A Search for Trends in Cometary Dust Emission C.M. Lisse a , M.F. A'Hearn b , Y.R. Fernandez c , S.B. Peschke d a Space Telescope Science Institute, Servicing Mission Office, 3700 San Martin Drive, Baltimore, MD 21218 lisse@stsci.edu b University of Maryland, Department of Astronomy, College Park, MD 20742 ma@astro.umd.edu c Institute for Astronomy, University of Hawaii , 2680 Woodlawn Drive, Honolulu, HI 96822 yan@ifa.hawaii. d ISO Data Centre, Villafranca Del Castillo, ESA Satellite Tracking Station, P. O. Box (Apdo.) 50727, E-28080 Madrid, Spain speschke@iso.vilspa.esa.es We present the results of searching for statistical trends in the properties of dust emitted by 9 comets as determined using mid-IR observations. 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 volatiles, although caveats concerning our currently small (but growing) dataset and the role of potentially important selection effects in our results.. These results are consistent, however, with a larger sample of 46 infrsred comet observations found in the literature. 1 Introduction Studies of the physical properties of cometary dust are important for understanding the phenomena of comets as a class, and for understanding the formation and evolution of comets and the solar system. The quantities we can derive from infrared observations of the dust include the dust composition, mass, particle size distribution, and emission history. Given a large enough statistical population of observations, we can use the results of the measurements to understand the role of comets in the present day solar system, including, e.g., the contribution of cometary dust to the interplanetary dust cloud, and the evolution and fate of highly aged comets. Since comets are among the most primitive bodies in the inner solar system, despite their evolution, we also want to use the results to understand the formation of the solar system from the proto-solar nebula, e.g., the total and relative abundance of rock forming material, the formation and evolution of the accreting icy planetesimals (comets), and chemical variations in the proto- solar nebula. We hope to learn more about the aging process of comets, e.g.: the effect of weathering and mantling on the cometary surface and the effect of collisions on the cometary population. Combined thermal infrared and optical observations are sensitive to the 0.1 - 100 um range of dust particle sizes [1],[2], [3]). The presence of a strong 8 - 13 µm silicate emission feature is also an indicator of relatively small dust grains (≤ 10 µm), as is a spectral color temperature elevated more than 10% above the local equilibrium temperature, or a negative deviation from greybody behavior at 30 - 100 µm [4]. A total albedo for scattering, defined as the ratio of the scattered luminosity to the total luminosity observed at a given phase angle, is ~6% for large cometary particles (e.g., comets Austin and Encke) and >12% for small particles (e.g. comets Levy, Hyakutake, and Hale-Bopp). Extensions of the dust tail morphology along the projected orbital velocity direction (i.e., "trails") are due to large, heavy particle emission, while morphologies with only anti-solar tails are indicators of small particle emission [3]. Trends in long term light curves yield independent estimates of the particle size distribution and emission rate [1],[2]).