Experiment challenges hypothesis of cell-like membranes on Titan

New experimental results have cast doubt on earlier proposals suggesting that spherical, cell-like membranes could form in the methane lakes of Saturn’s largest moon. Through results published in Science Advances, Tuan Vu and Robert Hodyss at NASA’s Jet Propulsion Laboratory suggest that exobiologists will likely need to explore alternative routes when considering the possibility of life on Titan.

Life on Titan?

Despite frigid surface temperatures of around –180 °C during the day, Titan is widely considered to be one of the most Earth-like bodies in the solar system. With a dense atmosphere composed mostly of nitrogen, its surface hosts lakes and seas of liquid methane and ethane, which flow, evaporate, and fall as rain in much the same way as water does on Earth.

For decades, this striking similarity to our own water cycle has inspired exobiologists to consider whether exotic forms of life could have evolved under these conditions. In 2015, researchers at Cornell University took this idea a step further through molecular-dynamics simulations designed to recreate Titan’s environment.

Their simulations revealed a remarkable possibility: that a type of cyanide known as acrylonitrile (which is known to form in Titan’s upper atmosphere) could self-assemble in liquid hydrocarbons. According to the study, the molecules might organize into stable, spherical membranes called “azotosomes,” directly analogous to the lipid bilayer membranes that form the boundaries of almost all living cells on Earth.

Yet in 2020 , another theoretical study, carried out by researchers at Chalmers University of Technology in Sweden, arrived at a less promising conclusion.

Using quantum-mechanical calculations, the team estimated that azotosomes would be thermodynamically unstable under Titan’s conditions and therefore unlikely to form. Without experimental evidence, however, there was no way to confirm which of these opposing theories was more likely to be correct.

Testing the azotosome hypothesis

In their new study, Vu and Hodyss shed fresh light on this question through the first experimental investigation of the 2015 azotosome hypothesis. In their lab, the duo began by preparing mixtures of acrylonitrile with methane and ethane, then subjecting these mixtures to temperatures and pressures similar to those on Titan’s surface.

After leaving the samples for long enough for the acrylonitrile molecules to assemble into more complex structures, the researchers analyzed them using several advanced techniques.

One of these was differential scanning calorimetry, which measures the amount of heat required to increase the temperature of a sample relative to a reference material. In addition, the team used Raman microscopy to produce high-resolution images revealing the molecular structures present in the mixtures.

Unlikely formation path

The results were not encouraging for the original 2015 theory. While the analysis revealed that acrylonitrile molecules formed a stable molecular cocrystal with ethane, the researchers observed virtually no changes at all in the acrylonitrile–methane mixture. Together, these findings strongly suggest that azotosome structures would be unlikely to form under the conditions present on Titan.

All the same, the result does not rule out the possibility of Titan-based life altogether. Exobiologists widely predict that, despite their ubiquity in Earth-based cells, spherical membranes may not be strictly necessary for functioning living structures. Instead, the new experiment may pave the way for alternative ideas—encouraging researchers to look beyond azotosomes when considering how life might arise on our closest Earth-like neighbor.

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