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BBC NEWS
Comets 'are born of fire and ice'
By Paul Rincon
BBC News science reporter, in Houston, Texas
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.
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.
Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/4801968.stm
Published: 2006/03/14 01:00:52 GMT
© BBC MMVI
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