Exothermic Chemical Reactions Can Drive Nonthermal Crystallization of Amorphous Silicate Grains

Details Exothermic Chemical Reactions Can Drive Nonthermal Crystallization of Amorphous Silicate Grains Exothermic Chemical Reactions Can Drive Nonthermal Crystallization of Amorphous Silicate Grains

Sep 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007apj...666l..57k&link_type=abstract

The Astrophysical Journal, Volume 666, Issue 1, pp. L57-L60.

Astronomy and Astrophysics

Astronomy

11

Astrochemistry, Comets: General, Methods: Laboratory, Radiation Mechanisms: Nonthermal

Scientific paper

To explain how cometary silicates crystallize yet still preserve volatile interstellar ices in their parent comets, we experimentally demonstrate the possibility of chemical-reaction-driven crystallization, which is called nonthermal crystallization, using laboratory-synthesized amorphous Mg-bearing silicate grains. Analog silicate grains ~50-100 nm in diameter covered with a carbonaceous layer consisting of amorphous carbon, CH 4, and other organics to a thickness of ~10-30 nm were used as models. The analog silicate grains crystallized via the direct flow of surface reaction energy, which is produced by the graphitization of the carbonaceous layer due to oxidation at room temperature in air, into the silicates. The experimental results imply that amorphous silicates are transformed into crystalline silicates as the grains leave the comet's surface, rather than as the comet was accreted 4.5 billion years ago. Thus, primordial ices and amorphous silicate grains are predicted to reside in most comets until they approach the Sun.

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