Shape, mean radius, gravity field, and interior structure of Callisto

Radio Doppler data generated by the Deep Space Network (DSN) from five enco unters of the Galileo spacecraft with Callisto, Jupiter's outermost Galilea n satellite, have been used to determine the mass (GM) and unnormalized qua drupole gravity coefficients in Callisto's external gravitational field. Th e results are GM (7179.292 +/- 0.009) km(3) s(-2), J(2) = (32.7 +/- 0.8) x 10(-6), C-22 = (10.2 +/- 0.3) x 10(-6), S-22 =(-1.1 +/- 10.3) x 10(-6), C-2 1 = (0.0 +/- 0.3) x 10(-6), and S-21 = (0.0 +/- 1.6) x 10(-6). Also, four s pacecraft images of Callisto have been used to determine its mean radius. T he result is R = (2410.3 +/- 1.5) km, with no detectable deviation from sph ericity. Derived parameters are Callisto's mean density of (1834.4 +/- 3.4) kg m(-3) and axial moment of inertia C/MR2 = 0.3549 +/- 0.0042. While the mean density indicates that Callisto is a mixture of rock and ice, the mome nt of inertia is too small for a homogeneous mixture. Accordingly, we prese nt a suite of possible two- and three-layer interior models that satisfy th e given constraints for radius, density, and moment of inertia. While not u nique, these models show that Callisto cannot be entirely differentiated, a nd that there must exist a region of mixed ice and rock-metal, possibly ext ending to the center of the satellite. (C) 2001 Academic Press.