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Stanford Report, April 12, 2000

New field of astrobiology has scientists pondering extraterrestrial origin of life

BY MARK SHWARTZ

The question of whether life exists elsewhere in the universe left the realm of science fiction last week, as NASA hosted the galaxy's first Astrobiology Science Conference.

The event was held April 3-5 at the NASA Ames Research Center in Mountain View.

The conference brought together researchers from Stanford and dozens of other institutions to discuss new developments in the burgeoning field of astrobiology, which NASA defines as "the study of the origin, evolution, distribution and destiny of life in the universe."


A great deal of astrobiology research focuses on the discovery of signs of life elsewhere in our Solar System -- specifically on Mars and one of Jupiter's moons, Europa. (Graphic: NASA Astrobiology Institute)

While several authors submitted papers on the search for extraterrestrial intelligence, most focused on the question of whether simpler life forms, such as bacteria, exist elsewhere in our solar system or in other galaxies.

Christopher F. Chyba, associate professor (research) of geological and environmental sciences, co-authored a study showing that life-giving organic molecules could have arrived on Earth and other planets aboard fast-moving comets.

"Life as we know it requires organic compounds, liquid water and a source of energy," wrote Chyba and Elisabetta Pierazzo of the University of Arizona.


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"Few places in the solar system appear to satisfy these requirements," they noted, except Earth, Mars and Jupiter's moon, Europa.

Previous studies showed that comets contain organic compounds such as amino acids, the building blocks of proteins.

Using computer simulations, Chyba and Pierazzo concluded that certain amino acids could have survived "large cometary impacts" billions of years ago, becoming part of a primordial soup that eventually gave rise to single-celled organisms on Earth -- and perhaps on Mars and Europa.

But is it possible that tiny creatures from another world somehow hitched a ride on a comet, crash-landed on Earth, then began to multiply and conquer the planet?

The idea is known as panspermia -- the theory that microscopic life was delivered to Earth billions of years ago from outer space.

First proposed in 1865, panspermia has gained widespread support in recent years with the discovery of organisms that thrive in extreme environments like those found elsewhere in space -- boiling or freezing temperatures, extreme acidity and even heavy doses of radiation.

One NASA Ames researcher pointed to recent experiments with a species of worm called C. elegans that is only a millimeter long. Miniature worms were placed in a centrifuge then spun at 10 Gs, or 10 times the gravitational force felt on Earth.

The little creatures survived four days of spinning -- no surprise to Stuart Kim, associate professor of developmental biology at Stanford.

"These worms will live at 100,000 Gs," said Kim, who is conducting additional studies with NASA Ames to see if extreme gravitational forces affect a worm's ability to regulate cell growth, metabolism and other genetic functions.

To find out how quickly invasive species adapt on Earth, conference organizers invited Stanford biologist Harold A. Mooney, the Paul S. Achilles Professor of Environmental Biology.

Mooney is an authority on how natural ecosystems are transformed by the introduction of exotic plants and animals.

"Our world, our planet, is now one world," he told the audience, noting that the oceanic walls separating the continents were breached long ago by Columbus and subsequent explorers.

Mooney said that improvements in global transportation have caused even more ecological disruption, pointing out that, between 1961 and 1995, a new marine species was inadvertently introduced into San Francisco Bay every week in the ballast water of cargo ships from Asia and other continents.

"That's just a blip in geologic time," Mooney noted, adding that even Earth-bound ecologists "can't predict which organisms will become successful invaders."

Stanford geophysicist Norman H. Sleep presented a paper arguing that the Earth's oceans contain just the right amount of water to sustain oxygen-breathing organisms.

According to Sleep, if the oceans were twice as deep as they are today, the continents would be submerged, exposing only a small amount of granite to the atmosphere.

When granite weathers, it turns into clay, which then absorbs excess carbon from the air as it becomes shale.

Without clay, too much carbon would remain in the atmosphere, stealing oxygen molecules to form carbon dioxide gas, which is toxic to most animals.

On the other hand, if the oceans were half as deep, atmospheric oxygen would be soaked up by iron-rich basaltic rocks that are now submerged under seawater.

"Getting just the right amount of water," Sleep concluded, "is a hurdle in having complex life forms on a planet." SR