Comet's ingredients surprise scientists

The first comet samples returned to Earth have forced scientists to change long-held views on the origins of our solar system.

Researchers expected samples from the icy celestial body to be mostly made of interstellar dust or a single mineral, as comets were long thought to have spun out of the outer rim of the early solar system.

But miniscule samples obtained from comet Wild 2 by the Stardust spacecraft and returned to Earth in January, 2006 have been found to contain a variety of mineral components, some of which may have formed close to the young Sun.

The findings, published on Friday in the journal Science, have forced scientists to rethink how the solar system and its planets came to be.

"We weren't expecting to find such widely-spread material in the sample of dust we were given to examine," said Dr. Phil Bland of the department of Earth science and engineering at Britain's Imperial College London.

"The composition of minerals is all over the place, which tells us that the components that built this comet weren't formed in one place at one time by one event. It seems that the solar system was born in much more turbulent conditions than we previously thought."

Particularly significant was the discovery of calcium aluminum particles held in the comet's minerals, which are among the oldest solids in the solar system.

Don Brownlee, a University of Washington astronomer who is the principal scientist for the $212 US million Stardust mission, estimated 10 per cent of materials in comets may come from the inner solar system.

The comet has been dubbed a frozen time capsule because it contains material preserved from the aftermath of the solar system's birth more than 4.5 billion years ago.

"Comets are likely to be the oldest objects in our solar system and their components have remained largely unchanged, so discovering more about what they have experienced gives us a snapshot of the processes that formed the planets over four and a half billion years ago," said Bland.

"Fundamentally we still don't know how you make planets from a cloud of dust and gas. Hopefully the Wild 2 samples will help us towards an answer."

The Stardust spacecraft, launched in 1999, obtained the samples in 2004 when Wild 2 came within range between Mars and Jupiter. Stardust flew within 240 km of the comet's nucleus, capturing microscopic samples in a mold of a special lightweight material called aerogel.

Aerogel, the lightest solid known, is a foamed glass with the density of air. As the comet particles hit the mold they slowed down and eventually came to rest within the aerogel.

An analysis also found Wild 2 differed from comet Tempel 1, which was studied in NASA's Deep Impact mission.

Last July, NASA crashed a probe into Tempel 1 and studied the dust and ice spewing from its core, but did not retrieve samples from the surface.

The dust collected from Wild 2 showed no interaction with water, leading Anton Kearsley of the Natural History Museum in London, England, to suggest comets may form in a variety of ways.

"This is a very interesting mismatch, and it seems that comets are not all the same," he said.