The European Space Agency’s Rosetta mission detected solid carbon dioxide on Comet 67P/Churyumov-Gerasimenko (1)—the first solid CO2 to be found on a comet. Pressure-temperature phase diagrams (2) show the conditions for a substance to be a solid, liquid or vapour. Comet 67P is cold enough for CO2 to solidify, despite the low pressure.
At Earth’s atmospheric pressure (1 atm, 105 Pa), CO2 solidifies at −78°C without becoming liquid. The background image shows how when dry ice is warmed it sublimes to produce vapour. CO2 can only become liquid at pressures greater than about 5 atm. Beyond its critical point (73 atm, 31°C) CO2 is a supercritical fluid, which expands to fill a container but has a density typical of liquids.
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