“condenses out directly onto an existing ice substrate.”
You mean like rime ice?
I have no feel how water acts at 7 millibars of atmospheric pressure, so do not have a clue.
Hmmm, thinking about it, rime ice forms on the carburetors of airplanes at extreme cold temperatures and low vacuums. You probably are right.
.......Bob
Being right is not a concern. The story must merely be plausible. :)