One site is a giant crater that may have contained a lake, while the other has deposits of an iron oxide mineral considered a "chemical signature of past water," the agency said in a statement.

"Landing on Mars is very difficult, and it's harder on some parts of the planet than others," said NASA's associate administrator for space science, Ed Weiler.

"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," he added.

The first of the two Mars Exploration Rovers that will carry out the geological studies is set for launch by Delta II rockets from Cape Canaveral, Florida, on May 30, headed for the Gusev Crater, some 15 degrees south of the planet's equator, where it is scheduled to land on January 4, 2004.

The second Rover will be launched on June 25 destined for the opposite side of Mars to the Meridiani Planum, some two degrees south of the equator. Its landing is scheduled on January 25.

The tricky Martian landings will be slowed by parachutes and cushioned by airbags in a method tested by the Pathfinder mission to Mars in 1997.

The solar-powered Rovers are expected to complete their primary missions over three months, after which their efficacy is expected to decrease because of dust accumulating on their solar arrays, the National Aeronautics and Space Administration communique explained.

SPACE.WIRE