These droplets (~10 µm across) show structures reminiscent of cells -- although they are not alive. They are from a chemically separated fraction of the bulk residue. Photo: NASA-Ames

The chemical building blocks necessary for the formation of life on Earth, as well as rudimentary structures that could have been the basis for the first cells, may have come from outer space, one group of scientists has concluded.

In a paper published Tuesday in a special issue of the journal Proceedings of the National Academy of Science, researchers from NASA's Ames Research Center and the University of California Santa Cruz said conditions in the early solar system were ripe for the formation of critical organic compounds and even cell-like structures, although they stopped short of saying that life itself originated in outer space.

Scientists took a mixture of simple ices, including water, carbon dioxide, carbon monoxide, and methanol and cooled it to just ten degrees above absolute zero. That supercold ice mixture was then exposed to strong ultraviolet radiation to mimic conditions in the dense molecular clouds from which solar systems form.

While scientists expected that the ultraviolet radiation would create new, more complex chemicals within the ice, the amount and complexity of the new compounds surprised them. "Sure, we expected that ultraviolet radiation would make a few molecules that might have some biological interest, but nothing major," said Lou Allamandola, a NASA Ames scientist. "Instead, we found that this process transforms some of the simple chemicals that are very common in space into larger molecules which behave in far more complex ways, ways which many people think are critical for the origin of life, the point in our history when chemistry became biology."

These droplets (small ones are ~10 µm across) glowing under black light in the microscope show internal structure and suggest chemical complexity. Photo: NASA-Ames

If such complex organic chemicals were formed in the early solar system, they would have been the primary source of the compounds on Earth that were the building blocks for the origin of life. "Every year more than a hundred tons of extraterrestrial stuff falls on the Earth, and much of it is in the form of organic material," explained NASA Ames space scientist Scott Sandford. "In the early life of our solar system, before the debris from its formation was fully cleared away, these materials were deposited on the Earth in far greater quantities than we see today. Thus, much of the organic material found on the Earth in its earliest years probably had an interstellar heritage."

Not only could such chemicals provide the raw materials for the formation of life, they may have helped provide the structure for the first cells, according to another member of the team. "Upon the addition of liquid water to the organics produced during ice irradiation, some of these new compounds, with no outside help, organize themselves into tiny vesicles with complicated structures," said Jason Dworkin, a SETI Institute researcher and lead author of the journal paper. Vesicles are hollow droplets that resemble cells in size, shape, and structure, but have inorganic origins.

Dworkin noted that the vesicles included other chemicals, including one that converted ultraviolet light into visible light. "Molecules that do these things are thought to be extremely important for the origin of life," he said. "Membrane structures are necessary to separate and protect the chemistry involved in the life process from that in the outside environment, and all known biology uses membranes to capture and generate cellular energy."

This is a vesicle (~10 µm across) glowing under black light in the microscope made from the bulk residue. Proof that it is a hollow vesicle, rather than a simple drop of oil, is the green pyranine dye which we have trapped inside of it. Photo: NASA-Ames

Such findings may boost the prospects of panspermia, a controversial school of thought that suggests that life may not have originated on Earth but instead started elsewhere and was transported to Earth by comets and asteroids. However, the researchers noted that while the chemicals and membrane-like structures found in the ice could be critical to support the formation of life, they alone were not necessarily sufficient for life to form.

"Membranes are like a house," Dworkin said. "Maybe these molecules were just the raw lumber lying around that allowed origin-of-life chemicals to move in and set up housekeeping or construct their own houses."

Some members of the team are keeping an open mind to the issue of the origins of life. "We are just now beginning to realize that we are only seeing the tip of the iceberg in terms of extraterrestrial molecular complexity," said Allamandola. "I know I hold a minority view on this nowadays, but I suspect that even deep inside a comet, which is mainly water ice after all, reactions are much further along than we think and the chemistry quite complex."

The discovery also raises the prospects of the formation of life in other solar systems. Irradiation of ices "happens all the time in the dense molecular clouds of space," said Allamandola. "Very complex organic molecules that might be important for the origin of life could well be falling on the surfaces of newly formed planets everywhere in the univers... This discovery implies that life could be everywhere in the universe."