Mars is under more intense scientific scrutiny than ever. The curiosity is about whether Earth's cold, barren neighbor was ever wet enough to support simple microbial life. Scientists speculate that liquid water once flowed there because U.S. satellites in recent years have observed channels and other land forms that appear to have been carved by water. Now, two U.S. robot rovers are on the Martian landscape to seek proof of this.

The quest for Martian water has resulted in a record five spacecraft visiting the red planet, the two U.S. rovers, a pair of U.S. satellites, and a European orbiter.

"What a time in the history of Mars exploration," says James Garvin, the chief scientist for Mars at the U.S. space agency, NASA. "We have more vehicles exploring Mars now than any time in human history. The opportunities for learning about this planet are enormous. If water were there for as long as we hoped it was, what can we learn? Can we follow the record from the water to the record of the building blocks of life, the record of carbon?"

The first step is to determine if there was liquid water on Mars in the first place. Using satellite images as a guide, NASA chose two very different rover landing locations that looked as if water had influenced them. The Spirit rover touched down in Gusev Crater, which appears to be an ancient lake bed. The twin rover Opportunity landed on a flat plain called Meridiani, which may contain the solar system's largest deposit of an iron compound called hematite known to form in association with water.

Mission scientist Joy Crisp says "Gusev Crater has evidence for past liquid water in the shape of the terrain. It's a crater that has a dry river bed leading into it, whereas Meridiani has evidence for past liquid water in the form of a mineral."

But finding water at these sites is not as simple as turning on a tap. Geologists will have to infer that it was present based on obscure, dry evidence on the Martian landscape.

To do so, Ms. Crisp says the two rovers are outfitted with a suite of sophisticated tools. "We think we have a really good set of instruments that does the kind of basic geology that a field geologist does and even a bit more," she says.

Both six-wheeled vehicles have a microscope camera for closeup pictures of rock structure and several instruments to determine the minerals and chemical elements that make up rocks and soil. With this gear, mission scientists hope to discern telltale signs that water flowed through the terrain.

At Meridiani, for example, they are hoping to learn whether the observed hematite was formed in an ancient ocean or underground springs heated by volcanoes. Mission scientist Ray Arvidson of Washington University in St. Louis explains the types of evidence the Opportunity rover will seek.

"What are the other minerals associated with hematite? Did they form at low temperature in water or high temperature in water? What we'll be looking for is evidence of sediments that may have been washed back and forth in an ancient sea or a lake by waves. And then we'll also be looking for the land forms. For example, are we going to find evidence for ancient vents where the ground may have fractured and there may have been hot steam coming out and extensive alteration? The hematite had to come from somewhere," he says.

Another clue about whether Meridiani was once wet will come from inspection of exposed bedrock. This normally underlying rock sticks out of the ground at this site, an unexpected gift to scientists who otherwise might never get to see it. Harvard geologist Andrew Knoll says it piled up either as sediments deposited by water or volcanic ash.

The Opportunity rover's microscopic camera will examine the rock texture to determine which. "If these are volcanic rocks that have simply fallen out of the sky or been transported by a gas-driven fluid, we would expect to see very parallel rows of those fine layers. If they are sediments, sediments tend to actually erode a little bit into the layers beneath them as they are deposited. So if these are sediments, I would expect to see at least very fine level discontinuities [unevenness] between adjacent beds," says Mr. Knoll.

The shapes of rocks also tell a story. If their edges are rounded, it usually means water eroded them. From these many types of evidence, Opportunity and Spirit will help researchers learn whether Mars ever had liquid water.

For NASA mission scientist Matthew Golombek, the answer cannot come too soon. "We're horribly impatient scientists because we want it now and we want to start going to look at these incredible things!"