Challenges on Mercury's Interior Structure Posed by the New Measurements of its Obliquity and Tides
Gregor Steinbrügge, Mathieu Dumberry, Attilio Rivoldini, G. Schubert, Hao Cao, Dustin M. Schroeder, K. M. Soderlund
Abstract
Abstract We present calculations of interior models of Mercury that are constrained to match Mercury's mean density, normalized moment of inertia factor (MoI), and 88 days libration amplitude. We show that models matching a MoI = 0.333 ± 0.005, based on a recent obliquity measurement require a new perspective on Mercury's interior. Specifically, we confirm the mandatory presence of a large inner core > 600 km in radius which, however, leads to lower mantle densities in comparison to previous models and implies a mantle with > 5wt.% C, > 10wt.% magnesium sulfide (MgS), or is partially convecting. Furthermore, we also show that the core radius is lower than previous estimates, making it inconsistent with current estimates from magnetic induction measurements. In addition, the requirement of low viscosities in the lower mantle to match recent estimates of k 2 imply a significantly weaker mantle than previously believed, potentially including partial melting.