• The largest satellite of Saturn, and in the solar system. Titan was discovered by Huygens in 1655 and is over 5000km across, just larger than Mercury. It takes 16 days to complete an orbit some 1.2 million km from Saturn. As the densest of Saturn’s satellites, with a density of 1.9gm/cc, Titan is thought to have a rocky interior. It is also the only solar system satellite known to have a dense atmosphere in which lightning has been observed. It contains hydrogen, methane, acetylene and other complex organic molecules and its chemistry may resemble that of the young Earth’s atmosphere. Titan appears to have a solid surface and the large amounts of hydrocarbons in its atmosphere have led to speculations that it may have lakes or even oceans of liquid hydrocarbons There are also large amounts of nitrogen in the atmosphere of Titan. The interior of Titan is less familiar. Titan may have been subject to important geological change and may contain significant amounts of ice and even water. Some observers regard Titan as a smaller version of the Earth, stalled at an earlier evolutionary stage by its position in the outer solar system away from significant supplies of solar energy. Titan was visited by the ESA’s Huygens space probe in the year 2004, and this probe sent back the first direct images of Titan’s surface.