8
$\begingroup$

Scientists strongly suspect that several moons in our solar system have frozen-over oceans of water-ammonia mixture. I've also read speculations on the possibility of surface water-ammonia oceans on exoplanets and exomoons. What I haven't seen a lot of is a discussion on how those oceans might behave differently than the water oceans of Earth.

More specifically, how would the surface water-ammonia ocean behavior differ from our water ocean behavior in terms of:

  • Ice(s) formation
  • Evaporation
  • Surface and underwater currents
  • Varying ratios of water-ammonia based on temperature or other factors

I'd like to know if this has been studied and if these behaviors have been described in papers, books, or in-depth articles on this topic. Especially anyting that goes into the specifics of the water/ammonia ratios.


I'll add some personal speculation here. While it is not part of the primary question stated above, it helps to explain the type of problem I'm trying to understand, so may be helpful to narrow down which resources may be most useful.

  • When subject to increasing levels of heat would the ammonia molecules in the ocean evaporate before the water molecules, causing warmer areas of the ocean to have an increased amount of H2O relative to NH3? (And the cooler areas like the poles having an increasing ratio of NH3 to H2O).
  • If the above was true, might the equatorial waters that have less NH3 facilitate the formation of water-ice? If so I could imagine exoplanets having water-ice-covered equators because they were warmer, and the cooler poles might be ice-free thanks to the greater amount of NH3 in the ocean.
  • Would the ice formed on the ocean be composed of a molecular mix matching that of the ocean (with H2O and NH3 in the same ratio as the ocean), or would some molecules freeze-out first, possibly causing different layers or areas of varying types of ices?
  • Might the poles of some water-ammonia planets have "continents" of water-ammonia ices that are frozen from ocean bed up to the surface (thanks to the fact that frozen ammonia is more dense than liquid, allowing for entire sections of ocean to freeze solid)?

If my speculation is correct (and I could be wildly wrong since I don't yet understand how a mix of water-ammonia actually behaves) then an exoplanet with an equatorial band of water-ice, mid-latitudes of liquid ocean, and solid polar-caps/continents of water/ammonia ices could be possible. The ammonia in the ocean would evaporate more quickly near the warmer equator, causing that area of a global ocean to have a greater ratio of H2O, and the ammonia precipitation and ocean might freeze at the poles, causing the build up of vast polar continents of ammonia/water ices. I would imagine the currents would function very differently than on earth, with surface and deep-level currents of varying ammonia-water ratios and temperatures. I I also see the possibility of frozen ocean beds in the more ammonia-heavy parts of an ocean, with the surface unfrozen.

Of course the ratio of ammonia to water will be very important. I'd love to see specific information on that!

Any informed clarification appreciated!

$\endgroup$
  • 1
    $\begingroup$ Have you researched the basic physical and chemical properties of water-ammonia mixtures? Things like viscosity, specific heat, melting & vaporization temperatures would be important. $\endgroup$ – Carl Witthoft Jul 16 at 18:32
  • 1
    $\begingroup$ Looking at the 1 atm water-ammonia phase diagram in the -80 to -100 C range changes in concentration may indeed cause melting/freezing. But I suspect the vapour pressure at these temperatures will be low. Warmer planets near 0 C may have interesting dynamics as pure water ice may become unstable in ammonia-water mixtures if the ammonia content goes up. $\endgroup$ – Anders Sandberg Jul 16 at 20:24
  • 1
    $\begingroup$ I think is too broad. Not only it requires knowledge of ammonia in water solutions / mixtures under various condition, but even modelling at planet size. $\endgroup$ – Alchimista Jul 17 at 10:41
  • 1
    $\begingroup$ @Alchimista I've lightly edited the question so that it clearly asks for information in existing research and publications, rather than for "specific individual knowledge" from users. Does this look better? $\endgroup$ – uhoh Jul 18 at 2:41
  • 2
    $\begingroup$ Nasty Question: duckduckgo.com/… I doubt there's a lot of grant money tied up in studies, but plenty of complicated graphs and equations: shodhganga.inflibnet.ac.in/bitstream/10603/37842/16/… We'll probably have to migrate to such a world be fore we get serious about understanding it. -Not a friendly environment. $\endgroup$ – Wayfaring Stranger Jul 19 at 1:13
3
$\begingroup$

It's a big question, but kind of a favorite subject of mine, thinking about exoplanets, so I can give a ballpark answer, and I invite anyone to give correction or give a more technical answer if they like.

Ice(s) formation

An ammonia-water ocean wouldn't be friendly towards ice formation because water ice would sink in the ammonia-water solution and Ammonia ice would sink in liquid ammonia. There's no range where the ice floats unless you remove nearly all of the ammonia and had it a high percentage of water.

That said, oceans are likely to have dissolved salts in them, especially on rocky worlds that have a very high rock to water ratio. A water world might have much less salty oceans, but, lets not get too sidetracked.

If there's enough dissolved salts or iron in the water-ammonia mix, then the density might be sufficient that water-ice could remain on top. Ice tends to form out of nearly pure water, with very little ocean salt, which increases it's buoyancy. It's possible, with enough saltiness, that ice could form in an ammonia-water ocean. It's also possible that ice would sink.

Density

I want to point this out now, because it's important. Using our oceans as a model, higher salt concentration sinks.

Salt concentration by density

enter image description here

That's kind of common sense and is likely to happen with a water-ammonia ocean as well. Higher ammonia concentration near the surface, higher water (and higher salt) concentration as you go lower. That's speculation on my part, but it seems reasonable.

Varying ratios of water-ammonia based on temperature or other factors

Higher water concentrations would sink and higher ammonia concentrations would rise and flow towards the lower. That's simple physics. Like on Earth, the variation of salinity of surface water isn't all that much. The same thing would likely happen with an ammonia-water ocean, the surface currents would try to equalize the different ratios. There would be some variation, but likely not as pronounced as the variation with depth. You're not likely to get a 90% water ocean in one region and a 40-60 ratio in another region because gravity abhors an imbalance like that. The 90% water region would be heavier and as a result, lower. You can have that kind of density imbalance with a solid crust, but not a flowing ocean.

The varying density of the ocean is important for another reason. On Earth, the salinity being relatively consistent across the ocean surface, the cold polar ocean is more dense and it sinks, and this is what's thought to drive the oceanic conveyer.

There's a range of uncertainty on whether an ammonia-water ocean would conveyer or not. You'd need a sufficient temperature and/or density variation, keeping in mind that if you begin with a ratio imbalance the ocean would work to correct it, so the planet needs an engine to maintain the conveyer. On Earth, that engine is the warm equator and cold poles. Different planets would have different temperature variations. Venus, for example, has practically no temperature variation at it's surface.

I looked, but couldn't find a table on ammonia-water solution by temperature. That would be a factor if anyone can find it.

Evaporation

Ammonia in water has a saturation point, based on temperature and pressure. If the ammonia-water solution exceeds that saturation point, say, on a hot day, then ammonia would bubble out of the ocean fairly quickly. Perhaps quickly enough to generate a weather system and trigger a density change, and in this case, warmer water sinking and a kind of reverse conveyer, driven by sinking water at the planet's warmest regions.

The phase transition, liquid to gas would remove heat from the water as well.

A planet with an ammonia-water ocean and a surface temperature that exceeds the saturation point could be quite dynamic.

If the planet is slightly colder and/or the ammonia percentage stays below the saturation point, then it's more of an equilibrium system. Water is a good solvent and it takes in many gases, such as CO2, in small amounts. Ammonia and Water, unlike CO2 and Water, mix very happily, so evaporation of an ammonia-water ocean below saturation wouldn't be that different to water on Earth, where evaporation is driven primarily by wind and sunlight. (contrary to logic temperature isn't as big factor in evaporation as direct sunlight). Wind is also a big player and on Earth, running water from rain or snow melt and transpiration from plants all return water to the atmosphere, which quickly rains or snows back to the surface. At any given time, Earth's percentage of water vapor in the atmosphere is about 1%, but it varies locally based on air temperature and relative humidity.

Ammonia would enter the atmosphere similar to water, but how long it would stay in the atmosphere is a factor. Air temperature and 100% relative humidity are the upper limits for how much water vapor can exist in the atmosphere. There are no such upper limits for Ammonia at Earthlike temperature and pressure because Ammonia, unlike water, is a gas at standard temperature and pressure.

As a result, a theoretical Earth-like planet with an ammonia-water ocean is also, very likely, going to have a lot of ammonia in the atmosphere, not the trace 1% of water that's in our atmosphere, but a lot more. You can't have an ammonia ocean without ammonia in the atmosphere at well, in high concentrations.

For example, to have ammonia raindrops at 1 ATM, the air temperature needs to be colder than -33 C because that's ammonia's boiling/liquification temperature. Unless the planet was really cold, a lot of the ammonia would just stay in the atmosphere, though, presumably the planet would have water-rain and that water rain would pull some of the ammonia back out and return it to the oceans. That's the equilibrium, and it's a complicated thing to try to calculate, but the question isn't just how fast would it evaporate, but also, how fast would it return and the return rate would depend on the method of return. Ammonia, being a gas at what we think of as normal planetary temperatures, means it would likely remain in the atmosphere as well, it wouldn't all go into the ocean. It would be some of both.

Perhaps over time, the ocean ratio and the rain-water/ammonia ratio would equalize, but again, that's just speculation.

If the planet has very cold poles, where the ammonia could rain out of the atmosphere, or perhaps tidal locking, and a colder night-side of the planet, then the ammonia could return from the atmosphere more quickly.

I'd like to know if this has been studied and if these behaviors have been described in papers, books, or in-depth articles on this topic.

I don't know of any papers. Theoretical planetary formation is fun to think about, but to do it right, requires some pretty serious computer modeling. What I've read on the subject is much less ambitious than your question. Models are done on water worlds and on tidally locked worlds and on plate tectonics for different mass planets. I've never seen a study on ammonia-water oceans, but I think it's a fun subject.

Earth had to have fairly abundant ammonia at some point because it's one of the primary "ices" in comets. Comets are mostly ammonia, water, CO2 and CH4, with smaller amounts of other elements. It's worth asking where Earth's ammonia went. It may be more chemically reactive than the other elements. If Earth lost it's ammonia, other planets may lose their ammonia as well in the process of planet formation and the chemistry that goes with it. It's also possible that ammonia-water oceans are common, perhaps more common than the pure water oceans like the ones we have on Earth.

Perhaps time and better telescopes will tell us more about this subject.

Take my answer with a grain of salt as I'm a hobbyist.

$\endgroup$
  • $\begingroup$ This is great! I think I'll have some follow-up questions for you if you're game. I've been doing my own research on the matter, and I have a few new insights I may be able to contribute. Also, FYI my interest in ammonia-water worlds skew a little more towards those that are cold(ish) (0C,-10C,-30C,-40C) than warmer, in part b/c it seems that NH3 oceans are more likely on worlds farther from their sun(s). At that distance "super-earth" hydrogen-dominated atmospheres are highly plausible, which means a "cold haber" bio-chemical process could generate ammonia from N2 and H2. Maybe a lot of it. $\endgroup$ – n_bandit Jul 21 at 22:29
  • $\begingroup$ @n_bandit Thanks. I do my best, though, my answer is more of an outline than a scientific answer. Try to follow the stack rules, such as, one question at a time and not too broad, or we can always move to chat for more of a back and forth discussion. $\endgroup$ – userLTK Jul 21 at 23:32
  • $\begingroup$ Are there not "mixed" ices. Crystals containing one or two water molecules for each ammonia molecule? They might form ices that could float. $\endgroup$ – Steve Linton Jul 22 at 7:43
  • $\begingroup$ @SteveLinton You're right that ammonia and water will freeze together (if it gets cold enough). I think I overlooked that completely. Water doesn't freeze out of a water-ammonia solution the way it freezes out of a salt-water solution or some (all?) water-alcohol solutions. That's why ammonia makes a good antifreeze. But - my chemistry should probably be checked. $\endgroup$ – userLTK Jul 23 at 4:27
  • 1
    $\begingroup$ @userLTK I found a diagram in a paper recently going over ice formation at various ammonia concentrations, temperatures, and pressures. If I recall correctly as temperature is lowered water will gradually freeze out of the ammonia as pure water-ice until, reaching ammonia's freezing-point, only ammonia is left. Then ammonia starts to freeze, reacting with the water-ice to form some new type of ice. The ices and temperatures were different when you increased the pressure. I'll need to dredge up the diagram and share it here. $\endgroup$ – n_bandit Jul 24 at 16:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.