Scientists have had a hunch that a distant moon experiences weather like Earth's, forming clouds that douse its craggy surface with rain.
If that's the case, it would make Titan the only other world in the solar system that has that in common with our home planet. Now researchers have one more clue that Titan, the largest of 274 known Saturn moons, has a climate cycle similar to what occurs on Earth.
The catch: Instead of evaporating and filling Titan's lakes and oceans with water, it's likely showering the moon with cold, oily methane.
Using two powerful telescopes — the James Webb Space Telescope and the Keck II telescope in Hawaii — astronomers watched clouds emerge and then climb higher in the sky over Titan, which is about 880 million miles away in space. For the first time, they saw clouds hovering in the north, where most of the moon's lakes and seas exist, at the tail end of its summer.
The discovery of cloud convection bolsters the theory that these bodies of surface liquid, comparable in size to the Great Lakes in the United States, are getting replenished through rain, perhaps like how summer storms work on Earth. The team's research appears in the journal Nature Astronomy.
"We were able to see methane clouds evolving and changing close to Titan’s north pole over multiple days, in the region where large seas and lakes of methane were discovered by the Cassini spacecraft," said Conor Nixon, a NASA research scientist and the paper's lead author, in a statement.
Titan is a strange world, at -300 degrees Fahrenheit, and shrouded in a jaundiced smog. Similar to Earth, the atmosphere is mostly nitrogen.
Scientists are interested in Titan because, despite its brutally cold temperatures, it appears to have the organic ingredients for life — the kinds that humans know about, at least. Whether the moon harbors any microbial aliens has become a top exploration priority, helping to spur NASA's $3.35 billion Dragonfly. The mission just passed its critical design review, a milestone that means engineers can begin constructing the spacecraft. The helicopter-like robot is expected to visit the moon in the 2030s.
The new Webb observations show that during summer in Titan’s northern hemisphere, clouds can rise higher and may be fueled by heat from the sun — sort of like Earth storms.
Scientists had seen this kind of cloud activity before, but never in the north. That's key because it suggests the northern methane and ethane seas are part of a process that maintains Titan’s atmosphere. On Earth, those chemicals are gases, but on freezing Titan, they are liquid like gasoline on Earth.
By using different kinds of light filters, the two telescopes helped astronomers determine how high the clouds were drifting. On Earth, the lowest part of the sky — the troposphere — goes up about 7.5 miles, but that same layer on Titan soars up to 28 miles, thanks to the moon’s lower gravity.
Webb found something else new: a tiny, fast-moving molecule, called the methyl radical, in Titan's atmosphere. This molecule is ephemeral, so its detection indicates that chemical reactions are ongoing in the moon's skies. Similar molecules are associated with the chemical origins of life on Earth.
"It’s possible that (methane) is being constantly resupplied and fizzing out of the crust and interior over billions of years," Nixon said. "If not, eventually it will all be gone, and Titan will become a mostly airless world of dust and dunes."
Though the discovered rising clouds suggest it could rain on Titan, no precipitation was actually observed. The team is planning follow-up studies to see how the weather patterns change, especially in the period after the equinox this month.
Over time, the methane in Titan’s air could vanish if it isn’t being replaced somehow from within the moon. If that happens, Titan could lose its atmosphere and become something more akin to Mars.
"For the first time we can see the chemical cake while it’s rising in the oven," said coauthor Stefanie Milam in a statement, "instead of just the starting ingredients of flour and sugar, and then the final, iced cake."
