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Abstract
The Mercury gravity field, spin-pole axis, rotation period, Love number, and ephemeris have been determined using the complete four years of MESSENGER tracking data from March 2011 to April 2015. The pole location and obliquity (1.99 ± 0.12 arcmin) is consistent with previous determinations. Since MESSENGER was in a highly elliptical orbit with periapsis in the far northern hemisphere, the gravity resolution over the surface of Mercury varies greatly from harmonic degree n = 12 at the south pole to n = 154 in a small region near the north pole which was covered with exceptionally low periapsis data near the end of mission. The gravity field MESS160A is determined to n = 160 and shows notable improvement in the correlation with topography. Three different constraint methods are used to generate the gravity field. The nominal method is a Kaula power law of 5 × 10−5/n2 to constraint the coefficients. One alternate constraint uses surface acceleration measurements, which only constrains the unobserved portion of the gravity field. The other constraint method sets the gravity uncertainties using the harmonic spectrum of the gravity derived from topography. The Mercury tidal Love number solution k2 = 0.53 ± 0.03 is larger than previous results but within suggested error bounds. In addition, the technique for estimating the Mercury ephemeris is discussed.
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