Near Infrared Spectroscopy of Liquid Hydrocarbon Mixtures : Application for In - Situ

Introduction: The presence of ethane in liquidfilled lakes on Titan was confirmed by the Cassini Visible and Infrared Mapping Spectrometer (VIMS) in 2008, and has been investigated in further detail by the Cassini radar instrument [1,2]. Radar sounding, infrared absorption, and modeling suggest that the lakes are predominantly liquid methane and ethane, with trace amounts of propane, butane, other complex hydrocarbons, and dissolved nitriles [3,4]. Measuring the composition of Titan lakes is essential for understanding the chemical interchange between Titan’s atmosphere and surface, and the complex pluvial, erosional, and climatological processes that govern the distribution of organic materials. Insitu measurements of lake chemistry, then, would provide a dynamic dataset on Titan meterology and limnology. Spectroscopy, as a non-destructive method requiring no sample preparation, is an ideal tool for investigating the bulk composition of Titan’s lakes, and can be achieved using robust, small-scale fiber optic probe devices. Prior work conducted by M.L. Cable and T.H. Vu demonstrated the utility of fiber optic probes in analyzing complex organic mixtures within the near and midinfrared (NIR and MIR; 0.9-2.5 μm and 2.0-9.0 μm, respectively) [5]. This study follows up on earlier results, and shows how distinct absorption features in NIR spectra (within the windows of Cassini VIMS) can be used to determine the relative abundances of ethane and methane in varying mixtures at Titan-like temperatures. Methods: Liquid hydrocarbon mixtures were prepared by condensing gaseous ethane or methane into a plastic pipette submerged in liquid nitrogen; measured amounts were added to a 20 mL beaker (for larger volumes) or a 6 mL test tube (for smaller volumes). The 20 ml beaker was set into a 100 mL beaker filled with steel ball bearings, and both beakers were anchored to a large dewar filled with liquid nitrogen [6]. For measurements using smaller volumes, the inner 20 mL beaker was filled with 16 mL of liquid ethane, and set to remain at 94 K using a silicon diode thermometer and nickel-chrome heating elements. The test tube was placed inside of the liquid ethane bath, and up to 6 mL of liquid hydrocarbons was added for subsequent measurements. Aliquots of hydrocarbons were prepared by cooling plastic pipettes filled with gasous ethane or methane in a liquid nitrogen bath. Mixtures were then formed by adding the appropriate volume of ethane (or methane) to the test tube, and adding aliquots of methane (or ethane) to create the desired ratios. Near infrared (NIR) spectra were taken from 0.9 to 2.5 μm using an Ocean Optics NIRQuest256-2.5 spectrometer with a Mikropack Tungsten Halogen HL2000-FHSA Lamp, and a fiber optic probe. All measurements were taken inside a glove bag purged with a constant flow of nitrogen gas to maintain a total oxygen concentration of less than 0.1% on average.