User:Tlw051/Atmosphere of Titan

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Similar to the hydrological cycle on Earth, Titan features a methane cycle. This methane cycle results in surface formations that resemble formations we find on Earth. Lakes of methane and ethane are found across Titan's polar regions. Methane condenses into clouds in the atmosphere, and then precipitates onto the surface. This liquid methane then flows into the lakes. Some of the methane in the lakes will evaporate over time, and form clouds in the atmosphere again, starting the process over. However, since methane is lost in the thermosphere, there has to be a source of methane to replenish atmospheric methane.

Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years — a short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. Most of the methane on Titan is in the atmosphere. Methane is transported through the cold trap at the tropopause. Therefore the circulation of methane in the atmosphere influences the radiation balance and chemistry of other layers in the atmosphere. If there is a reservoir of methane on Titan, the cycle would only be stable over geologic timescales.

Evidence that Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, because comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon. Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes.

Another possible source for methane replenishment in Titan's atmosphere is methane clathrates. Clathrates are compounds in which an ice lattice surrounds a gas particle, much like a cage. In this case, methane gas is surrounded by a water crystal cage. These methane clathrates could be present underneath Titan's icy surface, having formed much earlier in Titan's history. Through the dissociation of methane clathrates, methane could be outgassed into the atmopshere, replenishing the supply.