NASA's stardust spacecraft flew past the nucleus of Comet Wild 2, seen here, and collected tiny particles of comet dust.
Comet particles hint at Solar System's birth
4 Dec 2008
Using tiny particles from gathered from comet Wild 2, scientists are learning about the violent convulsions in the giant cloud of gas and dust that gave birth to our Solar System 4.5 billion years ago.
Those convulsions flung primordial material billions of kilometres from the hot, inner regions of the gas cloud that later collapsed to form the Sun, and out into the cold, nether regions of the Solar System, where they became incorporated into an icy comet.
'If you take a gas of solar composition and let it cool down, the very first minerals to solidify are calcium and aluminium-rich,' said Steven Simon, Senior Research Associate in Geophysical Sciences at the University of Chicago.
And comet Wild 2 does indeed contain these and other minerals formed at high temperatures. 'That's an indication of transport [of the minerals] from the inner Solar System to the outer Solar System, where comets are thought to have formed,' he said.
Simon presents his data in the November 2008 issue (expected to be published early next year) of the journal Meteoritics and Planetary Science. His 11 co-authors include Lawrence Grossman, Professor in Geophysical Sciences at the University of Chicago.
< A transmission electron microscope image (magnified 5,000 times) of a slice of the Inti comet dust particle, which NASA's Stardust spacecraft collected in 2004 and returned to Earth two years later.
Either turbulence within the gas cloud (nebula), or a phenomenon called bipolar outflow from the early Sun, could account for the long-distance transport of cometary material, according to Simon and his Meteoritics co-authors.
Bipolar outflows happen when the swirling, flattened cloud of gas that surrounds a developing star's equator forces gas to shoot or 'jet' out from the star's polar regions. 'That's part of the so-called X-wind model, which is somewhat controversial,' Simon said.
The contentious aspect of the X-wind model is the claim that the process would produce the kind of granules that Simon and his colleagues have now identified in comet Wild 2.
Another less likely possibility is that the cometary material in question may have formed around an entirely different star, with a composition similar to the Sun, and then drifted into the outer reaches of the Solar System, there to become incorporated into comet Wild 2.
The dust particles that Simon and his colleagues examined were among thousands that NASA's Stardust spacecraft collected from comet Wild 2 in January 2004. Two years later, Stardust became the first mission to return samples of a comet to Earth.
Simon, Grossman and collaborators identified all three particles described in the Meteoritics study as pieces of a 'shattered refractory inclusion', one of the most unusual and informative materials discovered in early analyses of the Wild 2 samples. Such inclusions, found in some meteorites, formed by condensation from the gas in the solar nebula at temperatures of more than 1,400 degrees Celsius early in the history of the Solar System.
The three particles were named Inti, Inti-B and Inti-C, after the Incan Sun god. The original, unbroken particle would have measured no more than 30 microns across, much narrower than a human hair.
A striking aspect of the studies is the similarity in chemical composition between the Wild 2 samples and particles from 'carbonaceous chondrite' meteorites. These meteorites contain material that has been unaltered since the birth of the Solar System 4.5 billion years ago.
The Stardust spacecraft's sample capsule, soon after landing back on Earth.
Equally striking is the complete lack of any water-bearing minerals in the cometary grains. Carbonaceous chondrites are rich in hydrated silicates, clay-like minerals that emit water when heated, 'but there's no hydrated silicate in the comet sample,' Grossman said.
Scientists organised the Stardust mission with the expectation that Wild 2's samples would reveal a bonanza of exotic minerals, including debris from stars that had met their demise long before the birth of the Sun. They might now need to rethink how comets formed, according to Grossman.
'Because they're loaded with ices we've always thought that these are outer Solar System objects,' he said. 'But maybe cometary ices formed much closer in, after the inner part of the solar nebula cooled off, and incorporated the high-temperature stuff that formed earlier.'
The Stardust mission was scientifically important because comets are usually out of reach, Grossman said. And yet aside from the Sun, they may be the most abundant material in the Solar System. 'There may be more stuff in the comets than in all the planets put together,' he said.
Adapted from information issued by the University of Chicago / NASA.
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