A procedure for determining the nature of Mercury's core

Abstract

We review the assertion that the precise measurement of the second degree gravitational harmonic coefficients, the obliquity, and the amplitude of the physical libration in longitude, C20, C22, θ, and Φ, for Mercury are sufficient to determine whether or not Mercury has a molten core. The conditions for detecting the signature of the molten core are that such a core not follow the 88-day physical libration of the mantle induced by periodic solar torques, but that it does follow the 250,000-year precession of the spin axis that tracks the orbit precession within a Cassini spin state. These conditions are easily satisfied if the coupling between the liquid core and solid mantle is viscous in nature. The alternative coupling mechanisms of pressure forces on irregularities in the core-mantle boundary (CMB), gravitational torques between an axially asymmetric mantle and an assumed axially asymmetric solid inner core, and magnetic coupling between the conducting molten core and a conducting layer in the mantle at the CMB are shown for a reasonable range of assumptions not to frustrate the first condition while making the second condition more secure. Simulations have shown that the combination of spacecraft tracking and laser altimetry during the planned MESSENGER orbiter mission to Mercury will determine C20, C22, and theta to better than 1% and Φ to better than 8%-sufficient precision to distinguish a molten core and constrain its size. The possible determination of the latter two parameters to 1% or less with Earth-based radar experiments and MESSENGER determination of C20 and C22 to 0.1% would lead to a maximum uncertainty in the ratio of the moment of inertia of the mantle to that of the whole planet, Cm/C, of ~2% with comparable precision in characterizing the extent of the molten core.