The northern pole has some of the lowest elevations on Mars and so air pressure may have been dense enough back then for perchlorate-rich liquid water to remain stable. It would have been sandwiched between the dust top coating and a hard ice-rock mix underneath the surface. Microbes may have flourished in a rich film of liquid water, like their cousins on Earth who live in a similarly thin sandwich in the high Antarctic dry valleys.
"The polar temperatures back then (about 15 degrees Fahrenheit) are within the range of biological possibility," McKay says enthusiastically. He envisions Mars microbes extracting oxygen from the perchlorate and eating iron oxides in the soil.
NASA's nuclear-powered rover the Mars Science Laboratory named Curiosity (look out you Martian cats!) has an experiment called the Sample Analysis at Mars (SAM) instrument. It will wash out organics if they exist in the Martian soil without the need for heating it. If there is a positive detection after it lands in 2012 it will end a three-decade long search for organic compounds on Mars.
The ultimate dream is to do a Mars sample return mission to bring microorganisms to Earth labs (with a price tag running as high as $10 billion a rock). McKay favors a quicker, cheaper technology demonstrator mission, like the 1997 Mars Pathfinder landing. A small probe would touch down, scoop up a soda can full of soil, and scoot back to the Earth.
To trace the history of life on the Red Planet, "ultimately you want to go to Mars and drill down 500 feet into the soil and rock," says McKay. This is very hard to do with robots that barely might penetrate down just couple of feet (unless NASA builds Transformers - not the electrical device but a real-life version of the 1980s Hasbro toy robots). It would call for brawny Mars astronauts who could jiggle the drilling apparatus and give it some good old-fashioned elbow grease. (At last we have a role for Bruce Willis in space!)