Ammonia World Formation & Biology
- From: "Logan Kearsley" <chrono.surfer@xxxxxxxxxxx>
- Date: Thu, 24 Nov 2005 05:56:33 GMT
I've been thinking about ammonia worlds. Specifically, how to get a nearly
pure ammonia ocean with minimal water, and then how things could live there.
The big problem seems to be that you not only need to get rid of water, you
need to make sure that there isn't any free oxygen laying around afterwards
to burn the ammonia, which means you need a highly reducing environment. The
best that I've come up with so far is as follows; alternative suggestions
are most welcome:
Start out something like Cannonball from _World-Building_ by Stephen
Gillett, but have the impact happen when there's still plenty of accretion
left to be done.
So, you've got a pretty normal-looking protoplanet shaping up, when all of
the sudden a gigantic impact blasts off most of the lithosphere, leaving the
iron core exposed. Further impacts plaster on a bunch of new volatiles and a
thin veneer of rock.
>From there, we've got N2, NH3, CO, CO2, CH4, and a bit of H2O in the
atmosphere, NH3 and H2O in the oceans, and Fe, C, Ni, and a bit of S in the
ground (plus all of that other rock-making stuff, but we'll ignore the
silicates and calcium and magnesium and aluminum and stuff for the moment).
Iron dissolves in the NH3/H2O solution and reacts with water to form
Fe(OH)3, locking away the oxygen and releasing excess hydrogen into the
atmosphere. The free hydrogen reacts with N2, C, S, and CO to produce H2CO
(formaldehyde), H2S, and more CH4 and NH3- so the lost water is essentially
replaced by more ammonia.
Some carbon dioxide also dissolves in the NH3/H2O solution and reacts to
form (NH4)2CO3 (ammonium carbonate), NH2CO2NH4 (ammonium carbamate), and
H2CO3 (carbonic acid). As water is destroyed by reaction with iron,
(NH4)2CO3 and H2CO3 decompose, but solid ammonium carbamate is left behind
in an equilibrium with liquid/gaseous NH3 and CO2. Some of it might end up
dehydrating to form (NH2)2CO (urea) and water, but the rest of it will form
a buffer for CO2, as an alternative to carbonate rocks, thus making the
carbonate/silicate cycle less important.
Now to consider consider biology...
I started working a lot of this out on my own, then came across ths:
http://groups.google.com/group/sci.space.policy/browse_frm/thread/39c8771cf2
a87012/061a90d7727f5c9f?lnk=st&q=C6H18N6&rnum=1&hl=en#061a90d7727f5c9f
which was very helpful on some matters, but not so much on others.
To start with, normal earthly photosynthesis has the net reaction
6CO2 + 6H2O -> C6H12O6 + 6O2
Which is endothermic. Burning glucose with the released oxygen gets the
energy back out again, to be used by animals and such.
If we try to do the exact same thing, but replace water with ammonia as the
hydrogen source, we get
6CO2 + 4NH3 -> C6H12O6 + 3O2 + 2N2
Which is also endothermic, but also still releases some oxygen into the air.
This is bad for an ammonia world, as oxygen will cause the ammonia to burn
and make water. This could be a track to transforming an ammonia world into
a regular water world. Or, since oxygen is toxic to primitive anaerobic
microbes anyway, and they would want some easy way to keep nasty oxygen out
of their way, maybe they combine the oxygen and nitrogen into N2O3, which
would also make it easier to get the nitrogens apart for respiration later
on.
But, as the post linked to above notes, glucose or other carbohydrates might
not be very good energy storage molecules for something using in ammonia as
a solvent. So, I've tried to come up with a few alternative mixed
oxygen/nitrogen and pure nitrogen carbon/hydrogen compounds. For all of
these, lacking any better data, I've just assumed a heat of formation
approximately equal to that of glucose. A dubious assumption, I know, but
I've not got anything better to go on at the moment.
The first one just replaces three carbons in glucose with three nitrogens
(C-N bonds instead of C-C bonds).
They could be arranged C-N-N-C-C-N or C-N-C-N-C-N, but it looks like putting
the bits together is easier with the latter structure.
3CO2 + 3NH3 -> C3N3H9O6
-1318.38 kJ/m -> -1273.02 kJ/m
Stil endothermic, but like making glucose out of CO2 and Methane, it doesn't
produce any waste products. You get the energy back out just by decomposing
it. Something using this energy molecule would have little need to breath.
The numbers of atoms aren't divisible by two, so this molecule would have to
be assembled in three sections, rather than glucose's two. There are three
different possible configurations of the sub-molecules which lead to three
slightly different arrangements of hydrogens and hydroxyls:
3[(CH2)(OH)-(NO)] -> (CH2)(OH)-N(OH)-(CH)(OH)-N(OH)-(CH)(OH)-NO
3[COOH-NH2] -> (NH2)-C(OH)2-NH-C(OH)2-NH-COOH (perhaps not the best one to
choose, as it results in an acid)
3[HCO-(NH)(OH)] -> (NH)(OH)-(CH)(OH)-N(OH)-(CH)(OH)-N(OH)-HCO
The next one only replaces two carbons with nitrogens. The numbers of atoms
are all disible by two, but not by three, so this molecule must be assembled
in three parts.
4CO2 + 4NH3 -> C4H10N2O6 + 2HNO
-1757.84 kJ/m -> -1073.86 kJ/m
Still endothermic, and it produces waste products. Very good. Something
using this molecule would breath (drink? don't know if it's liquid or gas at
the relevant temperatures) nitrosyl hydride and/or hyponitrous acid. Eh, the
hyponitrous acid might make this one difficult.
2[(CH2)(OH)-N(OH)-HCO] -> (CH2)(OH)-N(OH)-(CH)(OH)-(CH)(OH)-N(OH)-HCO
But what about eliminating oxygen entirely? In this case, I've just replaced
all of the oxygens in glucose with NHs, resulting in the formula C6H18N6.
First, I considered Cyanogen as the carbon source.
3(CN)2 + 6NH3 -> C6H18N6 + 3N2
33.36 kJ/m -> -1273.02 kJ/m
Very much exothermic. The equivalent of photosynthesis here would produce
energy, while respiration would consume it, which somewhat screws things up.
A possible solution is to use the energy generate by this reaction to power
the synthesis of hydrazine or diazine, which would serve as the actual
energy storage molecules, and conveniently avoids the problem of having to
split N2 (although there is less N2 to split than O2 in regular respiration,
which might make up for the higher activation energy required for each
molecule). On the other hand, it raises the issue of "why not get energy
back out by decomposing hydrazine into ammonia and free nitrogen, rather
than bothering with this whole respiration thing?"
Anyway, C6H18N6 could be assembled in either two or three parts, but the
fact that there are three cyanogen molecules needed at the start probably
makes it easier to assemble in three parts, rather than splitting the
cyanogen up into cyano radicals and distributing them to do a two-part
assembly. So:
3[(CH2)(NH2)-(CH)(NH)] -> (CH2)(NH2)-[(CH)(NH2)]4-(CH)(NH)
Next, I tried hydrogen cyanide as the carbon source:
6HCN + 4NH3 -> C6H18N6 + 2N2
629.24 kJ/m -> -1273.02 kJ/m
Even more exothermic, and produces less waste nitrogen. If the kinks can be
worked out of the hydrazine system, this might be a better process to use it
on. This process could make use of a two-part assembly scheme:
2[(CH2)(NH2)-(CH)(NH2)-(CH)(NH)] -> (CH2)(NH2)-[(CH)(NH2)]4-(CH)(NH)
So. Any thoughts? Corrections, suggestions, comments?
-l.
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