Mars lacks an internally generated magnetic field today. Crustal remanent magnetism and meteorites indicate that a dynamo existed after accretion but died roughly four billion years ago. Standard models rely on core/mantle heat flow dropping below the adiabatic limit for thermal convection in the core. However, rapid core cooling after the Noachian is favored instead to produce long-lived mantle plumes and magmatism at volcanic provinces such as Tharsis and Elysium. Hydrogenation of the core could resolve this apparent contradiction by impeding the dynamo while core/mantle heat flow is super-adiabatic. Here we present parameterized models for the rate at which mantle convection delivers hydrogen into the core. Our models suggest that most of the water that the mantle initially contained was effectively lost to the core. We predict that the mantle became increasingly iron-rich over time and a stratified layer awaits detection in the uppermost core—analogous to the E’ layer atop Earth’s core but likely thicker than alternative sources of stratification in the Martian core such as iron snow. Entraining buoyant, hydrogen-rich fluid downwards in the core subtracts gravitational energy from the total dissipation budget for the dynamo. The calculated fluxes of hydrogen are high enough to potentially reduce the lifetime of the dynamo by several hundred million years or longer relative to conventional model predictions. Future work should address the complicated interactions between the stratified, hydrogen-rich layer and convection in the underlying core.

Super-Earth and super-Venus exoplanets may have similar bulk compositions, but their surface conditions and mantle dynamics are vastly different. Vigorous convection within their metallic cores may produce dynamos and thus magnetospheres if the total heat flow out of the core exceeds a critical value. Earth has a core-hosted dynamo because plate tectonics cools the core relatively rapidly. In contrast, Venus has no dynamo and its deep interior probably cools slowly, potentially due to a lack of plate tectonics. It is not fully known how or if magnetic fields affect habitability, but the size of a magnetosphere might indirectly constrain the habitability of a surface. In this study, we developed scaling laws for how planetary mass affects the minimum heat flows required to sustain both thermal and chemical convection, which we compared to a simple model for the actual heat flow of both super-Earth and super-Venus exoplanets conveyed by solid-state mantle convection. We calculated three critical thresholds for heat flow based on varying the size of an inner core, the rate at which light elements precipitate at the core-mantle boundary, and the thermal conductivity of the core. We found that the required heat flows increase with planetary mass (to a power of ~0.8–0.9), but the actual heat flows of both super-Earths and super-Venuses could increase even faster (to a power of ~1.6) (Figure 1). Massive super-Earths are likely to host a dynamo in their metallic cores if their silicate mantles are entirely solid. Super-Venuses with relatively slow mantle convection could host a dynamo if their mass exceeds ~1.5 (with an inner core) or ~4 (without an inner core) Earth-masses. However, the mantles of massive rocky exoplanets might not be completely solid. Basal magma oceans may reduce the heat flow across the core-mantle boundary and smother any core-hosted dynamo. Detecting a magnetosphere at an Earth-mass planet probably signals Earth-like geodynamics. In contrast, magnetic fields may not reliably reveal if a massive exoplanet is a super-Earth or a super-Venus. We eagerly await direct observations in the next few decades. Published in JGR, doi:10.1029/2020JE006739