SIMULTANEOUS ESTIMATION OF THE MASSES OF MARS, PHOBOS, AND DEIMOS USING SPACECRAFT DISTANT ENCOUNTERS

The masses of Mars and its satellites, Phobos and Deimos, have been es timated from the Mariner 9 and Viking 1 and 2 Orbiter tracking data. T hese spacecraft were sensitive to the gravitational force of Mars as w ell as to its satellites. Although the satellite masses are eight orde rs of magnitude smaller than Mars, their regular effect on the orbits of the spacecraft is evident in the tracking data and has enabled us t o derive their masses simultaneously with that of Mars. Our method for estimating the satellite masses uses the many ''distant encounters'' of the spacecraft with these small bodies rather than the few ''close encounters'' used in previous studies. The mass estimate for Phobos le ads to a mean density of 1530+/-100 kg m(-3) based on a volume of 5748 +/-190 km(3) (Thomas, 1993), while the mass estimate of Deimos leads t o a poorly constrained mean density of 1340+/-828 kg m(-3) based on a volume of 1017+/-130 km(3) (Thomas, 1993). Our analysis confirms, with in the bounds of error, the anomalously low density of Phobos using an independent method and data set. If the result is valid within severa l times the estimated error (1 sigma), then factors other than composi tion, i.e., porosity, a thick regolith and/or a significant interior i ce content, are required to explain the observed mass of this body.