“First to drop out was the carbon dioxide … Next there’s the nitrogen … On top of that … there’s the oxygen … Finally, at the very top, there’s a slick of liquid helium …”
– Fritz Leiber, A Pail of Air
When someone says “ice,” we think of water. So at the news that Pluto has ice caps and low density, I thought good ol’ frozen H20 and frozen methane. That was mainly because of comets, which are mostly water, carbon dioxide, ammonia, and methane, all frozen into a solid until they get close enough for the Sun to heat up and they boil off and form the tail. Since Pluto’s way out there, an average of about 34.5 Astronomical Units from the Sun (one AU is the average distance from the Earth to the Sun), and since it has a relatively smooth surface and low density, my first thought was a composition similar to a comet. But then it turned out that the ice caps are mostly made of methane and nitrogen, two gases here on Earth, but a solid out where Pluto reigns. Well, sort of solid: Pluto has a nitrogen atmosphere, though that might be “seasonal” in an orbit of almost 248 years, falling back to the ground much like the air does in Fritz Leiber’s science fiction story, A Pail of Air.
Hearing of nitrogen in the ice caps made me think of A Pail of Air, an apocalyptic yarn where Earth is snatched from its orbit by a burned out star and dragged far from the Sun, far beyond where Pluto orbits, into the space between the stars themselves. As the air cools, the gases freeze. At standard pressure, (air at sea level), Carbon dioxide freezes at -56.4º C (-69.5º F); nitrogen at -210.0º C (-346º F); oxygen at -218.4º C (-361.1º F), and helium liquifies at -268.9º C (-452.0º F). In the fictional world of A Pail of Air, the earth, without atmosphere or the heat of the Sun, is colder than -268.9º C. On the real Pluto, scientists think it’s a more balmy -235º C to -210º C (-391º F to -346º F). One of the things New Horizons will do is to take the planet’s temperature. Pluto never – or shouldn’t – get so cold there’s a skim of liquid helium coating things, assuming there’s enough handy.
“Wait a minute,” you may be saying, “if the freezing point of nitrogen is -210.0º C, and Pluto’s colder than -210.0º C, what’s it doing with a nitrogen atmosphere?” To which I can only say “good catch.” One reason the temperatures listed above are higher is because we’re surrounded by air pressure. Just as water boils at a lower temperatures where the air is thinner, so do other liquids. The second reason is that this is for the pure stuff and not a mix where you have to deal with things like partial pressure (I think – chemistry was a long time ago). This means the point where it turns into a liquid is lower when there’s no atmosphere, or not much of one to write home about. This, in turn, means lower freezing points out in space, which is a pretty good vacuum. On both the real world of Pluto and the fictional world of A Pail of Air, gases freeze at a lower temperature than they would at standard pressure.
Pluto’s still plenty cold; cold enough for nitrogen and methane to essentially form rocks. And that’s less than 40 AU out. The Oort Cloud, where comets are thought to come from, starts about 1,000 AU, where it’s a cool -263º C to -253º C. That’s -441.4º F to -423.4º F. Absolute Zero, which is so cold that molecules stop vibrating, is about -273.2º C (-459.76º F). That’s cold. But notice that interstellar space is about 10º to 20º C above Absolute Zero, and Pluto, at its coldest, is only about 38º C from as cold as it can get.
While Mars has carbon dioxide snow, what about Earth? Antarctica can get down to -94.7º C (-138.5º F), and carbon dioxide freezes at -56.4º C. Does that mean we have CO2 snow at the South Pole? Unfortunately, no. This gets back to partial pressure (maybe: it’s been decades since I’ve had chemistry). Carbon dioxide only makes up about 0.04% of gases in the air, which puts the freezing point of CO2 in the air at around -140º C (-220º F), so no, we don’t have carbon dioxide snow in Antarctica, which would really be cool.
Make that cold. Brrr.