The Threshold for Life
If we’re to believe Hollywood storytellers about our first encounter with forms of life beyond mothership Earth, it will be a violent affair. Non-terran life forms always provoke a sort of panic in the popular imagination, but the reality of what our first meeting with ETs will look like is a lot more mundane.
In June, NASA announced the Dragonfly mission which will deploy a quadcopter craft to sample Titan, one of Saturn’s sixty-two moons. It’s so cold up there (94K or -290.5 °F) that elements that would be gases here on Earth (methane, ethane, and other hydrocarbons) instead form large liquid oceans upon and under Titan’s surface. Water mostly takes the form of frigid ice while the predominant nitrogen fills most of the atmosphere. Hydrocarbons, nitrogen, and water. Sounds like the perfect soup to conjure up life, no? That’s precisely why I am convinced our first encounter with non-Earth bound life will occur a mere 840 million miles away on Saturn’s largest moon.
As a physician, I’ve always defined life within the framework of consciousness and particularly in relation to death. Certainly, when consciousness dies, so does life. The brain death criterion used in the state of California fits within this ethico-medico-legal framework . But the astrobiologists at NASA define life in a very different manner, one that is less interested in the metaphysical dimensions of existence and more reliant on the biochemical. Life is a self-sustaining chemical system capable of Darwinian evolution. That’s it. A thing that is alive is made of chemicals and has the capacity to reproduce (as a species) and evolve in response to environmental stimuli. This is the definition NASA will use to decide whether what they encounter on Titan is alive.
But before we get to what we might observe there, it would be useful to look closer to home for precedents for what we may see on that alien moon. One clue lies where life exists in what would seem to be the most inhospitable environments on the planet. So called extremophiles survive in such geochemical hell-pits as the hydrothermal vents in the Mariana Trench or the frozen tundra under arctic ice beds. Despite high ionizing radiation, highly alkaline or acidic environments, and excessive atmospheric pressures, these species thrive. The best known among these is the half millimeter long tardigrade, a tiny invertebrate that has even survived in the anhydrous vacuum of space.
Another way to preview the type of life we will find on Titan is by looking for the most basic forms of life we have here on Earth. Since the astrobiological definition of life merely requires a set of chemicals capable of reproduction and evolution, what we find on Titan may resemble the simplest terran organisms. The bacteria Mycoplasma genitalium has just 525 genes and is considered one of the most basic organisms, but a group of scientists went further and stripped another mycoplasma species of what they thought were non-essential genes, reducing the number to just 473 genes. Even then, the function of a third of the genes remained unknown. Continued experiments like this, gutting simple organisms down to their bare essentials, may offer the best snapshot of what to expect of extra-terrestrial life. Just don’t expect a wow moment like the movies have been priming us to anticipate.