Experimental reflectance study of methane and ethane ice at Titan’s surface conditions

Abstract

Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) has provided evidence of several different hydrocarbons on the surface of Titan using seven atmospheric windows. Methane (CH4) and ethane (C2H6) are suggested to exist in both the liquid and solid states on Titan. Even if the average surface temperature (90–94 K) of Titan is clearly in the liquid stability field of both CH4 and C2H6, the particles can condense in the atmosphere (∼65 km for C2H6 and lower stratosphere for CH4 Anderson et al. in Icarus, 243:129–138, 2014) and precipitate allowing them to melt and/or sublimate. It is also suggested that these liquids can freeze on the surface due to evaporative cooling. We conducted a laboratory study at Titan surface conditions to determine the phase change of CH4 and C2H6 and to test if they would freeze on the surface of Titan. Using NIR reflectance spectroscopy, we calculated the reflectivity ratio ( $R_{\mathrm{solid}}/R_{\mathrm{liquid}}$ ) of CH4 and C2H6 of 1.08 and 1.36, respectively, suggesting an 8% increase in reflectivity for CH4 and a 36% increase for C2H6 during phase change. The low albedo in liquid phase for both CH4 and C2H6 is consistent with observations made by VIMS in both Titan’s northern and southern latitudes. We also find the evaporation rate of amorphous CH4 close to Titan conditions, which is $9.0 \pm 0.3 \times 10^{-5}~\mbox{kg}\,\mbox{s}^{-1}\,\mbox{m}^{-2}$ at 87 K and we estimated a sublimation rate of $0.22 \times 10^{-5}~\mbox{kg}\,\mbox{s}^{-1}\,\mbox{m}^{-2}$ at 83 K for a 1.5 bar N2 atmosphere. The freezing rate of ∼46 m/year for C2H6 was observed whereas, for CH4, we observed that CH4 does not freeze at 87 K due to a high N2 dissolution rate. However, the viscosity of CH4 increases with a decreasing temperature that results in amorphous CH4. The results show a remarkable difference between the formation of ice for two liquids with different N2 dissolution rates. Consequently, using the results obtained from the laboratory study we predict that the observed change is albedo during and after rainfall on Titan is caused by CH4 due to evaporative cooling processes.