“Do living creatures would need cell membranes to survive,” ask Hilda Sandström and Martin Rahm with the Initiatives on Cosmic Origins at Chalmers University of Technology in Gothenburg, Sweden, probing two of the most central questions in astrobiology –“what are environmental and chemical limits of life?” This unsolved mystery led them to explore the possibility of life on Saturn’s moon, Titan. Often described as harboring conditions similar to early Earth, they used computer modeling to determine whether “polarity-inverted” membranes dubbed “azotosomes” –hypothetical structures that lack the phosphorus and oxygen found in membranes on Earth but contain nitrogen– could self-assemble under Titan’s extreme environment.
Titan could harbor methane-based, oxygen-free cells that metabolize, reproduce and do everything life on Earth does, proposed James Stevenson, astronomer Jonathan Lunine and chemical engineer Paulette Clancy at Cornell University in 2015. Azotosomes (Cornell University illustration bellow) are made from nitrogen, carbon and hydrogen molecules known to exist in the Titan’s cryogenic seas but show the same stability and flexibility that Earth’s analogous liposome does.
Titan offers a unique opportunity to explore just how far toward chemical complexity evolution can advance without liquid water, at low temperature, at time scales nearing the age of the solar system, featuring rich atmospheric chemistry and a dynamic surface driven by seasonal rainfall and cycling of predominately methane and ethane, write Sandström and Rahm in Science Advances. “Hydrocarbon lakes and seas have been observed near the polar regions of Titan, drawing comparisons with the hydrologic cycle of Earth and its presumed relevance for the origin of life. However, the surface conditions of Titan are a frigid 90 to 94 K. Moreover, in contrast to Earth, Titan’s outermost surface is covered by products of the atmospheric photochemistry that are likely to be essentially free of oxygen. A frozen water ice crust is suspected underneath the outermost organic layer.”
“The lipid bilayer membrane is one of the central prerequisites for life as we know it,” they continued. “Previous studies based on molecular dynamics simulations have suggested that polarity-inverted membranes, azotosomes, made up of small nitrogen-containing molecules, are kinetically persistent and may function on cryogenic liquid hydrocarbon worlds, such as Titan.
Sandström and Rahm then evaluated the thermodynamic viability of azotosome formation. Quantum mechanical calculations, they predict that azotosomes are not viable candidates for self-assembly akin to lipid bilayers in liquid water. they argue that cell membranes may be unnecessary for hypothetical astrobiology under stringent anhydrous and low-temperature conditions akin to those of Titan.
“Biological macromolecules,” they conclude, “would be immobile given the extremely low temperatures on Saturn’s moon. Titan life forms, if they exist, would need to rely on the slow diffusion of smaller molecules. Any membrane, however, would hinder that diffusion, as well as the removal of waste products in the opposite direction. So, the scientists reason, any potential life on Titan may not need a membrane at all.”
Image credit top of page: Titan via NOAA