Dust Properties of Comets Observed by Spitzer

As comets journey into the inner solar system, they deliver particulates and volatile gases into their comae that reveal the most primitive materials in the solar system. Cometary dust particles provide crucial information for assessing the physicochemical conditions in the outer disk from which they formed. Compared to the volatiles and soluble organics, the refractory dust particles are more robust and may be traceable to other small bodies. Using data from the Spitzer Heritage Archive, we present thermal dust models of 57 observations of 33 comets observed spectroscopically with the NASA Spitzer Space Telescope. This comet spectral survey offers the opportunity to study comets with data from the same instrument, reduced by the same methods, and fitted by the same thermal model using the same optical constants. The submicron dust tends to be dominated by amorphous carbon, and the submicron silicate mass tends to be dominated by amorphous silicate materials. We discuss the implications of these findings as they relate to Mg-rich crystalline silicates, which are high-temperature condensates, as well as to potential ion irradiation of amorphous Mg:Fe silicates prior to their incorporation into comets. These results impact our understanding of the protoplanetary disk conditions of planetesimal formation. Lastly, we cannot definitively conclude that a distinct difference exists in the dust composition between the Oort cloud and Jupiter-family comet dynamical population as a whole.