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.
Smelling for Signs
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.
Did Mars Hold Life’s Trigger?
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."
Scientists had originally dismissed the results, chalking them up to quirky contaminants in the Martian soil, but some are now having second thoughts.
"There are a lot of factors to consider," said Dworkin. "The Viking spacecraft were just looking at the top bit of soil in a volcanic area. Also the detectors have come a long way since the 1970s — they're much more sensitive now."
Part of the reason scientists like Dworkin are interested in reassessing the nearly 30-year-old results is a growing theory that dormant life may have piggybacked on comets and meteorites to travel between planets in the solar system.
The idea has been bolstered by the discovery of primitive life in extreme locations on Earth, such as around smoldering deep ocean vents and within frigid ice in barren Antarctica.
"We know there are terrestrial bacteria that could survive in space for years," said Bill Irvine, a professor of astronomy at the University of Massachusetts in Amherst. "And calculations indicate if you blast something off from Mars, some of it could get here pretty quickly."
During the planets' early history, it's believed that Mars was once warmer and wetter and both Mars and Earth were bombarded by comets, which may have delivered water, carbon, nitrogen and other key components of life. Once life sprung up on one of the planets, it may have then spread (in primitive form) to the other planet on board an asteroid.
Learning where life may have first originated could also shed light on another, deeper question: how life arose in the first place.
Astronomers have shown that organic materials and water are prevalent in the universe but are still stumped about what trigger might have caused these components to form life.
"Were we extremely lucky and Earth provided the special conditions for life?" said Irvine. "Or is it possible that Mars once held that special ingredient? It's a big jump to go from proteins to DNA — just how that happened is a step we don't completely understand."