Cometary grains

CCD observations of cometary comas in the spectral continuum are reported. Radial brightness profiles of 14 comets and isophotal maps of three are presented. For 10 of the 14, the azimuthally averaged brightness in the vicinity of the nucleus falls off faster than rho exp -1, where rho is the projected radial distance from the nucleus in the plane of the sky. Simple modeling is used to show that radiation pressure alone cannot account for the observed departures from rho exp -1; it is postulated that the grains fade, i.e., decrease in albedo or size, while they glow outward. For only three of the comets no evidence was found of any grain fading at all. Cases with the most rapid grain fading occurred at small heliocentric distances, but so did some of the cases with no fading, from which large intrinsic differences from comet to comet in the nature of the grain population are inferred.