Experimental study of ethylene and benzene evaporites under titan conditions

Introduction: Titan’s dynamic lakes of liquid methane (CH4)/ethane (C2H6) have intrigued scientists for over a decade with their fascinating discoveries. Using the Visual Infrared Mapping Spectrometer (VIMS) onboard Cassini, probable evaporites with a 5-μmbright signature were identified in many dry lakebeds near Ligeia Mare at the north pole [1,2], Tui and Hotei Regios at the midlatitudes [3] and Ontario Lacus at the south pole [4,5]. In-depth studies of these 5-μm-bright regions have concluded that they are non-water ice materials. These observations combined have provided the foundation on which Titan’s evaporites can be studied. Recent evaporite studies have focused on models and theoretical work [6,7], but some experimental work is also being undertaken to constrain potential solvents and solutes that may be active in evaporite production [8-11]. This ongoing study expands the compounds studied by analyzing ethylene (C2H4), benzene (C6H6), and additional solutes in future experiments. Methods: The University of Arkansas owns a specialized Titan simulation chamber that is designed to reproduces Titan surface conditions [12]. This chamber is unique in that it provides real-time experimental data on the composition of simulated hydrocarbon samples. The chamber is made of stainless steel with a height of 2.08 m and internal diameter of 0.61 m. We maintain a 1.5 bar atmosphere with N2 and sustain temperatures of 90 K – 94 K with liquid nitrogen (LN2), which is circulated through cooling coils and cryogenic lines that surround the chamber and temperature control box (TCB). The TCB is cooled using this method, while the rest of the chamber serves to maintain the atmospheric pressure of 1.5 bar. Compounds in gaseous phase at room temperature (CH4, C2H6, C2H4) are contained in gas cylinders, and introduced to the condenser via gas lines. C6H6, however, is a liquid at room temperature, so an Erlenmeyer flask was modified and connected to the condenser. We bubbled N2 through the Erlenmeyer flask for ~10 min until the N2 was saturated with respect to C6H6. Then, we opened the line to the condenser, which allowed N2 to carry gaseous C6H6 into the condenser where C6H6 condensed in the solid phase. After the compounds were added to the condenser and given time to condense and dissolve into the solvent, a solenoid valve was turned on, which allowed the liquid sample to transfer from the condenser to the petri dish at the bottom of the chamber. A layer of Spectralon®, which serves as a background for two-way transmission infrared spectroscopic measurements, covers the petri dish. Here, the sample is analyzed via Fourier transform infrared (FTIR) spectroscopy probes connected to a Nicolet 6700 FTIR (wavelength 1–2.5 μm) using a TEC InGaAs detector, CaF2 beamsplitter, and white light source. Results and Discussion: Ethylene: Three different experiments were analyzed for C2H4 evaporites: CH4/C2H4 (Fig. 1), C2H6/C2H4, and CH4/C2H6/C2H4. Through band depth measurements, mass data, and spectral data, we determined that an C2H4 evaporite deposit only formed in the CH4/C2H4 experiment (Fig. 1). Since C2H4 only forms an evaporite with CH4 in the chamber, this suggests that C2H4 evaporites would form in similar way on Titan. We also observed horizontal band shifts in the characteristic C2H4 absorptions (1.64 and 2.12 μm) (Fig. 2A, 2B). This band shift represents a phase change from dissolved C2H4 to solid C2H4). Additionally, we observed the persistence of a band at 1.66 μm (Fig. 2C). This band was unusual in the fact that it was present in pure CH4, however band depth measurements confirmed complete CH4 evaporation by the end of the experiment.