Infrared observations of comets with the Spitzer Space Telescope

The scientific motivation for infrared studies of comets is reviewed and the capabilities of Spitzer for cometary observations are described. We present Spitzer Space Telescope images and spectra of several comet comae, tails, and debris trails. Spitzer observations of Comet 29P/Schwassmann-Wachmann 1 are reviewed. The 24 μm images of the inner coma reveal structure consistent with jet activity on a rotating nucleus that otherwise appears to be ablating according to the "standard model". Spitzer spectra of the coma and nucleus indicate that the nucleus is large (R = 27 ± 5 km) and that the grains released from the nucleus have compositions similar to that of the grains in some stellar debris disks. Debris trails of several comets have now been imaged with Spitzer. The debris trail of Comet 2P/Encke is particularly noteworthy because its uniform intensity is consistent with the interpretation that the entire orbit is well populated by sand to gravel sized particles. A thermal infrared search for debris trails of several comets is described. We present a Spitzer spectrum of C/2001 HT50 (LINEAR-NEAT) showing that the grains populating its coma at a heliocentric distance of 3.3 AU are substantially larger than the grains in the comae of more pristine comets. Spitzer 3.6-8 μm images of C/2002 T7 (LINEAR) reveal a high-quality image of the extended dust tail.