Optical reflectivity of solid and liquid methane: Application to spectroscopy of Titan's hydrocarbon lakes

Reflectance spectroscopy of outer solar system bodies provides direct observations for interpreting their surface compositions. At Titan, the Cassini spacecraft revealed dark patches in the surface reflectance at 2 and 5 μm, interpreted as hydrocarbon lakes forming seasonally through a methane cycle. Whereas the composition of planetary materials in the solar system has been inferred from characteristic absorption bands, identification of phase states (liquid versus solid) on dynamic planetary surfaces requires laboratory reflectance ratio measurements at relevant temperatures. Using visible and near-infrared radiation from the National Synchrotron Light Source (NSLS), reflectance ratios of solid (single crystal) versus liquid CH₄ were measured at temperatures from 50-100 K. Although the wavelength and shape of the characteristic methane absorption bands at around 1.7 and 2.3 μm are insensitive to temperature or phase state from 50-100 K, the broad-spectrum reflectivity R(λ) at 0.5-2 μm decreases dramatically upon melting by at least 25% at 87-94 K. Transition from solid CH₄-I to liquid states at ∼90 K displays a reflectance ratio R_solid/R_liq 1.3-1.6 at 2 μm. This darkening of CH₄ upon melting at 87-94 K is similar at visible wavelengths, and consistent with observations of hydrocarbon lakes in the far northern and southern latitudes of Titan.