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In the News this month... the puzzle of crystalline silicates in comets

Illustration of crystals forming in a planetary disk.
The image shows a young sun-like star encircled by its planet-forming disk of gas and dust. The silicate that makes up most of the dust would have begun as non-crystallized, amorphous particles. CREDIT: NASA/JPL-Caltech
Models of solar system formation show that comets form at large distances from their parent star. This makes sense as they are made up largely of frozen material, but a long-standing mystery is how they end up containing tiny silicate crystals which need very high temperatures to form. These crystals start out in an amorphous form where their atoms are arranged randomly. At high temperatures, the atoms in these crystals become more ordered, forming what is known as a crystalline lattice. Because they need high temperatures to form, these crystalline silicates were not expected to be found in comets, so how they come to be there is a puzzle. In the May 14th issue of Nature, a team led by Peter Abraham of the Hungarian Academy of Sciences, published the first evidence of the formation of crystalline silicates in the disk around a young sun-like star. Together with colleagues from Leiden Observatory and the Max Planck Institute for Astronomy in Heidelberg, Abraham used the Spitzer Space Telescope to observe the eruptive star EX Lupi in April 2008 during one of its outbursts. When they examined the spectra from these observations, the researchers discovered the infrared signature of silicate crystals in the disk of dust and gas around the star.

When they compared their results to similar spectra of EX Lupi taken between outbursts, they found that the older observatories only showed the presence of amorphous silicates rather than the crystalline form. In the newer observations taken during the star's outburst, a broad peak corresponding to amorphous silicates was present, but an additional narrow peakat a wavelength of 10 microns was also visible. This narrow feature is likely caused by the presence of forsterite, the magnesium-rich crystalline form of the mineral olivine.

The appearance of this additional feature in the spectrum during the star's outburst suggests that crystal formation was happening in the star's disk during the outburst.  The researchers think that this is the first time ongoing crystal formation has been observed.  They say that the crystals were probably formed on the surface layer of the star's inner disk by heat from the outburst, in a process known as thermal annealing where a substance is heated to a temperature where some some of its bonds break and then re-form altering the material's structure and physical properties.  The forsterite crystals detected are just like those found in comets in the solar system which could have been produced by similar outbursts from out own Sun when it was much younger.

Posted by Megan on Saturday 30th May 2009 (13:06 UTC) | Add a comment | Permalink


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