All life processes on Earth take place in an aquous environment. The origin of life lies in the oceans, and even those organisms living on land or in the air are internally mainly composed of water. On Earth, there is no life without water. But is life imaginable where something else plays the role of water?

The most likely candidate for such an alternative solvent is ammonia. There is no planet or moon where liquid ammonia exists in our solar system. However, such worlds might exist in other solar systems. If a large enough planet orbited the Sun at 3 A.U., it could have lakes or even an ocean of liquid ammonia. What makes ammonia a likely candidate is its similarity to water. To us, it is a poisonous gas with an evil stench, nothing like the liquid which, to us, means life. However, from a more objective, abstract viewpoint, it shows several commonalities. Ammonia, like water, is a polar molecule, with a positive charge on one end and a negative charge on the other. Liquid ammonia has similar chemical properties to water; salts (and water ice) dissolve in ammonia, hydrophobic substances do not. There are even equivalents to acid/base reactions. In light of this, it might be possible that life exists in liquid ammonia or mixed ammonia/water environments. However, these environments are very unlikely to spawn technological civilizations, because oxygen or similar `breathing' and fire-feeding gases are not stable in a liquid ammonia environment.

Methane and other hydrocarbons, as they are possibly present on Titan, however, are entirely different. These solvents are non-polar, i.e. they behave entirely different than water. Salts and other hydrophilic substances (e.g. carbohydrates and water ice itself) don't dissolve in them, but on the other hand, hydrophobic substances do. Biochemical reactions like those found on Earth, however, heavily rely on the presence of a polar solvent and could not work in a non-polar environment. If life exists in non-polar environments, its biochemistry is probably more alien than we can imagine today.

Might there be any other liquids? Science fiction books and movies (such as Alien) sometimes feature beings with acid-based biochemistry. Acids are polar solvents; thus, such beings might be possible -- if something provides significant amounts of a certain acid on a planet's surface. However, concentrated inorganic acids tend to destroy all organic matter (except some highly hydrophobic substances), but life based on water-diluted acids might still be possible -- or inorganic life forms (see below). Concentrated sulfuric acid has a boiling point of 3380 C at Earth-normal pressure; on a planet like Venus with its much denser atmosphere, it is much higher.

Alcohols are polar solvents, but to a lesser extent than water; they don't dissolve salt, but dissolve some hydrophobic substances. However, being organic in themselves, they are not very likely to occur in large enough quantities before biological evolution sets in on a planet. Molten salts or even lava are also sometimes mentioned in bad `science' fiction stories; however, these are too hot to co-exist with organic matter. If there are life forms with molten salt or lava for `blood', they must be of inorganic composition -- and this is a matter of speculation.