Hydrogen sulfide replacing water?

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.

posted almost 8 years ago

HDE 226868‭

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