By Kaleb Lyonnais
What does life require? The most common answer is carbon, water, and solar energy. Carbon is the basis of biochemistry: organic molecules are composed of functional groups bonded to carbon skeletons. Water is the solvent those molecules are dissolved in. Solar energy allows organisms to break apart and rearrange those molecules.
These work for Earth-based life, but circumstances may exclude these on other planets. To explore other options, it must be understood why carbon, water, and solar energy are important on Earth. This study is called xenobiology, and it helps the search for alien life by expanding the set of things being looked for.
Carbon can form large molecules because it can bond with four other atoms from a large set of elements. The obvious alternative is silicon, just below carbon on the periodic table, which also bonds with four other atoms. However, silicon cannot bond with as many elements as carbon, and double bonds are unstable, which reduces the diversity of silicon-based molecules. At the same time, silicon molecules are more stable than carbon molecules in low-temperature, high-pressure environments. While silicon-based life is improbable on Earth’s surface, other planets or extreme environments on Earth might support this form of life.
Another alternative to carbon is metal oxides. Some metals can, with oxygen, form complex crystals with the potential for functional groups. These crystals require very high temperatures to react, or else they act like rocks instead of biochemicals. A planet that is very close to its star or that has a thick, heat-trapping atmosphere might support this form of life.
Water is an excellent solvent because it is polar, can act as an acid or base, and has a large temperature range where it is liquid. Speculated alternatives include ammonia, methane, hydrogen fluoride, and hydrogen sulfide. Most of these are less polar than water, with methane being completely non-polar. Many molecules (usually other polar molecules) will dissolve more easily in water but are also more likely to destabilize. To an extent, this is good for life, because this instability leads to reactions, but it makes some molecules completely unable to form.
Ammonia and methane are of special interest because there are places in our solar system with high amounts of each. Titan, a moon of Saturn, has lakes of hydrocarbons and an atmosphere of methane. Uranus and Neptune each have large amounts of ammonia. Both of these require much colder temperatures than the Earth to remain liquid, but this can go well with the climate needed for silicon-based life.
Hydrogen fluoride is more polar than water, and as a result will dissolve other polar molecules easily. It is also highly reactive, to the extent that most theorists do not consider it a likely solvent for life. The specific reaction that discredits it is that hydrogen fluoride has a habit of causing fires, even in cases where oxygen-burning fires would be impossible.
Hydrogen sulfide is structurally and chemically similar to water, but only melts at incredibly high temperatures. It is unlikely it would exist anywhere but in volcanic chambers of sulfur-rich planets or moons, like Io, a moon of Jupiter.
The final requirement of life is energy. Earth life traces most of its energy to the Sun, making starlight the obvious choice for alien life, but even life on Earth sometimes lives where there is no light. Organisms living near underwater volcanic vents take energy from chemicals formed through processes dominated by nuclear decay in the core of the Earth.
In addition to radioactivity, there are also tidal forces. It is not noticable on Earth, but the gravitational pull between a planet and a moon can heat the cores of both, such as on Europa, another moon of Jupiter, or Triton, a moon of Neptune, where tidal forces have created liquid oceans underneath their frozen surfaces.
Another less likely energy source is black holes. In addition to the energy released from things being torn apart by a black hole’s gravity, there is also Hawking radiation, energy released from the event horizon as the black hole consumes matter.
Life can take many forms, and researchers have to keep this in mind as they reach for alien life. There may be silicon life in ammonia oceans, metallic life in sulfide volcanoes, or even familiar carbon life but in orbit of a black hole. Life as we know it may not be all life there is.