Pluto weather forecast

May 4th, 2013

It looks as if when NASA's New Horizons probe arrives at Pluto in 2015 it's going to find weather that is both relatively simple and yet quite difficult to predict:

To establish context: Pluto, like Earth and Titan, has a nitrogen-rich atmosphere. It's a very thin atmosphere, its pressure measured in microbars. Earth's atmospheric pressure is, of course, about one bar. Titan's is 1.6 bars. Mars' is a hundred times more tenuous, less than 10 millibars. Pluto's is about a hundred times more tenuous again, less than 100 microbars. Which is really thin; but it's way thicker than the essentially airless exospheres at Mercury and the Moon. Pluto has plenty enough atmosphere for the world to have wind and weather and clouds, just like Venus and Earth and Mars and Titan.

Nitrogen in Pluto's air is in equilibrium with nitrogen frost or ice on the ground. Broadly speaking, when Pluto warms up, ice sublimates to gas, and the atmospheric pressure goes up. When Pluto cools, you get frost and a lower atmospheric pressure. Changing seasons remove ice from the summer pole, and may re-deposit it at the winter pole.

Emily Lakdawalla's post goes into much more detail about why it's so hard to predict what New Horizons will find, even taking into account what we know from probes to destinations elsewhere in the solar system. Which, as she notes, is exactly why it's necessary to send a spaceship out to Pluto – to tell us which theories are right and which are wrong (and in turn to fuel a couple of decades-worth of scientific papers figuring out whether the theories that gave the right answers did so for the right reasons.)

In the meantime, New Horizons will be heading on out to the Kuiper Belt, which promises to be interesting in an entirely different way.

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