Sagan helped solve the reddish haze mystery of Saturn’s moon Titan: Titan is the answer.
Titan’s orange-red appearance comes from a thick atmospheric haze made of complex carbon- and nitrogen-bearing organic compounds. In laboratory experiments, Carl Sagan and Bishun Khare exposed methane-rich gas mixtures to ultraviolet light, producing reddish-brown materials resembling Titan’s haze.
Sagan and Khare coined the term “tholins” in 1979 for these difficult-to-characterize organic substances. The word comes from a Greek term associated with muddy or hazy material. Tholins are not one single chemical compound, but a broad mixture of complex molecules formed when sunlight drives chemistry in atmospheres containing methane and nitrogen.
A common mix-up is to confuse Titan’s haze with its clouds or surface lakes. Titan’s clouds can contain methane and ethane, while its stable surface lakes and seas are mainly liquid methane and ethane. The haze is higher in the atmosphere and helps conceal the surface from ordinary visible-light observation.