NASA Presses Search for Life Out There.

One is the world of cold dark matter; the other is the world of warm wet 
matter. One is majestic, terrible in its bright furies of exploding 
stars and galaxies, abstract and far away. The other is squirmy 
and slithery, often gross, and as close as your fingernails.

But without astronomy to provide the stage, in the form of 
galaxies, suns, planets, comets, sheets of cosmic radiation and 
that dark stuff that makes up most of the universe, there 
would be no place for biology, for life, to strut its stuff.

The pursuit of that ancient heavenly connection has lately 
moved near center stage at NASA, which assembled some 
100 astronomers, physicists, chemists, geologists and even a 
few biologiste at the Space Telescope Science Institute on the 
Johns Hopkins University campus recently to talk about 
extraterrestrial life.

"The Search for life has come up as a major issue," said Dr. 
Kenneth Nealson, a professor of geobiology at the University of 
Southern California. Money will be available, he said, meaning a 
lot of very smart people will be looking at the problem.

Three years ago, NASA set up an Astrobiology Institute without walls, 
centered at the Ames Research Center in California 
and composed of 15 dIfferent research teams tied together by 
videoconference and tėlephone.

Thė "Astrophysics of Life" meeting, last month, was top 
heavy with astronomers and cosmologists. Running through
the meeting was a constant tension between the ability to develop 
exquisitely sensitive tools to sense life as we know it and 
the difficulty of looking for life as we do not know it.

Though spacecraft have visited, at least briefly, almost every 
planet and sizable moon in the solar system, Earth is still the lone 
proven site of life, not to mention intelligence, in the universe.

"Everything we do here is hostage to our concept of what life 
is," said Dr. David Des Marais of NASA's Ames Research Center, 
and part of a committee charged by NASA with developing a road 
map of astrobiological research for the next decade.

Life as we know it starts with a planet. There are now 85 
known planets orbiting other stars, said Dr. Geoffrey W. Marcy, 
an astronomer at the University of California at Berkeley, 
who helped discover many of them. They are all massive 
gas giants like Jupiter, and so unlikely to support life.

But astronomers think that smaller planets may be abondant
as well. NASA's Kepler spacecraft, to be launched in 
2007, will look for the dips in starlight caused by planets 
passing in front of their parent stars, and could find "hundreds 
of Earths," said Dr. Charles Beichman of NASA.

Then there is the Terrestrial Planet Finder, planned for 2015, 
whose goal is to detect distant planets directly, using techniques 
to separate the planets dim lght from that of its much 
brighter parent star. Dr. Beichman compared the challenge to 
"finding a firefly next to a searchlight on a foggy night."

(The European Space Agency has its own planet finders, Eddington 
and Corot, in the works, culminating in a mission called 
Darwin, which may be combined with T.P.F.).

In planning their search, scientists think they can rule out 
at least parts of the galaxy and focus on so-called habitable 
zones. For one thing the elements like iron, oxygen and carbon, 
which form terrestrial planets and creatures, not to mention 
the radioactive elements that provide the internal heat for 
planetary geological activity, must be built up over eons by nuclear 
processing of primordial hydrogen and helium in stars.

In fact, Dr. Guillermo Gonzalez, an astronomer from Iowa 
State University, pointed out that the stars known to have 
planets tend to have higher abundances of these elements.

This nuclear processing happens faster at the cores of galaxies 
than on the outside, he said, leading to an abundance of 
metals in the center of the galaxy and a paucity at its edge.

Each star has its own habitable zone, defined as the region 
around it in which a planet can keep liquid water on its surface, 
said Dr. James F. Kasting, a professor of geosciences at 
Penn State University.

For Earth, the inner edge of  that zone, where water vapor 
from evaporating oceans would ramp up the greenhouse effect, 
lies at 95 percent of Earth's present orbital distance. The 
outer edge, where the planet would freeze solid, he said, could 
extend as far as twice Earth's orbital radius, beyond the orbit 
of Mars. Some scientists have suggested that Earth was a so-called snowball 
in its past when the Sun would have been cooler.

But eruptions of carbon dioxide from volcanoes or bursts of 
methane from emerging life would have released enough 
greenhouse gases into the atmosphere to warm things up, 
Dr. Kasting said.

What acted to keep Earth warm before life emerged is, a 
bit of a mystery. "We know Earth was not a snowball most
of its history," Dr. Kasting said. 

Life, according to the strongest version of the so-called Gaia 
principle, regulates conditions on Earth to maintain an optimal 
environment, but these regulatory powers have their limits, 
said Dr. Kevin Zahnle, an astro-physicist at the Ames Research
Center. Maybe a planet bas to be lucky, as well, he said. 

Consider the plight of Venus.  On Earth, he said, Gaia had 
many knobs to turn, including aerosols, ice sheets and clouds. 
On Venus, now sweltering under think greenhouse clouds at 800 
degrees, and dry as a bone, there was little evidence that 
any knobs were turned.

"You begin to wonder if Gaia happens," he said, pointing out 
that billions of years ago Venus had the conditions for oceans. 
What happened to the oceans? "What happened to Gaia?"

He ended his talk with a litany of questions. "Do small planets 
die young? Do big planets evolve animals?"
"Was Venus ever habitable?" "This is a sad question, I think," he said.