H2 and CO2 ventilated outboard of the ISS, could become ice flakes in the extreme temperatures outside the ISS?

Question

Outside the ISS in the orbital night, the temperature can reach -250 degrees Fahrenheit (-157 degrees Celsius). The ECLSS (Environmental Control System and Life Support System), more specifically the OGA sub-system (Oxygen Generation Assembly) vent overboard hydrogen in the electrolysis process to produce oxygen from water, and the sub-system CDRA (Carbon Dioxide Removal Assembly) vent CO₂ overboard ISS, too. At orbital night temperature, or even in the penumbra / terminator area, could these gases (I'm not sure if the H₂ is maintained gaseous or liquid before it is about to be vented) become flakes of ice when ventilated outboard? Could CO₂ become flakes of dry ice?

(EDITED by the comment of a friend below): The solidification point of the hydrogen gas is close to -259° C. And the solidification point of carbon dioxide is -56 ° C. But I am not sure if other factors (pressure, vacuum...) would interfere with this process.

I found this question here also refers to H₂ and CO₂ vent outboard. Why vent CO₂ and H₂ waste products to space on ISS?

Ammonia is also ventilated in some cases, and this one I have no doubt that forms flakes of ice, as seen here (vented) and here (leak)

Just for reference, before part of hydrogen was used in a Sabatier reactor, but it was removed and returned to ground tests (2017) Why was the Sabatier system removed from the ISS USOS?

Answer 1

Summary: The solidification temperatures given for CO₂ or H₂ in the original post are for atmospheric pressure. At the very low pressure at the altitude of the ISS, both CO₂ and H₂ will be gases at equilibrium. However, depending on the details of how they are released, they may form solid particles (likely due to rapid depressurization and evaporative cooling rather than the "cold" of space) that eventually sublime.

You are correct that pressure matters. At the altitude of the ISS (408 km), the pressure is at most about $10^{-7}$ Pa NASA. The phase diagram for H₂ below shows that at $10^3$ Pa ($10^{-2}$ bar) H₂ is a gas at even 20 K. The solid-gas coexistence curve drops with a high slope below 10 K, but space is no colder than about 3 K and a gas with most of its field of view covered by nighttime Earth will likely be much warmer than this.

Graph from: DOI: 10.1007/s00114-004-0516-x

I haven't been able to find a phase diagram for carbon dioxide that shows the low pressure behavior in fine detail, but it should also sublime at the pressure at ISS altitude for any reasonable temperature. We can at least say that CO₂ likes being a solid more than H₂. In general, as the pressure decreases toward an idealized perfect vacuum, the gas state becomes more and more thermodynamically favored at any nonzero temperature. (Of course, for large covalent network solids or ionic solids (rock), it could take many lifetimes of the universe to evaporate. We are talking about molecular solids here, though.)

Frozen particles of CO₂ or H₂ gas may form initially due to rapid depressurization and evaporative cooling. @OrganicMarble pointed out this SE answer showing solid O₂ forming outside of a thruster. This apparent solid O₂ persists for at least a few minutes (note the times on the video screenshots). However, the shape of this formation changes. It's not clear how much it might be accumulating new material and how much it might be subliming. How long would it take to sublime fully? I'm not sure how to predict that and it would depend a lot on the size and shape of the solid particles and other conditions.

So, what timescale are you interested in? The particles can probably last a few minutes, but probably do not last years. I don't yet have enough evidence to make claims beyond that.