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Stardust analysis reveals turbulent start to the Solar System

15.12.2006
Analysis of comet samples brought back to earth by NASA’s Stardust mission reveal that the start of the Solar System was a lot more turbulent than first thought.

Dust particles from comet Wild 2 are found to originate from both the inner reaches and outer edges of the Solar System indicating a massive amount of mixing of particles prior to the formation of the comet. The findings of the initial analysis of comet samples are published in Science today (15th December)

The precious cometary samples were collected by the Stardust spacecraft which journeyed 2.88 million miles during a seven year round trip from Earth to comet Wild 2 and back again. Following the capture of the particles from the comet in January 2004 the capsule containing the interstellar cargo was returned to Earth in January 2006. After its retrieval from the Utah desert grains of the comet were distributed to worldwide teams of scientists to begin the initial analysis.

In the UK scientists from The Open University, Imperial College London, the Natural History Museum and the Universities of Kent, Manchester and Glasgow are involved in the analysis.

The scientists have used a wide variety of sophisticated laboratory analytical techniques to study the samples and have found that they reveal a great deal not only about the composition of comets, but also provide key insights into the earliest history of our solar system.

Professor Monica Grady, from The Open University is a member of the Stardust Spectroscopy team, comments, “We are all very excited about what these results mean for our understanding of how the Solar System formed – and what this also means for other planetary systems. It seems that the cloud of gas and dust from which our Sun and planets grew was much more active and turbulent than had been appreciated, with mixing between different populations of grains taking place across the whole width of the disk.”

The returned samples consist of extremely small particles, from below 1 up to 300 micrometre (1/1000th of a millimetre) in diameter. The diameter of a human hair, for comparison, is 50 micrometres. Locked within the cometary particles is unique chemical and physical information that provides a record of the formation of the planets and the materials from which they were made.

The team from The Open University used a variety of instruments to determine the shape, composition and structure of the grains from the comet. They also analysed pieces of foil hit by the comet dust to measure the size and toughness of the grains.

Dr Simon Green, an expert in comets and asteroids, and a mission co-investigator, also from The Open University said, “I am intrigued by the unexpected pattern of impacts on the dust collectors, and what this may be able to tell us about the structure of cometary dust.”

Many of the grains were silicates – iron and magnesium rich grains that are the usual constituents commonly observed around newly forming stars where planets are being created. But one of the most exciting results was that some of the dust grains were not silicates, but were rich in calcium and aluminium, grains that are only produced at very high temperatures. This is important because it shows that some of the dust must have been formed very close to the centre of our Solar System. And it leaves a mystery as to how they managed to become mixed in with the rest of the dust and gas that made up the comets, forming on the outer edges of the Solar System.

Dr Ian Franchi from The Open University adds, “This is an astonishing set of data that we are still learning how to interpret. We have not yet begun to unravel the relationship between the organic and inorganic parts of the comet dust.”

Using spectroscopy technology, which does not damage the mineral content of the particles, the Stardust scientists from Imperial College London and the Natural History Museum have found that the comet dust is made up of many different mineral compositions rather than a single dominant one. This implies that the dust was formed in many different environments before coming together to make the comet, indicating a great deal of mixing in the early Solar System prior to the formation of the planets.

Of particular significance is the discovery of calcium aluminium inclusions, which are amongst the oldest solids in the Solar System and are thought to have formed close to the young Sun. This discovery suggests that the components of the comet came from all over the Solar System, with some dust having being formed close to the Sun and other material coming from the asteroid belt between Mars and Jupiter.

Dr Phil Bland, of Imperial’s Department of Earth Science and Engineering said, “We weren’t expecting to find such widely spread material in the sample of dust we were given to examine. 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.”

The researchers have also found evidence of surprising variety in cometary composition. NASA’s 2005 Deep Impact mission, which provide images of materials blasted from the nucleuss of comet Tempel 1, revealed evidence of aqueous activity within the comet. However, the dust from Wild-2 has none of those characteristics and apparently has not interacted with water at all. Dr Anton Kearsley of the Natural History Museum says, “This is a very interesting mismatch, and it seems that comets are not all the same. Perhaps they vary as much in their evolution as in the composition of dust from which they are made.”

The samples that have been analysed to date were obtained from only 10 of the 132 aerogel tiles in the Stardust collector, and a similar fraction of the exposed aluminium foils, that were exposed directly to the comet, leaving the remaining samples for future studies.

Seven papers on the initial findings of the Stardust mission appear in the December 15th edition of Science.

Gill Ormrod | alfa
Further information:
http://www.pparc.ac.uk

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