Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021
EPSC Abstracts
Vol. 15, EPSC2021-604, 2021
https://doi.org/10.5194/epsc2021-604
European Planetary Science Congress 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Modelling of thermal stratification at the top of Mercury’s core

Jurrien Knibbe and Tim Van Hoolst
Jurrien Knibbe and Tim Van Hoolst
  • Royal Observatory of Belgium, Planetary Science, Belgium (jurrien.knibbe@oma.be)

Introduction:

Thermal evolution studies of planet Mercury consistently predict that the heat flux at Mercury’s core-mantle boundary drops below the adiabatic heat flux early in Mercury’s evolution and thermally stratifies at least the outer part of Mercury’s core [1-3]. The deep dynamo explanation for Mercury’s broad-scale and low-intensity magnetic field suggests that a thick stratified liquid layer covers the convective dynamo region in Mercury’s core [4,5] and adds evidence that the top of Mercury’s core is stratified. In the simplest scenario that the liquid planetary core is well-mixed and that stratification of an upper liquid core layer is exclusively thermal in nature, the temperature profile satisfies the conduction equation in the stratified region of the core and it approaches the adiabat in the lower region of the core where heat fluxes are super-adiabatic.

Thermal evolution modelling of a core that is partly conductive and partly adiabatic involves a moving boundary problem in which the radius of the interface between the conductive region and the adiabatic region varies with time. One-dimensional thermal evolution models for the Earth have been performed that take thermal stratification into consideration by discretizing the core in many small spherically symmetric intervals [6]. For Mercury, Knibbe and van Westrenen (2018) have taken the development of a thermally stratified layer in consideration in a thermal evolution model [3]. They implemented a ‘steady-flux’ form of the temperature profile in the conductive region, by which we mean that they implemented the converged solution of the conduction problem where fluxes remain constant in time. This approach leads to a considerable numerical simplification, because discretizing the conductive part of the core is not needed. However, the approach is inconsistent with the time-variable fluxes that apply throughout Mercury’s evolution. We have recently improved the steady-flux approach of Knibbe and van Westrenen (2018) by the development of a numerical scheme that does not rely on the steady-flux assumption. Here, we present this method along with implications for Mercury’s thermal evolution.  

Method:

We developed a ‘piece-wise steady-flux’ (PWSF) numerical scheme, which solves one-dimensional thermal conduction problems up to arbitrary precision with increase of spatial resolution. This numerical scheme assumes a steady flux form of temperature in each of the intervals, which is continuous and differentiable in radial direction. These smoothness properties are convenient for implementing the numerical scheme in an energy-conserved thermal evolution approach for Mercury’s core, in which a conductive thermally stratified layer is considered that develops below the core-mantle boundary when the heat flux drops below the adiabatic heat flux. If a single interval is used by the numerical scheme, the method is equivalent to the steady-flux approach of Knibbe and van Westrenen (2018).

Results: