OBSERVATIONS OF STRUCTURES IN THE INNER COMA OF CHIRON WITH THE HST PLANETARY CAMERA

We have obtained a series of images of 2060 Chiron with the HST Planet ary Camera on 3 nights near its minimum geocentric distance in 1993 (D elta = 8.342 AU). Extended structure was visible within a radial dista nce of 0.3'' from the nucleus. We interpret the structure to be the bo und ballistic atmosphere and exopause boundary (at R(e) approximate to 1800 km) hypothesized by Meech gr Belton [AJ, 100, 1323 (1990)]. Near simultaneous ground-based photometry of the outer coma showed a steep surface brightness profile with a logarithmic gradient of -3.1 +/- 0. 03 (from 1-3.3'') and -2.2 +/- 0.01 (from 3.3-9.8''). The inner gradie nt is consistent with the HST observations. The color of the outer com a from the ground-based data is neutral at short wavelengths, and slig htly redder than the sun at the longer wavelengths. The outer coma sca tters neutrally at much shorter wavelengths than most comets, which su ggests that the outer coma is populated by smaller grains (lambda less than or equal to 0.6 mu m). Combining the grain size of particles esc aping into the outer coma with the HST observations, and constraints o n grain densities, we infer that the density of Chiron is relatively l ow, near rho(Nuc) < 10(3) kg m(-3). Within the exopause boundary the n ucleus contributes approximately 80% of the total light. (C) 1997 Amer ican Astronomical Society.