There are quite a few that have been suggested. Including liquid nitrogen, supercritical CO2, liquid methane, ... Earth cells have polar molecules on the inside which dissolve in water, and insoluble lipids in the cell wall. Cells in liquid methane might be the other way around, soluble non polar chemicals inside.
If restricted to polar liquids, that makes the list of candidates much smaller. But still you have quite a few candidates, see Hypothetical types of biochemistry - non polar solvents (good wikipedia article)
Some astrobiologists argue that water is so good for life processes that extraterrestrial life almost certainly also uses water. But you can also argue that we know so much about the behaviour of life related organics in water because
We have quite a few habitats in our solar system that are potential places for alternative biochemistry which we could explore to see if any exotic life has evolved there or not. We have no way to answer this question from orbit or by geological missions, but have to send life detection instruments, able to detect some of the chemical imbalances, the redox reactions, and the way that life makes use of a restricted set of chemicals from the wide variety possible, and so on. Or if lucky, maybe observe obvious lifeforms e.g. multicellular or whatever. Anyway - we just haven't looked yet, lots of places in our own solar system where we could look for exotic biochemistry, but so far, no missions to go there to look for it.
Top places to look for really exotic life biochemistry which doesn't use water as a solvent - or to find out what kinds of chemistry you get if life doesn't evolve in these conditions - might include
So far we just don't have data about them. And they are all places we could organize a robotic spacecraft to visit right now in our own solar system if there was enough interest and funding to do it.
See also my science20 blog post: As Philae Awakes - Where Might Life And Proto Life Hide In Our Solar System?
If restricted to polar liquids, that makes the list of candidates much smaller. But still you have quite a few candidates, see Hypothetical types of biochemistry - non polar solvents (good wikipedia article)
Some astrobiologists argue that water is so good for life processes that extraterrestrial life almost certainly also uses water. But you can also argue that we know so much about the behaviour of life related organics in water because
- We have many examples from nature to guide our investigations of organic chemistry which we'd maybe never think of studying if we didn't have those examples
- We are most comfortable at the temperature ranges of liquid water, and our laboratories are at those temperatures. We need special facilities to explore extremely high or low temperatures (for us). Perhaps life in a liquid nitrogen or liquid methane or supercritical CO2 ocean might similarly argue that water and organics based chemistry is uninteresting because they know of very few chemicals and chemical reactions that life could use in those conditions because they are hard for them to reproduce.
We have quite a few habitats in our solar system that are potential places for alternative biochemistry which we could explore to see if any exotic life has evolved there or not. We have no way to answer this question from orbit or by geological missions, but have to send life detection instruments, able to detect some of the chemical imbalances, the redox reactions, and the way that life makes use of a restricted set of chemicals from the wide variety possible, and so on. Or if lucky, maybe observe obvious lifeforms e.g. multicellular or whatever. Anyway - we just haven't looked yet, lots of places in our own solar system where we could look for exotic biochemistry, but so far, no missions to go there to look for it.
Top places to look for really exotic life biochemistry which doesn't use water as a solvent - or to find out what kinds of chemistry you get if life doesn't evolve in these conditions - might include
- Titan - is there any form of life in its methane / ethane oceans?
- Io - is there by any chance some form of life that can live in liquid H2S mixed with liquid SO2?
- Triton - can there be life in liquid nitrogen?
- On our own Earth, it's now known that there are reservoirs of supercritical CO2 deep below the sea bed (above 31.1 degrees centigrade and 78 or so atmospheres, CO2 becomes a solvent for organics and has been proposed as a possible habitat for life, with properties of both a gas and a liquid - "supercritical"). But most will be so deep that the liquid CO2 is denser than water so doesn't bubble upwards so very hard to detect. Is there any chance that some form of life has evolved to take advantage of these conditions, using supercrtical CO2 as a solvent - which would rapidly kill normal Earth based life? See Alien Life Could Thrive on 'Supercritical' CO2 Instead of Water
- Ammonia is abundant in our solar system. In very cold conditions, then Ammonia acts as an antifreeze. And it is a polar molecule. So what about very cold but liquid ammonia / water mixtures (water - ammonia "eutectics"?) We might find conditions like that for instance below the surface of Titan - or elsewhere in the outer solar system where it is very cold but ammonia mixed with water is still liquid The Limits of Organic Life in Planetary Systems. A lifeform with ammonia mixed with water as its solvent inside the cell would be quite exotic :).
So far we just don't have data about them. And they are all places we could organize a robotic spacecraft to visit right now in our own solar system if there was enough interest and funding to do it.
See also my science20 blog post: As Philae Awakes - Where Might Life And Proto Life Hide In Our Solar System?
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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