Ask A Scientist Environmental Earth Science Archive

name         Frank V.
status       educator
age          30s

Question -   I saw the answer to the Coldest Recorded Earth
Temperature at a chilly -128.6F (89.6C).  This is 10 degrees (C) colder
than the freezing temperature of Carbon Dioxide at atmospheric.
My question is:  "During Ice Ages, did the temperature at the poles ever
get cold enough to freeze CO2?"
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The vapor pressure of CO2 = 760 mmHg = 1 atm @ 194.7 K. At 184 K = -89.2 the
vapor pressure of CO2 = 300 mmHg = 0.4 atm  (approx.). So unless the partial
pressure of CO2 during the Ice Ages exceeded 300 mmHg = 0.4 atm
the CO2 would sublime. I doubt that the partial pressure of CO2 would be
that large.

Vince Calder
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Frank,

The freezing temperature of pure carbon dioxide at one atmosphere of pressure 
is -78.5 C (-109.3 F).  In the Earth's atmosphere, at sea level, carbon dioxide 
constitutes only about 0.0004 atmosphere of partial pressure.  Partial pressure 
refers to the amount of force (which is basically equivalent to weight in the 
Earth's atmosphere) that the gas exerts in air.  At such a low partial pressure, 
a temperature of less than about -140 C is needed for carbon dioxide gas to be 
converted to solid carbon dioxide.  At such low temperatures and one atmosphere 
pressure, carbon dioxide can not exist as a liquid, only as a solid.  Since the 
lowest recorded temperature on Earth is -89.2 C at Vostok Station, Antarctica, it 
can be safely said that carbon dioxide gas has not frozen out of the air anywhere 
on Earth in recorded history.  

Furthermore, since Vostolk is not at sea level, the atmospheric pressure there is
less than one atmosphere, resulting in less dense air, a lower partial pressure of 
carbon dioxide than at sea level, and thus an even lower temperature being required 
to convert carbon dioxide gas to a solid. 

The temperature being at "freezing", the temperature usually defined as resulting in 
a phase change from pure liquid to pure solid at one atmosphere of atmospheric 
pressure, does not imply that the solid phase of carbon dioxide will result at that 
temperature in the atmosphere.  The partial pressure of the gas must be high enough 
for "saturation" of the air to occur (where the air can no longer hold all of the 
element as gas without some changing to the liquid or solid phase).  At Vostok, 
Antarctica, the carbon dioxide content of the air is not high enough and the air 
temperature is not low enough for carbon dioxide gas to reach the saturation partial 
pressure (referred to as saturation vapor pressure for water); therefore carbon 
dioxide gas will not freeze out of the air.  

On Mars, the temperature is often low enough and the carbon dioxide content of the 
atmosphere is high enough (approximately 90% of the Mars atmosphere is composed of 
carbon dioxide gas) that carbon dioxide gas can "freeze out" of the atmosphere as a
solid, even though the surface atmospheric pressure is much lower than it is on the 
Earth.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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