Mercury's Annual and Long-Period Librations from Co-Registration of MLA Profiles

Abstract

The annual longitudinal physical libration and the mean rotation rate of Mercury have been determined independently based on observations from the Earth-based radar [1], the camera and/or laser altimeter [2,3], and the radio tracking system [4,5,6]. Unfortunately, the precision of the annual libration measurements does not permit the detection of a large solid inner core [7]. Moreover, the existing estimates of rotation rates are largely inconsistent. Additional forced libration terms can be present due to non-Keplerian characteristics of Mercury’s orbits which can be amplified due to resonance if their periods are close to that of a libration eigenperiod [8]. Constraints on the amplitudes and phases of these long-period librations would provide valuable insights into Mercury’s deep interior. We devise an alternative approach based on the co-registration of laser altimetric profiles to precisely track the temporal variation of the planetary rotation angle. We apply it to the MESSENGER Mercury Laser Altimeter (MLA) measurements with the aim to disentangle the superimposed forced librations with various periods.