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Would it be possible to have alien life use Hydrogen sulfide instead of Water? I assume they are still carbon based. Also for a sunlike star what distances should this planet or moon be located?

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  • $\begingroup$ Life isn't based on water. It's based on carbon $\endgroup$ – TrEs-2b Jul 6 '16 at 21:57
  • $\begingroup$ @TrEs-2b I think he means could hydrogen sulphide do waters job in a carbon based life form. $\endgroup$ – Bellerophon Jul 6 '16 at 22:02
  • $\begingroup$ @Bellerephon then he should word it that way $\endgroup$ – TrEs-2b Jul 6 '16 at 22:05
  • $\begingroup$ @TrEs-2b Yes, I agree. Think the edit 5mins ago made it clearer. $\endgroup$ – Bellerophon Jul 6 '16 at 22:06
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Yes, it's quite possible for hydrogen sulfide ($\text{H}_2\text{S}$) to replace water as the solvent for life. It already plays a major role in chemosynthesis in hydrothermal vents. The basic reaction is $$12\text{H}_2\text{S}+6\text{CO}_2\to\text{C}_6\text{H}_{12}\text{O}_6+6\text{H}_2\text{O}+12\text{S}$$ Interestingly enough, water is one of the products! However, we can't necessarily expect alien life with hydrogen sulfide as a solvent to use this reaction. Why? Check Table 8.4. Hydrogen sulfide is a liquid between 187.7 K and 212.5 K, which is absolutely frigid - especially compared to the extreme temperatures in and around hydrothermal vents. If we assume that the solvent must be a liquid - which I would think it must - then the environment these aliens live in will not be anything like a hydrothermal vent.

However, hydrogen sulfide has some drawbacks, as given in Life in the Universe: Expectations And Constraints:

  • The narrow temperature range in which it is liquid isn't good. By comparison, water is liquid in a range of 100 K. This page says

    Its narrow liquidity range (25 °C) means that it should be suitable, if at all, only for planets with heavy atmospheres and small daily temperature variations.

    This page, by the way, says that high pressures can keep hydrogen sulfide a liquid (which makes sense), so perhaps a larger temperature range is possible.

  • It fails to moderate temperatures well, given its low heat of fusion (roughly half that of water), heat of vaporization, and dielectric constant.
  • Its low dipole moment means that sometimes it's not an efficient solvent.
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Wikipedia is your friend. A quick google for "hydrogen sulfide based life" gave the Wikipedia article on Hypothetical types of biochemistry as a top hit.

Hydrogen sulfide is the closest chemical analog to water,[57] but is less polar and a weaker inorganic solvent.[58] Hydrogen sulfide is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface; and astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there.[59] On a planet with hydrogen-sulfide oceans the source of the hydrogen sulfide could come from volcanos, in which case it could be mixed in with a bit of hydrogen fluoride, which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live off of sulfur monoxide, which is analogous to oxygen (O2). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.

Thus, you would want your planet to be at a distance such that the -60 to -86C liquid range for hydrogen sulfide is reasonable. Mars' orbit might fit the bill.

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  • $\begingroup$ If the planet is Earth sized it would probably have liquid water on it's surface. Since Mars is really only that cold from the thin atmosphere not just distance from Sol, possibly a moon or planet like Io would work just larger. $\endgroup$ – Stephanie Jul 6 '16 at 22:20

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