Astrobiologists and planetary scientists have a fairly good idea of which chemicals might indicate the presence of oxygen-breathing, water-based life—that is if it is like us. When it comes to worlds such as Saturn’s moon Titan, however, where temperatures are too cold for aqueous biochemistry, it's much harder to know which chemicals could signal the existence of hydrocarbon-based life.
A Cornell University team may have found a plausible candidate chemical that future missions to Titan could search for. The computer-simulation study, which appeared in the February 27 Science Advances [http://advances.sciencemag.org/content/1/1/e1400067], found that acrylonitrile, a hydrocarbon known to form in Titan's atmosphere, can organize itself into a structure having the same toughness and flexibility characteristic of the membranes that envelop cells on Earth and form the boundaries of organelles like mitochondria and the nucleus.
This computational finding could have lasting implications for scientists who study Titan’s geochemistry. For many planetary scientists, it's their favorite moon. Like Earth, Titan has a dense atmosphere complete with clouds, mountains, riverbeds and liquid seas on its surface. In fact, Titan would probably be the most promising place, rather than Europa, to look for extraterrestrial life in the solar system if not for its frigidity.
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Titan is way too cold for life as we know it. At Titanian surface temperatures (–179 Celsius) phospholipids—the chemical compounds that comprise cell membranes—and the water-based solutions that fill cells would be frozen solid. Any life that evolved on Titan's surface would have to be made of a very different set of chemicals.
In the team’s computer model acrylonitriles formed hollow balls (called azotosomes) that behave, even in the cold, in much the way hollow balls made of Earthly phospholipids (called liposomes) that form membranes in our cells and organelles. Like liposomes, azotosomes can bend into many different shapes and could act as a barrier between the inside and the outside of the bubbles they form, keeping the ethane–methane mix of Titan's seas from penetrating the encapsulation. (Because this study is the first of its kind, we don't know much about which hydrocarbons would be inside the azotosome.)
The degree of similarity between the hypothetical azotosomes and Earth-based liposomes was a surprise to the researchers. “I'm not a biochemist, so I didn't really know what I was looking for [at first],” says James Stevenson, the chemical engineering grad student who ran the computer simulations. “And when I did the calculations—lo and behold!” The simulated azotosomes at Titanian temperature were just as stretchable as liposomes at Earth temperatures. Because flexibility and the ability to withstand poking and twisting are crucial for evolving complex cellular behavior, azotosomes could potentially be a very useful structure for hypothetical alien life in ethane–methane seas and lakes such as those on Titan.
This study demonstrates that “at least in a computer simulation, one can build structures of a size and geometry [roughly] equivalent to the containers that were on the Earth when life began,” says planetary physicist and study co-author Jonathan Lunine. “You can do it with materials that we know are present on Titan...So we've presented potentially one step toward the evolution of life under Titan conditions.”
Chemical engineer and co-author Paulette Clancy compares figuring out how life might form on Titan in the absence of liquid water to “trying to make an omelet without any eggs. It sort of redefines how you think about an omelet,” she says.
Scientists will not know whether the acrylonitrile on Titan's surface actually forms the azotosome structures, let alone whether those structures are components of life, unless a new we send another probe and investigate the hydrocarbon seas' chemistry in more detail. “Titan is literally awash with organics—but it's impossible to disentangle them remotely,” Ralph Lorenz, a NASA scientist who designs and builds planetary exploration probes and who was not involved in this study, wrote in an e-mail. “You need to land, sample the material and use sophisticated chemistry instruments (like those on the Mars rover Curiosity) to see how complex the compounds have become and whether they can execute any of the functions of life.”
Lorenz and others have proposed a few designs for automated submarines or torpedo-shaped probes that could remotely explore Titan's seas, but those missions are several decades away. Furthermore, even if the space agencies began building a craft for a mission to Titan right away, it would be impossible to get it there before Saturn's seasonal revolution renders the moon’s northern hemisphere inaccessible for direct-to-Earth communications. The hydrocarbons seas are clustered on Titan's northern hemisphere, and because that hemisphere will be facing away from the Earth, any missions to Titan during the 2020s will require an orbiter companion that can relay signals back to Earth. Orbiters are expensive, so we probably won't be able to probe Titan's hydrocarbon seas until the 2030s.
So for the time being Titanian azotosomes will remain a hypothetical. But on the bright side, when the next mission does reach Titan, it will have a much more precise idea of which chemicals it should try to find.