Washington

The wild rover: NASA is set to launch two probes to scour the surface of Mars for signs of life. Credit: JPL/NASA

After more than two years of deliberation, a committee of planetary scientists has picked the landing sites for NASA's two Mars Exploration Rovers, due to launch between May and July this year. The two locations — Gusev Crater and Meridiani Planum — have been chosen because of the high likelihood that water, and perhaps life, once existed there.

The rovers will use parachutes and protective airbags to bounce down on to the martian surface next January. Once deployed, they will spend at least three months rolling slowly from rock to rock, taking panoramic photographs and examining the local geology. Each rover is equipped with a high-resolution camera, a diamond-tipped grinder to expose fresh surfaces on dusty rocks, a microscopic imager to examine such surfaces, and three spectrometers to analyse the rocks' chemical make-up and mineral content.

Gusev's appeal has been obvious since the Viking orbiters took the first close-up pictures of Mars in the 1970s. The ancient impact crater shows strong evidence of once having been flooded with water because its bottom is flat and relatively smooth, like that of a lake bed, and a dry river channel seems to run into the structure at one end. Scientists first scrutinized the Meridiani site in 2000, when NASA's orbiting Mars Global Surveyor detected large deposits of the iron oxide mineral haematite. As most of the processes that form this mineral on Earth require water, its presence hints that water may have been present at the site in the past. Water is considered essential for life, and finding evidence of past biology is a priority for Mars exploration.

When the landing-site selection committee began work in 2000, it had a list of 155 possibilities. These were quickly whittled down to seven top contenders, says John Grant, a planetary scientist at the Smithsonian Institution's National Air and Space Museum in Washington DC, who co-chaired the panel. Gusev and Meridiani have long been front-runners in terms of scientific merit, but they also had to satisfy several engineering constraints. The elevation of the landing site cannot be too high, for example, as the atmosphere needs to be dense enough for the parachutes to work. Boulders are a landing hazard and smaller rocks can impede the rovers, so the site must be reasonably free from such obstacles.

Questions still remain over another constraint. Project scientists will continue to refine their predictions of windiness at the Gusev site before NASA commits to landing there. NASA's last Mars lander — the 1997 Pathfinder mission — landed early in the martian morning, when it was least windy. But to make sure that signals are sent to Earth during landing, the new rovers will touch down in mid-afternoon, when the winds are typically stronger and could tear the airbags designed to cushion their landing, says Grant.

NASA can make final decisions on the landing sites as much as a month after the first launch, currently scheduled for 30 May. The second rover will launch in late June. Both rovers will be given names before launch, based on suggestions from schoolchildren.