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Project Progress

Co-Investigators Robert Hodyss and Pat Beauchamp and Postdoctoral Fellow Mathieu Choukroun have sought to identify what crystalline organic compounds might result as a consequence of atmospheric organics being dissolved in the liquid hydrocarbons and then precipitated at the lake shore. Recent observations of the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft (J.W. Barnes et al., Icarus 216, 136-140, 2011) suggest the presence of unidentified evaporitic materials on the rims of Titan’s Northern lakes. Models of Titan’s lakes chemistry (D. Cordier et al., Ap. J., Vol. 707, L128, 2009) suggest that organic compounds formed by the atmospheric photochemistry may be close to their saturation point in these liquids.

A custom liquid nitrogen-cooled cryostat has been developed for this task. The cryostat sample chamber is filled with ~5 mL of methane or ethane, and saturated with the solute or solutes of interest, then held at 94 K while a stream of nitrogen gas is blown across the surface to hasten evaporation. The materials remaining after evaporation are deposited onto a glass slide, and analyzed by Raman spectroscopy within a Linkam LTS-350 liquid-nitrogen cryostage cooled to 94 K. The Raman spectrometer, a Horiba Jobin-Yvon LabRam HR, coupled to both a solid-state frequency doubled Nd:YAG 532 nm laser and a He-Ne 633 nm laser, has a spectral resolution of ~1 cm^-1 across the spectral range of interest (100-3500 cm^-1).

For identification purposes, acetylene and benzene (two major products of Titan’s atmospheric chemistry) are deposited onto glass slides, and the morphology of the solids and their Raman signature are investigated. Figure 1 shows microscopic images obtained in reflected light with a 50x magnification, and the corresponding Raman spectra. Acetylene is vapor-deposited onto the cold glass slide, and forms a very fine-grained frost, whereas benzene freezes from its liquid phase, which results in larger, sub-automorph crystals. Their respective Raman signatures are similar in peak positions up to 1000 cm^-1, with different peak intensities, and differ largely at higher wavenumbers, making them easy to distinguish from one another.

Preliminary evaporation-precipitation experiments have been conducted on benzene and acetylene in liquid ethane within the cryostat. During these experiments, water vapor was not precluded to enter the system because it is not perfectly sealed from the ambient air. This turned out to be a major issue, as only the water ice frost that formed on top of the precipitates could be detected optically and by Raman spectroscopy. A sealed glovebag, purged with gaseous nitrogen, has been purchased and the experiment placed within to counter this problem in future runs.