Titan is the largest moon of Saturn and the second-largest in the solar system (after Jupiter's Ganymede). It is the only moon known to have a dense atmosphere, and the only known extraterrestrial body with stable surface liquid. One face of Titan continuously faces Saturn and a Titan day (the time taken to orbit Saturn) is around sixteen Earth days.
It is inhospitable to human life, with an average daytime temperature of -180° Celsius and an atmosphere composed primarily of nitrogen and methane, without oxygen. The air is largely opaque, five times the density on Earth, resembling a thick, hazy, orange-tinted fog. This provides good protection against cosmic radiation. Atmospheric pressure is 1.5 times that on Earth.
Powerful winds stir the upper atmosphere and at heights around 25 miles above the surface and speeds of 50 miles per hour barrage balloons support pylons generating wind energy to supply the surface settlements. At ground level the winds reduce to a gentle two miles per hour.
The surface is soft and spongy, composed largely of loosely-packed methane snow. All the water is frozen but there are lakes and seas of liquid methane and ethane.
Gravity on Titan is much lower than on Earth due to its smaller size and lower density. At 14% of Earth's gravity this is less than the Moon. Colonists wear weighted clothing to counteract some of the adverse effects of low gravity but significant health risks remain requiring an extensive programme of exercise and medication to maintain physical fitness.