Titan is the largest moon of Saturn and is the only moon in the solar system with a substantial atmosphere. The main constituent of the atmosphere is nitrogen with methane forming several percent. Sunlight and electrons from Saturn's magnetosphere dissociate the nitrogen and methane and thereby start a cascade of reactions that produce organic compounds including the solid organic haze that fills the atmosphere and shrouds the surface. The organic chemistry on Titan may be a model for abiotic organic synthesis. However, the surface temperature is -180 °C, so no liquid water is present. Thus biological systems based on liquid water are not possible on the surface of Titan.
There is a liquid present on Titan; liquid methane and ethane are present in lakes in the polar region and as a moist film at the equatorial landing site of the Huygens Probe. Life in that liquid methane on Titan would be able to derive energy from atmospheric gases, in particular combining acetylene and hydrogen to form methane.
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