MASSES AND DENSITIES OF PLUTO AND CHARON

We have analyzed Hubble Space Telescope wide field camera CCD images of Pluto, Charon, and a background star, obtained on seven HST visits over a 3.2 day span in 1991 August, to observe Pluto's barycentric motion and to determine the individual masses and bulk densities of Pluto and Charon. The most fundamental new result was an accurate solution for a previously undetermined parameter, the Charon/Pluto mass ratio q; the new solution is q = 0.0837 +/- 0.0147. Significant accuracy improvements by almost a factor of 2 were obtained for Charon's orbital semimajor axis, a = 19 405 +/- 86 km and the Pluto system mass, M(sys) = (1.401 +/- 0.0 1 9) X 10(8) inverse solar masses. The Pluto and Charon m were M(P) = ( 13.10 +/- 0.24) X 10(24) g and M(C) = ( 1. 10 +/- 0. 18) X 10(24) g, respectively. Computed densities depend strongly on the assumed radius values. Assuming radii determined from mutual event observations by Tholen & Buie [BAAS, 22, 1129 (1990)] then the density of Pluto is rho(p) = 2.13 +/- 0.04 g/cm3 and the density of Charon is rho(C) = 1.30 +/- 0.23 g/cm3. Later analysis of Pluto stellar occultation data [Elliot & Young, AJ, 103, 991 (1992)] indicated that Pluto atmospheric transmission for mutual events and stellar occultations may have a significant effect on determination of the Pluto surface radius R(P). The R(P) value from their ''thermal-gradient'' atmospheric model implies rho(P) = 1.78 +/- 0.06 g/cm3 and the R(P) value from their ''haze'' model implies rho(P) greater-than-or-equal-to 1.90 g/CM3. All these values for rho(P) are significantly higher than the solution value for rho(C).