Planetary Dynamics From Laser Altimetry: Spin and Tidal Deformation of the Moon and Mercury

Studying the dynamics of planetary bodies can shed light on their interior structure and evolution. For example, the temperature variation with depth affects how a planet's surface deforms under the influence of gravitational tidal forcing. Also, the pole orientation, libration, and spin rate of a planet depend on its interior mass distribution and thermal evolution. Altimetric crossovers measured by orbiting spacecraft are ideal for observing these subtle dynamical effects because they have little sensitivity to local and regional relief, which can mask the signals of interest. On the Moon, the tidal surface deformation has an amplitude of only 10 cm, but Mazarico et al. (2014) were able to measure the radial Love number h2 using the highest quality crossovers from the Lunar Orbiter Laser Altimeter (LOLA). Building upon that work, we are incorporating more crossovers to improve the spatial and temporal sampling of the tidal signal. On Mercury, tidal torques from the Sun cause longitudinal librations about its 3:2 spin-orbit resonance with an amplitude of 450 m at the equator. This amplitude is significantly larger than the geolocation uncertainty of MESSENGER's Mercury Laser Altimeter (MLA) data ( 10/100 m in radial/horizontal), and could, thus, be detectable from MLA crossovers. Here we describe recent work using MLA crossovers to measure deviations of Mercury's rotation from the canonical IAU model. Careful accounting of the spacecraft orbital errors and MLA pointing biases will allow an estimation of libration amplitude, pole position, and mean spin rate.