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 to sustain 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 it is a gas in Earth-like conditions, a sufficiently cold world would allow liquid methane.
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). 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), which is only just matched by the coldest temperatures in the world. The larger alkanes such as ethane and propane have higher boiling points, but they would likely exist only as traces in a methane ocean.
The cold temperatures mean that there's no risk of organic molecules breaking apart, and there's no risk of attack by water, so methane can support quite complex chemistry. There are quite a few things about terrestrial life that wouldn't work in a methane solvent or other liquid hydrocarbon, though! A methane-based organism would need to work very differently. The most important issue is that two large categories of molecule, charged molecules and polar molecules, won't dissolve in hydrocarbons. That includes proteins, nucleic acids and other vital molecules, unfortunately.
However, lipids such as oils and fats will dissolve in a hydrocarbon solvent, though it is weaker than water so more volume may be required. Any life in a hydrocarbon solvent might be made from giant lipids, or it might anchor lipids to other molecules to increase their solubility. The lipids in our cell membranes, however, could not work in the way they do now, which relies on a head that is attracted to water and a tail that is repelled by water forming bilayers with the tails back-to-back. Without water or a polar solvent that works in a similar way, this wouldn't work and would need to be adapted. When thinking about the alternatives to the way our cell membranes work, it's worth looking at Clay Theory for ideas.
A lot of reactions wouldn't easily work in a hydrocarbon solvent, such as acid-base reactions and reactions such as respiration which rely on the flow of hydrogen cations. The use of simple salts, such as the (charged) potassium ions in our nervous system, is restricted as well. Methane can't be split for photosynthesis, either. These functions are pretty important, and the lack of charged molecules and ions is a major drawback to hydrocarbons and other non-polar solvents. Life can find a way, no doubt, but the results might not be quite as good as the water-based system or any system that uses polar solvents.
There are benefits to life in a methane solvent, though. The lower temperatures allow a wider variety of molecules to exist without breaking apart, and faster reactions which can't be controlled at normal temperatures could be tamed. It is often said that life in methane would be simple and slow, but the ability to control these faster reactions, such as free radical reactions which are damaging to our cells, might allow them to increase their speed. Methane does not expand when frozen, so it would not burst cells with the ice crystals, either. It also does not interfere with hydrogen bonds or attack molecules directly, though the latter might make digestion a little harder. In all, we know that non-polar solvents can support complex organic chemistry, so even if they lack some useful aspects of water, as biological solvents they might just be able to work.
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.
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).