A Mars invasion, by a fleet of rovers

Efforts to explore Mars - a planet that has captivated the human imagination for millenniums - represent one of the few bright spots in a space program overshadowed by the loss of the shuttle Columbia and its crew last month.

Now, US and European scientists are poised for a return to the red planet late this spring in an unprecedented effort to deliver two rovers and a lander to the surface, while a new orbiter takes up station high above to gather stereo images of the planet's surface in extraordinary detail.

The projects, NASA's Mars Exploration Rover mission and the European Space Agency's Mars Express, will help determine whether Mars could once have hosted simple forms of organic life - and whether such forms still may exist there.

A key to solving this mystery is determining whether Mars's ancient climate was warm enough and wet enough to harbor life. This history is etched into Mars's parched surface and locked up inside its rocks and soils. NASA's two rovers will spend 90 Martian days touring the surface, imaging and analyzing rocks in two broad regions, while the Mars Express's Beagle 2 lander focuses on soil chemistry and its orbiter maps minerals and tracks the planet's climate.

If both missions succeed, "man, it's going to be a heck of a good year for Mars exploration," says Steven Squyres, a Cornell University astronomy professor who is the lead investigator for the instrument packages the two rovers carry.

Indeed, if the hardware arrives intact and operates as advertised, it will join Mars Global Surveyor and the Mars Odyssey orbiters already on Mars, leading to what Brown University geologist John Mustard terms an "incredible" array of scientific assets there. On Thursday, Mars Odyssey will have completed its first year of mapping minerals and chemicals on the Martian surface, while the Mars Global Surveyor has been imaging the surface since March 1999.

Within the past few weeks, the science team for the Mars Exploration Rover mission has nominated four regions of the planet for exploration. NASA is slated to pick the final two on April 10.

For their money, the mission's researchers are hoping for a nod to visit two intriguing locations near the Martian equator - one dubbed the Hematite site, the other Gusev Crater.

Hematite is iron oxide, a compound that gives both bricks and the Martian surface their red hue. But the site, spotted by the Mars Global Surveyor orbiter, represents "a really unusual deposit," Dr. Mustard says. "This is the hematite you'd see in a jewelry story. That's what's weird."

He notes that on Earth, the shiny black mineral typically forms under intense heat. But it also forms under intense pressure, yielding formations such as the 2.5 billion year-old banded-iron formations in Minnesota. Early in Earth's history, these formations were layered sediments on the floor of an ocean that contained large quantities of dissolved iron. As marine microorganisms carried on photosynthesis, they gave off oxygen as waste. The oxygen bonded to the iron, and the hematite precipitated onto the ocean floor, where successive layers compressed it until it took on its jewel-like form. Hematite also appears around deep-sea hydrothermal vents.

The second site high on the research team's list is Gusev Crater.

"This is a large crater with a flat floor that has been breached by a river channel" says Harry McSween Jr., a planetary geologist at the University of Tennessee. "It may be an ancient lake bed with lake deposits on the floor of this crater."

While the Mars Exploration Rover mission is not designed to look for evidence of life, an ancient lake bed "certainly is the right place to go to see if you can see any features that might be indicative of life," he says.

Yet for all the scientific excitement these missions hold, they also carry risks. By one estimate, the US, Russia, and more recently Japan, have mounted 33 missions to the red planet. Only 10 have been accounted a complete success.

"NASA has put a tremendous amount of scrutiny behind everything going into the two rovers. They have turned over absolutely every rock to see if there was anything problematic," Mustard says. "For the health of the program, these have to work."