Pluto has blue skies and water on its surface, but don’t think about a vacation there quite yet.
Pluto’s blue sky as seen above is really more of a haze. On Earth the sky is blue due to light scattering off molecules such as nitrogen in the atmosphere, an effect known as Rayleigh scattering. Since blue light scatters more strongly than red, our sky takes on a bluish color. Pluto doesn’t have a thick atmosphere, but the atmosphere it has is filled with soot-like particulates known as tholins. These particles also scatter light, but by a different effect known as Mie scattering. While Rayleigh scattering tends to occur in all directions, Mie scattering varies with scattering angle. Longer wavelengths (reds) tend to scatter more uniformly, while shorter wavelengths (blues) tend to scatter at slight angles. When Pluto’s atmosphere is backlit, as in the image above, it’s mostly blue that we see.
While there’s water on Pluto, it’s frozen as ice. When earlier images of Pluto showed large mountains on the world, we knew that they were most likely water ice, since other ices likely to be found on Pluto aren’t stiff enough to support mountains. But simple images weren’t enough to rule out other possibilities, such as some kind of rock. Infrared spectroscopy from the Linear Etalon Imaging Spectral Array (LEISA) found regions where water ice is exposed on the surface, including the mountain region. What’s interesting is that most of the surface is not exposed water ice, which means it is covered with a layer of other materials. The fact that Pluto is covered with other “volatiles” is more evidence that Pluto is a dynamic world.
Of course these new observations haven’t told us what we didn’t already suspect. We figured tholins would make Pluto’s sky look blue, and we suspected Pluto’s mountains were ice. But with science a suspicion isn’t enough. As data from New Horizons continues to trickle in, we’ll continue to confirm suspicions and we’ll likely get a few more surprises.