Life as We Don’t Know It on Titan
Cell membranes may be made of different materials on Saturn’s large moon.
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Saturn’s moon Titan is like no other world in our Solar System. Imagine an oil spill in the Arctic, then drop the temperature below 100 Kelvin (-280 Fahrenheit), remove all free oxygen and all carbon dioxide, and add an occasional downpour of methane rain. Yet, in spite of the alien conditions, the organic-rich hydrocarbon lakes on Titan’s surface may contain life—even if a very different kind of life from what we see on Earth.
A team of researchers from Cornell University—James Stevenson, Jonathan Lunine and Paulette Clancy—recently offered one suggestion for how cell membranes, which are critical for life, might be formed in Titan’s methane lakes. Azotomes—compounds that contain polar nitrogen groups—could play the role that liposomes, sphere-shaped structures made of phospholipids, fulfill in cell membranes on Earth. Liposomes are unlikely to form on Titan because they are adapted to interact with polar solvents such as water, rather than non-polar solvents like methane. Also, phosphorus and oxygen are thought to be extremely rare in the hydrocarbon lakes of Titan, so any life that exists there would have to find chemical substitutes.
The Cornell researchers showed that azotosomes, particularly the compound acrylonitrile, has many of the same physical characteristics that terrestrial phospholipid cell membranes have when exposed to methane under Titan’s frigid conditions. In that deep freeze, acrylonitrile is thermodynamically stable, doesn’t decompose easily, and has a very similar rigidity. Based on their modeling results, Stevenson and his co-authors conclude that azotosomes could be an option for life on any planetary body with a nitrogen-methane atmosphere.
These kinds of studies of how life might function under extremely exotic conditions are exciting, and very much needed. However, we have to realize that they’re only baby steps toward understanding how life might function on an alien world. Just pointing out the physical similarity to terrestrial membranes is not enough. A membrane in an alien organism would also have to fulfill the function of a selective, semipermeable membrane—one that lets in certain molecules, particularly compounds to be metabolized, and discards others as waste. Being able to form a vesicle doesn’t do it all by itself: Soap bubbles on Earth form vesicles, but they aren’t used as membranes in biology. In addition to studies like this one based on modeling, future researchers need to conduct experimental testing under Titan’s cryogenic and oxygen-depleted conditions, and such tests will be expensive and very time-consuming.
The best way to study life, if it exists, on Titan, is to go there. Fortunately, people are working on concepts to do just that, including a Titan submarine that could explore the moon’s hydrocarbon lakes.
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