Making Sure Martian Life Hasn't Hitched Ride on Earthly Craft

Recent discovery of "could-be" fossils in a Martian meteorite has sharpened an old issue - interplanetary contamination.

The prospect that Mars may once have had life - and may still have it - whets explorers' appetites. But before they launch missions to look for it, they have to be sure they won't make a biological mess. Scientists don't want to "discover" life on Mars that hitched a ride from Earth on their space probes. They also must plan to isolate Mars samples brought here.

Daniel Goldin, administrator of the National Aeronautics and Space Administration (NASA), has said the agency may want to speed up its time table for collecting Mars samples. That means sending a sample-return mission a few years earlier than the 2005 date now provisionally targeted as part of a 10-year Mars exploration program. But Mr. Goldin also has said that NASA "would rather not have a mission" at all until issues of contamination are satisfactorily dealt with. He added that this policy "is non-negotiable."

That puts the bee on NASA to update its biological safety standards quickly. Kenneth Nealson of the University of Wisconsin at Milwaukee says this would be an "urgent" problem if NASA hadn't gotten a jump start on the work. Last year, the agency asked the National Research Council of the National Academy of Sciences to review the standards. Dr. Nealson, who chairs the council committee, says he expects to have recommendations late this year.

This issue is more complicated than just sterilizing everything sent to Mars and locking away everything brought here. Earth and Mars have exchanged material for eons. On average, some 50 kilograms (110 pounds) of Martian meteorites arrive every year. Earth probably has returned the favor. Knocked off the parent planet by the impact of an asteroid or meteorite, material orbits through the inner solar system until captured by the host planet's gravity. Encounters with other planets or with the home planet can alter the material's course.

Some Martian meteorites wander for millions of years. Some make the trip to Earth in six months, according to Mars researcher Joseph Burns of Cornell University. No one knows the time scale for the Earth to Mars journey.

Scientists now know that Martian organic material can survive the trip to Earth. Likewise, Dr. Nealson says he thinks "it's 100 percent assured" that Earth has contaminated Mars. That being the case, many Mars researchers are relaxed about sterilizing space craft sent to Mars on nonbiological missions. They would reserve costly complete sterilization, which can be hard on spacecraft, for missions that will seek out past or present Martian life. They wouldn't want to confuse their instruments by bringing along earthly microbes. But since Earth contaminates Mars anyway, it should suffice to clean up the hardware with alcohol as is being done with two missions, including one landing craft, due to launch this fall. However, until the Nealson committee reports, we won't know what new standards the scientific community will finally recommend.

All of this was examined for the Apollo moon program. At that time, moon rocks were quarantined even though many scientists considered this unnecessary. There's no water and, hence, no chance for life where Apollo astronauts took samples. Now you can see moon rocks in museums. Equipment sent to the moon before astronauts landed was well cleaned. Yet, when astronauts brought home part of a Surveyor landing craft, scientists found live terrestrial microbes that had survived both the cleaning and the lunar environment. We may need tougher standards for Mars.

Meanwhile, we're left with an intriguing question raised by Richard Zare of Stanford University, one of the Mars meteorite researchers. He asks, "Who is to say we are not all Martians" if Mars seeded the early Earth with organic chemicals. Or, he adds, "how do we know life on Mars didn't come from Earth?" Stay tuned.