When NASA's current pair of Mars rovers, Opportunity and Spirit, open their airbags and bounce onto the surface of Mars next January, chances are they will be carrying more than just equipment.
As they prowl the dusty surface, scratching rocks and searching for evidence of life on the red planet, the twin rovers may themselves be carrying life from our green and blue planet. Nooks and crannies on space equipment can harbor Earth microbes and some may be capable of surviving a trip in space.
Scientists don't know if exotic Earth microbes can take root in the Martian environment. Compared to Earth, Mars is frigid and bathed in ultraviolet radiation strong enough to kill most known life-forms. Yet some worry Earth's microbes could survive there, perhaps sheltering under the soil or getting into underground Martian water. If we were to contaminate a pristine planetary ecosystem, they say, the implications would be huge.
"We know if you move organisms from one place to another on Earth you can cause ecological disruption," says Margaret Race, a consultant ecologist who specializes in the field of space contamination. "So when we go to Mars we have to do our science responsibly."
The concerns are heightened, Dr. Race says, because the rovers are going to Mars specifically to look for signs of Martian life. Scientists don't know if it's there or it's alive, she says. It could be merely remnants mixed with the Martian soil. "In order to look for those," she adds, "you don't want to add on a lot of other materials like contaminants from Earth."
But preventing biological contamination of space equipment is difficult. Every surface of every object on Earth is crawling with a microbial film. Scientists have found microbes living miles down in the Earth's crust. They are in hot volcanic vents in the ocean, in sea ice, and in the radioactive cores of nuclear-power plants. And recent research shows they can survive in a patch of the Canadian Arctic that resembles the Martian landscape.
The Sisyphean task of keeping those bugs off NASA spacecraft destined for Mars takes place at the Jet Propulsion Laboratory in Pasadena. Since sensitive space equipment can be easily damaged by heat, technicians can't boil or steam clean. In clean rooms, with hairnets and robes, they swab spacecraft parts with hydrogen peroxide and use ultrasound to shake microbes loose, employing the same technology used in the ultrasonic toothbrush.
NASA says it's not taking any chances. "The number of microbes you'll find attached to a spacecraft is fewer than you'd find in half a bottle of spring water," says John Rummel, NASA's planetary protection officer. That sounds reassuring, except that it means there could be as many as 300,000 microbial spores per spacecraft. NASA hasn't yet sent up a completely sterile spacecraft and Dr. Rummel says the agency doesn't plan to absolutely sterilize its equipment until there is a strong chance of a mission encountering actual water or ice.
On planetary bodies with water the danger of contamination is much higher because many microbes need water to multiply. NASA recently incinerated the dying Galileo spacecraft in Jupiter's atmosphere rather than allowing it to crash onto Europa, one of that planet's moons, which is believed to have an ocean under a thick crust of ice.
While scientists have recently discovered evidence of much ice near the Martian poles, the Opportunity and Spirit missions are destined for Mars' equator, making the chance of finding water very unlikely.
Some say NASA's contamination precautions are ridiculous. Robert Zubrin runs the Mars Society, a Mars-booster organization that has funded some of its own research. He points out that there is a natural transfer of material between Mars and Earth all the time. When asteroids hit planets they knock off pieces of rock which go flying around the solar system and sometimes land on other planets as meteorites. Earth and Mars have been hit many times in the past by asteroids and pieces of both planets have presumably already landed on the other.
"This natural transfer between Earth and Mars is one reason why the efforts to stop contamination by NASA are almost certainly futile," Zubrin says. He thinks bacterial spores can survive space travel attached to meteorites.
At a simulated Mars research station on Devon Island, in the remote Canadian Arctic, Andrew Schuerger has been studying what might happen if we did send humans and their microbes to Mars.
In a polar desert, and with high ultraviolet radiation, Devon Island is perhaps the closest approximation of the Martian environment that can be found on Earth. There NASA is one of the sponsors of a mock-up Mars habitat on the rim of Haughton Crater, an ancient meteorite crater the size of London. Looking like a huge white water tank plunked in the middle of mushy brown permafrost, the "hab" has drawn research scientists from all over the world.
In the Arctic cold, Dr. Schuerger, a plant pathologist from the Kennedy Space Center in Florida, has spent two summers scratching up soil samples looking for the human microorganisms that might have fallen off those scientists onto the ground. Sure enough, his results showed that microbes associated with humans, such as coliform bacteria, can survive from year to year on the barren Arctic soil.
The finding makes it all the more likely that a manned mission to Mars will be put off until NASA can find a way to send humans there without leaving behind microbes that might elbow out any possible Martian natives. "If Mars has life, from what we know about Mars now it probably would be rare and endangered," she says. "Do we have a right to interfere with that and its existence and evolutionary trajectory? I don't know the answer to that."