- No Place Like Home
Saturn’s Largest Moon Would Host Really, Really Weird Life
Ah, Titan. Saturn’s largest, haziest moon had a brief starring role in last night’s Cosmos: A Spacetime Odyssey. Toward the end of the episode, Neil DeGrasse Tyson eases his spaceship into one of the moon’s dark, oily seas. He wanted to see what was down there—more specifically, what kind of life might be down there.
After spending most of an hour describing the evolution of life on Earth, it was time to turn toward alien terrains and chemistries—to a place that, while not so very far away, could host some very, very strange lifeforms.
“There’s a world I want to take you to, a world far different from our own, but one that may harbor life. If it does, it promises to be unlike anything we’ve ever seen before,” Tyson says, in the episode.
Titan is deceptively Earth-like. It has a thick, nitrogen atmosphere. Seasonal rainstorms produce wet patches that are visible from orbit. It has lakes. In fact, Titan is the only place in the solar system, besides Earth, with stable liquids on its surface. Those liquids flow through rivers and streams, pool into lakes and seas, sculpt shorelines and surround islands, just like on Earth.
But Titan’s puddles aren’t filled with water—the moon is soaked in hydrocarbons. Methane and ethane, compounds that are gassy on Earth, are liquid on Titan’s frigid surface. Here, temperatures hover around -179 Celsius (or -290 Fahrenheit). It’s so cold that water ice is rock-hard—in fact, the rocks littering the moon’s surface are made from water. Water is everywhere on Titan, but it’s locked in a state that’s inaccessible for life-sustaining chemistries.
Ask an astrobiologist about the prospect of finding life on Titan, and they’ll say the shrouded, orange moon is the place to go if you’re looking for bizarre life. Life that’s not at all like what we know on Earth. Life that, instead of being water-based, uses those slick, liquid hydrocarbons as a solvent. Life that, if we find it, would demonstrate a second genesis—a second origin—and be suggestive of the ease with which life can populate the cosmos.
Life that’s worth taking a chance to find?
“We will never know if liquid water is the only special solvent in which life can form and propagate unless we go and sample these damn lakes and seas,” planetary scientist Jonathan Lunine of Cornell University said during a recent astrobiology conference. Lunine has spent years studying Titan; at one point, he and his colleagues designed a spacecraft that could land on the moon and float in one of its hydrocarbon seas [pdf].
Thinking about life on Titan isn’t new. In the 1970s, Carl Sagan and chemist Bishun Khare, then at Cornell University, were already publishing papers describing the organic chemistry that might be taking place on the Saturnian moon. At that point, though, the large bodies of liquid on the moon’s surface hadn’t yet been spotted, so Sagan and Khare were thinking about the types of reactions that might be taking place in the moon’s atmosphere (in 1982, Sagan and Stanley Dermott proposed that such lakes might exist). Later, Sagan and Khare would show it was possible to make amino acids using the elements found in the moon’s haze.
In the 1990s, the Hubble space telescope offered hints of a wet world, but it wouldn’t be until NASA’s Cassini mission that scientists got a good look at the moon. In 2004, the spacecraft began peering beneath Titan’s cloudy shroud; in 2005, Cassini sent the Huygens probe parachuting through the haze to a spot on Titan’s equator. Data sent back to Earth revealed a world that looks very much like ours—just with a completely different chemistry.
What that different chemistry means for the possibility of life is still speculative.
“Think about life on Earth—we’re all either in water or we’re fancy bags of water,” says astrobiologist Kevin Hand of the Jet Propulsion Laboratory. “On Titan, life in the lakes would be ‘bags’ of liquid methane and/or ethane. That 90[Kelvin] liquid would be the solvent and then whatever is dissolved into the lakes would be the material that’s used to build the other components needed for life, and to power metabolism.”
Powering metabolism is tricky at those temperatures, though, which is one of the reasons why some scientists are hesitant to focus on sending a probe to Titan. Nonetheless, astrobiologists are studying the reactions and pathways that life might use to gain some traction on Titan—including things like breathing hydrogen and eating acetylene.
“Which elements are easy and which elements are hard to access if you’re a ‘weird’ microbe living in Titan’s lakes?” Hand says. “At this point we don’t really know—work is ongoing.”
I had a few questions after watching the Cosmos depiction of Titan’s alien seas. First, if I were a weird life form on Titan, would I be able to see Saturn through Titan’s hundreds of kilometers of haze? Or would the most spectacular planetscape in the solar system be hidden behind that smoggy curtain?
“Even with the human eye, Saturn would be visible as a faint, bright-ish blob in the nighttime haze,” Lunine says. “And if you have eyes that extend even a bit beyond human sight into the nearest part of the infrared, the ringed world would be clearly seen floating ethereally in the skies of Titan.”
Second, the scene with Tyson in the spacecraft shows a craggy, chaotic seafloor, with things that look like hydrothermal vents. How much do we really know about Titan’s seafloors?
Turns out, we know quite a lot about Titan’s seashores, and slightly less about its seafloors. Until now, scientists had mostly used seashore shapes and surrounding topography to infer what the seafloors might be like. But in May 2013, the Cassini spacecraft aimed its radar at the depths of Ligeia Mare, the second largest sea on Titan (Kraken Mare, which Tyson took a swim in, is the largest). Using the radar data, the team created a map of the sea’s floor—its bathymetry—and saw that Ligeia Mare plunges to a depth of 160 meters (524 feet). The northern seabed is gentler and smoother than the southern, which is riven with flooded valleys and punctuated by steep peaks.
Getting the depth profile meant that scientists could estimate how much liquid hydrocarbon rests in Ligeia Mare: As much as 100 times more than the oil and gas reserves on Earth combined.
Next up? Peering into the depth of Kraken Mare, which covers an area of at least 400,000 square kilometers, or approximately equal to the size of Germany. “Kraken appears to consist of no fewer than three distinct basins, each about the size of Ligeia Mare,” Lunine says. “So there’s a lot of sea to see on Titan.”