by Edna DeVore - Deputy CEO
When we study life in the universe, we have a handy example of where we know life occurs: the Earth, our home. Understanding how life began here on Earth can help us understand how life might occur on other worlds. This is the domain of scientists at SETI Institute's Center for the Study of Life in the Universe as well as the NASA Astrobiology Institute.
In research about life on Earth, scientists seek sources for the building blocks of life, the carbon-based chemistry that makes up every living thing. Today, we find the ingredients for life (carbon, nitrogen, oxygen, hydrogen, and so forth) everywhere on our planet. All the basic elements that make up trees, flowers, birds, bacteria and us are abundantly available on the surface of the Earth.
But where did the organic compounds that are the basis of life arise? Were they part of the original planet? Did they form slowly in shallow pools and oceans as the Earth cooled? Did they arrive from space? These questions have occupied scientists seriously for more than 50 years.
Today, there is strong evidence that comets delivered these molecules after the Earth had formed. Can we obtain samples of these early materials? You might not think so, as the solar system formed about 4.5 billion years ago. But each time a comet swings by the Sun, we are seeing a miles-wide sample of that nebula. Using that data, some researchers are trying to simulate the early Solar System in the lab.
SETI Institute scientists Dr. Max Bernstein and Dr. Jason Dworkin work at NASA Ames Research Center in cooperation with Dr. Lou Allamandola and Dr. Scott Sandford in the Astrochemistry Laboratory. Together, the team builds interstellar clouds in simulation chambers at very low temperatures and pressures like those in the outer regions of the solar nebula. They work with chemicals we know are present in comets to learn how complex organic materials form in icy bodies like comets. Their experiments have produced complex organic compounds like those we find in the debris of comets, the meteorites. When added to water, some of these materials form tiny (10 micron diameter) capsule-like droplets similar to cell membranes (photo of droplets). Extracts of organics from some meteorites also form these capsule-like droplets when added to water. This intriguing result points to the possibility that comets and meteorites could have salted the Earth with organic materials that were a springboard for life.
There was a time in the early history of our planet when it was a molten globe, too hot for liquid water, too hot for the complex organics of life, and too hot to hold an atmospherenot a good place to live. Any organic substances that fell into that inferno would have been quickly destroyed.
As the Earth cooled, it began to hang onto an atmosphere of carbon dioxide and nitrogen, the volcanic exhalations of the planet and debris of comets striking the planet. Shallow oceans formed, condensing from water vapor spewed out by volcanoes and from water arriving as giant snowballs from spacecometswhich bombarded all the planets of the early solar system. Just look at the geologically frozen surfaces of the Moon or Mercury to see the evidence of this early bombardment; craters cover every inch of them. There is little evidence of the early cometary strikes on Earth because its surface is recycled as the continents move about, collide, sink and risethe plate tectonics we sometimes feel as earthquakes and see as active volcanoes.
But these organic compounds would have been safe in the smaller colder clumps of matter left over from the original nebula, such as comets.
Comets are some of the oldest bodies in our solar system, falling toward the Sun from the distant reaches of the original solar nebula. From ground-based, airborne, and space-based observatories we know that comets are big dirty, loosely formed snowballs, made of water, carbon dioxide (dry ice), ammonia, and dust. (See "Building a Comet")The dust contains lots of complex organics. As the Earth sweeps up the debris left behind by the comets, we see it burn high in our atmosphere as meteors or "shooting stars." Some survive to crash land as meteoritesan estimated 1,000 to 10,000 tons of dust and rock land on Earth each day from space. These are samples of the early solar nebula.
A well-known experiment shows what might have happened after Earth had cooled enough for water to remain liquid, and was re-seeded with complex molecules.
In 1953, Dr. Stanley Miller and Dr. Harold Urey at the University of Chicago conducted an experiment to see if the basic building blocks of lifeamino acids and sugarscould be formed from the basic chemicals they believed were readily available on the early Earth: methane, hydrogen, ammonia, and water. They built a closed system that cycled the chemistry between liquid and gas phases, and added energya sparkto mimic lightning. After some time passed, many compounds formed, including some of the amino acids found in our cells. This experiment catapulted them to world fame, and created the new scientific field of exobiology, the study of life beyond Earth, because it appeared that the basic building blocks of life could arise from inorganic materials, water, and energy. More simply, given the right stuff and the right conditions, planets can make the primordial soup where the chemistry of life can form. And, scientists reasoned, it could happen elsewhere. More recently, NASA has coined the term, astrobiology to encompass the study of life beyond Earth. Internationally, this field of study is also known as bioastronomy. Regardless of the term, all encompass astronomy, biology, chemistry, physics, geosciences, and space sciences in learning about life beyond Earth.
The Miller-Urey experiment did not provide an entirely satisfactory answer to the question of "Where did the building blocks of life arise?" Today, scientists are seeking evidence that these building blocks of life may have arrived from space. In other words, the basics for life came from elsewherenot as walking, talking gray humanoids, but as tiny bits of organic material delivered by comets and meteorites. Astronomers have identified many kinds of organic moleculescarbon compoundsin the space between the stars, floating about in clouds of gas, or bound up in small particles of dust. These sorts of materials form new stars and their planetary systems. Similar material formed the early solar nebula, and when our Sun and planets condensed from that cloud of dust and gas, some organics were already present. But the molten condition of early Earth would have broken up complex molecules, so Miller and Urey started with simple chemistry. Today, the next generation of scientists are considering that the organics could have arrived later (when the Earth had cooled a bit). They could have arrived as comets and meteorites that rained down water for oceans, carbon dioxide for the atmosphere, and organics to contribute to the origin of life.
Like the Miller-Urey experiment, the Astrochemistry Laboratory results are only suggestive; they point to a cosmic source for some of the building blocks of life. Bernstein, Allamandola, and Sandford described their research in Scientific American and rightly concluded,
Of course, a huge gap still yawns between even the most complex organic compounds and the genetic code, metabolism and self-replication that are crucial to the definition of life. But given their omnipresence, if organic molecules from space had something to do with life here, that means they wereand always areavailable to help with the development of life elsewhere. "Life's Far Flung Raw Materials" (http://www.sciam.com/1999/0799issue/0799bernstein.html)
As we seek to understand life here on Earth and beyond, each observation and each experiment contributes a piece to the puzzle. With enough pieces, the whole picture may emerge.