Date: 26 OCT 1994 13:53:21 GMT 
From: Joerg Rhiemeier <rhiemeir@ips.cs.tu-bs.de>
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.