As recently as 2004, we knew Saturn‘s largest moon Titan basically as a fuzzy orange blob, a cold, Mercury-sized satellite of a distant gas giant. Scientists knew it had a dense, nitrogen-rich atmosphere — the only one that we knew of, other than our own — but they weren’t sure about how the atmosphere was organized, nor how the planet’s surface dynamics went. Now Cassini has changed all that. In a recently released study worthy of NASA’s Titan Hall of Fame, Cassini peers through the nitrogen fog at the surface, and specular highlights and radar measurements confirm our predictions. Titan is covered in surface features that are just like what we have on Earth… but a whole lot different, too.
It’s hard to describe Titan without getting a little breathless — the place is a little like Earth’s “upside down.” Earth is a wet, temperate planet, with continents bounded by surface water, clouds, and a water cycle. Titan is so cold that it has liquid methane, and yet it also has clouds and surface weather that imply a hydrocarbon cycle. There are lakes of methane near Titan’s north pole, and closer to its equator lie vast deserts of hydrocarbon dunes, made of granules of water ice coated in dark hydrocarbons that fall from the sky like rain. Perhaps upside-down-iest is the fact that there’s a hidden interior ocean of water and ammonia that covers the entire moon — submerged beneath the thick rime of frozen organic chemistry. It’s all topsy-turvy.
As hydrocarbon rain turns into rivers and carve through the surface, canyons proportionate to our Grand Canyon run with flowing methane. New photos from Cassini show us rivers of methane and ethane that stretch for hundreds of miles before they empty into Titan’s northerly sea, Ligeia Mare. And Cassini has been watching the transition from fall to winter at Titan’s south pole: Seasons on Titan last for seven years or so, and winter is coming. In fact, this is the first time anyone has ever seen the onset of a Titan winter. “We’re monitoring the weather on Titan, watching for predicted methane rainstorms at the north pole,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory.
“I am intrigued by how many features on Titan’s surface are remarkably Earth-like,” said Spilker, “including hydrocarbon rivers, lakes and seas, and equatorial dunes, with liquid methane playing the role on Titan that water plays on Earth.”
Observations like the ones we get from Cassini and Huygens are important because they bear out our expectations about what we can safely infer from spectrographic readings of remote worlds. Before Cassini, we knew very little about Titan, except that we had seen nitrogen in its atmosphere with a spectrograph. Cassini’s instrumentation not only confirmed our predictions of nitrogen, but gave us the power to peer through the obfuscating atmosphere to the surface below — and beamed back a gallery of visual imagery to bring it all together. With a robust theory of Titan’s composition and chemical dynamics, we’ll be able to keep digging deep into our solar system’s history — and also rest more securely on the idea that we can point a prism at a planet light-years away, and still gather useful data that reflects reality.
Titan’s planetary chemistry also poses some fascinating questions about the origin of life, and what extraterrestrial life might look like on the fine scale. Think about this. The entire chemistry of life on earth depends on how life probably arose in seawater, a polar solvent. What sort of fantastical information-transfer biopolymer might life on a nonpolar planet employ? Imagine a DNA analog from a hydrocarbon planet, where the information is encoded in benzene rings and stereochemistry. If we can use Titan to advance our own understanding, we could put forth answers to some big questions about life, the universe, and everything.