If a Mars rover bumped into signs of life on Mars, would it know it — and would that life look anything like life here on Earth?
Geological and remote readings have suggested life may exist on Mars and some think it's highly possible that NASA's Odyssey or Spirit could have stumbled across evidence. Planetary scientists also say if primitive life exists on Mars, it could very well share traits with life on Earth.
"There is transport from Mars to Earth by meteorites," said Jason Dworkin, a biochemist at the SETI Institute in Mountain View, Calif. "So it's reasonable to suggest that perhaps we're all Martians and life started on Mars and traveled to Earth."
The problem is, neither of the rovers now rolling around the Red Planet is very well equipped to confirm the existence of life or past life.
Telescopes on Earth recently confirmed readings from the European Space Agency's Mars Express, now orbiting Mars, which detected traces of the gas methane in the Martian atmosphere. Because methane lives for only a short time, its existence on the planet suggests it may be coming from volcanoes or microbial life.
So far, no active volcanoes have been located on Mars.
But neither NASA's Spirit, nor Opportunity have the tools to seek out a microbial culprit.
The rovers were designed to hone in on another key component on Mars — water. Last week, NASA scientists announced the rover Opportunity had found evidence that a salty pool of liquid water once sloshed on Mars' surface.
The mission's main scientist, Steve Squyres of Cornell University, described the former salty, wet conditions as "very suitable for life." Still, recognizing conditions for life is one thing; spotting evidence of life itself is another, more complicated challenge.
"I would not be surprised if the team has a number of odd-looking things and are puzzling over whether there is a possible organic connection," said Victor Baker, a planetary geologist at the University of Arizona in Tucson.
"But they'd have a great deal of difficulty with the instruments they have to identify them. They can only look at structure and shape, and shape alone is not completely diagnostic."
One of the rovers aimed for the Red Planet was actually equipped to test for organic molecules. But engineers lost contact with the European Union's Beagle II lander as it began its descent to Mars on Christmas Day.
The loss of the Beagle II means researchers eager to find definitive signs of Martian life may have to wait until 2007, when NASA plans to send another lander to the planet's northern latitudes. The Phoenix rover is slated to arrive on the planet, dig a trench and analyze soil and ice samples for organic molecules.
This would provide a direct test for life. But it wouldn't be the first time a lander has tested for life on Mars.
During the Viking missions in 1976 and 1977, two landers were equipped with an instrument called a gas chromatograph mass-spectrometer to identify organic molecules on Mars. Neither lander found signs of organic molecules.
But recently, some have revisited the results from one of Viking's other instruments. The so-called Labeled Release life-detection tool mixed Martian soil with a nutrient "soup" to see if any living organism would digest the solution and then release labeled gases. Experiments on both landers signaled the release of the labeled gas — suggesting that something had possibly eaten the "soup."