Basic Information
Terrestrial life uses water as a solvent, keeping the various chemicals of life dissolved, providing a medium for aqueous reactions, and transporting them within and between cells. Water also has a high specific heat, helping moderate temperatures. However, life on another world might use a different liquid as a solvent.
One of the most likely alternatives (as it is known to exist in seas and lakes naturally, on Titan) is methane (possibly mixed with other alkane hydrocarbons such as ethane, as it is in the lakes of Titan). Though a gas in Earth-like conditions, a sufficiently cold world would allow liquid methane.
Necessary Conditions
First, the hydrocarbon must exist in large quantities on the planet in question. Methane is common in the universe. It exists in some quantity (from 0.3% in Jupiter to 2.3% in Uranus) in all the Solar System's giant planets. Titan has seas and lakes of methane. Methane has been detected in the atmosphere of the extrasolar planet HD 189733 b. Methane exists in small quantities in Earth's atmosphere, but it is not stable there, being easily oxidized, and remains in the atmosphere only because it is continually produced by life. Methane has also been detected on comets and Pluto.
Ethane is much rarer, but exists in trace quantities in most of the above locations, since the Sun's ultraviolet light can split methane into a methyl radical and a hydrogen atom; if methyl radicals combine, the result is ethane.
Second, it must be liquid most of the time. Methane has a relatively narrow liquid range, from 91 to 112 Kelvin (-297 to -259 Fahrenheit). So a planet with methane oceans must be both very cold and with a fairly stable temperature (mild seasons).
Any life that existed using methane as a solvent would boil to death at any temperature humans could survive at, even the coldest Antarctic winters. Under intense pressure, the boiling point of methane would increase, but even the critical temperature of methane is 190 K (-118 Fahrenheit)! Ethane, propane, and the larger alkanes have higher boiling points, but they would likely exist only as traces in a methane ocean.
Use in Fiction
Hal Clement's Mesklin series (Mission of Gravity, Star Light, and the short stories "Lecture Demonstration" and "Under") centers around the adventures of the Mesklinites (methane-breathing aliens) and the humans who work with them, and how their inability to come into physical contact or inhabit the same environment affects the relationship between the two species.
Sources
Game and Story Use
- Titan might have methane life even in our own solar system. It has lakes and seas of methane (with some ethane).
