MadSci Network: Astronomy

Re: Other worlds where H2S serves in place of H20?

Date: Tue Apr 13 19:00:36 1999
Posted By: Nick Hoffman, Oil and Gas Exploration Geophysics - Melbourne, Australia
Area of science: Astronomy
ID: 923851588.As

Hi Jackson,

I didn't see that programme - we don't get NOVA here in Australia :-)

I can guess what it said though, because I've been researching this sort of thing in my spare time recently. One thing to get clear is that either you or NOVA have something a bit wrong about how the Bacteria uses H2S. If you look at these sorts of bacteria (and algae like Cyanidium caldarium that have equivalent chemistry) you find that their bodies are just like "normal" organisms, with body fluids based on water. They aren't full of liquid H2S. What they do is USE H2S as an energy source, instead of photosynthesising or eating "normal" energy foods like sugars and fats, and oxidising them. What they do is take in H2S and combine it with other chemicals to make sulphates, plus energy in the form of hydrogen ions.

All organisms on Earth have a chemistry based on water. Some use different energy sources like sunlight, H2S, or eat other organisms and oxidise their structural components to get energy. If we look at early life on earth, it seems to have evolved originally as sulphate-reducing metabolisms, with photosynthesis coming later. To many organisms, and to some key parts of the chemical cycle of all cells, oxygen is a virulent poison! It might be possible on another planet for life to have evolved based on another liquid than water, but water is special. it is stable as a liquid over a wide temperature range, and takes lots of energy to freeze or boil. it is a powerful and diverse solvent (it even dissolves gold, in small amounts), and it is very common in the Universe. Hydrogen is the most abundant element in the Universe. Oxygen is the third, so H2O is probably the commonest molecule in the Universe. Sulphur has 1/40th the abundance as oxygen. It would be hard for life to evolve based on another liquid than water, but if it did, it would be weird.

See WebElements for more info on the Elements:

As to the main question: Are there other planets with oceans, ice, and rain of other chemicals than water? The answer is yes. At different distances from the sun, different chemicals will be near their "triple point" (the pressure at which liquid, solid, and vapour can coexist). On Earth, water is near its triple point everywhere, so we have water oceans and rivers, rain and snow, and polar icecaps, plus clouds and weather. On another, cooler planet a different volatile could play that role, like H2S or CO2. There are complications, though. When both water and another volatile are present at reasonable pressures, they form a mutual framework ice which locks the small gas molecules into cages of water ice. This is so stable in the case of H2S that it can persist to +30 Centigrade. On Earth, these GAS HYDRATES or CLATHRATES can be a problem in oil and gas producing wells and processing plants. There is a lot of information on these, for example AquaLibrium:

The effect of hydrates is to reduce the occurrence of liquids, so sometimes even if we expect unusual liquids, we just get clathrates instead. Some of the Jovian and Saturnian Ice Moons probably have clathrate ices at their surface. Planetary ices of CO, N2, and other low-temperature ices are thought to exist in the outer solar system - see Bill Arnett's excellent nine planets tour:

Most of the Moons have little atmosphere and a surface composed of solid, refractory non-volatile ices (all the more volatile stuff has boiled off to space). Titan, however, has a a significant atmosphere of 1.5 bars (standard atmospheres) of Nitrogen, plus some argon and methane. Titan MAY have Ethane/Methane clouds and rain, and rivers, lakes and oceans, but the evidence is not strong.

Finally, as a bit of speculation, I'm putting together the story of early Mars. Everyone says that early Mars must have been warmer and wetter because we see clear signs of water flowing on its surface. I disagree. I can explain everything on Mars by using liquid CO2 rivers and lakes in its youth and very strange cold avalanches/eruptions of gaseous CO2 for the later "outburst floods". These ideas are not yet generally accepted, but I'm convinced they will be. If you are interested in the idea, send me an email and I'll discuss it further.

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