, the Netherlands.

The region appears to be a sedimentary basin where traces of life could have been preserved, if primitive life really did exist at some time on Mars. "This is the best site given the landing constraints and scientific aims of Beagle 2," said John Bridges from the Natural History Museum, London, who has been assessing several sites on behalf of the project.

Isidis Planitia. Photo: ESA

Isidis Planitia lies between 5 degrees and 20 degrees north. The specific site chosen lies close to 10 degrees north, which is the maximum latitude for a site to be warm enough for Beagle 2 to function properly during early spring, the season at which it is due to land on Mars. The number of rocks on the surface seems to be about right - not too many to threaten a safe landing, but enough to provide an interesting landscape for the experiments. The site is also at a low enough elevation to allow the parachutes sufficient atmosphere to brake the lander's descent, has few steep slopes down which the tiny probe may have to bounce as it lands, and doesn't seem to be too dusty.

Sites previously under consideration include the Chryse Planitia, Candor Chasma and the Elysium plains. Further studies, however, showed that the probe would be unable to function properly at these sites because their latitudes make them too cold in early spring. One channel south of Chryse, Simud Valles, would have been warm enough, but it is too narrow to ensure a safe landing.

Another possibility might have been areas of the layered terrain imaged by the camera on NASA's Mars Global Surveyor (MGS) and made public earlier this month. The layering, which is extensive, especially in low-lying areas around the equator, was most likely formed by sediment deposited at the bottom of long-standing lakes or seas, an ideal environment for preserving traces of life. "Unfortunately, this layered terrain is revealed in steep, narrow canyons which are unsuitable because of the landing ellipse size," Bridges told the meeting.

Beagle 2 landing site map. Photo: ESA

The landing ellipse is an area up to about 500 km long by 100 km wide in which Beagle 2 will land. The size of the ellipse will depend on the angle at which the probe enters the Martian atmosphere, which has yet to be determined: the steeper the angle, the smaller the ellipse. However, a landing site must be chosen to accommodate the maximum likely ellipse size and that rules out the bottom of many valleys.

When Beagle 2 has landed, its precise position will need to be measured. One way of doing this will be to time the movement of the shadow of Mars's tiny moon, Phobos, as it passes over the lander during a partial eclipse of the Sun. "In February 2004, the Sun will have a partial eclipse by Phobos. By timing the eclipse, we will be able to pinpoint the lander's position quite accurately," said Tom Duxbury from the Jet Propulsion Laboratory, Pasadena, California, USA, who is helping to characterise the landing site using MGS data.

The timing of the landing site decision has been determined by the need of ESA's Mars Express and Starsem's launcher teams to work out a trajectory to enable the spacecraft to deliver Beagle 2 to the desired site. However, further detailed characterisation of the site will continue as new data becomes available from MGS.