In a new paper published in Science Advances, Hilda Sandström and Martin Rahm from Chalmers University of Technology in Gothenburg, Sweden, consider the possibility of life on Titan—and, more specifically, whether living creatures would need cell membranes to survive.
Membranes are essential for life as we know it because they protect the integrity of a cell’s interior by keeping unwanted substances out. At the same time, they allow critically needed nutrients to come in, and waste products to be discharged. On Earth, cell membranes are built in such a way that they are polar to the outside (to interact well with water, which is a polar solvent), and non-polar to the inside. It’s generally thought that the first membranes were able to self-assemble under Earth’s early environmental conditions.
On Titan the conditions are quite different. Temperatures are extremely cold—between 90 and 94 degrees Kelvin (about -180o C). There is no liquid water anywhere near the surface, and no free oxygen. There are, however, lakes on Titan’s surface consisting of methane and ethane. These hydrocarbons are non-polar. That has led scientists, including me, to suggest that membranes on Titan would be inverted compared to Earth—non-polar on the outside to interact with methane as a solvent and polar to the inside. In an eyebrow-raising paper, James Stevenson and co-authors from Cornell University suggested azotosomes as possible membranes for life on Titan. Azotomes are hypothetical structures that lack the phosphorus and oxygen found in membranes on Earth but contain nitrogen.
Here’s where the new work by Sandström and Rahm comes in. They used computer modeling to determine whether these “polarity-inverted” membranes could self-assemble under the environmental conditions that exist on Titan. The answer was no. Also, the azotosomes were not stable, which at first glance is bad news for life.
However, according to the authors, biological macromolecules 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.
Personally, I doubt their conclusion. The presence of membranes still appears to me to be essential for maintaining the disequilibrium between the interior of a cell and the outside. Without it, there would be nothing to prevent the loss of valuable organic molecules to the environment. I strongly believe that life—here, on Titan, or anywhere else—requires that the integrity of the smallest unit of life, the cell, has to be maintained.
But I might be incorrect. In any exotic environment like Titan’s, we have to question assumptions that appear to us on Earth as common sense. And I like that the authors have used a computational astrobiological approach to try and push the boundaries of our knowledge. Of course, the best way to answer these questions, as always, is to send missions like Dragonfly to explore Titan up close.