The surface energy balance at the Huygens landing site and the moist surface conditions on Titan

Abstract

The Huygens Probe provided a wealth of data concerning the atmosphere of Titan. It also provided tantalizing evidence of a small amount of surface liquid. We have developed a detailed surface energy balance for the Probe landing site. We find that the daily averaged non-radiative fluxes at the surface are 0.7 W m^-2, much larger than the global average value predicted by McKay et al. (1991) of 0.037 W m^-2. Considering the moist surface, the methane and ethane detected by the Probe from the surface is consistent with a ternary liquid of ethane, methane, and nitrogen present on the surface with mole fractions of methane, ethane, and nitrogen of 0.44, 0.34, and 0.22, respectively, and a total mass load of ∼0.05 kg m^-2. If this liquid is included in the surface energy balance, only a small fraction of the non-radiative energy is due to latent heat release (∼10^-3 W m^-2). If the amount of atmospheric ethane is less than 0.6×10^-5, the surface liquid is most likely evaporating over timescales of 5 Titan days, and the moist surface is probably a remnant of a recent precipitation event. If the surface liquid mass loading is increased to 0.5 kg m^-2, then the liquid lifetime increases to ∼56 Titan days. Our modeling results indicate a dew cycle is unlikely, given that even when the diurnal variation of liquid is in equilibrium, the diurnal mass variation is only 3% of the total liquid. If we assume a high atmospheric mixing ratio of ethane (>0.6×10^-5), the precipitation of liquid is large (38 cm/Titan year for an ethane mixing ratio of 2×10^-5). Such a flux is many orders of magnitude in excess of the photochemical production rate of ethane.