Astrobiology is cursed with the problem of a “sample size of one”. We only have one example of life--Earth--to examine, which creates difficulty in predicting what to expect of extraterrestrial life. Without any independent examples of life to examine, how can we know what to search for? Among other considerations, water has been used as a proxy in searches for life on Mars, namely because all forms of life on Earth use water at some point in their life stage. Likewise, experiments to search for organic carbon are based on observations of carbon utilization by life on Earth. Nevertheless, some astrobiologists have also taken a broader scope and examined some of the possibilities of weirder, exotic forms of life that we might someday discover.
Water is indeed critical to all living organisms on Earth, and water’s weak molecular polarity gives it many useful properties that help in creating complex biological structures. Even so, we could imagine a form of life where water is replaced by a different fluid, perhaps an alcohol, that also functions as a solvent. The chemistry of such systems would necessarily be different, owing to the different composition and properties of the alcohol solvent, but perhaps if alcohols are more prevalent on a habitable world then they might be selected for use in biology instead of water. (This makes me wonder: would alcohol-based organisms become inebriated when drinking water?)
Other possibilities for exotic biochemistry include genetic structures analogous to DNA that are based on arsenic instead of phosphorus. In DNA, phosphate is needed along with sugars to create the double-helix backbone of the genetic molecule, which suggests the importance of phosphorus in the development of life on Earth. But could another form of life use a different set of molecules to construct a similar genetic code? Arsenic provides one possibility that could, in principle, be used to construct a molecule similar to DNA--perhaps an extraterrestrial environment rich in arsenic would develop hereditary molecules along these lines. A controversial study published in 2010 even suggested that a microbe named “GFAJ-1” actually uses arsenic in place of phosphorus in its DNA, but this study has since been refuted, finding that the microbes use phosphorus like all other Earth life.
Yet another possibility rests on the skeleton of our biological molecules. We say we are “carbon-based” organisms because the molecular structures that make up our cells and our sources of energy are based upon the carbon atom (with plenty of hydrogen, oxygen, and nitrogen along to help). Carbon is so special because it can create bonds with up to four other atoms, which allows it to create long molecular chains that have a multitude of uses in biology. But do any other molecules offer a similar advantage as carbon? Silicon is the next heaviest atom that similarly provides the same atomic utility as carbon, and some biochemists have started to experiment with silicon-based organic molecules. No silicon-based life forms have yet been discovered on Earth, but this again is worth keeping in mind as the search for extraterrestrial life continues.
Finally, exotic forms of life could exhibit different metabolisms than we typically see on Earth. We are accustomed to deriving our energy from organic materials (i.e. the food we eat), while we also encounter plants that derive their energy from sunlight. While most organisms on Earth use some combination of sunlight and organic material to stay alive, certain microorganisms have found a different strategy and use inorganic materials to get their daily energy. Microorganisms known as extremophiles are highly adapted to specific environments, and some such organisms have evolved the ability to extract energy from ferrous iron and sulfur in highly acidic environments. These organisms have found a way to thrive in conditions that are otherwise inhospitable to other forms of life and represent possible strategies that extraterrestrial life may use to cope with harsh conditions.
No life has been discovered on Titan yet, but Titan provides a rich oasis of organic materials that perhaps contains all the right ingredients. In a future where the Saturnian moons have been partially terraformed, perhaps organisms that have been introduced to a foreign environment will eventually learn to cope with these changes and even develop new mechanisms for survival.