Mercury: Surface, Space Environment, Geochemistry, Ranging (MESSENGER)

The MESSENGER mission to Mercury (Mercury: Surface, Space Environment, Geochemistry, Ranging (MESSENGER)) has been selected in the latest round of NASA's Discovery Program of low-cost planetary missions. The acronym, which stands for Mercury: Surface, Space Environment, Geochemistry, Ranging, describes the comprehensive nature of the mission, which will provide global mapping of the closest planet to the sun. MESSENGER will be the first mission to Mercury since Mariner 10 flew by the planet in the early 1970's. In contrast to Mariner 10, MESSENGER is an orbiting spacecraft that will return detailed observations of the planet for one Earth year. Mercury represents an especially important target from the perspective of understanding planetary formation and evolution. The MESSENGER mission objectives were designed to address the principle outstanding questions about this enigmatic body while operating within the challenging constraints of the Discovery Program.

The Principal Investigator of MESSENGER is Sean Solomon of the Department of Terrestrial Magnetism of the Carnegie Institution. The spacecraft will be built and operated by the The Johns Hopkins University Applied Physics Laboratory. MIt's involvement in the mission will be in geophysics, of course, where we will measure and interpret observations of the planet's gravity and topography fields, rotational and orbital dynamical motions, and precision spacecraft orbits. Colleagues at the Goddard Space Flight Center will provide the laser altimeter. Another Goddard Group, including Greg Neumann (MIT), will compute precision orbits. In addition to determining the global crustal and mantle structure of Mercury from analysis of gravity and topography observations, we will address the fundamental question:
Does Mercury currently have a molten outer core?

A molten core has been suggested by the presence of a magnetic field detected by Mariner 10, but there are alternative geophysical scenarios that can explain the data. Whether or not Mercury has a present-day liquid core has significant implications for the planet's thermal evolution and internal dynamics, as well as for the accretion of planets in general. So answering this question is a high priority in planetary science.

The existence or not of a liquid core can be determined by measuring some geophysical parameters (like the flattening of the gravitational field and the planetary obliquity) that are currently known for Mercury but not well, as well as one parameter for which there is no measured estimate: the planet's forced physical libration. The physical libration is an oscillation in longitude of the planet's principal axis of minimum inertia with respect to its mean orientation. It arises due to a periodically reversing tidal torque about the mean angular velocity as the orbiting body rotates relative to the central body. The amplitude of the libration is generally very small compared to the circumference of the body. However, if detected, it can be used as a diagnostic indicator of the body's internal structure (i.e. core size and physical state), because fluid material, such as might be present in the planet's core, responds differently to the irregular rotational motion than the solid mantle surrounding it.

We have developed a new method for the detection of planetary librations from a spececraft in orbit that uses the long wavelength terms of the planet's gravitational and topographic fields. We have undertaken a simulation of the MESSENGER mission to demonstrate the recoverability of the libration and other physical parameters needed to distinguish whether or not Mercury currently has a liquid core using our approach. The analysis uses a priori geophysical models for Mercury and the MESSENGER mapping mission scenario. We have simulated the recovery of the planet's gravity and topography fields as well as the pole position, rotation rate and libration. Our results indicate that the libration and other parameters can be measured from the orbital geophysical measurements at a level adequate to distinguish Mercury's present-day core state.