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C. G. Mason, R. D. Gehrz, T. J. Jones (U. Minnesota), M. S. Hanner (JPL), D. M. Williams (Harvey Mudd College), C. E. Woodward (U. Wyoming)
Comet nuclei are believed to contain, in the frozen state, samples of the primary solid and condensible constituents that were present in the early solar system during the epoch of planet formation. During perihelion passage, the characteristics and composition of this primordial material can be analyzed as it is ablated from the nucleus and is dispersed by radiation pressure to form the coma and tails. The primary source of infrared (IR) radiation from comet comae and tails is thermal emission from dust grains. The spectral energy distributions (SEDs) derived from IR filter photometry can be used to deduce a variety of physical attributes of comets including the size distribution of the grains and the nature of the nuclear ablation mechanism. Observations of other comets has demonstrated the importance of measuring the entire 1 - 20 \micron \ SED with frequent temporal coverage to document the characteristics of transient events and dust evolution as a function of heliocentric distance r and comet nucleus solar insolation exposure times.
We present 1.2 - 18.5 \micron \ photometry of comet C/1995 01 (Hale-Bopp). Our observations spanned nearly 4 months pre- and post-perihelion. We compare our results with those predicted by the steady-state model for IR emission from coma dust grains. In addition, we obtained data in several regions of the coma. We found significant temperature gradients suggesting differences in the dust grain size distribution throughout the coma. Finally, we compare several attributes of comet Hale-Bopp with those of 11 other bright comets. The color temperature, 10 \micron \ silicate strength, and albedo of comet Hale-Bopp exceeded those of all the other comets, suggesting the coma of Hale-Bopp either had a different dust grain size distribution and/or was composed of dust grains with different optical properties than those associated with the other comets.
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