Titan—Beyond the Orange Haze (14): Chapter 5: Alien Analogies—Comparison with Mars (Part 2)
The Possibility of Life—Two Realms
The possibility of life on Titan is discussed in two distinct realms. One is the subsurface ocean, and the other is the methane seas on the surface.
Life in the Subsurface Ocean
Cassini's gravity measurements revealed that Titan possesses a liquid water ocean beneath its icy outer shell. It is estimated that a liquid layer, ranging from tens to hundreds of kilometers in thickness, exists at depths of several tens of kilometers or more beneath the ice shell. The liquid likely contains ammonia and, theoretically, could contain salts (such as sulfates).
This subsurface ocean is an environment where life as we know it—water-based life—could potentially exist. On Earth, it is known that rich ecosystems exist around deep-sea hydrothermal vents. Even in dark environments where sunlight does not reach, microorganisms that obtain energy through chemosynthesis thrive. A similar environment might exist in Titan's subsurface ocean.
However, there are several issues. First, it is unclear whether Titan's subsurface ocean is in contact with a rocky core. If the bottom of the ocean consists of high-pressure ice (Ice II, III, V, VI, etc.), hydrothermal activity is unlikely to occur. Without a supply of energy sources and chemical substances, the emergence and maintenance of life would be difficult.
Second, Titan's interior is cold. While it has a rocky core like Earth and radioactive decay occurs, the internal heat source may be weaker than Earth's. The temperature of the subsurface ocean may be at most near the melting point of water (0 degrees Celsius) or even lower. Low temperatures slow down chemical reactions and constrain biological activity.
Nevertheless, the possibility is not zero. On Saturn's other moon, Enceladus, geysers erupt from the subsurface ocean, and organic molecules have been detected within them. A similar process might exist on Titan, with chemical exchange occurring between the subsurface ocean and the surface.
Life in the Surface Methane Seas
A bolder hypothesis is the possibility that a completely different type of life from Earth's exists on Titan's surface—in the liquid methane environment.
Earth's life is based on water. Cell membranes are composed of phospholipids, which form flexible bilayers in water. However, in a liquid methane environment, phospholipids do not function. If methane-based life exists, it must be based on entirely different chemistry.
In 2015, a research team at Cornell University proposed a theoretical model for a cell membrane that could function in liquid methane. They named this an "azotosome"—a coined term derived from Greek meaning "nitrogen body." Azotosomes are composed of a molecule called acrylonitrile (C₃H₃N), which contains nitrogen, carbon, and hydrogen, but lacks the phosphorus and oxygen found in Earth's cell membranes.
According to the research team's computer simulations, azotosomes exhibit properties surprisingly similar to Earth's phospholipid membranes—they form sheet-like structures through self-assembly, are flexible, and are stable. Moreover, they function even at Titan's cryogenic temperatures (94 Kelvin, -179 degrees Celsius). Cassini observations confirmed that acrylonitrile actually exists in Titan's atmosphere, and it is estimated that the amount of acrylonitrile in the Ligeia Mare is sufficient to construct a number of cells comparable to or exceeding the bacterial density of Earth's oceans.
However, another study in 2020 poured cold water on this hypothesis. A research team at Chalmers University of Technology in Sweden showed through quantum mechanical calculations that azotosomes cannot self-assemble under Titan's conditions. Instead of forming membranes, acrylonitrile crystallizes as molecular ice. With the addition of each constituent element, the energy of the azotosome increases, making its formation thermodynamically increasingly difficult. In other words, while azotosomes might be able to survive on Titan, it was suggested that their spontaneous formation is thermodynamically difficult.
However, in 2025, a new possibility was proposed. Astrobiologists Christian Mayer and Conor Nixon suggested that the interaction between small mist droplets and the surface of methane lakes might provide a mechanism to overcome the thermodynamic barrier. The debate continues.
Furthermore, other researchers point out that cell membranes may not be universally necessary for life. On Earth, one of the important functions of cell membranes is to prevent cellular contents from being diluted and destroyed by water. But in Titan's cryogenic environment, biomolecules are already restricted in their movement. Methane and ethane are not as reactive as water. In such an environment, "naked" molecular life without membranes might be possible—without membranes, the diffusion of small energy molecules required for metabolism (such as hydrogen, acetylene, and hydrogen cyanide) would be easier, and the removal of metabolic waste (such as methane and nitrogen) would also be easier. Titan is a place that forces us to rethink the very "definition of life" as we know it. If the Dragonfly lander, to be launched in 2028, discovers a depletion of acetylene (evidence of respiration) or an unusual bias in complex organic molecules on Titan's surface, biological interpretation may be debated, and it could mark the beginning of the greatest paradigm shift in human history: "life without water."
(Published on note, March 6, 2026)