A comparison of the continuum spectra of four comets

The continuum spectra of comets carry information concerning the physical and chemical properties of solid coma grains. Although it is not feasible to use the continuum spectra to uniquely characterize the solid grains, variations among the continua of different comets may reveal subtle differences in their respective grain populations. We have taken and reduced optical spectra of four comets in the wavelength range 3700–7300 Å using a single observing system and reduction procedure. The continua all appear reddened with respect to the solar spectrum. The amount of reddening is consistent with a prevalence of ～2-μm-sized grains in all four comets, if the refractive indices of the grains are approximately equal to those of terrestrial rocks. Significant color differences were measured among the comets. Different intrinsic grain properties are suggested since the scattering geometries were very similar. The amount of reddening does not appear to be correlated with the amount of dust in the coma.