But carbon dioxide is a different matter. Because this gas makes up only 0.04 percent of Earth’s atmosphere, “If you measured carbon dioxide [on Earth], you would find that it was highly variable,” McKay says. The life-caused variation in carbon dioxide is large enough to affect the total. On Titan, carbon dioxide’s analogue might be molecular hydrogen, which makes up less than 0.5 percent of the moon’s atmosphere. If biological processes on Titan consume hydrogen, this gas may fluctuate, offering researchers a sign of life.
Actively searching for living organisms on any other world is difficult. This is especially true in an environment as alien as Titan. Unlike Mars and Enceladus, where we know that Earth-like life will operate with analogs of terrestrial metabolism, detecting life on Titan will require many different approaches. First, a probe would need to search for biomarkers in the environment, such as imbalances or cycles of gas levels.
Second, in examining surface materials, a lander would go on to search for new structures or unfamiliar, repeating, non-geological patterns. Boats or submarines could sniff out biosignatures in Titan’s methane seas. Orbiters also could detect biosignatures from above. One probe under consideration, a drone called Dragonfly, aims to chart hydrogen levels by scooping up material around it. It will then study the material with an advanced gas chromatograph mass spectrometer, which will separate and analyze specific types of molecules within the sample. On Mars and Enceladus, scientists are looking for water-based life, so they will search for molecules that work well in water like amino acids and lipids. But on Titan, we have no idea which molecules to look for. So researchers will be looking for anything that sticks out and makes them say, “Huh, that’s odd.”
The discovery of life on Titan would be a watershed moment in the biological sciences. Though life on Mars or Enceladus will likely be chemically similar to that found on Earth, on Titan, any life capable of surviving in liquid methane will tell us that more than one kind of life exists in the universe. As McKay puts it, the news would tell us, “Not only do we have neighbors, but they are strange, or we are strange, we don’t know which. It would be remarkable.”