Minerals: Stardust and the make up of a comet
Dept of Minerology, Natural History Museum
The Stardust mission flown by NASA and JPL/Caltech, to collect and analyse the materials which comprise a comet, was the first collection of extraterrestrial material for 30 years. It cost in excess of 200 million dollars and has involved the expertise of 200 international scientists. The specimens collected are regarded as possibly the most precious extra-terrestrial material ever obtained. Hence, they are being subjected to every available analytical technique. These include analytical scanning electron microscopy at the Natural History Museum. This is the same technique as routinely used on thin sections of meteorite material, using X-ray analysis under the JEOL 5900LV microscope to detect Mg, Al, Si, Ca and Fe. For example, this technique clearly shows that asteroid-derived meteorite materials, formed relatively close to the sun in the early solar system 4.5 billion years ago, contain melted droplets or chondrules rich in the minerals olivine and pyroxene, as well as other grains rich in Al and Ca. These meteorite materials provide a comparative background for the interpretation of returned comet samples.
Why collect from Comets?
The least processed dusty material lies within the interstellar medium. The Hubble telescope has shown the colossal clouds of dust and gas, with evidence of the formation of new stars and dense accretion disks, probably similar to the earliest phase of our own solar system’s development. This has provoked the question, are short period comets unaltered remnants of the cooler outer part of early nebular disk formed 4.5 billion years ago? Pre-solar, largely amorphous dust was extensively altered close in to the Sun, but within the Kuiper Belt, the comets might be expected to contain the raw unprocessed material. This material is believed to be different from other bodies. The bulk of the Solar System mass is represented by the Sun (rich in Mg, Si, Fe and S as well as Hydrogen and Helium), and although there is a fair understanding of the composition of the Earth (the core and mantle are inaccessible) it is clear that this planet is not primitive in structure and composition (for example the crust has higher proportions of Al, Ca and Na). Most asteroids (Gaspra being a good example) are also probably not as primitive as comets. Even the carbonaceous chondrite meteorites which are believed to be from the most primitive asteroids have often undergone metamorphism or alteration by fluids.
The Stardust spacecraft was launched on top of a Delta 7426 rocket in 1999 and flew >4 billion kilometres, to reach a comet in 2004 from which it collected dust and returned to Earth in 2006. The aims of this project were to examine the geochemistry, petrology and mineralogy of cometary dust. The capture process involved the use of aerogel, a solid substance of very low density. It is comprised of tiny spheres of bonded silicon and oxygen joined in long strands and separated by voids. The latest and lightest version weighs less than 3mg/cm3. The dust collector resembled a large tennis racket holding translucent blocks of aerogel, supported in an aluminium framework, sheathed by thin aluminium foil. As Stardust passed at 6.1km/sec through the comet dust, particles, which varied both in size and shape, were embedded into the aerogel.
Dust grains were later extracted using very fine needles and were then examined by numerous techniques including optical microscopy, X-ray projection microscopy, laser Roman spectroscopy, synchrotron (particle accelerator) IR spectroscopy and X-ray fluorescence maps to show the elements along the aerogel impact tracks and to measure the bulk composition of the cometary sample. Transmission electron microscopy revealed grains of olivine. Forsterite Mg2SiO4 was detected, although the range of composition suggests that these grains are probably not of interstellar origin, but are similar to olivines from solar system meteorites. X-ray maps of other material captured in the aerogel showed them to rich in Mg, Al & Ca; very similar to very high temperature materials for example thosefound in the Allende meteorite! These were both surprises.
Crater studies were performed to assess number, size, mass, shape, density and composition of dust impacted onto the aluminium foil. Using a light gas gun at the University of Kent , it was possible to simulate the effects of impact at velocity 6.1kms -1 on the aluminium sheets between the aerogel blocks. On impact most materials shatter and partially melt, but are quite well preserved, however sulphur (e.g. in pyrrhotite residue) evaporates, making measurement of this element difficult. Olivine, pyroxene and sulphides were again found in the Stardust material. The metal foil also proved to be a good medium for isotope analysis, and one tiny particle was discovered with an enrichment of an unusual isotope of oxygen, suggesting that this probably really was a grain of pre-solar, interstellar dust. However, in most grains the oxygen isotopes were similar to those found in meteorite dust, suggesting a common history.
The cometary dust ranged in size from 60 micrometres down to less than 40 nanometres. Many larger dust grains were aggregate clusters of smaller grains, may have resembled stratospheric dust particles, and created complex crater shapes. Mg silicates (olivine and pyroxene) were the most common minerals, with some Fe-rich sulphides. Some captured grains were single minerals, others were mixtures of minerals, including odd silicates and oxides. Organic material was found, but it is poorly preserved around craters and difficult to distinguish in the aerogel. The results of study so far have concentrated on coarser mineral grains which seem to have been carried from the inner part of the solar system outward to where the comet formed. Little of the finer-grained, and probably really more primitive, material has yet been examined thoroughly. Evidence from the four cometary nuclei that have now been examined by spacecraft suggests comet nuclei differ from one another. Most people did not expect the high-temperature minerals that were found in comet dust, and possibly little of the real ‘star-dust’ has yet been identified.
Further information can be found on the official NASA website at: http://stardust.jpl.nasa.gov/home/index.html
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