I read that without the Sun's heat, the Earth's atmosphere would precipitate out into oxygen and nitrogen snow. What colour would this be?
The short answer: Solid oxygen is clear with a very pale sky-blue color, and solid nitrogen is clear and colorless. If our atmosphere froze solid and turned into snow, it would probably look much like normal snow, since water ice is clear and colorless or light blue. Though over time the color would slowly change as the ice is bombarded by micrometeorites and the solar wind, assuming that the atmosphere has condensed on to the surface.
The long answer: Now we know what color the atmosphere would be, but let's find out more. How much snow would there be? The earth's atmosphere has a mass of about 5000 trillion metric tons (this can be estimated using atmospheric pressure, newton's law Force = Pressure / Area = Mass x Acceleration, and the Earth's gravitational acceleration and the surface area). If we assume that the "snow" formed when the atmosphere freezes has a density comparable to freshly fallen water snow (about 100 kg/m3), then we can find the depth of the frozen atmosphere.
Depth of "Snow" = Atmosphere's Mass / Density of Snow /Surface Area
Using numbers that I looked up online, and making sure all the units work out, I calculate about 100 meters of frozen atmosphere will snow out onto the surface.
Thankfully, the Sun will keep shining for another 5 billion years or so. Some places in the solar system still have to worry about their atmospheres freezing solid, though. Triton, a moon of Neptune may have wispy clouds of frozen Nitrogen in its tenuous atmosphere, while Pluto has an atmosphere primarily of methane that condenses on to its surface during its "winter" months when it moves further from the Sun. These frigid worlds are so distant that the Sun looks like a bright star in their skies.
This page was last updated on June 27, 2015.