Titan's lakes chemical composition: Sources of uncertainties and variability

Abstract

Between 2004 and 2007 the instruments of the CASSINI spacecraft, orbiting within the Saturn system, discovered dark patches in the polar regions of Titan. These features are interpreted as hydrocarbon lakes and seas with ethane and methane identified as the main compounds. In this context, we have developed a lake-atmosphere equilibrium model allowing the determination of the chemical composition of these liquid areas present on Titan. The model is based on uncertain thermodynamic data and precipitation rates of organic species predicted to be present in the lakes and seas that are subject to spatial and temporal variations. Here we explore and discuss the influence of these uncertainties and variations. The errors and uncertainties relevant to thermodynamic data are simulated via Monte-Carlo simulations. Global Circulation Models (GCM) are also employed in order to investigate the pos- sibility of chemical asymmetry between the south and the north poles, due to differences in precipitation rates. We find that mole fractions of compounds in the liquid phase have a high sensitivity to thermodynamic data used as inputs, in particular molar volumes and enthalpies of vaporization. When we combine all considered uncertainties, the ranges of obtained mole fractions are rather large (up to ∼ 8500%) but the distributions of values are narrow. The relative standard deviations remain between 10% and ∼ 300% depending on the compound considered. Compared to other sources of uncertainties and variability, deviation caused by surface pressure variations are clearly negligible, remaining of the order of a few percent up to ∼ 20%. Moreover no significant difference is found between the composition of lakes located in north and south poles. Because the theory of regular solutions employed here is sensitive to thermodynamic data and is not suitable for polar molecules such as HCN and CH 3 CN, our work strongly underlines the need for experimental simulations and the improvement of Titan's atmospheric models.

Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake

“…While much theoretical work has been reported concerning the solubility values of organic materials in Titan fluids (Cordier et al, 2013(Cordier et al, , 2012(Cordier et al, , 2009Shock, 2013a, 2013b;Dubouloz et al, 1989;Raulin, 1987), little experimental work has been done. The bulk of the experimental work has examined the solubility of hydrocarbons in nitrogen-equilibrated and pure methane solutions (Szczepaniec-Cię ciak et al, 1978;Preston et al, 1971).…”

“…These change the fluid ratio and may modify the solubilities. Previous calculations by Cordier et al (2012) Table 3 in their text) suggested that a 10% decrease in atmospheric pressure over a Titan lake of mixed hydrocarbons and nitrogen would have a <2% increase in benzene solubility. This is driven primarily by the effect of pressure changing the overall composition to a more ethane-rich fluid in equilibrium with the atmosphere as the volatiles, methane, and nitrogen leave the solution.…”

“…Nitrogen is the dominant component of Titan's atmosphere. While normally a gas, at Titan's low temperatures it will become incorporated to a significant extent into the precipitable fluids and surface liquids if they are at equilibrium with the atmosphere Shock, 2013a, 2013b;Cordier et al, 2012Cordier et al, , 2009Graves et al, 2008;Tokano et al, 2006). Methane is the next most dominant atmospheric component and is a volatile liquid at Titan's temperatures.…”

“…Benzene has been identified on the surface by Cassini's VIMS instrument and has been estimated to account for up to 5% of the surface materials on Titan (Clark et al, 2010). The ability or inability of benzene to dissolve in hydrocarbon solution would help constrain the surface processes and lake compositions (Cordier et al, 2013(Cordier et al, , 2012. Benzene will also serve as a benchmark for other organic compounds of interest with either higher or lower theoretical solubilities (Malaska et al, 2011).…”

Titan's Methane Weather

“…Nearly all the surface liquids discovered to date are contained in the polar lakes, which are largely confined to north of latitude 55 • and range in size from 10 to 10 5 km 2 (Hayes et al 2008). Titan's lakes are a mix of methane, ethane, and dissolved nitrogen and heavier hydrocarbons, although the exact ratio of ingredients is uncertain (Cordier et al 2012). The rest of the surface is devoid of lakes, except the south polar region, which is covered in considerably fewer lakes than the north polar region but notably has Ontario Lacus.…”