THE GEOPHYSICS OF MERCURY: MESSENGER's VIEW FROM ORBIT

On March 18, 2011, MESSENGER entered an elliptical, near-polar orbit about Mercury, becoming the first spacecraft to orbit that planet. As part of MESSENGER’s geophysics investigation, the Mercury Laser Altimeter (MLA) is ranging to the surface of Mercury when the spacecraft slant range is less than 1500 km, largely over the northern hemisphere. MLA profiles have sampled a variety of landforms, including volcanic plains, tectonic structures, and impact craters and basins as well as longer-wavelength variations in topography. MLA has also profiled several polar impact craters previously identified by Earth-based observations as hosting radar-bright deposits in their interiors. Tracking of radio signal transmissions from MESSENGER’s low- and high-gain antennas is yielding new information on the gravity field of Mercury. Regional variations in Mercury’s gravity field, together with topographic information, provide important constraints on mechanisms of topographic compensation. Importantly, MESSENGER is also improving estimates of the second-degree spherical harmonic gravity-field coefficients C₂₀ and C₂₂. Prior to MESSENGER orbital operations, C₂₀ was the least constrained parameter necessary for determining the size of Mercury’s core. Ultimately, the determination of the planet’s normalized polar moment of inertia (C/MR²), where M and R are Mercury’s mass and radius, and the ratio of the polar moment of inertia of the mantle and crust to the planet’s polar moment of inertia (C_m/C) provide crucial constraints on Mercury’s internal structure. Observations by MESSENGER’s Magnetometer indicate a dipole-dominated planetary magnetic field, centered on the spin axis, with a tilt of less than 3° and a northward displacement along its axis of 0.2 R. These observations are consistent with a core dynamo as the origin for Mercury’s internal magnetic field, albeit one with an unusual geometry or boundary conditions.