Ultraviolet Albedo of Cometary Grains

Several dusty comets, including two at heliocentric distances greater than 3 AU (Bowell 1982 I and Cernis 1983l), have been observed with the International Ultraviolet Explorer. Ultraviolet albedos have been determined for the wavelength range 2600–3050 A by comparison of the IUE data with both recent solar flux measurements and with IUE observations of solar analogue stars. All of the comets show a similar reddening within this spectral range as well as with respect to the visible and infrared. No spectral signatures of possible ices are evident in the data. The ratio of gas to dust production rates, evaluated for 15 comets observed by IUE, shows a well defined dependence on heliocentric distance.