This chapter is concerned with the properties of Mars as a planet, that is, as an individual member of the solar system. The first of these properties to be considered is the mass. Two methods have been commonly used in the past to determine the mass of a planet. One is dependent on the disturbances (or perturbations) produced by the planet on the orbital motion of an adjacent planet or, preferably, of an asteroid. The other is based on a study of the orbit of a natural satellite (or moon) of the planet. In addition, a third method, which has recently become feasible, is to observe the influence of the planet on the trajectory of a spacecraft in its vicinity.
Because Mars has two satellites, and was approached by the spacecraft Mariner IV in July 1965 (ch. XIII), all three procedures outlined above have been applied in calculating the mass of the planet. Of the three, the last one is probably the most accurate and it has led to the following value of the mass of Mars:
6.419 X 1026 grams or 6.419 X 1020 metric tons where 1 metric ton is 1000 kilograms (2204.6 pounds). This is 0.1073, or a little over one-tenth, of the mass of Earth.
It is of interest that the mass of Mars obtained in 1877 by A. Hall from observations of the satellites of Mars, which he discovered, was 6.430 X 1026 grams. This is the value which has been used since 1895 in "The American Ephemeris and Nautical Almanac," published annually by the U.S. Naval Almanac Office. It is difficult to understand, however, the discrepancy between the Mariner IV value for the mass of Mars and that published in 1961 by D. Brouwer and G. M. Glemence, 6.4411 X 1026 grams, a figure based on some 30 observations of the satellites over a period of years. Furthermore, the mass recommended by G. M. Clemence of the Naval Almanac Office to the International Astronomical Union in 1964 was 6.521X10-'1 grams.
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