Forward Contamination of Space: The Aliens May Be Us

Humans searching for extraterrestrial life might be surprised someday to find organisms in a distant corner of the universe that originated on Earth. This could have several causes, one of them being us.

Readers of science fiction may be used to the scenario in which humans returning from space are contaminated with the beginnings of a fearsome alien creature that proceeds to knock down power lines and have its way with the locals. But we are just as likely to send Earth-based life forms into outer space, a process known as forward contamination.

When we send a mission into space, we can never be certain that no microscopic life form travels with it. Scientists take great efforts to eliminate any tiny hitchhikers, but the process is imperfect. Scientists weren't even aware of some extremophile organisms—which can survive, and even thrive in extreme conditions of temperature and radiation—when first sending probes to the moon and other planets.

The dangers of forward contamination are twofold. We might discover an “alien” of terrestrial origin, or we might inadvertently infect an extraterrestrial life form with a bacterium from Earth, with possibly catastrophic results.

The Viking landers that touched down on Mars in 1976 were baked for a couple of days at 230 degrees Fahrenheit. You'd figure that would kill anything, but, as Caleb A. Scharf notes in his Scientific American blog, a single-celled creature called Strain 121 actually reproduces at such temperatures.

Life forms can also spread through the universe by a process called impact transfer. This occurs when bodies such as asteroids and comets collide with a planet, causing bits of its surface to escape its atmosphere and hurtle through space. This may rarely occur, but it will happen many times over billions of years in an incredibly large universe. Life forms may have migrated incredible distances from time to time.

NASA's Apollo 12 mission in 1969 returned to Earth with a camera that the Surveyor 3 probe had deposited on the moon two and a half years earlier. A post-mission analysis of the camera detected the bacterium Streptococcus mitis. It was thought that NASA had not properly sterilized the camera before launch.

NASA scientist Leonard D. Jaffe later reported that a breach of sterile procedure could have contaminated the camera after its return to Earth. The camera was stored in a duffel bag after recovery, which also could have contaminated it. Three researchers concluded that the contamination was most likely caused by poor clean room conditions.

Spacecraft heading to Mars must be cleaned to exacting standards. This typically means fewer than 300 heat-resistant bacterial spores per square meter of the craft's surface. If the spacecraft might enter a “special region” where there might be liquid water, that number must be reduced by another four orders of magnitude, to just 0.03 spores per square meter. That's equivalent to about 230 spores on an area the size of a football field. This is necessary because every known organism on Earth requires water to survive.

Life can take forms that scientists don't anticipate. The Viking landers found no sign of life on Mars, although they only penetrated 10 centimeters (4 inches) below the planet's surface. Back on Earth, scientists surmised that Mars was much deader than our planet's deep-sea bottoms, according to a 2001 paper written by John D. Rummel and published by the National Academy of Sciences. Seven months after the first Viking landing, scientists discovered a previously unknown form of life 2,500 meters below the ocean's surface along the Galápagos Rift in the Pacific Ocean.

Could Mars also harbor a surprising form of life? After a three-year analysis of Martian soil samples collected by the 2008 NASA Phoenix mission, astronomers say that after a super-drought that may have lasted hundreds of millions of years, any life on Mars would have to be deep underground. But the surprising discovery of deep-sea life might cause one to wonder.

At the AstroSurf site, Jose Ribeiro discusses some of the surprising properties of extreme microbes. Extreme thermophiles in volcanic areas grow at temperatures above 100 degrees—the boiling point of water. Bacterial endospores can survive for thousands of years in freezing temperatures and vacuum conditions. They can even survive moderate amounts of ultraviolet radiation. Microorganisms have been found 70 meters below the Earth's surface. Some can repair their DNA, implying the ability to resist radiation and survive for long periods in space. The deep-sea organisms mentioned above can resist great pressures. Endoliths live in Arctic sandstone at extremely low temperatures.

Who knows? Some badass Earth bug could hang out on Mars for a few millennia before waking up to cause interplanetary havoc.

As we sow seeds of Earth-based life throughout the universe, one can't help but wonder what form a resultant life form might take. As we aim electronic ears at the heavens to listen for signs of extraterrestrial life, will we someday encounter a welcoming message from a distant galaxy, or will we hear a static-laden appeal to purchase a Big Mac? I hope they've invented fries as well. The drink I can do without.

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