Beneath the orange haze cloaking Titan, Saturn’s largest moon, it’s raining methane. While not much falls — a couple of meters every few centuries — it’s enough to fill seas 200 meters deep in the moon’s northern hemisphere (SN: 12/13/14, p. 13).
Despite having an average temperature of about – 180° Celsius, Titan is the only world in the solar system other than Earth with liquid on its surface (SN: 1/25/14, p. 14). Researchers are starting to understand how the alien seas, mapped by the Cassini spacecraft, help drive a global process strikingly similar to Earth’s hydrological cycle.
Two of Titan’s seas probably serve as a conduit to shuttle methane from north to south, says Ralph Lorenz, a planetary scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md. Ligeia Mare in the rainy north is like a pristine mountain lake. “This stuff is pure, clear methane,” he says. Liquid from Ligeia then trickles south through channels to the much larger Kraken Mare, Lorenz reported in August in Geophysical Research Letters. There, the methane evaporates in the drier air, leaving behind high concentrations of ethane and propane.
Substitute water for methane and salt for ethane and these seas resemble two on Earth, Lorenz says, with the low-salinity Black Sea flowing into the saltier Mediterranean.