The Gravity Signal of Mercury's Inner Core

In a reference frame rotating with Mercury's mantle and crust, the inner core and fluid core precess in a retrograde sense with a period of 58.646 days. The precession of a triaxial inner core with a different density than the fluid core induces a periodic gravity variation of degree 2, order 1. Elastic deformations from the pressure that the precessing fluid core exerts on the core-mantle boundary also contribute to this gravity signal. We show that the periodic change in Stokes coefficients Delta C-21 and Delta S-21 for this signal of internal origin is of the order of 10(-10), similar in magnitude to the signal from solar tides. The relative contribution from the inner core increases with inner core radius and with the amplitude of its tilt angle with respect to the mantle. The latter depends on the strength of electromagnetic coupling at the inner core boundary which in turn depends on the radial magnetic field B-r; a larger B-r generates a larger tilt. The inner core signal features a contrast between Delta C-21 and Delta S-21 due to its triaxial shape, discernible for an inner core radius >500 km if B-r > 0.1 mT, or for an inner core radius >1100 km if B-r < 0.01 mT. A detection of this contrast would confirm the presence of an inner core and place constraints on its size and the strength of the internal magnetic field. These would provide key constraints for the thermal evolution of Mercury and for its dynamo mechanism.

Automated summary

This study investigates the precession of Mercury's inner core and fluid core and their resultant gravity signal in a reference frame rotating with the mantle and crust of Mercury. The research shows that the contribution from the inner core depends on its radius and tilt angle, which can be discerned through the contrast in the observed signal. This contrast can provide key constraints on the size and internal magnetic field of the inner core.