Io's Triaxial Figure

Timings from photoelectric observations of four immersions and two emersions of the recent occulation of Beta Scorpii C by Io have made it possible to derive Io's apparent equatorial radius to an accuracy of 2 km, an improvement of nearly two orders of magnitude over previous results. However, when we consider the distortion of Io attributable to rotation and tides raised by Jupiter, the derived mean radius becomes several kilometers less than the observed equatorial radius. If Io were a homogeneous fluid body in hydrostatic equilibrium and in synchronous rotation, which we consider to be reasonable assumptions, it would show a bulge along a line to Jupiter about 20 km in radius greater than the polar radius. The mean radius thus derived is (1818 ± 5) km and mean density (2.88 ± 0.34)g/cm ³, the largest uncertainty being attributable to the value of Io's mass. This timing residual of one immersion suggests the existence of a surface irregularity about 3 km below the mean limb of Io. The new value for Io's radius is higher than those previously determined and suggests about 30% lower values for mean densities of all Galilean satellites.