Non-water Solvents
In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussion about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.
Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist; however on another occasion he said he was a carbon chauvinist but "not that much of a water chauvinist". He considered hydrocarbons, hydrofluoric acid, and ammonia as possible alternatives to water.
Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity useful for temperature regulation, a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H+ ion allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. Additionally, water has the unusual property of being less dense as a solid (ice) than as a liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds) then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.
Not all properties of water are necessarily advantageous for life, however. For instance, water ice has a high albedo, meaning that it reflects a lot of light and heat from the sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.
There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Since boiling points can vary based on the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (9.2 MPa) of pressure, it can indeed exist in liquid form over a wide temperature range.
Read more about this topic: Hypothetical Types Of Biochemistry