Earth and Titan atmospheres

Comparison between the atmospheres of Earth and Titan.
The descent of the Huygens probe has allowed the first detailed study of the atmosphere of Saturn's moon Titan, revealing startling parallels — and stark contrasts — with that of Earth. Both atmospheres are nitrogen-dominated, but the low temperature of Titan means that the carbon-carrying gas in its atmosphere is methane (1.6% of the total) rather than carbon dioxide (present at only 345 parts per million). Photochemical reactions involving this methane produce a smog at middle altitudes, and an organic rain of methane and nitrogen-containing aerosols falls steadily onto the satellite's surface, creating an Earth-like terrain of extended river networks. Radiogenic argon (argon-40), which makes up 1% of Earth's atmosphere, is in short supply on Titan (just 43 parts per million). The still smaller amount of primordial argon (argon-36) suggests that the nitrogen in the atmosphere must have arrived in the form of compounds such as ammonia, rather than as molecular nitrogen (image and text: Nature 438 756).

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