Titan

Titan is Saturn's largest moon, the sixth from Saturn, and the second largest moon in the solar system. Its radius is 2575 kilometers, about 50% larger than the Moon's and its mass is 1.8 Moon masses or 0.022 Earth masses. Its density suggests a significant amount of water in its makeup, perhaps underground oceans.

Titan is tidally locked: its orbit and rotation are 16 days (close to the same timespan as a Titan solar day, aka, a Tsol), and it orbits aligned with Saturn's equator and rings, so its seasons (variation in radiation from the Sun) are due to Saturn's 26.7-degree tilt and orbit, a tilt similar to Earth's but a "year" of about 30 Earth years. Its atmosphere has a pressure of about 1.45 atms and its atmospheric circulation tends to a single Hadley cell extending from the equatorial to the polar region.

The thick atmosphere was first suggested by observations of limb darkening in 1903. It has an opaque layer (clouds) obscuring the surface. It extends much further than Earth's atmosphere (~1000 km) due to the lower gravity. Its stratosphere has been analyzed, presumably through absorption lines, and is more than 98% molecular nitrogen, with much of the rest being hydrogen and ammonia. Evidence has been found in its ionosphere for heavy ions, thousands of times the mass of hydrogen. Its atmosphere and surface also have methane, which at Titan's temperatures (around 98 K) can be solid, liquid, or gaseous, and cycles through the atmosphere (a hydrologic cycle), fitting a role similar to water in Earth's hydrology. On the surface are lakes and seas of methane. Some of the landscape is water-ice (including some mountains), and there are signs suggesting cryovolcanism.