Mariner 9 Celestial Mechanics Experiment: Gravity Field and Pole Direction of Mars

Abstract

Analysis of the Mariner 9 radio-tracking data shows that the Martian gravity field is rougher than that of Earth or the moon, and that the accepted direction of Mars's rotation axis is in error by about 0.5°. The new value for the pole direction for the epoch 1971.9, referred to the mean equatorial system of 1950.0, is right ascension α = 317.3° ± 0.3°, declination δ = 52.6° ± 0.2°. The values found for the coefficients of the low-order harmonics of Mars's gravity field are as follows: J ₂ =(1.96±0.01) × 10 ^-3 , referred to an equatorial radius of 3394 kilometers; C ₂₂ = -(5 ± 1) × 10 ^-5 ; and S ₂₂ = (3 ± 1) × 10 ^-5 . The value for J ₂ is in excellent agreement with the result from Wilkins' analysis of the observations of Phobos. The other two coefficients imply a value of (2.5 ± 0.5) × 10 ^-4 for the fractional difference in the principal equatorial moments of inertia; the axis of the minimum moment passes near 105°W.