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Saturn's Moon Titan
This image of Saturn's largest satellite Titan was taken from a distance of 4000 km by the space probe Voyager 1.
It was long thought that Titan was the largest satellite in the solar system but recent observations have shown that Titan's atmosphere is so thick that its solid surface is slightly smaller than Ganymede's. Titan is nevertheless larger in diameter than Mercury and larger and more massive than Pluto.
Our knowledge of Titan is frustratingly incomplete. Titan is surrounded by a thick, opaque atmosphere; the surface cannot be seen at all in visible light. (The Cassini mission will map Titan's surface with radar as Magellan did at Venus.) All that the Voyager images show is a slight variation in colour between the northern and southern hemispheres.
Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky centre surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity compresses its interior.
Alone of all the satellites in the solar system, Titan has a significant atmosphere. At the surface, its pressure about 50% higher than Earth's. It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few per cent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started.
Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere. It is therefore directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.
At the surface, Titan's temperature is about 94 K (-290 F). At this temperature water ice does not sublimate and thus there is little water vapour in the atmosphere. Nevertheless, there appears to be a lot of chemistry going on; the end result seems to be a lot like a very thick smog.
There are probably two layers of clouds at about 200 and 300 km above the surface. Other more complex chemicals in small quantities must be responsible for the orange colour as seen from space.