DUST IN COMETS .1. THERMAL-PROPERTIES OF HOMOGENEOUS AND HETEROGENEOUS GRAINS

The equilibrium grain temperatures (Teq) for heterogeneous grains were calculated using the Maxwell-Garnett and Bruggeman effective dielectric medium theories for admixtures of magnetite, water ice, and a silicate. Admixtures of magnetite and a silicate were analyzed as a function of the total volume fraction occupied by each mineral. The equilibrium grain temperature depends upon the volume fraction and which theory is applied but is not a volume-weighted average of the temperatures of the two homogeneous grains. Spectra of a size distribution of heterogeneous grains show differences which depend on the theory and the relative volume fractions, although these differences may not always be detectable. The Maxwell-Garnett theory was applied to an admixture of water ice and magnetite or a silicate. The presence of an inclusion raises the equilibrium temperature, but sublimation reduces the magnitude of this increase. The spectrum of dirty ice shows spectral features similar to that of pure ice in the 10 μm region. At 1 AU, the grains are never warm enough to excite the O-H vibration modes seen at 3.1 μm. Small (a ≤ 100 μm) dirty ice grains have lifetimes less than 15 minutes at 1 AU, although larger or less dirty grains may exist long enough to form an icy grain halo surrounding the nucleus of a comet.