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In a solar system much like ours, would there be the possibility of a planet of our size that would have lakes, rivers, oceans, etc. that would be composed of either a mixture of water and ammonia (plus various impurities like salt, other minerals, and - in our present day and age - pollutants) or just ammonia (again, plus various impurities) rather than just water (plus the impurities) as is the case here on Earth, and in which life would be having a water-ammonia mixture (or just ammonia) as a biochemical medium and as something to drink as opposed to pure water for earthlings?

If so, then would such worlds of water-ammonia (and certainly just ammonia) be more common in the colder end of the Goldilocks zone, given that water-ammonia and ammonia would both melt/freeze at much lower temperatures than pure water? (Or alternatively, with much higher pressures more in the heart of the Goldilocks zone or at its hotter end?)

How likely is it for an Earth-sized planet (not just the size of our own but also the size of a super-Earth - smaller than Neptune - or a mini-Earth - bigger than Mars) to have a water-ammonia mix (in which H2O>NH3) as opposed to pure water the way we do? Would such a H2O-NH3 mix be more likely indeed in the colder end of the Goldilocks zone, or could it be even in our own position in the Goldilocks zone or warmer?

Or is it less likely all the above and more likely that pure water would be present in planets with oxidized atmospheres while ammonia-water or pure ammonia would be present in planets with reducing atmospheres, no matter the planet's size or its position relative to its parent star?

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Just ammonia is not terribly likely, for the simple reason that water is so stinkin' common, which in turn is because oxygen is way more common than nitrogen. Just look at the moons of Jupiter and Saturn: there's oodles of water ice there, but no "ammonia moons".

So, if you want a pure-ammonia ocean, you have to come up with some excuse for why water was excluded, which is gonna be kinda tricky to manage. Just freezing it won't work, because water will dissolve into ammonia to form a eutectic mixture; getting it cold enough to freeze out all of the water means you'll freeze all of the ammonia, too!

A water-ammonia mixture, on the other hand, is perfectly plausible, and reasonably common in a certain type of SF. Hal Clement, for example, has several works which assume that life-bearing water-ammonia worlds are in fact more common than straight water worlds like ours, and humans are the odd ones out in the galactic community for that reason.

Ammonia worlds generally, and especially small ammonia worlds, would be more common around smaller, cooler stars, with less UV output. And yes, they would also be more common on the cooler end of the "Goldilocks zone"- especially since ammonia is a greenhouse gas, which shifts the whole zone outwards, and because (as noted in the question) ammonia-water mixtures have lower melting points and wider liquid ranges than pure water, or pure ammonia, so the "Goldilocks zone" for a water-ammonia planet is in fact wider, and extends farther into the cooler end, than for a plain water planet.

Warm ammonia worlds are, however, also still possible, especially around cooler stars with less UV output (in which case they'd need much smaller orbits than ours to end up hotter than Earth is despite the cooler star). And there are a lot of cooler, K-type stars out there! Eventually, though, even with UV-suppression, you run into the problem of hydrogen loss, which becomes a problem for retaining ammonia at lower temperatures than it does for water. Most warm ammonia worlds will thus probably need to be super-Earths, not mini-Earths, with higher escape velocities so that they can retain hydrogen in the upper atmosphere. So, yeah, ammonia worlds could occur in our position in the "Goldilocks zone", and even warmer, but it's easier to form them, and you can get a greater variety, out in the cold.

Meanwhile, the oxidation status of the planet absolutely also matters. A world with an otherwise oxidizing environment will lose its ammonia, and end up with a nitrogen atmosphere instead, while a world with a lot of free hydrogen will generate ammonia, removing hydrogen and nitrogen from the atmosphere. If the world is large enough to hold on to hydrogen against thermal Jeans escape, it'll probably remain reducing, and you can keep your ammonia-water mixture oceans. Life is unlikely to develop oxygen photosynthesis in that sort of situation, since it'll be a lot easier to just grab free, pre-reduced hydrogen from the environment. A world that cannot retain excess hydrogen, but which is cold enough to retain ammonia anyway due to an atmospheric cold-trap such as helps retain water on Earth could be threatened by oxygen photosynthesis... except oxygenic photosynthesis is also unlikely to develop there, since it's a lot easier to pull hydrogen off of ammonia than it is to pull it off of water! As a result, you are likely to end up nitrogenic photosynthesis, which would be a major extinction event like the development of oxygenic photosynthesis was on Earth, but for different reasons--it'll shift the balance between ammonia and atmospheric nitrogen, changing the water/ammonia ratio that organisms have to deal with, but not eliminating ammonia completely.

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  • $\begingroup$ Sounds like in practice, the likelihood of an ammonia-water world (in analogy to our plain water world) is the greatest when the planet in question is either a) a warm super-Earth where hydrogen is retained in large amounts or b) a cool/cold world of any size (from mini-Earth to super-Earth) where hydrogen does escape in large amounts but somehow nitrogen-based photosynthesis (rather than oxygen-based photosynthesis like here on Earth) develops. Is that about right? $\endgroup$ – user42533 Sep 19 '17 at 15:19
  • $\begingroup$ @user42533 It misses some details, but yeah, that's a reasonable summary. $\endgroup$ – Logan R. Kearsley Sep 19 '17 at 15:43
  • $\begingroup$ As a slight aside, it seems that all planets start out as having reducing atmospheres, and yet over time some planets (e.g. ours) evolve oxidized atmospheres while others retain reducing atmospheres one way or another? $\endgroup$ – user42533 Sep 19 '17 at 16:44
  • $\begingroup$ Another question: Would any water-ammonia world end up being a eutectic mixture, or would some such worlds be eutectic and others non-eutectic? $\endgroup$ – user42533 Sep 20 '17 at 12:58
  • $\begingroup$ @user42533 I'm not absolutely certain (there may be extra processes I'm not thinking about that would push it towards a eutectic), but I would not expect eutectic mixtures to be common at all. It would require either extreme luck, or a very precise, very thin temperature range that freezes out excess components leaving a liquid eutectic behind. Whether all planets start with reducing atmospheres kind of depends on when you start counting; when is planetary formation "finished"? What is definitely true is that all planets form out of an initially reducing medium (the protoplanetary disk). $\endgroup$ – Logan R. Kearsley Sep 20 '17 at 18:26
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There was more ammonia in our oceans prior to the great oxygenation event. Ammonia is reduced nitrogen. Earth's atmosphere was more reducing when it was young.

So, yes, you could have Earth with more ammonia in the water. This has actually happened. And yes, life lived there.

But oxygenation allows new chemistries and biochemistries (O2 is the strongest electron acceptor around, or close to it), and I believe you lose a lot of ammonia if oxygenic processes (like some forms of photosynthesis) evolve.

Your last paragraph is most relevant to my understanding of the chemistry. However, an absence of liquid water makes the presence of life challenging.

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  • $\begingroup$ I think I may have read that there are two theories as to what the atmosphere of the Earth consisted of (although that source is from some 25-30 years ago and may thus be out of date). One is that it consisted mainly of CO2, the way that the atmospheres of Venus and Mars still do. The other is that, as you say and from many sources (including relatively recent ones) that I've read, it was a reducing atmosphere rather than an oxidizing one. Seems to me that the latter theory is more valid than the former theory? $\endgroup$ – user42533 Sep 19 '17 at 15:11
  • $\begingroup$ I haven't read the primary literature on it. Reducing atmosphere is current dogma, in my understanding. $\endgroup$ – DPT Sep 19 '17 at 16:42

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