"Saturn During Equinox" by NASA / Public Domain

Atmospheric Mystery on Saturn’s Largest Moon

Jan 10, 2019

By Erik Larson

Titan, Saturn’s largest moon, is bigger than the planet Mercury and has the thickest atmosphere of any known planetary satellite. Its blanket of gases is incredibly similar in some ways to our own: it is made predominantly of nitrogen, for example, and the surface pressure—roughly 100 times greater than that of Mars and 100 times less than that Venus—resembles Earth’s as well. It even has clouds and precipitation.

In other ways, however, Titan’s atmosphere is utterly alien. At an average temperature of -290 degrees Fahrenheit, the clouds and raindrops are made not of water but of liquid methane and ethane that evaporate from hydrocarbon lakes and seas near the poles. The water ice that makes up the moon’s surface is so cold that it acts like bedrock. Combined with Titan’s low gravity—just 1/7th of Earth’s—the thickish atmosphere has led some to speculate that humans could fly under their own power with synthetic wings strapped to their arms.

A particularly fascinating aspect of Titan is the thick haze of organic chemicals that dominates the atmosphere and obscures the surface. It is reminiscent of the smog in Los Angeles, but hundreds of miles thick. The haze is composed of microscopic, oil-like snowflakes and is formed high in the atmosphere through the interaction of atmospheric nitrogen and methane with ultraviolet light and high energy particles. Although this photochemistry happens in other oxygen-poor atmospheres including those of Pluto, Jupiter, Saturn and, billions of years ago, even the early Earth, the sheer abundance of Titan’s haze and the extended length of the Cassini mission, which orbited Saturn from 2004 until its plunge into the planet in 2017, make it the most striking and well-studied example of this process.

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