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January 2003

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GeographicaWho Knew?
Strange Life
Who Knew

Strange Life

Would we even recognize it?

In coming years, NASA and other space agencies will intensify the search for life on Mars and elsewhere in the solar system. But the search is complicated by a fundamental mystery: What is life, anyway?

NASA has been using a fairly simple working definition: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution." Brevity forbids any mention of love, friendship, or ice-cold beer.

Other scientists have circulated their own definitions, such as, "Life is a chemical system able to replicate itself through autocatalysis and to make mistakes that gradually increase the efficiency of the autocatalysis."

But of course.

Life, squirrelly thing that it is, tends to elude capture by any single definition. Maybe life, for example, doesn't have to evolve. Imagine creatures that have no information-bearing molecule like DNA. They might reproduce but not replicate. The parent would be no more biologically related to the child than to a complete stranger. (Actually, I think that has happened in my own family.)

One obvious flaw with the NASA definition is that it doesn't apply to an individual organism. Let's say a space probe to Jupiter's moon Europa drilled through the surface ice and found a deep, dark ocean. And let's say that, in that ocean, the probe encountered Marlon Brando. Is he capable of reproducing? Not by himself, dare we point out. And there'd be no way of knowing if he was capable of evolving. How long would you wait to find out?

Some attributes of life may be non-negotiable. Water probably need to be in the mix, because living things need a solvent for biochemical reactions, though ammonia might also work. And we can anticipate that life will be carbon-based, since carbon is so useful for making chemical bonds. Also, a living thing probably has to have an inside and an outside. Without a membrane, an organism can't distinguish itself from its environment. It's hard to thrive if you're just undifferentiated mush.

Philosopher Carol Cleland and astrobiologist Chris Chyba argue that trying to define life right now is like trying to define water when nobody knew that water was H2O. They say we'll never have a good definition of life as long as we have only one example, Earth life. That's why NASA wants so badly to find a scrap of something, even the humblest microfossil, on Mars. Since we don't know precisely what life is, the search will focus on what life does. Life alters its environment. It creates "biosignatures." A really compelling biosignature would be a footprint. But it could also be just a pattern of chemicals in soil, a chemical gradient that can't be explained nonbiologically.

CHEMICAL GRADIENT FOUND ON MARS isn't the grabbiest headline, but you have to start somewhere.

—Joel Achenbach
   Washington Post Staff Writer

Web Links

Rough Draft
Writer Joel Achenbach's column is gaining a cult following. It takes a sometimes humorous, sometimes eye-squinting, but always intelligent look at today's headlines, personal interests, and the little life-annoyances we all live with.

Astrobiology Institute, NASA
NASA's Astrobiology Institute not only seeks to learn whether there is life beyond Earth but also aims to understand how life originated and evolved on Earth. Check out the "Ask an Astrobiologist" page, where you can find answers to questions about life on Earth and beyond, and submit your own questions too.

SETI Institute
SETI stands for Search for Extraterrestrial Intelligence, and the SETI Institute is a private nonprofit organization that is seeking signs of extraterrestrial technological civilizations. Visit this site to learn more about the hunt for life, including the ambitious Project Phoenix, a targeted search that focuses on a particular group of stars.

European Space Agency (ESA)
Learn about European efforts to find evidence of life beyond Earth, including the Huygens probe, which ESA hopes to land on one of Saturn's moons.
Free World Map

Achenbach, Joel. "Life Beyond Earth," National Geographic (January 2000), 24-51.

Dyson, Freeman. Origins of Life, 2nd edition. Cambridge University Press, 1999.

Gross, Michael. Life on the Edge: Amazing Creatures Thriving in Extreme Environments. Plenum Press, 1998.

Koerner, David, and Simon LeVay. Here Be Dragons: The Scientific Quest for Extraterrestrial Life. Oxford University Press, 2000.

Koshland, Daniel E., Jr. "The Seven Pillars of Life," Science (March 22, 2002), 2215-2216.

Overbye, Dennis. "NASA Presses its Search for Extraterrestrial Life," New York Times, June 2, 2002.


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