Conclusions

Our infrared observations of dust emission from 9 comets can be explained by particles with similar composition ratios but two very different PSD's. While both these PSD's are mass dominated by the largest (radius >100 urn) particles, the emitted surface area (and optical brightness) is dominated by either 0.1-10 um particles in "dusty comets" or > 10 um particles in "gassy comets". There is good agreement between the results of our small/large particle dominated classification and the high/low polarization comets of Levasseur-Regourd et al. [16] and the strong/weak silicate feature comets reported by Hanner et al. [18] and Williams et al. [12]. Trends in the aggregate dataset of dust PSD, total emission rate, and emission rate vs time were found versus dynamical class, with the short period comets slowly emitting mainly large, dark dust particles while the LP comets emitted most of their dust surface area (but not mass) rapidly in small, high albedo particles. These differences may be due to the effects of cometary evolution on the structure of the cometary surface and/or the depletion of cometary volátiles, although caveats concerning our currently small (but growing) dataset and the role of potentially important selection effects in our results. To partially address these caveats, we have found 46 infrared studies of cometary dust emission in the literature, which, when classified by cometary class, showed evidence of small or large particle dominated dust emission consistent with the trends found in our database.

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