Figures: Surface of Titan. Image of the surface obtained by the Cassini/Huygens lander as it was descending through Titan's atmosphere Images from during the probe's descent show light and dark areas on the surface, with a whole network of what appear to be drainage systems (photo: ESA) - like you may see if you were to fly over the Lake District . Huygens landed in one of the dark areas, what appears to be a beach, and due to the gas chromatograph we know that there is methane in the sub-soil. The SSP also detected this with its thermal properties instrument. So we believe that this drainage network was formed by rivers and lakes of liquid methane which have since dried up.
It's density is about one-third that of water (1.9 gm/cc) so this suggests that the planet is rich in rocks that contain lots of hydrogen. The material is probably similar to what astronomers have studied in carbon-rich, chondritic meteors. Titan has an atmosphere whose pressure at the surface is an incredible 1.5 times that of Earth's atmosphere at sea level. The temperature at the surface is estimated to be -179 C (94 K). The atmosphere has been spectroscopically studied and it seems to be a witch's brew of molecules you might expect to find at your local gasoline pump. Besides nitrogen, methane and hydrogen, you can find propane, acetylene, ethylene, hydrogen cyanide and cyanoacetylene. Its surface receives about one percent the sunlight that Earth does, and it is expected from chemical models that ethane probably condenses out of the atmosphere like rain, and has formed an ocean perhaps one mile deep with a sediment of solid acetylene up to 600 feet thick at its bottom.
In reality, given the over-abundance of theoretical models, the oceans are probably little more than large lakes or puddles spread across the surface. Titan is not a habitable place even with its solid surface and thick atmosphere, but it shows that Mother Nature likes to build some very large factories for cooking organic molecules under almost impossible conditions. Just think what she cooked up on Earth under warmer conditions about 4.5 billion years ago. The Hubble Space telescope has taken intriguing images of the surface of Titan such as Plate 7 that show hot and cold regions that are perhaps continents surrounded by oceans of some kind. On Christmas Day, December 25, 2004 the Cassini spacecraft will have deposited the Huygens probe on the surface and we will have an even better idea of what this fascinating world is really like.
This answer was updated in 2011.
See my books:
The Astronomy Cafe (1998) and
Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011
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