To the eye, Saturn's largest moon is enshrouded in a thick, reddish-brown photochemical smog. Like Earth, Titan's atmosphere is primarily nitrogen, but its second most abundant constituent is methane, not oxygen. Ultraviolet sunlight and energetic electrons streaming in from Saturn's magnetosphere act directly and indirectly to dissociate methane and nitrogen and form a suite of organic molecules, both hydrocarbons (molecules composed of hydrogen and carbon) and nitriles (molecules containing carbonand nitrogen linked by a triple bond). These organic molecules react to produce more complex and heavier molecules and ultimately the photochemical hazes that envelop the moon, although much of the detail remains obscure. As Griffith et al. report on page 1620 of this issue (1), detection of a vast ethane cloud in Titan's arctic region is another step in unraveling the complex nature of its atmosphere.
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