Pluto has been Melting!

Sure, astronomers might not call it a planet anymore, but every schoolchild knows how badass Pluto really is. It’s got a giant moon, Charon, and two smaller ones, Hydra and Nix.

In addition to being colder than ice with an average temperature of 44 Kelvin (that’s colder than liquid nitrogen), I’m here to bring you the news that despite the fact that it’s so cold and so far from the Sun, Pluto has been melting!

Since its discovery by Clyde Tombaugh in 1930, astronomers have been fascinated with Pluto, making very careful observations of it, trying to learn more about this bizarre, icy sphere so far-flung from the Sun. Recently, a team of scientists led by Bradley Schaefer took a look at 75 years of accumulated data on Pluto, and found something incredible.

You see, much like our Moon, Pluto has one hemisphere darker than the other. For our Moon, the side that faces us is darker than the side that faces away:

Astronomers call the difference between the brightness (or reflectivity) of a planet albedo. Pluto’s northern hemisphere has a darker albedo than its southern hemisphere, and that explains why Pluto appeared to get darker from 1954 to the 1980s.

But from 1933 to 1954, we saw the same latitudes of Pluto. Why, then, did it appear to get darker over those times as well? Some portion of Pluto’s poles, just like Earth’s poles, receive sunlight constantly for months (or in Pluto’s case, possibly years) at a time. They conclude that there must have been a real albedo change by about 5% over Pluto’s southern hemisphere over those 21 years.

And their best explanation is that since Pluto has a very thin atmosphere (made of Nitrogen, just like ours), when one of the poles is continuously exposed to the Sun, the light and heat causes the frost on the poles to sublimate (or boil off directly from the solid phase). Because frost has a different reflectivity than the rest of the reddish Plutonian crust (which is mostly solid methane), that causes the observed change in albedo. And hence, Pluto is melting!

Now, where will the Plutonians go to live? And will we have to wait until the New Horizons mission reaches Pluto in 2015 to find out?



  1. #1 Richard Helmich
    May 15, 2008

    Great information about Pluto. You’re very good at writing about something and make it fun to read!

    Melting and sublimation are different changes, however. Melting is a phase change from solid to liquid, while sublimation is solid to gas and so much more awesome.

    I’m curious, when the ice at the poles sublimated was Pluto’s gravity strong enough to hold the atmosphere to the surface or was it lost to outerspace?


  2. #2 ethan
    May 15, 2008

    Thanks for clarifying the melting/sublimation details. People actually argue whether it’s nitrogen, methane, or water-ice that forms the bulk of Pluto’s crust, and sublimates.
    Although it does get lost to space, it isn’t clear how quickly that happens vs. the rate of creation of the atmosphere. It appears to depend on Pluto’s distance from the Sun, which varies greatly. When it’s closer, it makes atmosphere faster but also loses it faster, and when it’s farther, vice versa.

  3. #3 giovanna
    August 5, 2008


  4. #4 and
    March 31, 2009

    thiz stufF iz da bom

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