INFO 01-2000: The crystalline revolution: ISO's finding opens a new research field, 'astro-mineralogy'
3 February 2000Silicate crystals, the most abundant minerals on Earth, are also found in great quantities around old stars and in protoplanetary discs -the discs where planets form. This finding, presented today at a press conference at ESA's Villafranca station in Madrid, is considered by experts in space chemistry as one of the main results of ESA's infrared space telescope, ISO.
Silicate minerals were known to be a main component of dust in space, but detecting them in a crystallised state has been a surprise. It allows the identification of precise silicates in astronomical objects, which will open "a totally new field in astronomy: astro-mineralogy. This is the crystalline revolution." said the author, Dutch astronomer Rens Waters of Amsterdam university.
"It's really fantastic, this possibility of identifying the silicates. Before ISO everybody thought that all silicates in space were amorphous, without a well-ordered internal structure; that means you cannot differentiate among the many different silicates existing. Now we can try to identify them and track their presence in different regions. A whole new research field is starting," added Waters, who brought to the press conference samples of several terrestrial crystalline silicates: olivine and pyroxene, the most common silicates on Earth.
Crystals give key clues about the physical conditions and evolutionary history of crystal-bearing objects. The precise mechanisms for crystal-making are now being researched now very actively in the laboratories, although some working-hypotheses are already being used. For instance, crystals can be made by heating the material to temperatures above 1300 °C and then cooling it down slowly. Those found so far by ISO are at -170 °C, both in stellar envelopes and in protoplanetary discs.
"The crystals detected by ISO in these discs have a size of about a thousandth of a millimetre. They collide with each other, forming bigger and bigger bodies. Models predict that in about ten to one hundred million years they will make planets," Waters says. "In fact, crystalline silicates are very common in our own Solar System. You also have them in the comet Hale Bopp!"
The reason why crystalline silicates had not been detected before in stars has to do with their low temperatures. Cold material emits mostly infrared light, which means an infrared space telescope like ESA's ISO was needed. The two high-resolution spectrometers on-board the satellite, able to detect the 'chemical fingerprint' of the crystals, did the rest.
Astronomers are sure about the discovery because those chemical fingerprints, the spectra, can be compared in laboratories with spectra from crystalline silicates found on Earth. This method has demonstrated the crystalline structure and has even already allowed the identification of some of the crystals, such as forsterite and enstatite. However, crystalline silicates are a large family and their chemical signatures can be very similar; to enlarge the list of precise crystals more work will be needed, say experts in space chemistry.
That is just one of the open questions requiring lab work. There's at least another one: crystalline silicates are found around old stars, in protoplanetary disks and in our own Solar System, but not in the space among the stars; astronomers can't explain it yet.
"Crystalline silicates are synthesised around the stars; then that dust goes into the interstellar space, and enriches the raw material out of which more stars and planets will form. So you would expect crystals also to be in the interstellar medium! Crystals will certainly make us learn a lot..." says Waters.
"This finding shows that ISO is really unveiling the chemistry of the Universe," says ESA astronomer Alberto Salama, chairman of the workshop about ISO results in spectroscopy held this week at ESA's Villafranca station in Madrid where the results were presented to the scientific community. "This is becoming more and more a 'hot field' of research. Initially we intended to organise a modest workshop, but we have had 150 astronomers coming from all over the world!"
Footnote about ISO
The European Space Agency's infrared space observatory, ISO, operated from November 1995 to May 1998, almost a year longer than expected. An unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO made nearly 30 000 scientific observations.
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ESA Communication Division
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Martin F. Kessler
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