Identification of Acetylene on Titan's Surface

Abstract:

Titan’s atmosphere is opaque in the near infrared due to gaseous absorptions, mainly by methane, and scattering by aerosols, except in a few “transparency windows” (e.g., Sotin et al., 2005). Thus, the composition of Titan surface remains difficult to access from space and is still poorly constrained, limited to ethane in the polar lakes (Brown et al., 2008) and a few possible organic molecules on the surface (Clark et al., 2010). Photochemical models suggest that most of the organic compounds formed in the atmosphere are heavy enough to condense and build up at the surface in liquid and solid states over geological timescale (Cordier et al., 2009, 2011). Acetylene (C₂H₂) is one of the most abundant organic molecules in the atmosphere and thus thought to present on the surface as well. Here we report direct evidence of solid C₂H₂ on Titan’s surface using Cassini Visual and Infrared Mapping Spectrometer (VIMS) data. By comparing VIMS observations and laboratory measurements of solid and liquid C₂H₂, we identify a specific absorption at 1.55 µm that is widespread over Titan but is particularly strong in the brightest terrains. This surface variability suggests that C₂H₂ is mobilized by surface processes, such as surface weathering, topography, and dissolution/evaporation. The detection of C₂H₂ on the surface of Titan opens new paths to understand and constrain Titan’s surface activity. Since C₂H₂ is highly soluble in Titan liquids (Singh et al. 2015), it can easily dissolve in methane/ethane and may play an important role in carving of fluvial channels and existence of karstic lakes at higher latitudes on Titan. These processes imply the existence of a dynamic surface with a continued history of erosion and deposition of C₂H₂ on Titan.