Gravity Fields

Doppler radio tracking of deep space probes near extra-terrestrial bodies has provided information on the internal mass distribution for Jupiter, Mars, the Moon, Venus, and Mercury. The more detailed results have been obtained for Mars and the Moon where orbiting spacecraft (Mariner 9 and Apollo) have returned a vast amount of gravity data The instrumentation to acquire the data consists of a spacecraft transponder and an Earth-based ultrastable frequency oscillator, along with appropriate transmitters, receivers, and antennas. The necessary data reduction to extract the gravity parameters from the Doppler signal variations requires a very complex computer program. The major achievements have been the detailed results for Mars and the Moon where global gravity fields reveal high correlation with topography. Present investigations on isostatic conditions are constraining viscosity estimates, and it appears that viscosity at various eras may be determined. Apollo data at very low altitudes (15-20 km) have provided gravity profiles for frontside mascons, craters, and mountain chains. The determinations of the first three even zonal spherical harmonic coefficients for Jupiter places constraints on the interior as well as the outer envelope. A proposed cross section of Jupiter is shown, composed primarily of hydrogen and helium in different states. Some comments are made on Mercury, Venus, and Jupiter´s Galilean satellites.