On the asphericity of the figures of Pluto and Charon

摘要

The NASA spacecraft New Horizons allowed us to refine many physical, chemical, topographic, and geological characteristics of main bodies of the Pluto - Charon system significantly. Their sizes have been clarified. Their shapes were approximated by triaxial ellipsoids. But measurement errors of the axes differences a - c, a - b have exceeded their values. So the asphericity of the figures was not revealed. The same concerns the asphericity of the gravitational fields of Pluto and Charon. So measurements during future space missions must be more accurate. To evaluate the necessary precision we need bilateral theoretical boundaries of the possible values of polar and equatorial oblatenesses alpha = (a - c)/a, beta = (a - b)/a, as well as the second zonal and sectorial harmonics I-2, I-22 for both bodies. In the present paper we have obtained them under the assumption of the hydrostatic equilibrium of the Pluto and Charon figures. Pressure, centrifugal forces, and gravity (tidal interaction included) were taken into account. It turns out that the influence of rotation and tides are comparable; tides change the Charon shape more than the Pluto one; dimensionless quantities alpha, beta, I-2, I-22 are of the order of 10(-5) divided by 10(-4) for Pluto, and 10(-4) divided by 10(-3) for Charon.