Evidence for frozen hydrocarbons on Titan

Cassini Visual and Infrared Mapping Spectrometer (VIMS) and Imaging Science Subsystem (ISS) have twice, now, observed widespread darkening of Titan's surface that has been interpreted as evidence of rainfall (Turtle et al., 2009, GRL 36; Turtle et al., 2011, Science 331) followed by an increase in albedo, well beyond the pre-darkened albedo (Barnes et al., 2013, Planet. Sci. 2; Soderblom et al., 2014, DPS). Based on the timescale and magnitude of the albedo changes, and the correlations between the timescale and temperature (inferred from latitude), we favor a thermodynamically controlled process to explain the brightening. Herein, we present a detailed comparison of the IR spectra of the bright materials of these two events. We also discuss the implications on the interpretation of these data from recent laboratory work investigating the freezing of ethane at Titan-like conditions (Farnsworth et al., 2016, LPSC 47).