Hazy days on Titan (Image: NASA/JPL/Space Science Institute)

Saturn’s hazy moon Titan may sometimes morph into a ball of ice.

Titan is already a frigid moon made mostly of ice. But methane gas in its atmosphere causes just enough of a greenhouse effect to keep a scattering of lakes filled with liquid hydrocarbons.

Scientists have puzzled over Titan’s atmospheric methane because sunlight causes the molecule to react readily with other chemicals in the air, producing the moon’s dense smog. Calculations suggest that the amount of methane now found in Titan’s atmosphere should have been used up within tens of millions of years – a blip in the moon’s roughly 4-billion-year lifetime.


Clear the air

Adding to the mystery, the methane reactions create hydrocarbon compounds that rain over the surface. If fresh methane emissions steadily replaced used-up methane over the course of Titan’s history, this process would happen constantly, and Titan would be covered not by lakes, but by a global ocean hundreds of metres deep.

Michael Wong at the California Institute of Technology says snowballs may be the missing piece of the puzzle. Scientists suspect Earth went through multiple snowball phases, including one about 2 billion years ago, when the planet became covered in ice.

A similar event could have taken place on Titan, says Wong. Methane levels may rise and fall if the gas is periodically released from inside the moon, which would explain how Titan has so much in its atmosphere today. If at some point the methane dropped by a factor of 100, temperatures would fall, and surface liquids would ice over. The haze produced by methane reactions would also freeze out, leaving the atmosphere clear and exposing the snowy surface.

Pluto proxy

But how would this explain the moon’s missing ocean? This clear atmosphere would produce a different mix of molecules, so the cold snap would leave the moon’s surface covered in lots of compounds called nitriles, which would be solid rather than creating an ocean. The absence of deep oceans suggests that Titan spends more time as a snowball than as a smogball.

Getting good enough readings of the surface composition to check this would require a future mission to Titan. In the meanwhile, the New Horizons mission to Pluto could offer early clues.

“Like Titan, Pluto has an atmosphere that is mostly nitrogen with some methane,” says Wong. Pluto’s atmosphere is much thinner and colder, but the physics are similar enough that examining its composition could test the snowball model.

Wong poses an interesting notion, says Ralph Lorenz at the Johns Hopkins University Applied Physics Laboratory in Maryland. If the snowball idea bears out, our image of Titan becomes very different, says Lorenz. “We are now likely to glimpse that Titan has a much more complex history than we had thought.”

Journal reference: Icarus, DOI: 10.1016/j.icarus.2014.05.019