Comet 17P/Holmes in Outburst: The Near Infrared Spectrum

Jupiter family comet 17P/Holmes underwent a remarkable outburst on UT 2007 Oct. 24, in which the integrated brightness abruptly increased by about a factor of a million.We obtained near infrared (0.8 - 4.2 micron) spectra of 17P/Holmes on UT 2007 Oct. 27, 28 and 31, using the 3.0-m NASA Infrared Telescope Facility (IRTF) atop Mauna Kea. Two broad absorption bands were found in the reflectance spectra with centers (at 2 micron and 3 micron, respectively) and overall shapes consistent with the presence of water ice grains in the coma. Synthetic mixing models of these bands suggest an origin in cold ice grains of micron size. Curiously, though, the expected 1.5 micron band of water ice was not detected in our data, an observation for which we have no explanation. Simultaneously, excess thermal emission in the spectra at wavelengths beyond 3.2 micron has a color temperature of 360 +/- 40 K (corresponding to a superheat factor of ~ 2.0 +/- 0.2 at 2.45 AU). This is too hot for these grains to be icy. The detection of both water ice spectral features and short-wavelength thermal emission suggests that the coma of 17P/Holmes has two components (hot, refractory dust and cold ice grains) which are not in thermal contact. A similarity to grains ejected into the coma of 9P/Tempel 1 by the Deep Impact spacecraft is noted.