Titan’s Big, Smoggy Secret Is Unlocked

Planetary scientists say they have solved the mystery of Titan, Saturn’s exotic moon, and its hydrocarbon-rich, smog-like atmosphere.

Writing in Thursday’s edition of the journal Nature, the team led by Jonathan Lunine of the University of Arizona and Gabriel Tobie of the University of Nantes, France, said the methane was spewed into the atmosphere during three distinct events in the moon’s evolutionary history.

The first occurred soon after its formation 4.5 billion years ago, a second event took place 2 billion years later, then another began 500 million years ago. The first two events resulted from heating in the moon’s core, caused in part by radioactive elements. Cooling in Titan’s solid ice crust caused the third event.

In January 2005, NASA’s Cassini spacecraft sent a probe to the surface of the giant moon, the second-largest in the solar system and one of the few bodies with its own atmosphere. The European Space Agency’s Huygens probe picked up whiffs of methane from the damp surface. Because the sun destroys methane over time, scientists wanted to understand why methane was shrouding the surface.

The latest findings show that methane locked up in surface ice has been released over the moon’s history. The most recent episode continues.

“We are now in an era where there’s enough out-gassing to add methane to the atmosphere, but not enough for widespread seas of methane,” Lunine said.

The current event is expected to end in the next few hundred million years. After that, the sun will slowly destroy the methane, stripping Titan of its atmosphere.

“There’ll be no further such events until billions of years in the future, when the sun goes red giant and cooks Titan,” Lunine said.