Life has not yet been found on Mars, and no one is sure whether it will be. But some researchers say it is not too early to consider the possibility that humans could do irreversible damage to indigenous Martian life.
A group of international experts will meet as early as this September to discuss whether it is time to revise policies that protect Mars from contamination.
At issue is the ethics of exploring the Red Planet - in particular whether hitchhiking Earth microbes could harm Martian habitats.
Past missions, including NASA's twin rovers, have already ferried hundreds of thousands of bacterial cells to the Red Planet. Most of the microbes on the exterior of these craft were quickly destroyed by intense ultraviolet radiation, which passes easily through Mars's thin atmosphere.
But dormant microbes might survive for tens of thousands of years on the interior of the crafts. And in the case of the Mars Polar Lander, which crashed into the planet's south pole in 1999, its interior surfaces may have come in direct contact with soil rich in water ice, which could potentially provide a habitable environment for the hitchhikers.
"The option of not contaminating Mars is an option that's no longer available to humanity," says Christopher McKay, a planetary scientist at NASA's Ames Research Center, who wrote a commentary about the need to protect any Martian life in the current issue of the journal Science. "Mars already has earthlings. We know that for a fact."
He warns that Earth life could be reawakened if weather conditions on the planet change. This could happen as a result of periodic swings in the planet's tilt, or if humans purposely alter the Martian environment, which, ironically, they might do to make conditions cosier for any Martian life they might discover. Microbes on subsurface drills in search of liquid water could also contaminate potential Martian habitats.
A 1967 United Nations treaty generally requires countries avoid "harmful contamination" of celestial bodies.
The specific rules that govern Mars exploration are set by a Paris-based group called the Committee on Space Research (COSPAR), which advises the UN and provides guidelines followed by NASA and other space agencies.
COSPAR's current policy sets limits on how many microbes can be present on spacecraft bound for Mars. To accommodate these rules, for example, the robotic arm on NASA's Phoenix probe was sterilised and kept in a protective wrapping until landing.
But McKay says COSPAR's policy needs to be changed, since it was crafted to protect the integrity of scientific investigations and not potential Martian life.
This might involve changing the policy to require future exploration to be "biologically reversible", ensuring that any contamination can be undone. This could be accomplished by returning to the sites of previous lander missions and recovering or destroying the spacecraft components, while also exposing any potentially contaminated dirt to the sterilising effects of the Sun's UV radiation.
It might also mean preventing humans from establishing bases in darkened caves, since sunlight would not be able to kill any terrestrial microbes loosed there.
Martian 'rabbit problem'
Since the current policy promotes sterilisation of equipment that could potentially come into contact with water, it effectively protects potential Martian life, says NASA's planetary protection officer Cassie Conley.
But Conley agrees that the ethics of Mars exploration should be discussed. "We're not sure that life is present on Mars, but we do know we have dramatically screwed things up by transferring life around on Earth," Conley says.
She said irresponsible exploration could produce the Martian equivalent of Australia's rabbit problem, wherein rabbits shipped to the country for hunting in 1859 then bred exponentially and stripped the land bare.
COSPAR has already noted that future missions to Jupiter's moon Europa would run the risk of coming into contact with any life there. A lander could one day make its way though the moon's icy shell to contaminate any life in what is thought to be a subsurface ocean of liquid water, says John Rummel, who heads up COSPAR's planetary protection panel.
Momentum for the ethical discussion began building even before 2006, when a US National Research Council committee recommended that COSPAR convene the workshop in light of the accelerating pace of Mars exploration.
NASA and the European Space Agency both plan to send large rover missions to the Red Planet in the next few years. The SUV-sized Mars Science Laboratory, which is set to launch in 2011, will restrict itself to the top several centimetres of the Martian surface.
But ESA's ExoMars rover, now set to launch in 2016, will be able to drill some 2 metres below ground to look for signs of life.