6.1 Introduction

Titan appears alluringly familiar. Its surface is shaped by weather, with lakes, fluvial channels, and dunes (Tomasko et al., 2005; Lorenz et al., 2006; Stofan et al., 2007; Barnes et al., 2007; Lopes et al., 2010). Its atmosphere sports clouds that can grow to over four times the height of terrestrial thunderstorms (Griffith et al., 1998; Brown et al., 2002; Roe et al., 2002; Schaller et al., 2006a). These features result from an uncanny resemblance to Earth; similar to the terrestrial hydro-logical cycle, Titan has a methane cycle, with methane clouds, rain, and seas. On both Earth and Titan, the condensable is supplied by the surface; evaporates into the atmosphere, where it condenses into clouds; redistributes in the atmosphere; and precipitates back to the surface. These processes depend on the partitioning of solar insolation, the atmospheric structure and temperature, the condensable inventory and properties, and the circulation, all of which differ between Earth and Titan (Table 6.1).

On Earth, the equivalent of 2.7 km of water covers the surface and supplies the atmosphere with the equivalent of 2.6 cm of precipitable water. This largely wet surface (70% of the globe) is heated by, on average, 60 percent of the incident sunlight, which passes through the mostly transparent (when cloudless) atmosphere. Sunlight powers weather. Its effects are direct – for example, through the evaporation of surface liquids. In addition, there are indirect impacts – for example, through differential heating across the globe, which ultimately steers the general circulation of the planet, with conditions altered locally by the variable heating associated with surface topography, land-water contrast, and other terrain heterogeneities.