An infrared color gradient in the inner coma of Comet Halley

Near-infrared images of Comet Halley were obtained in the standard J, H, and K bandpasses, on 1985 November 3.5 UT with a HgCdTe camera at the University of Arizona 1.54-m telescope. A well-defined gradient in the J-H and H-K colors within ∼8000 km of the nucleus has been discovered with the bluest colors at the photocenter. Surface brightness profiles steeper than the "expected" 1/ρ{variant}, where "ρ{variant}" is the projected distance from the nucleus, are observed in the same region. A preliminary analysis indicates that the color gradient and the brightness profiles can both be explained by the presence of volatile (dirty ice) grains in the inner coma. An outburst of very small (Rayleigh scattering) dust particles could also account for the observations; however, this second model is not supported by the spacecraft measurements. No obvious jets or other structures are observed.