Spectral detection of complex organics in the hydrocarbon lakes of Titan

Abstract:

The chemical composition of the lakes on Titan is still poorly determined. Liquid ethane and methane may be abundant with the presence of other organic species from atmospheric precipitation, such as complex organic materials similar to Titan's tholins.

While VIMS is still providing data on the surface of Titan, it is important to study how the deposits of the atmospheric organic aerosols may interact optically and chemically with the hydrocarbons lakes.

In this work, we present the results of laboratory experiments examining the spectroscopic signatures of a liquid ethane, liquid methane and the mixture of both in contact with laboratory analogs of Titan’s aerosols (Titan's tholins). Experiments were performed in the Titan's lakes simulation chamber of the W.M Keck laboratory at the University of Arkansas, at temperatures relevant to Titan's surface (90-94 K) and under a pressure maintained at 1.5 bar N₂ atmosphere. The optical behavior of the sample was monitored via FTIR spectroscopy, in the near-infrared from 2.5 to 1.0 µm (4000-10000 cm^-1).

The results for tholins in liquid ethane show modifications of the ethane bands essentially through flattening and broadening of bands at 1.7 - 1.75 µm and 2.0 µm. In liquid methane, the presence of tholins is observed through a broadening of the CH₄ bands between 1.8 and 1.6 µm and a positive slope at 2.0 μm These features might allow tholins identification in Titan's lakes. Results in mixture of liquid CH₄-C₂H₆ will be also presented. During the experiments, we have observed that tholins when in contact with these solvents are not remaining in suspension. In this case, the refractory material of Titan’s aerosols would not be dissolved in the Titan's surface lakes and seas but would rather sinks. Our experimental approach and its resulting data are relevant in regards to current VIMS observations of Titan’s lakes and proposed future missions to Titan like the ESA’s Titan Saturn System Mission (TSSM).