THE INFRARED-SPECTRUM OF COMET BRADFIELD (1987S) AND THE SILICATE EMISSION FEATURE

Infrared (1-20 μm) observations of comet Bradfield (1987s) from three observatories are reported. Silicate emission is prominent in all the data, from heliocentric distance r = 0.87 to 1.45 AU. A CVF spectrum at r =145 AU shows a peak at 11.3 μm identified as crystalline olivine; the spectral shape is similar to that in Halley. Dust optical properties are similar to those of the grains in Halley's jets. Dust production rate near perihelion was ∼106 g s-1 and varied ∝r-2. We suggest that some differences in grain properties among comets could result from differences in the thermal history of the nuclear surface and the relative fraction of the dust particles originating in the subsurface icy layer versus the devolatilized mantle.