January 15, 2016

Pluto’s Blue Haze

The sky on Pluto is blue! Kind of. This is Pluto in an Minute.

So it’s not exactly the case that the sky on Pluto is blue, rather, what the New Horizons science team has found in recent images downloaded from the spacecraft is that the haze around Pluto is blue, and they saw this in these gorgeous backlit images.

So what’s really going on in this image? Well, the haze particles themselves aren’t actually blue. The haze particles are probably grey and red, but they are very good at scattering blue light. It’s similar to what happens here on Earth, why we see our own sky as blue. On Earth sunlight scatters off tiny nitrogen molecules and gives the sky a bluish appearance. On Pluto, instead of nitrogen molecules, the scatterers are small haze particles likely made of tholins. These tholins form when ultraviolet sunlight breaks apart and ionizes the methane and nitrogen that makes up Pluto’s atmosphere. They react with one another to form complex macromolecules. These molecules then clump together and fall to the surface as reddish gunk—giving the planet its overall reddish hue.

So, of course, the question everyone wants to know now is: if we were to land humans on Pluto, would they see a blue sky? Well, not exactly, but the sky near the sun when it’s either rising or setting would look blue to astronauts on Pluto’s surface.

Credit: NASA

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