We're not quite ready to hunt for life itself yet, and the MSL rover isn't designed to do so, say researchers taking part in the $2.5-billion mission to the red planet.
But after a decade of "following the water" – a necessary ingredient for life as researchers currently understand it – planetary scientists are moving to take the next critical step: see if water co-existed with other critical environmental conditions that could have allowed simple forms of life to emerge.
Organisms on Earth take the forms they do because they are adapted to their environments, MSL researchers explain. If humans eventually hunt for evidence of life itself on the Red Planet, or anywhere else, for that matter, knowing something about the environment organisms inhabit will yield clues about what the organisms were or are like.
"If a Tim Allen, 'Galaxy Quest,' alien rock creature were to come up and bang us on the head, we don't want to ignore it. That would be the 'Ah ha!' moment we'd regret having missed," says Steve Brenner, director of the Foundation for Applied Molecular Evolution in Gainesville, Fla.
For Mars, the incremental Holy Grail is finding organic carbon, the stuff of complex molecules that form the building blocks for life, according to John Grotzinger, a planetary scientist at the California Institute of Technology in Pasadena, Calif., and the mission's project scientist.
"It's a long shot, but we're going to try," he said during a prelaunch briefing this week..
Meteorites deposit organic compounds on the Martian surface all the time, but today's conditions are so harsh that the compounds are quickly destroyed, he explains.
Finding organic carbon captured in the layered rocks that the rover Curiosity will explore would indicate that at the time the layers were deposited, conditions on the surface at that location could well have been far more benign, allowing organic compounds to exist at the surface.
Set for launch at 10:02 a.m. Eastern Standard Time Saturday, Curiosity holds a TripTik that sets the rover into Mars' Gale Crater next August.
The oversized ding in Mars' crust is 96 miles across, about 3 miles deep, and sports a gently sloping mountain in its center that rises to a height comparable to California's Mt. Whitney, the highest peak in the lower 48 states.
Some researchers crudely estimate the impact crater's age at between 3.5 billion and 3.8 billion years old.
After peering at images and sifting through mineral-composition data gathered from various orbiters circling Mars, mission planners settled on the crater and the mountain that vaults from its center because the crater walls and outcrops on the mountain's slopes bear Grand Canyon-like bands of different rock layers.
Clays are abundant at the mountain's base, testifying to a prolonged wet environment, Dr. Grotzinger explains. Higher up the slope, rock types change to minerals, such as sulfates, that form through evaporation, suggesting a transition from wet to periodically wet. Finally, these minerals give way to those that don't need water to form, pointing to the final shift to the kind of conditions Mar's exhibits today.
If all goes well, Curiosity will spend at least 98 weeks (one Martian year) exploring Gale Crater with a suite of 10 instruments that gather weather data and sample the atmosphere's chemistry, snap pictures, zap rocks with lasers, drill into rocks, and analyze the drilling samples.
In addition, instruments will measure the different isotopes of chemical elements found in the samples as well as in the atmosphere for clues about changes in the atmosphere's composition over Mars' history. Researchers will hunt for methane in Mars' atmosphere.
Ground-based observations as well as tantalizing hints from the European Space Agency's Mars Express orbiter have suggested that methane is present, at least temporarily. Termites, cows, and rice paddies give off methane; so do hydrothermal vents. So the source could be biological or geological. Curiosity has the ability to tell the difference.
Researchers say Curiosity's travels at Mars seem glacial at times. It might take a couple of weeks to finish gathering and analyzing samples at one spot, then identify and travel to the next.
As the rover moves, cameras atop a seven-foot-tall mast will take images of a proposed new spot at increasing levels of detail.
Once Curiosity approaches to within about 30 feet of a feature scientists find interesting, a laser atop the mast will zap a pin-head-sized spot on the rock, turning its minerals into a puff of hot, electrically charges gas that an on-board spectrometer can analyze for its chemical make-up.
Researches also can use the laser as a dust brush, sending pulses that can clear surface dust to expose the rock underneath.
Each of these steps allows scientists to pause and ask: Do we still want to look even more closely at this feature?
If the composition the spectrometer identifies looks interesting, the rover will pull within its arm's reach, drilling will begin, and the rover's internal chemistry labs will turn dust into data.
Finding organic carbon would be only the beginning of a debate on its source – delivered raw and preserved, or processed through a living organism? That question could only be answered by bringing samples back to Earth, where more-capable labs could settle the question.
And if Curiosity digs up no organic carbon? A disappointment, perhaps, but it would hardly close the door on Mars as once hospitable for life. Researchers point to mineral evidence of ancient life on Earth – layered, mineralized byproducts of microbial life in rocks that nevertheless contain no organic carbon.
During Curiosity's initial 98 months on the Martian surface, the rover will climb only about the first 1,300 feet of the mountain's height. But if the rover holds out, the researchers don't get bored, and extended-mission money is available, the rover could reach the summit, following "an appropriately circuitous route," Grotzinger says.
For Curiosity, it would represent a retirement spot with an out-of-this-world view.