By Helen Briggs BBC News Online science reporter

Gusev crater: Could a river once have flowed through it?

One is a giant crater that could once have been filled by a lake; the other a plain with layers of ancient rocks that may have been laid down by running water.

The twin rovers, which are designed to explore terrain up to a mile or so from where they touch down, are due to reach Mars in January.

The British-built Beagle 2, carried by Europe's Mars Express, will arrive at around the same time but will be dropped off a little further north on a large basin just above the equator.

The aim of the Nasa mission is to search for evidence that Mars was once warmer, wetter and more hospitable to life than it is now.

To this end, more than 100 Nasa scientists and engineers have been sifting through a list of 155 possible landing sites for the past two years.

Selection is based on two main criteria: how risky the landing will be and what the rovers might find when they get there.

Balancing risk

Dr John Bridges of London's Natural History Museum, a member of the Beagle 2 landing site team, believes both targets are exciting.

"They're great sites in addition to the Beagle landing sites," he told BBC News Online. "It's a fantastic opportunity to go to three sites."

If all landers make it to the surface of Mars, they will represent three more flags on a largely unexplored map.

Only a handful of spacecraft have landed on the Red Planet; the last attempt ended in failure.

America's Mars Polar Lander was supposed to touch down on 3 December, 1999. Ground crews were unable to contact the probe and it is unclear what happened to it.

"Landing on Mars is very difficult, and it's harder on some parts of the planet than others," says Dr Ed Weiler, associate administrator for space science at Nasa in Washington, DC.

"In choosing where to go, we need to balance science value with engineering safety considerations at the landing sites. The sites we have chosen provide such balance."

Warmer, wetter

The first rover, scheduled for launch on 30 May, should land at Gusev Crater, just south of the equator of Mars.

There has been much interest in the crater because it appears that a river may once have poured into it.

"There is the inferred presence of water," says John Bridges, "as ever no clinching evidence, but it's an exciting site."

The second rover, due for take-off on 25 June, will be targeted to land at Meridiani Planum, about two degrees south of the equator on the other side of the planet.

Again, it shows evidence of past water, but for different reasons. The plain is thought to contain some of the oldest rocks on Mars, including deposits of an iron oxide mineral that could have formed in water.

"People have been inferring the presence of water billions of years ago on Mars in the region," says John Bridges.