Habitability of Hydrocarbon Worlds: Titan and Beyond

Introduction: Titan is an ocean world, an icy world, and an organic world. Recent models of the interior suggest that Titan’s subsurface ocean may be in contact with an organic-rich ice-rock core, potentially providing redox gradients, heavier elements, and organic building blocks critical for a habitable environment. Farther above, at the contact of the ice shell and ocean, Titan’s abundant surface organics could be delivered to the aqueous environment through processes such as potential convective cycles in the ice shell. Our work investigates the pathways for atmospheric organic products to be transported from the surface to the ocean/core and the potential for ocean/deep ice biosignatures and organisms to be transported to the shallow crust or surface for interrogation and discovery. Our major objectives are: (i) Determine the pathways for organic materials to be transported (and modified) from the atmosphere to surface and eventually to the subsurface ocean (the most likely habitable environment). (ii) Determine whether the physical and chemical processes in the ocean create stable, habitable environments. (iii) Determine what biosignatures would be produced if the ocean is inhabited. (iv) Determine how biosignatures can be transported from the ocean to the surface and atmosphere and be recognizable at the surface and atmosphere.

Summary of Progress: Examining Titan’s atmosphere, we have coupled two atmospheric models that cover different altitudes provide a comprehensive integrated model of the entire atmosphere of Titan. On the observational side, analysis of ALMA data resulted in the first observation of the CH₃D molecule at sub-millimeter wavelengths [1]. Analysis of NASA IRTF data resulted in the first detection of propadiene (CH₂CCH₂) in Titan’s atmosphere [2]. Spatial and seasonal changes in Titan’s gases from the final years of the Cassini mission were the subject of several papers, using data from ALMA [3] and CIRS [4, 5].  In order to understand how materials falling from the atmosphere are transported across the surface, we are developing a landscape evolution model, based on the DELIM code that is used for Mars. We have published the first global geomorphologic map of Titan [6], which will serve as a constraint for the landscape evolution model by showing how sedimentary and depositional materials are distributed over the surface. We obtained an updated estimate of the amount of organic materials on Titan, which is important as a constraint on the amount of chemical energy and building blocks available for potential life. To investigate the molecular pathways from surface to subsurface ocean, we have performed a series of numerical simulations on the effect of a clathrate layer capping Titan’s icy crust on the convection pattern in the stagnant lid regime [7]. In the investigation of habitats resulting from molecular transport, we have modeled the accretion of Titan to understand the effects of thermal evolution on the rocky interior, and to constrain the composition of volatiles exsolved from the interior and that may have migrated vertically to build up the ocean early in Titan’s history [8]. We have also published results of modeling water-hydrocarbon mixtures using the CRYOCHEM code, which now successfully allows chemical modeling of both the hydrocarbon-rich condensed fluid phases and the water-rich condensed fluid phases (and vapor phases, too) simultaneously [9]. Preliminary results for our investigation of ocean habitats led to new insights into the origin of methane and nitrogen (N₂) on Titan by modeling D/H exchange between organics and water, as well as high pressure C-N-O-H fluid speciation in Titan’s rocky core [10]. Results suggest an important role for organic compounds in the geochemical evolution of Titan’s core, which may feed into the habitability of Titan’s ocean. A novel experimental high pressure culturing chamber has been developed to investigate high pressure biosignatures which could survive in Titan’s ocean [11].   Our aim is to demonstrate that earth organisms can survive and build biomass in Titan’s subsurface conditions.

Acknowledgments: Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. This work was funded by NASA’s Astrobiology Institute grant NNN13D485T.

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