Date: 26 OCT 1994 13:53:21 GMT From: Joerg Rhiemeier Newgroups: rec.games.frp.misc Subject: Re: Hard SF: Alien Atmospheres Well, monitoring this discussion of alien atmospheres, I have decided to come up with a list of substances which might -- or might not -- take the role of oxygen on alien planets. Many oxidants have been suggested in science fiction. On Earth, oxygen is created by photosynthesis. Other oxidants might come into play the same way. Photolysis of water wapour in the outer atmosphere creates oxygen, which in turn might create other oxidants. (On Earth, ferric oxide and sulfates were created by oxygen.) However, I think oxygen is still the likeliest choice. Each oxidant has a non-oxidant counterpart which it is formed from in photosynthesis. Photosynthesis is basically reduction of CO2 to organic carbon, and it needs a reducant(sp?). The correspondence is the following: Reducant Oxidant Water Oxygen Ammonia Nitrogen (*) Hydr.sulfide Sulfur (*) Nitrogen Nitric acid, nitrates Water Hydrogen peroxide Chlorides Chlorine Sulfur Sulfuric acid, sulfates (%) Ferrous oxide Ferric oxide (%) (*) No oxidizing properties. (%) Too weak an oxidant for fire. The ammonia/nitrogen and hydrogen sulfuride/sulfur cases are one-way reactions, there won't be nitrogen or sulfur `breathers'. All other may have something analogous to Earth oxygen breathing process, and therefore be circuit processes. The most common reducant is of course water, so oxygen is the likeliest oxidant. Note that none of these oxidants are likely to coexist with a reducing atmosphere. Besides the oxidant itself, you have the common redox-neutral components: nitrogen, carbon dioxide, water vapour, noble gases. Oxygen: We have this on Earth. Nitrogen: Everybody knows what it's like, too. Nitric acid: Liquid, forms acidic oceans and rivers. There will be nitric oxides and nitric acid vapour in the atmosphere. The sky is deep reddish-brown. Little light reaches the surface. Oxygen probably will also be present as a photolysis by-product. Metal oxides turned into nitrates which are highly water-soluble -- most metals will be in the oceans. Carbonates don't exist because nitric acid turns them to nitrates, releasing CO2. This CO2 could create a nasty, Venus-like greenhouse. If this doesn't happen, adapted lifeforms might exist. Fire is possible, and therefore technology. However, nitrates are likelier. Nitrates: Dissolved in water, don't affect planetary chemistry much, though atmosphere won't be reducing and will probably hold smaller quantities of nitric oxides and oxygen. Dry nitrates might occur in arid areas. These can be used to create fire. Technology might be possible. Hydrogen peroxide: Liquid, exists dissolved in water. There will also be oxygen, and some hydrogen peroxide vapour in the atmosphere rendering it poisonous to humans. Otherwise, not much weirdness as compared to Earth. Life, fire and technology possible. Chlorine: The sky is green, the oceans are acidic (HCl and HOCl). Metal oxides and carbonates turned into chlorides (mostly soluble). Life might evolve, fire and technology possible. Sulfuric acid: Liquid, strong acid, but weak oxidant. Acidic oceans, most metals dissolved as sulfates. (Not that heavy metal sulfates mostly dissolve well.) Life might exist, but H2SO4 won't be strong enough for fire. Sulfates are likelier. Sulfates: Most sulfates dissolve well in water (important exception: calcium sulfate). No strong effect on planetary chemistry. No fire possible. Ferric oxide: Solid. Weak oxidant, low influence on planetary chemistry. No fire possible. Sulfur: Some bacteria create this in photosynthesis instead of oxygen. No oxidizing properties. Everything else doesn't seem likely for miscellaneous reasons (rare, exotic elements involved, not stable enough, etc.) One of these losing candidates, which is still quite popular, is fluorine. This CANNOT occur naturally! The only way leading to it is electrolysis. Photosynthetic organisms can't produce it. It is strictly impossible. Liquid water immediately *bursts into flame*, as does organic material. Therefore, fluorine released somehow won't survive long.