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I was watching a video of what would happen if the earth got kicked out of the solar system and at one point it was mentioned that oxygen and nitrogen snow would form. So would such snow be any different physically to good ol' water snow? Would it be more fine or less dense? I'm asking this because I'm thinking of writing a story or something that uses that. Curiously, there seem to be almost no related results on the internet.

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    $\begingroup$ You may be interested in the short story "A Pail of Air", which has a very similar setting. $\endgroup$ Dec 5 '20 at 21:36
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    $\begingroup$ I guess our ordinary water snow may vary widely in that how fine or dense it is. Then again, a shape of individual snowflake is an interesting question in and by itself. $\endgroup$ Dec 5 '20 at 21:49
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    $\begingroup$ Have you ever discharged a CO2 fire extinguisher? How "different " would you say is that from regular snow? $\endgroup$
    – AndreKR
    Dec 6 '20 at 8:48
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    $\begingroup$ It is all physics - there aren't any chemical reactions going on here. Why is this question on this site? $\endgroup$ Dec 6 '20 at 15:17
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    $\begingroup$ @PeterMortensen Chemistry does bother with phase changes where there is no chemical reaction either. Similarly for the all the thermodynamics of ideal gas. Domains of interest are not sharp-cut, separated, boxed and labeled, but there are fuzzy boundaries, covered by interdisciplinary science. $\endgroup$
    – Poutnik
    Dec 7 '20 at 10:53
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I don't know if there would actually be oxygen and nitrogen snow. I would think under those circumstances, there might be carbon dioxide snow, but the rest of the atmosphere would probably liquify first, there may be oxygen and nitrogen rain with some hail or sleet, but snow? Not sure it would happen.

If it did, oxygen and nitrogen snow would be different from our normal water based snow because of the absence of hydrogen bonds.

Individual snowflakes hexagonal in shape would not form, as this is a function of hydrogen bonds and this would not happen with oxygen or nitrogen. Water ice is also less dense than liquid water because of the way hydrogen bonds cause "voids" in the solid structure. This would not be the case with oxygen and nitrogen. So you'd probably get something more like needles than snowflakes.

Also, solid oxygen is blue as noted by Ed V below, so blue snow if it happens at all.

Qualifier: If the earth is out there (beyond the orbit of Pluto) long enough, enough of the atmosphere would eventually freeze and the pressure may drop below the triple point of nitrogen. If that happens nitrogen would sublimate, then fall as snow as temperatures fluctuate. But even Pluto's surface temperature and pressure is above the triple point of oxygen most of the time (temperature does sometimes drop below though) so any oxygen in the atmosphere would probably fall as rain (blue rain) or possibly hail/sleet rather than snow.

Another thing to remember is that if the atmospheric pressure is that low, there won't be much drag/resistance either, so no "softly drifting" snow. Those needles will reach terminal velocity pretty damn fast and shatter on the ground.

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    $\begingroup$ I also thought most of the atmosphere would come down as rain rather than snow. However, depending on how the atmospheric pressure changes as it cools, it may be possible to go below the triple point temperature/presssure of nitrogen and oxygen, at which point any further cooling would trigger precipitation as snow. $\endgroup$ Dec 6 '20 at 1:35
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    $\begingroup$ “Solid, liquid, and compressed gaseous oxygen is blue.” Reference: First sentence of S.C Tsai, G.W. Robinson, “Why is Condensed Oxygen Blue?”, J. Chem. Phy., 51(8), 1969, 3559-3569. $\endgroup$
    – Ed V
    Dec 6 '20 at 2:19
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    $\begingroup$ For both oxygen and nitrogen the triple point pressure is practically a vacuum, so that won't really come into effect. The triple point temps are both under -200°C but not absolute zero so I suppose it's not entirely impossible, but rather unlikely $\endgroup$
    – Gwyn
    Dec 6 '20 at 2:30
  • $\begingroup$ Nitrogen's triple point is 63 K, ​13 kPa (on the order of 0.1 bar). That is a bit more than vacuum. I don't think there is any liquid nitrogen on Pluto - it either sublimates or snows. $\endgroup$ Dec 6 '20 at 15:16
  • $\begingroup$ @Peter Mortensen Sure. But compared to cutrently? Earth would need to lose a large chunk of its atmosphere to get to that. And again, sure, Pluto's surface temperature is below the triple point temp of nitrogen anyway, even if the high atmosphere isn't, and its atmospheric pressure maxes at only 1-2Pa so yeah, nitrogen snow could be a thing there. Not so much oxygen, since it doesn't really feature in Pluto's atmosphere. That doen't mean the same holds for earth. $\endgroup$
    – Gwyn
    Dec 6 '20 at 15:59

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