Titan, Saturn’s largest moon, hides its surface beneath an orange haze. The haze, scientists believe, results from the breakdown of methane in Titan’s atmosphere by ultraviolet light from the sun. Yet the ultraviolet radiation should have destroyed Titan’s methane long ago. Some researchers have speculated that evaporating gas from methane oceans on the moon’s surface might replenish its atmosphere. But Ralph Lorenz, a planetary scientist at the University of Arizona, says recent data seem to rule out this scenario. It’s quite clear from radar data and Hubble Space Telescope images that Titan is not completely covered with liquids. If there are any methane oceans there, he notes, they don’t contain enough methane to solve the problem. Lorenz says he may have a better explanation for the moon’s mysteriously persistent atmosphere. The moon may periodically lose and rebuild its atmosphere.
Lorenz and a colleague created a computer model of Titan’s atmosphere. The model shows that as sunlight eats away the methane over several million years, Titan’s greenhouse effect would weaken, causing nitrogen, which makes up the bulk of Titan’s atmosphere, to condense and rain onto the surface.
At the end of such a nitrogen downpour, which would probably last about a thousand years, Lorenz says that Titan would be left with seas of liquid nitrogen enveloped by a thin, clear nitrogen atmosphere, with traces of heavier gases like methane and other hydrocarbons dissolved in the sea or frozen at its bottom. So why isn’t the moon that way now? Lorenz speculates that comet impacts and volcanic eruptions may periodically add methane to its atmosphere. Such events would probably suffice to reinflate the atmosphere even if it had collapsed completely.
Lorenz hopes radar images from the Cassini spacecraft, scheduled to begin a visit to Saturn in 2004, will support his theory by revealing signs of volcanoes or impact craters. Cassini will be the first probe to Saturn since Voyager II flew by Titan in 1981. But Voyager’s cameras couldn’t penetrate the moon’s haze. So nobody has thought about the geology of Titan. It’s been the plaything of atmospheric chemists, but all that will change seven years from now.