No Evidence of Morning or Large-Scale Drizzle On Titan

Widespread methane precipitation on Titan has been predicted by several authors since methane saturation in the upper troposphere was measured by the Huygens probe over its landing site. Recently, observational evidence of widespread morning drizzle has been suggested based on detection of a dark morning site in surface-subtracted 2 mu m spectroimages obtained from ground-based observations. We have obtained similar new spectroimages of Titan observed at Gemini-North Observatory at several rotational phase angles and have analyzed them using the same image processing technique. By comparing images at different phases, we show that the presumed dark drizzling regions in the surface-subtracted images are mainly negative renditions of bright surface regions. No evidence of spectroscopically verifiable drizzling is apparent in our cloudless Titan images. At present, constraining models of Titan's hydrological cycle to produce a diurnal response would be premature.