Measurement of the diameter of Pluto by speckle interferometry

Abstract

For the diameter of Pluto the values of 3000 + or - 400 km, assuming no limb darkening, and 3600 + or - 400 km, assuming limb darkening following a cosine law are obtained. The presence of a degree of limb darkening is possibly indicated by earlier work. The data also show evidence of surface structure, and possibly a strong surface mottling, strong central enhancement in brightness (as would be produced by a semispecular surface), or a combination of both. Values for the average geometric albedo are 0.36 + or - 0.14 (no limb darkening) and 0.25 + or - 0.08 (limb darkening included), consistent with a surface having a large covering of methane frost. From a determination of Pluto's mass using recent data, mean densities of 0.8 (+0.6, -0.3) g/cu cm (no limb darkening) and 0.5 (+0.3, -0.2) g/cu cm (limb darkening included) are obtained. Such low densities suggest that Pluto consists mainly of frozen volatiles.