Saturday, March 21, 2015


Earth’s atmosphere has an equivalent thickness – the thickness if you compress the atmosphere to uniform pressure and density – of about 10 kilometers, or six miles. Compare this with the radius of Earth, 6,370 kilometers, and you see that the razor-thin thickness of Earth’s atmosphere is about 0.17% of its radius. Even if you consider the “outer limit” of Earth’s neutral atmosphere, what we call the exobase, that reaches about 600 kilometers altitude, the atmosphere’s equivalent thickness is only 10% of Earth’s radius—still very thin. So the volume of Earth’s atmosphere is tiny compared to Earth’s volume.

But now consider Pluto. Its atmosphere has a near-surface equivalent thickness of about 40 kilometers, which is almost 4% of its 1,200- kilometer (or so) radius. But the “outer limit” of Pluto’s atmosphere is very difficult to define, although we know that it is very far from the surface. If one defines it similar to the way we define the exobase of Earth’s atmosphere, then Pluto’s atmosphere has an outer limit of at least seven times Pluto’s radius above it surface. This means that the volume of Pluto’s atmosphere is over 350 times the volume of Pluto itself! This illustrates what a strange and wonderful new kind of world we are about to visit and explore.

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