A SPECK of dust that drifted into the Earth鈥檚 atmosphere from the edge of the solar system shows that complex organic molecules can form even in the chill near-emptiness of interstellar space.
The dust particle is a remnant of the material that filled our cosmic neighbourhood before the sun and planets formed 4.5 billion years ago. The material is carried in from the outskirts of the solar system by passing comets, and thousands of tonnes fall to Earth every year.
Simple organic molecules have been detected in this dust before, but the latest measurements prove that they pre-date the solar system. This means dust particles could have seeded the young Earth with organic matter. 鈥淚t鈥檚 another hint that extraterrestrial carbon may be implicated in the origin of life,鈥� says John Bradley, a chemist at Lawrence Livermore National Laboratory in California.
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His work with a team at Washington University in St Louis, Missouri, revealed that the particle 鈥� dubbed Benavente and caught during a flight through the stratosphere in 1994 鈥� had an unusual history. The chemical reactions that produced the organic material in it took place in temperatures only a few degrees above absolute zero, before the sun warmed up.
The clue lies in the molecules. Although they are made of carbon, nitrogen and hydrogen like organic molecules on Earth, the atoms differ. On Earth, most carbon atoms have six neutrons in their core, while roughly 1 in 90 has seven neutrons. Nitrogen usually has seven neutrons, but the occasional atom has eight.
Chemical reactions are generally unaffected by which isotope is present, so they turn up with the same frequency no matter what molecules they are in. But in extremely cold conditions, when there is very little energy to power reactions, one isotope becomes more reactive than the other and the ratio changes. The team found that the molecules in Benavente contain fewer heavy carbon atoms and more heavy nitrogen atoms than their terrestrial equivalents (Science, vol 303, p 1355).
Telescopes cannot tell the difference between the isotopes, so this is the first direct probe of the chemistry of interstellar molecular clouds. 鈥淚t is very tantalising to see chemical analysis performed on such material in the lab,鈥� says Ian Sims, an astrochemist at the University of Rennes 1 in France.
Christine Floss of Washington University measured the isotopes in the dust particle by vaporising a small portion of it with an electron beam and weighing the atoms that came out using a mass spectrometer. She had only a femtogram of material to work with 鈥� a millionth of a billionth of a gram. That was enough to detect the anomalies in the isotopes, but not to work out exactly what organic molecules are present.
Bradley believes the molecules are quite complex, however, and suggests they could even contain adenine, a base of DNA, although he admits such speculation is 鈥減rovocative鈥�. Adenine has been found in meteorites, but could it emerge from deep space? The team hope to find out for sure when NASA鈥檚 Stardust mission returns pristine dust from a comet in 2006.