The role of tidally driven mixing on early Martian habitability

Abstract

Abstract
Ocean tides are an important source of vertical mixing in Earth’s oceans. They influence heat transport, stratification, large-scale circulation and therefore climate. Tidal mixing has also been shown to shape ocean circulation and ventilation on the early Earth, raising the possibility that tides could similarly have influenced the climate of other planetary oceans. However, the role of tidal mixing in the climate and habitability of ancient Mars remains unknown. This study therefore examined whether tidal mixing was necessary to maintain habitable surface conditions on early Mars during the Noachian and Hesperian periods by comparing fully coupled ROCKE-3D GCM atmosphere-ocean simulations. Two distinct climate regimes were investigated: a colder Hesperian (R1) scenario, characterised by a 1-bar CO2 atmosphere with 10% H2 and a 150 m Global Equivalent Layer (GEL) inventory, paired with a shallow, restricted northern ocean covering approximately 25.7% of the surface; and a warmer Noachian (R8) scenario, configured with a 2-bar total pressure, 20% H2 and a 1000 m GEL inventory, which produced a deeper, more connected ocean covering approximately 48% of the surface. These scenarios were selected because they represent the two leading reconstructions for early Mars, a “cold and wet” Hesperian state and a “warm and wet” Noachian state and therefore bracket the range of proposed atmospheric and hydrological conditions. To isolate the impact of internal ocean mixing from overarching climate drivers, each scenario was tested across Control and Reduced vertical tidal diffusivity configurations. The findings of this study demonstrated that ocean tidal mixing was a secondary modifier rather than a primary driver of ancient Martian climate stability. In the Hesperian R1 regime, the northern circumpolar ocean maintained stable surface temperatures between 4˚C and 8˚C. It was found that halving vertical diffusivity produced negligible changes to surface conditions, confirming that Hesperian ocean stability was robust against the reduction of tidal mixing. Similarly, in the Noachian R8 regime, robust CO2-H2 collision-induced absorption and extensive thermal buffering drove near-total surface habitability (97.36%). Reducing vertical diffusivity within this vigorous overturning regime (13 Sv) yielded identical fractional habitability (97.36%) alongside only localized deep-ocean cooling. These results indicated that baseline atmospheric greenhouse forcing, total water inventory and basin geometry dominated climate stability across both Hesperian cold-and-wet and Noachian warm-and-wet regimes. Consequently, interior ocean mixing dynamics did not threaten the established habitability frameworks, highlighting that ancient Martian climate evolution was governed primarily by atmospheric greenhouse forcing, basin geometry and water inventory rather than interior ocean transport efficiency.

Date of Award	1 Oct 2026
Original language	English
Awarding Institution	Bangor University
Supervisor	Mattias Green (Supervisor)