Methane drizzle on Titan

Saturn's moon Titan shows landscapes with fluvial features(1) suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole(2-4) or cloud streaks at southern mid-latitudes(5,6) and the mechanisms of their formation. It is not known, however, if the clouds produce rain or if there are also non-convective clouds, as predicted by several models(7-11). Here we show that the in situ data on the methane concentration and temperature profile in Titan's troposphere point to the presence of layered optically thin stratiform clouds. The data indicate an upper methane ice cloud and a lower, barely visible, liquid methane-nitrogen cloud, with a gap in between. The lower, liquid, cloud produces drizzle that reaches the surface. These non-convective methane clouds are quasi-permanent features supported by the global atmospheric circulation, indicating that methane precipitation occurs wherever there is slow upward motion. This drizzle is a persistent component of Titan's methane hydrological cycle and, by wetting the surface on a global scale, plays an active role in the surface geology of Titan.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62668/1/nature04948.pd

Temperature, Clouds, and Aerosols in the Terrestrial Bodies of the Solar System

International audienceThis chapter is intended to provide a concise overview of the state of knowledge regarding the temperature, clouds, and aerosols of the terrestrial bodies of our Solar System, namely Mars, Venus, and Titan. These bodies are the planetary objects that most resemble the Earth. The atmosphere of each body is described in terms of composition and vertical structure. We distinguish and compare the extent of the various atmospheric compartments that form the atmospheric column, from the troposphere up to the thermosphere. The temperature structure is then presented, and the main causes known for explaining its variations on each body are listed. The specific roles of waves, radiation, as well as convection in shaping temperature profiles are then discussed. In a second part, the particulate components of these atmospheres, clouds and aerosols, are described in terms of their physical properties (composition, optical properties) and of their variability in both space and time. Mars , Venus, and Titan exhibit a remarkable variety of clouds and aerosols. Our knowledge about them has made considerable progress thanks to the success of space missions during the last two decades, while in parallel theoretical models have improved to the point that three-dimensional Global Climate Models now include the detailed physics of clouds and aerosols. As a result, it is now widely recognized that particulates play a key role in forcing the climate and the evolution of these bodies