Mid-infrared Spectroscopic Observations of Comet 17P/Holmes Immediately After Its Great Outburst in 2007 October

Abstract

Dust grains of crystalline silicate, which are rarely present in interstellar space, were found in cometary nuclei. These crystalline silicates are thought to have formed by annealing of amorphous silicate grains or direct condensation of gaseous materials near the Sun in the solar nebula (SN), and incorporated into cometary nuclei in the cold comet-forming region after radial transportation of grains in the SN. Abundances of the crystalline silicate dust grains were therefore expected to be smaller farther from the Sun. We aim to better understand the formation mechanism of minerals incorporated into comet 17P/Holmes based on its mineral abundances. To derive the mineral composition of comet 17P/Holmes, we applied a thermal emission model for cometary dust grains to mid-infrared spectra of comet 17P/Holmes taken with the Cooled Mid-Infrared Camera and Spectrograph mounted on the Subaru Telescope a few days later the great outburst in 2007 October. The resulting mass fraction of crystalline silicate, f cry , and an olivine-to-pyroxene abundance ratio, f OP , are f cry  = 0.31 ± 0.03 and f OP  = 1.20 +0.16 / −0.12 , respectively. Based on a simple consideration of the mixing of dust grains originating in both the interstellar medium and SN, the minerals of 17P/Holmes formed by nonequilibrium condensation. This result is consistent with theoretical and experimental predictions for vaporization and condensation of olivine in the SN.