- Lunar Clinopyroxene Abundance Retrieved from M3 Data Based on Topographic Correction

Abstract

MESSENGER magnetometer data show that Mercury's magnetic field is not only exceptionally weak but also has a unique geometry. The internal field resembles an axial dipole that is offset to the North by 20% of the planetary radius. This implies that the axial quadrupol is particularly strong while the dipole tilt is likely below 0.8• . The close proximity to the sun in combination with the weak internal field results in a very small and highly dynamic Hermean magnetosphere. We review the current understanding of Mercury's internal and external magnetic field and discuss possible explanations. Classical convection driven core dynamos have a hard time to reproduce the observations. Strong quadrupol contributions can be promoted by different measures, but they always go along with a large dipole tilt and generally rather small scale fields. A stably stratified outer core region seems required to explain not only the particular geometry but also the weakness of the Hermean magnetic field. New interior models suggest that Mercury's core likely hosts an iron snow zone underneath the core-mantle boundary. The positive radial sulfur gradient likely to develop in such a zone would indeed promote stable stratification. However, even dynamo models that include the stable layer show Mercury-like magnetic fields only for a fraction of the total simulation time. Large scale variations in the core-mantle boundary heat flux promise to yield more persistent results but are not compatible with the current understanding of Mercury's lower mantle.

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

“…A Y 0 2 CMB heat flux pattern combined with internal heating may trigger an unstable odd convective mode which may explain the hemispheric field of Mercury (Cao et al 2014;Wicht and Heyner 2014) as observed by MESSENGER (Anderson et al 2012). While their dynamical scenario is convincing, the justification for a Y 0 2 thermal heterogeneity at the mantle of Mercury is somewhat vague.…”

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

“…This so-called iron snow regime has already been suggested to apply to the present cores of Ganymede (Hauck et al 2006;Christensen 2015;Rückriemen et al 2015) and Mercury (Chen et al 2008;Vilim et al 2010;Wicht and Heyner 2014). It has also been suggested that Mars' core may enter the iron snow regime (Stewart et al 2007) and that the lunar core may have gone through an iron snow episode (Zhang et al 2013;Laneuville et al 2014).…”

“…Other models that predict a strong quadrupole component (e.g., Christensen 2006;Christensen and Wicht 2008) imply a large dipole tilt, which has not been observed. A recent review of the various dynamo models for Mercury can be found in Wicht and Heyner (2014). We only provide a summary here.…”

Deep magnetic field stretching in numerical dynamos

“…The dynamo may be generated exclusively at depth due to stable stratification at the top of the core. For example, the very weak, large-scale, and axisymmetric magnetic field of Mercury (Anderson et al 2011;Oliveira et al 2015) can be explained by a skin effect across a stably stratified layer (Christensen 2006;Christensen and Wicht 2008;Wicht and Heyner 2014). As mentioned above, such a layer was also proposed for the Earth (Pozzo et al 2012;Gubbins and Davies 2013).…”