Scientists examining the first dust samples collected from a comet have found complex carbon molecules, supporting the theory that the ingredients for life on Earth originated in space.

The organic material was found in early studies of samples from the comet Wild 2 brought back to Earth by the Stardust space probe seven weeks ago.

Stardust collected hundreds of grains of dust as it flew through the tail of the comet two years earlier. Analysis suggests a high concentration of complex molecules of the kind thought necessary for the evolution of life.

"About 10 of this comet is made of organic materials. We don't know exactly what they all are but it is very exciting," said Don Brownlee, professor of astronomy at the University of Washington, who is NASA's principal investigator for the Stardust project.

Stardust was launched by NASA in February 1999 and flew twice around the sun as it matched its speed to the comet's. Then, in January 2004, somewhere between the orbits of Mars and Jupiter, it slipped into the comet's tail of dust and exotic gases, passing within 235 kilometres of Wild 2's nucleus.

Stardust swept up particles in a collector shaped like a tennis racket and packed with an absorbent material called aerogel, then spent two years lining itself up in an orbit that would return it to Earth. On January 15 it dropped a canister containing the precious comet dust, which landed by parachute.

NASA's Johnson space centre carved the aerogel into thin slices, each containing particles, and sent them out to researchers around the world. Next week they will share their findings at the Lunar and Planetary Science conference in Houston, Texas.

The samples will be a treasure trove of organic material, possibly including amino acids, the building blocks of proteins.

"What we want to know is how organic molecules actually form in comets and whether they helped deliver organic material to the Earth before life began," said Brownlee.

The idea that comets delivered the basic components needed for life has growing support among astronomers. The theory is that the sun and planets began to form from a vast disc of interstellar dust, gases and debris about 5 billion years ago.

The sun would have formed first. Its radiation and gravity would then have had a powerful influence on the rest of the solar system, driving lighter molecules of compounds such as water, sulphur dioxide and carbon dioxide out from the inner solar system.

The process would also have produced billions of comets and meteorites. The Earth was formed 4.6 billion years ago and, as it cooled, these bodies, some of them huge, bombarded it, bringing organic matter and water. The first stirrings of life appeared 3.5 billion years ago.

The Earth's atmosphere is still showered in dust, meteorites and other debris every day. This carries water and organic material including amino acids. But scientists are not sure whether this modern material has the same composition as the comets and meteorites that hit the young Earth.

Phil Bland, a Royal Society research fellow at Imperial College, London, who is working on the Stardust samples, said that comets deep frozen for billions of years were like time capsules. "We can compare what's in them with what we see now, to work out the processes that have shaped our planet and all the others," he said.

Monica Grady, professor of planetary and space science at the Open University, is a member of one of the teams examining the Stardust samples.

"Organic material delivered by comets and meteorites between those dates (4.6 billion to 3.5 billion years ago) is likely to have played a part in starting life on Earth," she said.

Source: China Daily