PHOTOMETRIC MODELS FOR GALILEAN SATELLITE ASTRONOMY

Models for the Galilean satellites have been used to determine the distribution of brightness on their surfaces. The results indicate that the distance between a satellite's photocenter and its center-of-figure can reach 209 km for Io, 221 km for Europa, 303 km for Ganymede, and 255 km for Callisto. Astrometry of the satellites can achieve much greater precision than this, so a correction for the photocentric offset should be applied. This paper presents tables of photocentric offsets for the Galilean satellites as functions of orbital longitude and solar phase angle. The estimated accuracy of these corrections is 32 km for Io, 45 km for Europa, 49 km for Ganymede, and 40 km for Callisto, but the latter two may be optimistic. The satellite ephemerides derived from astrometry are essential for interpreting observations of variable phenomena on Io, such as volcanic activity inferred from IR observations taken during occultations of the satellite. Accurate ephemerides are also critical to spacecraft navigation.