Buckyballs found in a huge impact crater near Sudbury, Ontario came from space nearly two billion years ago and arrived on Earth intact, a paper in the April 12 issue of the journal Science shows. The paper offers perhaps the most compelling evidence yet to support the theory that as comets and meteorites bombarded Earth early in its history, they helped bring about life by fertilizing our planet with vital organic compounds.
Scientists at the University of Rochester and at Scripps Institution of Oceanography at UC San Diego analyzed buckyballs (soccer-ball-shaped molecules of 60 carbon atoms) found at Sudbury, and concluded that they formed outside the solar system, yet survived the fiery impact as the meteorite or comet on which they hitched a ride crashed into Earth nearly two billion years ago.
Proof of origin comes in the form of the helium stowaways that have been locked inside the buckyballs since they were created nearly five billion years ago. The helium carries a telltale extra-terrestrial isotopic signature.
"Many scientists feel that when a large bolide [an object from space, such as a meteorite or comet] hits the Earth, everything is vaporized, and that whatever remains was somehow created during the impact," says Robert Poreda, associate professor of earth and environmental sciences at the University of Rochester, who made the helium measurements. "Our results show clearly that this helium, and these buckyballs, are of extra- terrestrial origin. If a meteorite or comet can deliver intact carbon molecules to the Earth's surface, then it's likely that other organic compounds can also survive an impact."
The Scripps scientists say they were surprised by the results. "It just didn't make sense to either one of us that fullerenes [another name for buckyballs] could actually survive an impact event like this because it is a such a large bolide -- it's about the size of Mt. Everest," says first author LuAnn Becker, a post-doctoral fellow at Scripps and the NASA Ames Research Center in California.
"The dogma right now is that because the Earth never had an atmosphere where supposedly you could have an effective terrestrial-based synthesis of organic compounds, the raw material for the origin of life must have been supplied from outside the Earth by impact events, such as comets and asteroids," says co-author Jeffrey Bada of Scripps, director of a NASA specialized center of research and training in exobiology at UC San Diego. "I have to admit I was a very strong opponent of this view -- I didn't think that this would be a viable way to get organics on the Earth because I believed, as most people did, that these events were just too energetic for the stuff to survive. Now, all of a sudden, I have a different view."
Scientists have long debated how Earth's atmosphere formed, enabling life on the planet to begin. The origin of the element carbon is one key to unlocking the puzzle. "Two billion years ago, there were no higher-level plants and no concentrated sources of carbon on Earth," says Poreda. "The question has been, where did all this carbon in the Sudbury complex come from? We have shown that it has an extra-terrestrial origin."
The Sudbury buckyballs were created nearly five billion years ago as part of the material that later condensed to form the solar system. The scientists believe the fullerenes most likely were synthesized inside relatively bright and cool stars known as red giants, then became mixed in with meteoritic or cometary material that eventually smacked into Earth.
"The journey from space to Earth is not a very friendly one for molecules like buckyballs," says Poreda. "Ultraviolet radiation, the solar wind, and energetic particles can all destroy compounds. But if the buckyballs were encased in a meteorite and insulated from these forces soon after formation, it could happen."
Becker and Bada discovered the Sudbury buckyballs two years ago, but it was unknown whether the buckyballs came from space or were created in the bolide's fiery impact, which created a crater that covers nearly 700 square miles. According to Bada, the energy released in such an impact event like Sudbury would be a thousand times greater than that released if every nuclear weapon on Earth were detonated.
Much like police use DNA fingerprinting to pinpoint a felon, scientists can use the isotopic ratios of elements to pinpoint their origins. The team extracted the helium from the buckyballs and used a mass spectrometer to measure the ratio of the rare isotope of helium, helium 3, to helium 4 (isotopes are atoms of the same element that have the same chemical behavior but slightly different weight). "The ratio of the helium inside the buckyballs is what we typically find in meteorites," says Poreda. "It's much higher than the ratio found anywhere on Earth, now or throughout Earth history."
Buckyballs are exotic molecules of carbon first discovered in 1985; they were nicknamed buckyballs because their structure resembles the geodesic domes designed by the engineer and architect Buckminster Fuller. Their carbon atoms are linked together in the shape of a soccer ball, forming a tiny cavity where molecules of such light elements as helium can nestle. Three years ago Poreda, working with Martin Saunders and colleagues at Yale, discovered that it was possible for buckyballs to lock helium inside.
This work was funded by the National Science Foundation, the National Research Council, and NASA. tr