Reading: "Saturn's Titan" by Chris Impey

Astronomers once thought Titan was the largest satellite in the solar system. When the Voyager 1 spacecraft arrived at the Saturn system in 1980, however, scientists realized that Titan’s uniquely dense atmosphere was making it appear larger than it really is. (Titan is actually a bit smaller than Jupiter’s Ganymede.) Although Titan is only 40% the diameter of the Earth, the pressure of its atmosphere on the surface is 1.6 times greater than that on Earth! Of all the satellites in the solar system, Titan is the only one with a substantial atmosphere.

Titan’s atmosphere is a nearly featureless orange haze, which, like smog, is caused by sunlight triggering reactions among molecules in the atmosphere. Intriguingly, the atmosphere is composed mostly of nitrogen, the same gas that makes up most of Earth’s atmosphere. This doesn’t mean that Titan’s environment is like ours, however. Because of the large distance from the sun, the temperature is very low – around 93 K (-292 °F). Minor constituents detected in Titan’s atmosphere include organic molecules such as methane, ethane, acetylene, ethylene, and hydrogen cyanide.

Meteorological calculations suggest the conditions on the surface of Titan may be very unusual. The temperature and pressure are near the unique “triple point” where methane can exist either as gas, liquid, or solid. This allows Methane to play a role like water on Earth, existing as vapor, "rain" drops, or ice. Rain and snowflakes made of methane and complex, gasoline-like compounds may fall from a dark sky.

In 2005, ESA's Huygens probe descended through the clouds of Titan to image the world's surface before living only briefly on the surface. It was able to image what appear to be river delta like formations and experienced varied weather patterns. Based on how it landed, scientists describe the consistency of Titan's surface as similar to a very crunchy creme brulee. In the succeeding years, Cassini has repeatedly imaged Titan in multiple wavelengths and has been able to determine that Titan experiences changes in the levels of the moon's lakes.

One of the more confusing factors with Titan is why it still has methane. This substance is readily broken down in sunlight. In order for Titan to have and maintain a thick methane atmosphere something must be constantly replenishing the gas. It could be geologically produced via volcanism, or there is a slight possibility that it could be biologically related. This slight possibility is substantiated by various chemicals being found out of equilibrium, but there is no where near enough evidence for this to be more than a hint of a possibility.

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Author: Chris Impey
Co-Author: Pamela Gay
Editor/Contributor: Ingrid Daubar-Spitale

Taken from: http://www.teachastronomy.com/astropedia/article.aspx?qid=312