Why are there no CO2 snowfalls on Earth?

Question

The CO₂ phase diagram shows that at atmospheric pressure and about -78 °C temperature CO₂ becomes solid:

Wikipedia confirms this:

At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below $−78.5\ \mathrm{^\circ C}$ ($−109.3\ \mathrm{^\circ F}; 194.7 ~\mathrm{K}$) and the solid sublimes directly to a gas above $−78.5\ \mathrm{^\circ C}$.

The lowest temperature measured on Earth is about -90 °C (Vostok station). Does this not mean that CO₂ should fall as snow at the poles?

Answer 1

There is more to phase change than just what you see on that graph. At temperatures and pressures that support multiple states (e.g. solid and gas) you have to look at the saturation vapor pressure of the gas and the actual vapor pressure (partial pressure) of the gas -- the line on the graph represents equilibrium between the states. The differences between the two will tell you if you have net sublimation or net deposition and determine whether you end up with a solid or a gas. For solid CO₂ to form you need to have more deposition (transition from gas to solid) than sublimation. The transition to solid may also need help nucleating, though I'm not familiar enough with CO₂ to say for sure.

To summarize, in your example Vostok at -90 C and 1000 hPa you have a rate of CO₂ molecules depositing and a rate of CO₂ molecules sublimating. The partial pressure of CO₂ in the atmosphere is low enough that there is more sublimation than deposition, e.g. net sublimation. To deposit CO₂ ice on the ground and have it stay there you would need atmospheric CO₂ concentrations at least at saturation, which we do not have.

@Tristan provided some numbers in the comments that atmospheric CO₂ vapor pressures (partial pressure) are <1 mm Hg (or less than about 133 Pa). To get net sublimation you need to either reach saturation via increasing the vapor pressure (add more CO₂) or reducing saturation vapor pressure (lower the temperature). He provided an estimate that given CO₂ partial pressures temperatures approaching -140 C would be needed for net deposition and CO₂ snow.