Particles returned to Earth last January by the Stardust spacecraft from comet Wild 2 are yielding precious information about the origin of the solar system, thanks to the brilliant X-rays produced at several of the world's synchrotron facilities, including the ESRF. Although the particles are tiny, the X-ray beams available at synchrotrons can be even smaller, enabling researchers to illuminate the cometary material and in some cases determine the distribution of elements within the particles without damaging them. These results describe the overall composition and chemistry of the samples returned by Stardust, and are published as part of a special series of papers in the 15 December 2006 edition of the journal Science.
Our Solar System is about 4.5 billion years old, and the details of its origin are still a mystery to researchers. Scientists theorize that large, interstellar dust clouds give rise to new stars and planetary systems. As these dust clouds collapse, a central star forms surrounded by a rotating disk of dense gas. The planets of our Solar System likely coalesced from one of these disks.
Wild 2 is believed to have originated within a cloud of comets just beyond the orbit of Neptune called the Kuiper Belt. Because Kuiper Belt objects spend most of their time far away from the Sun, researchers suspect they remain unchanged by radiation, heating and aqueous alteration and therefore likely carry intact material from the earliest ages of the solar system.
The cometary samples were collected from the comet Wild 2 by the Stardust spacecraft, which travelled 2.88 billion miles during its seven-year odyssey before returning to Earth. Stardust returned about one microgram of cometary dust, the largest of which are about 10 microns—about a tenth the diameter of a human hair.
The samples of the Stardust mission examined by the scientists were compared with the most primitive meteorites found on earth, which are believed to be samples left over from the formation of the solar system. The samples contain a wide variety of minerals and organic materials that look similar to those seen in primitive meteorites. But the Stardust samples also revealed the presence of new materials not previously found in meteorites. The chemical analysis of the Stardust samples could therefore improve our understanding of the chemistry of the early solar system.
The researchers also discovered that the samples contained minerals similar to compounds in meteorites known to form at high temperatures. These compounds, called Calcium Aluminum-rich Inclusions (CAIs), are believed to have been formed in the innermost part of the solar nebula, well inside the orbit of Mercury. This discovery challenges the belief that comets are formed only beyond the orbit of Jupiter, and suggests that these cometary materials must have somehow been transported to the edge of the solar system where Wild 2 formed. The results also suggest that the materials that formed our solar system underwent considerable mixing as the sun and planets formed.
A pinch of dust holds the answer
"We have taken a pinch of comet dust and are learning incredible things," said Stardust principal investigator Donald Brownlee, a professor at the University of Washington and lead author of an overview technical paper, one of seven reports in Science about the mission's initial findings.
During preliminary examination, over 200 samples from approximately 35 impact tracks were distributed to the 175 members of the Preliminary Examination Team around the world. The samples represent only a small fraction of the total collected material returned by the Stardust spacecraft. The rest will be preserved for study by future scientists as tools and techniques improve.
The diverse techniques needed to study the returned cometary material required the use of six synchrotron facilities around the world. Two European teams, one French from the Institut d’Astrophysique Spatiale in Orsay and The Ecole Normale Supérieur in Lyon, and the other from the Universities of Frankfurt (Germany), Antwerp and Ghent (Belgium), came to the ESRF to carry out experiments on a total of 7 samples. The minute size of the samples and their entrapment deep within slices of aerogel, called "keystones," made the brilliant X-ray radiation produced by synchrotron light sources ideal for peering into the particles. At the ESRF, they combined diffraction technique with high- and low-energy microspectroscopy to analyse the tracks in keystones. Due to the penetrating nature of the X-ray beams, the elemental distribution along the tracks could be mapped without removing the particles from the aerogel. Thus, crucial information was obtained which will be of use to subsequent researchers who wish to study the same particles.
Participating institutions included the European Synchrotron Radiation Facility in Grenoble, France; the Advanced Photon Source at Argonne National Laboratory, USA; the Stanford Synchrotron Radiation Laboratory at the Stanford Linear Accelerator Center, USA; the Advanced Light Source in Lawrence Berkeley National Laboratory, USA; the National Synchrotron Light Source at Brookhaven National Laboratory, USA; and Spring-8, Japan Synchrotron Radiation Research Institute.
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