Saturn's moon Titan is the second largest natural satellite in the solar system, and the only one that possesses a substantial atmosphere. With a surface temperature of 93.7 K at the equator, Titan's water is almost completely frozen out of the atmosphere; water ice comprises between 35% and 45% of the mass of Titan depending on the interior model. But methane seems to play many of the roles on Titan that water does on Earth: clouds have been observed, fluvial and dendritic features have been imaged suggesting episodic heavy rainfall, and there is compelling but circumstantial evidence for near-polar lakes or seas of methane and its atmospheric photochemical product, ethane. However, whereas Earth possesses a massive global ocean of water, Titan lacks a global methane ocean, and on Titan, low-latitude rainfall appears to be an occasional process limited by the small amount of available solar energy compared with that of Earth. Titan is therefore distinct from the Earth, but is also different from Venus in retaining an active cycle of precipitation and evaporation, and from Mars in the preponderance of active fluvial and pluvial processes in the present day.
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We thank Chris McKay for improving the manuscript. The Cassini-Huygens project and NASA Planetary Atmospheres Program supported the work of the authors. J.I.L was a visiting professor at IFSI-INAF, Rome, Italy, during the final revision of the manuscript, and is grateful to its director Angioletta Coradini for the Institute's hospitality.
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