Comet Grain Albedo and Coma Structure

Spectrophotometric analysis of materials released from comet nuclei during perihelion passage through sublimation, outgassing, and nucleus fragmentation provides insight into the physical conditions in both the environment beyond the "frost line'' (>5 AU) where icy planetesimals aggregated and the hotter dust formation zones closer to the proto-sun where refractory materials condensed. We are initiating a concerted effort to measure comet dust albedos, a basic parameter characterizing the size distribution and physical properties of comet dust, with SOFIA (+FORCAST), targeting in CY2 comet C/2012 K1 (Panstarrs). Only ~12 comets have measured IR grain albedos at various phase angles, and multi-epoch observations of individual comets show considerable scatter about the expected phase function. Scattering in albedo is very important as it reflects: (1) heliocentric variations of dust properties; and (2) difference in dust produced by comet outbursts and other activities. Thus, scattering in albedo is diagnostic about inhomogeneity of comets. Our science goals are to: (1) develop an empirical study that combines compositional information from the mid-IR with albedo and dust color to determine the mineral and physical properties of comets; and (2) increase the number of comets with well-determined albedos.