Scientists have found evidence that the seeds of life may indeed have fallen from the sky.
Analysis of a meteorite that fell onto the frozen Tagish Lake in Canada in 2000 has shown the space rock to be riddled with organic material that is at least as old as the solar system itself. Researchers speculate that this kind of matter could have played a vital role in the development of early life on Earth.
The Tagish Lake meteorite is unusual because it is so well preserved. Most meteorites, although usefully frozen in space, thaw or become contaminated when they arrive on Earth. This has frustrated researchers' attempts to test the hypothesis that organic material could have arrived on the primordial Earth on comets, asteroids, and meteors.
A team of NASA scientists, led by Keiko Nakamura-Messenger, scanned the Tagish meteorite in slices with a transmission electron microscope. This revealed sub-microscopic "globules", which consisted largely of carbon, hydrogen, nitrogen, and oxygen, the researchers report.
The ratio of isotopes of each of the chemicals shows that the globules formed at near absolute zero. To have captured such exotic isotope mixes, the meteorite must have formed much further from the sun than the Earth, Nakamura-Messenger says, which confirms that the chemicals are not contaminants, but are native to the space rock. They were most likely part of the cloud of material from which the planets themselves formed.
The team suggests two possible ways that the globules formed: in both cases they began life as icy grains that formed on the rock in the outer reaches of a very young solar system. It is possible that the grains then formed a hardened shell when bombarded by radiation. The centre would later have evaporated, leaving a hollow shell of organic matter. Alternatively, the grains were exposed to alkali compounds in the meteor itself, which could have hollowed out the centres.
The research, which is published in the journal Science, cites 26 such globules. But Nakamura-Messenger says the meteorite could contain billions of them. The team restricted its analysis to such a small sample because of the complexity of processing the rock.
"We're sure that these [globules] are not alive," Messenger told Scientific American. "But they may have been important ingredients for the first life-forms." ®