Temperature of Pluto

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These thermometers show different temperatures that you might be familiar with plus some you might not know. Click on each thermometer to find out more. The scale for the thermometers is on an absolute scale. The bottom corresponds to absolute zero (0 degrees Kelvin, -273 degrees Centigrade, or -469 degrees Farenheit). The top corresponds to 325 K, 50 C, and 120 F. Each tick is a change of 25 K, 25 C, and 45 F.

A - Phoenix, Arizona

This isn't the hottest place on earth but a good hot summer day in Phoenix can get to 120 F (49 C, 322 K) and is certainly plenty hot to make you very uncomfortable. Pluto is certainly much cooler than this but I wanted to show off something that just about everyone would agree is about as hot as people can stand (and not for very long at that). At this temperature, water is halfway between freezing and boiling. Some people even set their hot water heaters to this sort of temperature.

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B - Room temperature

At 70 F (21 C, 294 K), most people feel comfortable in a room or house heated (or cooled) to this temperature. 50 F lower than a hot summer day in Phoenix sounds like a big difference but the distinction on the thermometers is very small.

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C - Ice and Snow

This is the temperature at which water freezes, 32 F (0 C, 273 K). Again, this is a relatively small change in temperature from where we feel comfortable if you put it in relation to other colder places in the solar system.

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D - Dry Ice

Ever played with dry ice? This stuff is cold and you certainly don't want to touch the stuff with your hands. You can get frostbite very quickly in the process. At sea level, when you cool carbon dioxide down to -110 F (-79 C, 195 K) you will get dry ice. This is the stuff that makes up quite a bit of the polar caps on Mars. Now this is really cold compared to our comfort zone.

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E - Coldest spot on Earth

Did you know that -128 F (-89 C, 184 K) has been recorded somewhere in Antartica? This is even colder than the dry ice temperature! It's no wonder why traveling and working in Artartica is such a challenge. Now, you might ask (as did I) if there is carbon dioxide (dry ice) "snow" in places in Antartica. After all, the Martian polar caps have a great deal of dry ice in them. Well, it turns out that as the atmospheric pressure drops, the temperature at which dry ice is stable also drops. I don't know where the cold spot on Antartica was measured but I'd guess it was on a mountain top where the air is quite a bit thinner; probably thin enough to prevent CO2 from condensing.

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F - Liquid Nitrogen

At sea level, once you cool the air (which is mostly nitrogen) to -321 F (-196 C, 77 K) the air turns into a liquid. Many modern astronomical detectors are cooled with this croygenic liquid and the extreme cold is also used by the medical field to preserve biological samples. This is much, much colder than even the coldest spot on Antartica and yet it's still warmer than Pluto.

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G - Surface of Pluto

So now we come to Pluto. We're not exactly sure what the surface temperature is on Pluto but most scientists agree the number must be somewhere near -378 to -396 F (-228 to -238 C, 35 to 45 K). Some theories suggest that the atmosphere of Pluto must distribute the surface heat around such that all areas are the same temperature. Other theories would allow darker regions that did not have any nitrogen or methane frost to be warmer than the surrounding regions. The thermometer shows the range of temperatures estimated for Pluto and you can see that it is indeed a very cold place. So cold that water ice would act like rock and most gases have condensed out on the surface.

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Huan-Jim Ngo