Comets 'are born of fire and ice'

By Paul Rincon
BBC News science reporter, in Houston, Texas

The first images of a comet particle were revealed last month

Comets are born of fire as well as ice, the first results from the US space agency's (Nasa) Stardust mission show.

In January, Stardust's sample-return capsule landed in Utah, carrying over a million tiny comet grains inside.

Some of these grains contain material that formed at extremely high temperatures, scientists have found.

This is a surprise. Comets formed in the cold, outer-reaches of the early Solar System, and were never exposed to such extreme heating.

The Sun and the planets began forming out of a gaseous cloud called the solar nebula about 4.6 billion years ago.

This "accretion disc" consisted of a hot inner region and a cold outer region where ice was able to survive.

When these grains formed, they were incandescent - they were red or white hot Dr Donald Brownlee, Stardust chief scientist

The high-temperature minerals found in the Stardust samples may have formed in the inner part, where temperatures exceeded 1,000C.

But something must then have transported them out to the cold, comet-forming region known as the Kuiper Belt.

"These are the hottest minerals found in the coldest place, in the 'Siberia of the Solar System'," said Donald Brownlee, chief scientist on the Stardust mission.

"When these grains formed, they were incandescent - they were red or white hot."

Abundant samples

Details of the analysis were presented here at the Lunar and Planetary Science Conference in Houston, Texas.

The Stardust spacecraft encountered Comet Wild-2 in January 2004. It swept up particles from the frozen body of ice and dust, passing some 240km (149 miles) from the comet's core, or nucleus.

Dr Brownlee said further analysis should resolve some issues

It then released its sample-return capsule as it flew back to Earth at the beginning of this year. The US-built capsule touched down in the Utah desert on 15 January. They are the first cometary dust samples ever returned to Earth.

The high-temperature minerals discovered in the Stardust samples are not oddities.

They appear to be abundant, having been found in about one in four of the particles examined so far.

One of these minerals known as forsterite, which condenses at temperatures in excess of 1,120C, has been detected in a comet before.

But other minerals found in the Stardust samples resemble so-called calcium-aluminium inclusions (CAIs), which form at even higher temperatures.

"This raises as many questions as answers. We can't answer them all just yet," said Stardust co-investigator Dr Mike Zolensky.

Longer distances

There are two main possibilities currently being considered to explain the finding.

If the high-temperature minerals formed at the centre of our solar nebula, the molten droplets could have been blasted out to the cold outer region by powerful magnetised jet called the X-wind.

It's perhaps indicative that the X-wind model is a good one Dr Caroline Smith, Natural History Museum, London

"It's perhaps indicative that the X-wind model is a good one," Caroline Smith, meteorite curator at the Natural History Museum in London, told the BBC News website.

But it means that these bursts must have carried the minerals much further distances than has previously been suggested.

Alternatively, the minerals may have been formed in the hot regions of other stars before finding their way into the solar nebula, where they were incorporated into comets.

"These are fascinating possibilities," said Dr Brownlee.

"In the lab, we can study these at atomic-level resolution and use the chemical, mineralogical and isotopic properties."

He said that analysis of the different isotopes, or forms, of elements in the mineral should resolve where they originated.