2010 Annual Science Report
NASA Jet Propulsion Laboratory - Titan Reporting | SEP 2009 – AUG 2010
Task 3.3.2 Solubility in Lakes
The solubility of organics in hydrocarbon lakes is a key limiting factor to the extent of chemistry that can occur in Titan lakes.
Graduate student Joshua Kretchmer, in conjunction with Prof. Thomas Miller, is using atomistic simulations to study the chemical environment present in the hydrocarbon lakes on Titan. Due to the dearth of polar molecules in the hydrocarbon lakes, the solution-phase energetics and dynamics are governed by weaker interactions usually overshadowed in common aqueous environments. To gain insight into the nature of these interactions, enhanced sampling techniques are being used to study the pair-wise free energy of association, solvation free energies, and radial distribution functions of small non-polar solutes in the bulk solution of the hydrocarbon lakes. Surface properties of the hydrocarbons lakes are also of central interest. Simulations of methane at liquid-vapor phase coexistence and the Titan surface temperature are being performed. A decrease in the relative density of methane leads to the formation of a methane-vacuum interface. The tendency of the methane molecules to aggregate in slab-like configurations is quantified based on local densities and is compared to the relative fraction of methane molecules present. The equilibrium between aggregated and free molecules is studied to elucidate the strength of methane surface interactions. Finally, to explore the effect of solute and solvent polarizability in the non-polar lakes, molecular dynamics simulations that employ polarizable force fields (adiabatic shell model) are being used to model the aggregation and self-assembly of polarizable solutes in the hydrocarbon lakes. Preliminary simulations are being run on mixtures of polarizable and non-polarizable argon atoms under liquid conditions. By tuning both the relative concentration of polarizable molecules and the degree of polarizability, the conditions that allow for self-assembly can be studied. Following the study of polarizable atoms, more complicated molecules with polarizable heads and non-polarizable tails will also be studied to examine the possibility of micelle formation. The conditions necessary for these phenomena can be compared to the conditions present in the Titan lakes to draw conclusions as to the existence of these phenomena in the hydrocarbon lakes.
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Outer Solar System exploration
Sources of prebiotic materials and catalysts
Origins and evolution of functional biomolecules
Origins of energy transduction