Methane rain on Titan

We use a new set of absorption coefficients for methane, hydrogen, and nitrogen, as well as refractive indices of haze and methane clouds, in a multiple scattering radiative transfer model to reexamine the Voyager IRIS infrared spectra of Titan. We find, in agreement with previous studies, that hydrogen composes 0.2 to 0.6% of the atmosphere. We also find that the methane relative humidity at the surface may be in excess of 60%, that methane clouds probably exist from altitudes below 10 km up to about 30 km, that the cloud particle radius is probably larger than 50 μm so that the particles should be classified as rain, and that the optical depth of the clouds that are present is typically of the order of 2 to 5 in both the infrared and the visible. We infer that the clouds are patchy. The clouds have only a small effect on the infrared spectrum of Titan, and probably are not very significant for visible radiation either. The large particle sizes and low cloud optical depths on Titan relative to Earth can be understood if Titan's high altitude haze is the major source of condensation nuclei. Then particle growth is not limited by low supersaturation until raindrop sizes are reached. We find, in agreement with previous investigators, that hydrogen and the high altitude haze play an important role in the IRIS spectra in the wavenumber region from 500 to 600 cm −1. We suggest that latitudinal variations in the brightness temperature at these wavenumbers may be due primarily to observed variations in the temperature of the haze layer, rather than variations in the temperature of the surface. We find that the presence of argon is not required to interpret the IRIS spectra. Further measurements of the pressure-induced absorption of methane at conditions appropriate for Titan are needed, since these data are crucial for our analysis.