Mercury’s shape from radio occultations

Abstract

To support studies of Mercury’s internal structure, a MESSENGER mission goal is to measure the shape of the planet. Radio-frequency occultation observations contribute to this objective, particularly in most of the southern hemisphere where there are no altimeter data. We describe the techniques used to derive radius measurements from occultations and report results to date on the long-wavelength shape of Mercury. Before the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, Mercury’s shape was poorly constrained, primarily by Earth-based radar observations at low Mercury latitudes. During MESSENGER’s 12-month primary mission, the Mercury Laser Altimeter (MLA) acquired an extensive data set on the topography of Mercury’s northern hemisphere [1]. However, most of the southern hemisphere is beyond MLA range because the periapsis of MESSENGER’s highly inclined, eccentric orbit is at high northern latitudes. Along with limb measurements and global stereo mosaics, occultation-derived radius measurements are essential for understanding the shape of Mercury’s southern hemisphere. As viewed from Earth, the MESSENGER spacecraft passed behind Mercury every twelve hours for most of the primary mission. This geometry caused Mercury to occult the radio frequency (RF) transmissions, and we used an open-loop receiver to record RF power at the ingress and egress of each occultation. Incorporating the effects of diffraction, we extracted the time of occultation and used it to determine the RF path that grazed Mercury’s surface. The point on that RF path that is tangent to the surface defines a unique latitude, longitude, and radius. Since the highest point along the RF path provides the occultation edge, the radius measurements are biased high relative to the surrounding terrain. We corrected for this bias by evaluating topography local to the tangent point. Digital-elevation models (DEMs), derived from surface images acquired by MESSENGER’s Mercury Dual Imaging System (MDIS), contain the necessary topographic data. We compared northern-hemisphere occultation results to MLA data to