Rain and hail can reach the surface of Titan

We have calculated the condensation and evaporation of ternary CH4–N2–C2H6 liquid drops and solid CH4 hail as they fall through Titanʹs lower atmosphere to determine the likelihood that precipitation reaches the ground. Assuming the humidity profile determined by the Huygens probe, binary liquid CH4/N2 condensate grows in the region from ∼8 to 15 km in Titanʹs atmosphere because the combined humidity of CH4 and N2 exceeds saturation. These drops evaporate below ∼8 km. We determine the fate of 10 μm seeds composed of ethane, which is expected to provide condensation sites. In addition, we study the fate of already formed raindrops with radii of 1–4.75 mm falling out of the growth region. High (50%) and low (0%) ethane relative humidities (RH) are considered in the calculation. We find that drops with radii ∼3 mm and smaller dropping from 8 km reach the ground in compositional equilibrium with the atmosphere in the high ethane RH case as a result of the stabilizing influence of the ethane, and evaporate in the atmosphere in the low ethane RH case. Large drops (>∼3 mm) reach the surface large and cold because the latent heat loss due to the evaporation of methane cools the drop and slows the evaporation rate. Pure methane hail hits the ground if its radius is initially more than 4 mm at 16 km above the surface and sublimates in the atmosphere if its radius is smaller.