Liquid ammonia in space

Liquid ammonia in space

Liquids on planetary surfaces are interesting in themselves, but also
as possible media for life - possibly the only place where complex
life could arise. Only a few liquids are reasonable candidates to
exist on a planetary surface. One, we know, is water - it exists on
Earth, and is likely on any (sufficiently oxidised; see below) planet
with approximately the same temperature as Earth. The other that
exists in our solar system is liquid methane, on Titan. This is also
likely very common in the galaxy; and at still lower temperatures
liquid nitrogen is possible (which will contain substantial amounts of
methane and/or CO). [1] But perhaps more interesting are those
possible liquids that could exist at 'high' temperatures, let's say
-100C/-150F, which are more likely to possibly host life - though, if
extraterrestrial life is at all common in the galaxy, I wouldn't be
surprised to find methane-based life somewhere.

Liquid ammonia is the most commonly proposed alternative and I here
examine its possibilities for existence. What sort of planets might
have an ammonia ocean? The first problem is that water is far more
common than ammonia in space, less volatile, and much more stable to
photodecomposition, so one would expect any planet that had surface
ammonia to have it only as a solution in water. This would be normally
10-15% ammonia; it could be concentrated to the eutectic (33-35%) by
freezing but this would be locked under ice. To have ammonia on the
surface, then, it is necessary for ammonia to be able to exist on the
planet, but for water to not exist. The main condition affecting the
chemical composition of terrestrial planets is the C/O ratio, though
perhaps we should better refer to the effective C/O ratio as planetary
accretion is not homogeneous.

The stable form of C in a nebula at T700 K (at which the non-volatile
portion of terrestrial planets condenses) is CO, and then graphite. If
C/O1 (actually ~0.95 due to refractory metals taking some O), then
graphite will exist and condense due to its low volatility. The stable
form of C at low temperatures is always CH4, of course, but the
reaction between solid graphite and H2 is far too slow for conversion.
The solar nebula had a mean C/O~0.5, and for C/O~0.85 chemistry is
basically the same - there is enough excess oxygen to fully oxidise
all metals more electropositive than H, and thus form considerable H2O
also. At intermediate C/O ratios, roughly 0.85C/O0.95, the formation
of graphite is (at equilibrium) prevented but reduced minerals are
still present, and water does not form at high temperatures. Water and
ammonia form at lower temperatures at _any_ C/O ratio due to the
reactions between CO/N2 and hydrogen, which is always in excess as
long as the nebula exists. Thus we will always have water and ammonia
and methane in the solar system, capable of being delivered to an
accreting planet. The mineral assemblages formed under intermediate C/
O ratio do have an analogue in our solar system, this being the
enstatite chondrites. They are not wholly in equilibrium but they give
an idea of the solid phases that will form.

Now this chemistry will affect whether this water and ammonia will
survive on the planet. Under Earth-like chemistry, both water and
ammonia are stable; though ammonia can of course be lost to
photodecomposition as it has on the terrestrial planets. If reduced
minerals are present, water is not stable due to reaction with them.
If graphite is present (and it will be on the surface due to its
density), ammonia is not stable as it is dehydrogenated by graphite;
this will be rapid under mantle conditions. Thus we see that only at
intermediate C/O ratio can we have liquid ammonia, for then ammonia is
stable but water is not. The geochemistry of such a planet would be
somewhat familiar as it, like Earth, would be differentiated into a
silicate mantle and an iron core. Because of the highly reducing
conditions, nearly all Fe,Co,Ni and chalcophiles would partition into
the core; any of these elements in the crust and mantle would likely
be part of the 'late veneer' which may account for ~0.1% of a
terrestrial planet. Thus iron would be about a thousand times rarer
than on Earth; however, it would occur as the free metal (or rather an
alloy with Co and Ni). Sulphide minerals would be common but they
would be almost entirely of the active metals, not chalcophiles.
Indeed nearly all the alkalis, Ca, Sr, Ba, rare earths would be in
sulphides, along with enough Mg to use up the rest of the sulphur.
Nitrides of Ti and Si are also stable. The silicate phase of the
planet would be in the main similar to Earth's but rather limited
chemically, due to the depletion of active metals to sulphides (and
halides) and of transition metals to the core. [2]

The second problem with an ammonia ocean is photodecomposition.
Ammonia decomposes more readily than water, and the reverse reaction
is much slower due to the stability of N2. The thermal loss of
hydrogen to space makes this difference significant; on the gas
giants, where no H escapes, ammonia is in equilibrium. On the present
Earth the loss of water by this mechanism is held to an extremely low
value by a 'cold trap' mechanism where little water rises to the upper
atmosphere where it can be photolysed and thus lost. However, this
'cold trap' is relatively ineffective for ammonia as it more easily
decomposes in the lower atmosphere and does not rapidly re-form. Thus,
for ammonia to be retained on geological time-scales it is likely
necessary for the formation of ammonia from H2 to compete with its
escape, and since light of sufficient energy to cleave N2 is found
only at the top of the atmosphere, escape rates of H there must be
quite low. This could be achieved by a sufficiently massive planet
combined with low exospheric temperature, which is not unlikely in
reduced atmospheres around M-type suns. This formation of ammonia is
also necessary for another reason: the source of the NH3 is likely to
be volcanic emission (see below) and at the conditions in a volcano of
high temperature and low pressure ammonia is mainly decomposed and
thus must form in the atmosphere anyway.

The third problem with liquid ammonia, which turns out only to be a
problem under certain conditions, is methane accretion. Methane is
more stable than ammonia under all conditions; thus any ammonia
accreted is accompanied by a much larger (7-10x) amount of methane. If
a sufficient amount of ammonia were accreted to form oceans, hundreds
or thousands of bars of methane would accompany it. This methane could
not be lost without losing the ammonia also, and further, photolytic
loss of the methane would produce compounds that dehydrogenate
ammonia. However, if the methane were retained, the planet would
become in fact a miniature gas giant. These 'failed gas giants' may
actually exist in systems with C/O0.85. They would be easily
distinguished from true gas giants by their radius, and the very low
levels of H2 and He in their atmospheres. However, we know from
studying the history of the Earth that its initial atmosphere was lost
to space, and that its current atmosphere is almost entirely from
outgassing subsequently. Hence, the large amounts of methane discussed
above could be lost during this initial loss, and some ammonia would
be saved by being dissolved in the silicate mantle (the solubility of
NH3 in silicates should be nonpolar CH4, due to the formation of
silicamines). This is of course how Earth retained its water; and the
mechanism works if the loss of primary atmosphere happens after
accretion is largely complete, which is thought to be the case.

I have not considered the possible chemistry of life in liquid
ammonia, to avoid making this post longer than it already is.
Nevertheless, in pure liquid ammonia, it seems likely that rough
analogies to the important processes of terrestrial biochemistry can
exist, making ammonia-based life not so different. The situation, I
think, is actually worse for life in ammonia water, as the mixture is
quite corrosive and many of the molecules stable either in pure water
or in pure ammonia rapidly hydrolyse. It is interesting to
contemplate, if there were chemists on an ammonia-based world, the
developement of chemistry. For example, before electrolysis, how would
our hypothetical chemists obtain acids? There are no convenient acidic
minerals; I decided that to make any acid stronger than guanidine, the
practical solution would be ammonium chloride from volcanoes. In an
ammonia world, they would emit much higher levels of ammonia, due to
the high concentrations in the mantle, and about the same level of HCl
as on Earth. Of course, if this civilisation wanted to produce it on
an industrial scale, electrolysis would be used I think - NaCl
solution and a suitable membrane - as on Earth - would produce
chlorine and sodium solutions, which give strong acid and strong base
respectively.

Of course there are other possibilities than ammonia and water for
high-temperature liquids; however, I don't think I have enough
knowledge to predict their occurrence. The likely candidates seem to
be liquid CO2 and liquid sulphur. The former would require special
conditions to avoid its reacting with silicates, but seems to be a
possibility for life, though considerably different due to its non-
polarity. [3] Sulphur, though, does not seem to be suitable for life
due to its reactivity with organic substances. [4] Its presence would
require some level of oxidation, as primordial sulphur exists
exclusively as sulphide, but this can be provided by the familiar
method - loss of water to photolysis. This must be what happened on
Io, which has lost its water entirely.

Are any further liquids a possibility? I mentioned guanidine above -
how stable is that? I can almost imagine a dying ammonia world, now
tectonically dead, having finally lost its ammonia to photolysis, with
guanidine remaining behind ...

[1] Almost every cold planet or moon that can hold a sufficient
atmosphere should have liquid N2/CO/CH4 if the temperature is right.

[2] In fact, the only elements which would be concentrated in the O-
rich phase are Be, B, Mg, Al, Si, Ti, Zr, Hf. B and Ti are even
questionable as they may occur most frequently as the nitrides. All
other elements would be partitioned out.

[3] Membranes in liquid CO2 I believe could be made from amphiphiles,
but turned inside out - these would not have sufficient solubility in
the cold liquids (N2, CH4).

[4] Any alternatives to carbon-based chemistry would require lower
temperatures, not higher, due to lower stabilities. Sulphur melts at
113C/235F.

Andrew Usher