Is an earthlike planet with ammonia oceans even plausible?

Question

The idea of a planet where ammonia replaces water as the dominant liquid in the environment is quite popular in science fiction. I tried to develop a scenario where these kind of oceans are plausible from an astrophysical point of view. There seem to be several problems with the development of such an environment.

The big issue is water and how it interacts with ammonia. Any planet with ammonia will also have gotten water during its development as these ices tend to hang around together. Ammonia now acts as an antifreeze for water. While water would be solid at the temperatures one would expect to find liquid ammonia at, the antifreeze situation will lead to below freezing point water oceans saturated with ammonia. This is interesting and probably what is happening in the subsurface-oceans of several ice-moons, but it isn't and ammonia ocean.

So getting rid of the water is key. This is, however, harder than one might think. We cant get rid of it by removing hydrogen as we need it for the ammonia. Removing oxygen sounds like a good idea. This is however impractical as it is the third most abundant element in the universe. And to make matters worse most of what we call rock is oxygen.

The mass-abundance of the nine most abundant elements in the Earth's crust is approximately: oxygen 46%, silicon 28%, aluminum 8.2%, iron 5.6%, calcium 4.2%, sodium 2.5%, magnesium 2.4%, potassium 2.0%, and titanium 0.61%. Other elements occur at less than 0.15%.

This leaves me with carbon planets, which could be expected around white dwarfs and pulsars, or iron planets without a mantle. Both are bound to have their own, very interesting biochemistries and will probably be unsuitable for ammonia oceans. Gas-giants are also not an acceptable solution.

So how could a planet with ammonia oceans developed plausibly?

Answer 1

I see no particular reason why an iron world or a carbon world should be unsuited to ammonia oceans.

The big problem with an iron world is how you get one that has significant quantities of any volatiles at all. But if you posit, e.g., that shallow oceans were deposited by cometary bombardment after if formed, you're fine. Iron will rip the oxygen right out of water given enough time and good catalysts... and ammonia would speed up that process, as it's really good at solvating metal ions. Iron does not react readily with nitrogen or ammonia, and every bit of hydrogen liberated from water that the iron destroys is more hydrogen that is now free to bind with nitrogen and form more ammonia. What would be really cool is if you could get a layered ocean, with iron/nickel carbonyl on the bottom, and ammonia floating over it... but I have no idea if that would be chemically stable, or if the ammonia would react with carbonyls to form isocyanates instead.

With suitable time, heat, and/or catalysis, carbon will also rip the oxygen right off of iron oxide, as well as water. So, in the absence of ammonia, a carbon planet is going to end up with a lot of CO and CO2 in its atmosphere, along with methane, and possibly some formaldehyde--but probably more CO than CO2, since the whole point of a carbon planet is that the C/O ratio is tilted away from oxygen and towards carbon. With the addition of plenty of hydrogen to also react with nitrogen and form ammonia, all of whatever CO2 is still around will bond with ammonia to form solid ammonium carbamate, and you may end up with reactions of ammonia and CO to produce isocyanates--but once all of the available oxygen is locked up in those solids, excess carbon won't destroy ammonia like it will water. It will, however, provide plenty of geochemical opportunities to form hydrogen cyanide, aminomethylene, aminoacetonitrile, and full-on amino acids--so as long as there is enough ammonia to go around, to make sure all of the oxygen that isn't locked up in rocks gets locked up in complex carbon-nitrogen compounds, you'll have an ammonia ocean full of all kinds of good stuff for making life out of.

Answer 2

You don't need to remove the oxygen from the planet, just take it out of easy circulation. The oxygen trapped in rocks is actually exactly where you want your oxygen. Now, the question becomes how do you trap most of your atmospheric 02?

One possible solution is something like these oxygen absorbing crystals. https://www.sciencedaily.com/releases/2014/09/140930113254.htm

Now these particular crystals release the oxygen under heat or pressure, so they aren't quite right. They do however have high storage density, and the article talks about how the exact mixture of Cobalt and organic compounds changes the uptake and release properties. It isn't too much of a stretch to imagine a metal rich planet forming crystals like these that then sequester most of the oxygen before being buried.