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What I've found out is that Ammonia has (in the relevant temperature/pressure regime) cubic crystal structure. But what would that mean for ammonia snowflakes? Would they have fourfold symmetry? But then, you can also find threefold symmetries in a cube (just look at it from the corner), and cutting a cube in the right way you even get a hexagon.

Also, I don't know what causes water snowflakes to be planar rather than three-dimensional structures. That mechanism might well be special to water, so maybe ammonia snowflakes wouldn't be planar at all?

Or maybe ammonia wouldn't form snowflakes, but only solid crystals? That would mean that on an ammonia world, it would hail, but never snow.

So does anyone know what ammonia snowflakes would look like, or if they could exist at all?

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    $\begingroup$ wouldn't physics.SE get a better answer? $\endgroup$
    – ratchet freak
    Nov 15, 2014 at 18:28
  • $\begingroup$ FYI: I'd call this a bounty-worthy question. . . $\endgroup$
    – HDE 226868 ♦
    Nov 16, 2014 at 1:13
  • $\begingroup$ Dude, what's up with all the ammonia? :P $\endgroup$
    – Shokhet
    Nov 16, 2014 at 1:14
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    $\begingroup$ I think you need an answer by someone who knows a bit more crystallography for a full answer, but I believe "I don't know what causes water snowflakes to be planar rather than three-dimensional structures." is based on too much PR for the prettier forms of snowflakes. Snow comes in a variety of forms, and the iconic six-fold-symmetric flat crystals are just one type. Most of those have some variation in thickness affecting the pattern, too. $\endgroup$
    – Neil Slater
    Nov 16, 2014 at 17:38
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    $\begingroup$ @HDE226868: It would be whatever ammonia could be expected to fall from the sky on an ammonia-based world. So if you can make a good argument why that ammonia would not be pure, then also non-pure ammonia would be fine. BTW, thank you for putting a bounty on the question. $\endgroup$
    – celtschk
    Aug 22, 2015 at 6:10

3 Answers 3

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Expanding on Twelfth's answer:

It is still possible to see snowflake-like behaviour from ammonia in the right conditions.

In low/zero gravity situations, ammonia crystals would float down in a similar way to how water snowflakes fall down on Earth.

On the ISS when there was an ammonia leak, the astronauts described the ammonia crystals as "snowflakes".

Those "snowflakes" would probably not look any more impressive than carbon dioxide "snowflakes":

Carbon dioxide snowflakes

Source: http://www.exo.net/~pauld/Mars/4snowflakes/martiansnowflakes.html

Not very impressive snowflakes, huh?

Now, if we were talking about ammonia based substances rather than pure ammonia, ammonium chloride makes pretty nice snowflake-like crystals:

Ammonia Chloride crystals

Source: Wikipedia: Ammonium Chloride

They're not as nice looking or uniform as water snowflakes, but it's something.

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Gonna give a shot at the answer, though this might make a better comment...my university chem is a bit out of date.

The answer is I don't believe it's possible...one of the unique aspects of water is its ability to expand when freezing creating a less dense ice. This expanding while freezing is a key component to the snowflake structure you see. Additionally, the expansion of this snowflake adds to its surface to mass ratio, allowing it to 'float' seamlessly down to the surface.

NH3 (ammonia) forms into its crystalline state by seperating into alternating layers of NH4+ and NH2−...it would form by contracting creating small crystalline strcutures (could be a hexagonal structure given the way NH3 bonds). The result would be tiny pieces of salt-like crystals falling to the ground as ammonia hail (ice shards?) as opposed to what we would recognize as snow.

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I cannot pretend to give an authoritative answer, but the nature of snowflake formation should be kept in mind. The canonical snowflake shape (approximate hexagonal symmetry, flat plate, branching) is actually produced in only a small portion of the possible temperature/relative humidity phase space.

enter image description here

Without a very detailed analysis, and probably a great deal of experimentation, it's simply not at all clear how ammonia snowflakes would form.

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  • $\begingroup$ Yeah, you need props for digging out that graphic. I remember reading about this when looking at the set-up the one dude used to make his own perfect snowflakes one at a time for taking pictures. $\endgroup$
    – KalleMP
    Oct 31, 2020 at 8:53

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