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 k2 imply a significantly weaker mantle than previously believed, potentially including partial melting.

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We computed interior structure models of Mercury and analyzed their viscoelastic tidal response. The models are consistent with MErcury Surface, Space Environment, GEochemistry, and Ranging mission inferences of mean density, mean moment of inertia, moment of inertia of mantle and crust, and tidal Love number k2. Based on these constraints we predict the tidal Love number h2 to be in the range from 0.77 to 0.93. Using an Andrade rheology for the mantle the tidal phase‐lag is predicted to be 4° at maximum. The corresponding tidal dissipation in Mercury's silicate mantle induces a surface heat flux smaller than 0.16 mW/m2. We show that, independent of the adopted mantle rheological model, the ratio of the tidal Love numbers h2 and k2 provides a better constraint on the maximum inner core size with respect to other geodetic parameters (e.g., libration amplitude or a single Love number), provided it responds elastically to the solar tide. For inner cores larger than 700 km, and with the expected determination of h2 from the upcoming BepiColombo mission, it may be possible to constrain the size of the inner core. The measurement of the tidal phase‐lag with an accuracy better than ≈0.5° would further allow constraining the temperature at the core‐mantle boundary for a given grain size and therefore improve our understanding of the physical structure of Mercury's core.

A raft of papers in the last decade has advanced our understanding of the physics of floating sea ice covering tens of millions of kilometers of Earth's ocean. Seeding this research has been the need to quantify sea ice's role in global ocean circulation, a need to understand the stability of the ice through time, and interest in the ice itself as an ecological niche. One recent 1D model, by Buffo et al. (2018), captures the gravity drainage of brines initially entrained within forming ice, which flow through interconnected pores and melt channels within the ice. In follow-on work that was just published, Buffo et al. (2020) have extended their model to estimate the entrainment and transport of salts in the ice covering Jupiter's moon Europa. This type of work is timely, as the community of planetary scientists studying Europa and related ocean worlds sets its sails for NASA's Europa Clipper mission (Howell & Pappalardo, 2020), and ESA's JUpiter ICy moons Explorer (JUICE) mission (Grasset et al., 2013), planned to arrive at the Jupiter system toward the end of this decade. However, the work poses many further questions that will need to be addressed in the coming years. Europa's ice is global, and at least 3 km thick-possibly 30 km or more-covering an ocean as deep as 180 km (Anderson et al., 1998, Turtle & Pierazzo, 2001; Schenk et al., 2002). Sparse crater counts among the fractured ice suggest the average age of surface materials is less than 200 Myr (Zahnle et al., 2003). Materials retained in the ice could provide clues to the ocean's thermal and chemical evolution, its current composition, and the possible presence of life. Demonstrating mechanisms for extensive material entrainment into the ice would also support the existence of brine reservoirs near Europa's surface, hypothesized to explain the chaotic terrains covering much of Europa's surface (