Masters Thesis

Tharsis formation from density driven thermo-chemical plumes during planetary differentiation

The formation of Tharsis rise on Mars is a highly debated topic. Tharsis topography is very localized within the global terrain of Mars. The timing of Tharsis formation likely occurred early in Mars history in the Noachian but post dates the hemispheric crustal dichotomy which it overprints. Several models have been proposed to explain a localized orogeny, but use of boundary layer theory or convection studies result in multiple, evenly spaced plumes or have instability growth and rise times which are longer than estimates for Tharsis formation. We propose a new model which couples the early differentiation processes of the Mars interior and Tharsis formation. Here we conduct physical fluid dynamic experiments of core formation using high viscosity glucose syrup and an emulsion of liquid, metal gallium. Melting due to impacts during accretion may induce silicate-metal separation and form a liquid iron pond that settles temporarily at the base of the magma ocean. Thus we design our experiments as a two component fluid system with a metal pond at the interface. Previous experiments using a smooth liquid diapir observed three regimes for conduit formation where 1) low density conduit material is entrained and descends through the conduit, 2) reverses flow direction once the metal diapir reaches the base, and 3) travels back to the surface emptying out the conduit causing it to collapse. In our experiments with an emulsion diapir, we find that only regimes 1 and 3 are observed within the conduit. This is due to the fact that the return flow of buoyant conduit material does not occur within the conduit, but is forced to exit the conduit by the second stage of falling emulsion drops and travels to the surface as a buoyant thermochemical plume along its own path. A 2nd stage of small thermo-chemical plumes are observed to rise to the surface as they separate from the 2nd stage of sinking metal emulsion drops. This study suggests a thermo-chemical plume, which forms as a secondary effect of core formation, may explain the formation of a localized orogeny such as the Tharsis rise on Mars. Fluid experiments scaled to the planetary mantle interior indicate that the time for a metal-silicate plume to sink to the core, separation of silicate from liquid metal, growth of a buoyant thermo-chemical plume, and subsequent rise to the surface is on the order of 1-8 My. This model is consistent with several observations for Tharsis formation including rapid formation within the Noachian, the requirement of a single large upwelling soon after core formation, arrival of a plume after formation of the crustal dichotomy, as well as later effusive stages of volcanism on Tharsis observed up to the Amazonian.

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