How is Mercury's dynamo powered?

Tuesday, 16 December 2014
Grace Alexandra Cox1, Brent G Delbridge2, Jessica C E Irving3, Hiroaki Matsui4, William F McDonough5, Ian Rose2, Anat Shahar6 and Sean M Wahl2, (1)University of Leeds, Leeds, LS2, United Kingdom, (2)University of California Berkeley, Berkeley, CA, United States, (3)Princeton University, Princeton, NJ, United States, (4)University of California Davis, Davis, CA, United States, (5)Univ Maryland, College Park, MD, United States, (6)Carnegie Institution of Washington, Geophysical Laboratory, Washington, DC, United States
One of the more surprising findings of the MESSENGER spacecraft is the confirmation that the smallest terrestrial planet has an internally generated, dipolar magnetic field, which is likely driven by a combination of thermal and compositional buoyancy sources. This observation places constraints on the thermal and energetic state of Mercury’s large iron core and on mantle dynamics because dynamo operation is strongly dependent on the amount of heat extracted from the core by the mantle. However, other observations point to several factors that should inhibit a present-day dynamo. These include physical constraints on a thin, possibly non-convecting mantle, as well as properties of liquid iron alloys that promote compositional stratification in the core.

We consider a range of self-consistent internal structures, core compositions and thermal evolution models that are also consistent with observational constraints, and assess the circumstances under which a dynamo is permitted to operate in Mercury’s core. We present the thermal evolution models, 1D parameterized convection models and planetary entropy calculations. We attempt to account for the large uncertainties on some parameters by considering various end member cases.

We examine the thermal and magnetic implications of a long-lived lateral temperature difference resulting from Mercury’s orbital resonance and how it may play a role in driving the planetary dynamo. We compare simulations of mantle heat flow using the ASPECT convection code to predictions from the parameterized models and produce heat flow maps at the CMB. To represent fluid dynamics and magnetic field generation inside Mercury’s core, a numerical dynamo model is performed by using the obtained heat flux maps.

Lastly, we also investigate the seismic observability of the different structural models of Mercury to determine the extent to which any future single-seismometer mission will be able to provide alternative insights into Mercury's internal dynamics.

This study was initiated at the 2014 CIDER summer program on the dynamics of planetary interiors.