Titan is the fifteenth and largest of the known satellites of Saturn. In Greek mythology, the Titans were a family of giants. They were the children of Uranus and Gaia and they sought to rule the heavens. They were overthrown and succeeded by the family of Zeus. Titan was discovered by Christian Huygens in 1655. It was originally thought that Titan was the largest moon in the solar system. This belief was held for a long time. Recent observations have revealed that Titan's atmosphere is so thick, that its solid surface is slightly smaller than Jupiter's Ganymede. Nevertheless, Titan is larger in diameter than Mercury and larger and more massive than Pluto.
Titan is composed of half water ice and half rocky material. It is believed to be distinguished by several layers with a 3400 kilometer rocky center. The rocky center is surrounded by several layers composed of different crystalline forms of ice. There are very few if any craters visible, which is an indication that the surface is very young. If there are any "lakes" they are more likely to be slushy than liquid. The basins are not filled with liquid at all times. The interior of Titan is still believed to be hot. The surface of Titan has a temperature of about 94 Kelvin. Water does not sublimate at this temperature. Sublimation is when molecules changes from a solid to a gas without becoming a liquid first. Thus, there is very little water vapor in the atmosphere. Scientists have made note that there is a lot of chemistry going on on Titan. This has resulted in a very thick smog.
Of all the satellites in our solar system, Titan is the only one that has a substantial atmosphere. This atmosphere is composed mostly of molecular nitrogen with no more than 6% argon and a few percent of methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds such as ethane, hydrogen, cyanide, carbon dioxide, and water to name a few. The organics are formed as methane which dominates the upper atmosphere of Titan. It is destroyed by the sunlight which leaves this haze surrounding Titan which is very similar to the smog found over large cities on Earth but much thicker. These conditions are similar to the conditions on Earth early on its history when life was just beginning. It is this thick hazy, smog like atmosphere that makes it very difficult to see Titan's true surface. In addition to the deep smog like haze, there are scattered clouds in Titan's atmosphere. (
See Figure 5)
These clouds are believed to be composed of methane, ethane, or other simple organic materials. It is believed that other more complex chemicals in small quantities must be responsible for the orange color as seen from space.
Titan was observed by the spacecraft Voyager1. One of Voyager 1's principal foci was Titan. Voyager 1 came within 4000 kilometers of the surface. In just a few minutes of observation, more was learned about Titan than in the previous 300 years. In late 2004, the Cassini orbiter spacecraft began to record data of Titan with many instruments. The Cassini orbiter had a series of close encounters with Titan resulting in all types of data. Then, in January 2005, the Cassini orbiter launched a probe named the Huygens probe. The Huygens probe actually landed on the surface of Titan and sent back images from the surface of Saturn's largest satellite.
(image available in print form)
Image Courtesy of NASA
Scientists are beginning to get an understanding of Titan's surface. They are combining all of the data that they are receiving from various resources. Large ground based observatories, the Hubble Space Telescope, the Cassini orbiter, and the Huygens probe have all combined to provide valuable data, but it is still incomplete. The Cassini orbiter has instruments that can penetrate the thick haze. It has a radar mapper that can see right through the haze and the Huygens probe shows the surface clearly. However, the Cassini orbiter images are very vague and the Huygens images are so few in number and only cover a very, very small area. There have been some exciting images that have been recorded. The Hubble Space Telescope has recorded a huge bright "continent" on the hemisphere of Titan that faces forward in its orbit. This "continent" has a preliminary name of Xanadu. There are some darker regions that suggest oceans or lakes. The Huygens probe has shown images of what could be drainage channels and possible shorelines.