Dear friends Of Science and Astronomy,

Comets are born of fire as well as ice, the first results from the US space
agency's (Nasa) Stardust mission show.
http://news.bbc.co.uk/1/hi/sci/tech/4801968.stm
The first images of a comet particle were revealed last month

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. 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,
flying to within 240km (149 miles) of 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
melts at 2,000C and condenses at 1,127C, 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," 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.

Forwarded By yours Dr.BHUDIA-Science Group Of INDIA.
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