We present infrared images and spectra of comets 2P/Encke , 67P/Churyumov-Gerasimenko , and C/2001 HT50 ( LINEAR-NEAT ) as part of a larger program to observe comets inside of 5 AU from the sun with the Spitzer Space Telescope .
The nucleus of comet 2P/Encke was observed at two vastly different phase angles ( 20° and 63° ) .
Model fits to the spectral energy distributions of the nucleus suggest comet Encke ’ s infrared beaming parameter derived from the near-Earth asteroid thermal model may have a phase angle dependence .
The observed emission from comet Encke ’ s dust coma is best-modeled using predominately amorphous carbon grains with a grain size distribution that peaks near 0.4 µm , and the silicate contribution by mass to the sub-micron dust coma is constrained to < 31 % .
Comet 67P/Churyumov-Gerasimenko was observed with distinct coma emission in excess of a model nucleus at a heliocentric distance of 5.0 AU .
The coma detection suggests that sublimation processes are still active or grains from recent activity remain near the nucleus .
Comet C/2001 HT50 ( LINEAR-NEAT ) showed evidence for crystalline silicates in the spectrum obtained at 3.2 AU and we derive a silicate-to-carbon dust ratio of 0.6 .
The ratio is an order of magnitude lower than that derived for comets 9P/Tempel 1 during the Deep Impact encounter and C/1995 O1 ( Hale-Bopp ) .