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Combined Geochemical and Geophysical Constraints on Titan Interior Composition

Presentation #216.09 in the session Titan III: Surface and Interior (Oral Presentation)

Published onOct 23, 2023
Combined Geochemical and Geophysical Constraints on Titan Interior Composition

Ocean worlds have large subsurface reservoirs of water, but their habitability may be constrained by the volatile and organic compounds that can be transferred into those reservoirs from the surface or the interior. Here, we investigate the amount and type of organic compounds that can be delivered to Titan’s subsurface ocean from its rocky interior. Outgassing from Titan’s interior may have contributed significantly to Titan’s atmosphere and surface volatile reservoirs. Atmospheric measurements place constraints on Titan’s initial composition in terms of carbon and nitrogen chemistry, isotopic composition, and noble gas abundances. Geophysical measurements place constraints in the form of density and mass balance of the hydrosphere and rocky interior. We utilize data from chondritic and cometary materials to derive starting compositions for Titan that are consistent with both geochemical and geophysical constraints. We include combinations of five different materials: hydrated CI chondrite dust, anhydrous CI chondrite dust, L chondrite dust, insoluble organic matter (IOM), and ices. Starting compositions provide inputs for thermodynamic calculations of the evolution of Titan’s interior composition using the Perple_X and Rcrust codes and thermodynamic data from the Deep Earth Water model. We utilize a recently reported Titan thermal gradient that includes IOM in the interior to calculate the temperature profile. Our resulting density and hydrosphere mass are compared to present-day Titan, release of organics from the interior is considered, and implications for habitability of Titan’s interior are discussed. We find that an equal mixture of cometary and chondritic materials provides the best fit for existing constraints. For example, in the hydrated CI chondrite endmember, approximately 1.5e22 kg of fluid is volatilized from hydrated mineral phases over 4.5 Gyr, which may account for 30-40% of Titan’s hydrosphere but suggests Titan’s building blocks included more volatile-rich components. We also find that the mass of organics released from the interior may be equal to or up to 1000x greater than recent calculations of organics incorporated from the exterior by impact processes (Neish et al., this meeting).

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