Submitted on June 8, 2007
Accepted on October 5, 2007
Widespread Morning Drizzle on Titan
Máté Ádámkovics 1*, Michael H. Wong 2, Conor Laver 2, Imke de Pater 1
1 Department of Astronomy, University of California, Berkeley, CA 94611, USA.; Center for Integrative Planetary Science, University of California, Berkeley, CA 94611, USA.
2 Department of Astronomy, University of California, Berkeley, CA 94611, USA.
* To whom correspondence should be addressed.
Máté Ádámkovics , E-mail: mate{at}berkeley.edu
Precipitation is expected in Titan's atmosphere, yet it has not been directly observed and the geographical regions where rain occurs are unknown. Here we present near-infrared spectra from the Very Large Telescope and W.M. Keck Observatories that reveal an enhancement of opacity in Titan's troposphere on the morning side of the leading hemisphere. Retrieved extinction profiles are consistent with condensed methane in clouds near 30 km altitude and concomitant methane drizzle below. The moisture encompasses the equatorial region over Titan's brightest continent, Xanadu. Diurnal temperature gradients that cause methane relative humidity variation, winds, and topography may each be contributing factors to the condensation mechanism. The clouds and precipitation are optically thin at 2.0 µm and models of `subvisible' clouds suggest that the droplets are 
0.1 mm.
