Evolution of Titan and implications for its hydrocarbon cycle

Cage occupancy of methane clathrate hydrates in the ternary H2O-NH3-CH4 system

The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution. Ammonia's presence decreases methane occupancy in the large cages, without definitive Raman spectroscopic evidence for its incorporation inside the structure.

Protein Stability in Titan's Subsurface Water Ocean

Models of Titan predict that there is a subsurface ocean of water and ammonia under a layer of ice. Such an ocean would be important in the search for extraterrestrial life since it provides a potentially habitable environment. To evaluate how Earth-based proteins would behave in Titan's subsurface ocean environment, we used molecular dynamics simulations to calculate the properties of proteins with the most common secondary structure types (alpha helix and beta sheet) in both Earth and Titan-like conditions. The Titan environment was simulated by using a temperature of 300 K, a pressure of 1000 bar, and a eutectic mixture of water and ammonia. We analyzed protein compactness, flexibility, and backbone dihedral distributions to identify differences between the two environments. Secondary structures in the Titan environment were found to be less long-lasting, less flexible, and had small differences in backbone dihedral preferences (e.g., in one instance a pi helix formed). These environment-driven differences could lead to changes in how these proteins interact with other biomolecules and therefore changes in how evolution would potentially shape proteins to function in subsurface ocean environments.

Composition and chemistry of Titan's stratosphere

Our present knowledge of the composition and chemistry of Titan's stratosphere is reviewed. Thermal measurements by the Cassini spacecraft show that the mixing ratios of all photochemical species, except ethylene, increase with altitude at equatorial and southern latitudes, reflecting transport from a high-altitude source to a condensation sink in the lower stratosphere. Most compounds are enriched at latitudes northward of 45 degrees N, a consequence of subsidence in the winter polar vortex. This enrichment is much stronger for nitriles and complex hydrocarbons than for ethane and acetylene. Titan's chemistry originates from breakdown of methane due to photodissociation in the upper atmosphere and catalytical reactions in the stratosphere, and from destruction of nitrogen both by UV photons and electrons. Photochemistry also produces haze particles made of complex refractory material, albeit at a lower rate than ethane, the most abundant gas product. Haze characteristics (vertical distribution, physical and spectral properties) inferred by several instruments aboard Cassini/Huygens are discussed here.