Experiments funded by NASA’s Jet Propulsion Laboratory suggest that the potential life-supporting ingredients found in the frozen methane lakes and seas of Saturn’s moon Titan are unlikely to support the formation of stable closed membranes like cells. Instead, the experiments showed that the ingredients in Titan’s lakes and seas are likely to form lifeless, crystalline structures.
If proven current, the experiments would effectively refute a previously proposed concept, the azotosome hypothesis, which posited that Titan’s hydrocarbon-rich lakes might support rudimentary life.
Discovery of Hydrocarbon Lakes and Seas Sparks Extraterrestrial Life Theories
Most of the planets and moons of the Solar System have little to no atmosphere, and only one besides Earth has liquid lakes and rivers on its surface. However, unlike our home planet’s watery lakes and rivers, these same flowing features that appear on Saturn’s largest moon, Titan, are made of methane and ethane. These liquid features are also extremely cold, with temperatures well below freezing
The moon also has a dense atmosphere, including periodic hydrocarbon rainfall. It also boasts atmospheric pressures virtually equal to Earth’s. Due to these and other similarities, astrobiologists have targeted Titan as a possible home of extraterrestrial life.
“Since the discovery of hydrocarbon lakes and seas on Saturn’s moon Titan, there has been much speculation on whether these could serve as suitable environments to host exotic life,” the researchers write in a published study.
According to the JPL research team, one of those theories suggests that carbon compounds found in these lakes, called acrylonitriles, could assemble into self-enclosed structures called azotosomes. Similar in design to a cellular membrane, azotosomes can host biochemical reactions that support the formation of life.
“A decade ago, molecular dynamic simulations suggested that amphiphilic cyanide species such as acrylonitrile could self-assemble in these cold, nonpolar liquids to form stable closed membranes known as azotosomes, potentially compartmentalizing complex biochemical reactions,” they explain.
Still, subsequent models have failed to reach a consensus on the ability of Titan’s liquid lakes and seas to support azotosome formation, and ultimately, life. This included a 2020 thermodynamic analysis of Titan’s surface conditions that the study authors note “concluded that azotosomes cannot exist under Titan’s conditions.”
“One often encountered argument against Titan’s surface as a habitable environment is the presumed slow rate of chemical reactions at cryogenic temperatures,” they explained.
Experiments Reveal Formation of Lifeless Crystaline Structures
To further investigate what they described as “incongruent results,” the research team decided to test the azotosome hypothesis experimentally.
“This work undertakes the first experimental test of the azotosome hypothesis, where we characterize acrylonitrile- methane and acrylonitrile- ethane mixtures under simulated Titan conditions using a combination of differential scanning calorimetry and Raman microscopy,” they explained.
According to a statement announcing the study’s results, acrylonitriles in a simulated Titan environment tended to crystallize within liquid methane and ethane. Notably, the same compounds did not form any structures resembling an azotosome.
“We did not observe direct evidence of the formation of acrylonitrile- based vesicles in this fluid (nor in liquid methane), at least under experimental timescales,” they explain. “The results indicate that acrylonitrile forms a stable molecular cocrystal with ethane while exhibiting little changes in the presence of liquid methane under experimental timescales.”
The study authors said that this ‘null outcome’ is consistent with previous quantum-mechanical calculations that suggested the azotosome’s structure is inherently unstable.
“These findings suggest that the acrylonitrile- based azotosome structure would be unlikely to form in Titan’s lake fluids,” they conclude.
Titan’s Surface Habitability ‘Remains an Intriguing Notion’
When discussing the implications of their experiments, the researchers noted that their work does reduce the possibility of finding life on Titan “from a thermodynamic standpoint.” However, they also countered, the lack of support for azotosome formation in the moon’s hydrocarbon-rich lakes and seas “does not rule out the possibility that they might exist temporarily or be transiently stable.”
The JPL team also noted that their experiments simulated only “the conditions in which solid acrylonitrile comes into contact with liquid methane and liquid ethane,” and that not all variables were tested. Furthermore, since their work was finished, the team noted that a new mechanism for vesicle formation that involves aerosol droplets on Titan’s surface “has been proposed.
“Regardless of whether Titan has ever harbored vesicles, the chemistry of these systems has deepened our understanding of the history and distribution of organic compounds on this body,” they add.
For those still hoping to find life on Titan, the JPL team offered some additional hope. For example, they note that several past and current studies have showcased “intriguing, unexpected chemistry that is not only possible but also enabled by such frigid environments,” like those found on Saturn’s largest moon.
“Titan’s surface habitability thus remains an intriguing notion,” they conclude, “whether by way of vesicles or other means.”
The study “Experimental Insights into the Azotosome Hypothesis in Titan’s Lake Fluids” was published in Science Advances.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.
