Contamination fears over Nasa's rover machine on the surface of Mars

For all the hopes Nasa has pinned

on the rover it landed on Mars last month, one wish is unspoken: please don't find water.

Scientists don't believe they will. They chose the cold, dry equatorial landing site in Mars' Gale crater for its geology, not its prospects for harbouring water or ice, which exist elsewhere on the planet.

But if by chance the rover Curiosity does find water, a controversy that has simmered at Nasa for nearly a year will burst into the open. Curiosity's drill bits may be contaminated with earth microbes. If they are, and if those bits touch water, the organisms could survive.

The possible contamination of the drill bits occurred six months before the rover's launch last November. The bits had been sterilised inside a box to be opened only after Curiosity landed on Mars.

But that changed after engineers grew concerned that a rough landing could damage the rover and the drill mechanism. They decided to open the box and mount one bit in the drill as a hedge to ensure success of one of the most promising scientific tools aboard Curiosity. The drill is to bore into rocks looking for clues that life could have existed on the planet. Even if a damaged mechanism couldn't load a drill bit, at least the rover would have one ready to go.

Under agency procedures, the box should not have been opened without knowledge of a Nasa scientist responsible for guarding Mars against contamination from earth. But planetary protection officer Dr Catharine Conley was not consulted. "They shouldn't have done it without telling me," she said. "It is not responsible for us not to follow our own rules."