The Influence of Curvature on Mantle Convection featuring a Temperature-dependent Viscosity

Convection in the silicate mantles or ice layers of terrestrial planets and moons is characterized by a strongly temperature-dependent viscosity. Mantle convection regime is first examined in 2D and 3D spherical geometry with varying mantle curvature (core-to-surface radii-ratio, f) and constant heating conditions and viscosity contrast across the mantle depth (ΔηT). The onset of the stagnant-lid regime occurs for different conditions for bodies with relatively small cores and relatively thin shells. Differences in mean temperature and flow fields observed between 2D and 3D geometry are amplified as relative core-size is decreased. This finding has implications for modelling the thermal evolution and evaluating heat extraction from the cores of small moons or dwarf planets. Next, I consider evolving conditions for a body the size of the Earth's Moon with f = 0.5. Typically, modelling methods employ a ΔηT that does not encompass the entire effective range of values for ductile flow. This may result in spurious changes in convective regime and overestimate heat extraction from the core. Improving upon conventional methods for viscosity modelling, I employ a novel constant-viscosity contrast modelling method that emulates a large effective viscosity contrast ΔηT = 10^10. With this adjustment, regime change is suppressed in the calculations tested. Moreover, the modelling approach I propose may be less computationally intensive and save resources. The generation of a magnetic field is associated with plate-tectonics because efficient heat extraction is required at the core-mantle boundary. However, the planet Mercury, which does not have plate-tectonics, features a magnetic field. The work presented concludes by investigating the evolution of the Mercurian mantle. I propose that an increased mantle thermal diffusivity due to an iron-poor composition results in a conducting mantle at present that is compatible with the heat extraction required by Mercury's magnetic field strength. The possibility of higher core heat extraction as follows a mantle transitioning from stagnant-lid convection to conduction may explain the origin of the Mercurian magnetic field.

Department dc:contributor.department

Physics

Year dc:date.issued

2020

Author dc:creator

• Guerrero, Joshua Martin

Advisor dc:contributor.advisor

• Lowman, Julian P

• curvature
• mantle convection
• Mercury
• secular cooling
• stagnant-lid convection
• viscosity

Statement dc:rights

• Attribution-NonCommercial-NoDerivatives 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/4.0/

Handle dc:identifier.uri

http://hdl.handle.net/1807/103289

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/103289