Why does the atmosphere of Pluto have a blue tint?

A near-true-colour image of Pluto's bluish hazes taken by New Horizons. Credit: NASA/JHUAPL/SwRI, Source.

The New Horizons eclipse photo of Pluto shows the atmosphere as a blue ring, while a similar picture of Earth would show ours with a red hue. What causes the difference since they both have a nitrogen atmosphere?

Thank you for your question. It’s a good one! You are correct in that the primary constituent of both Pluto’s atmosphere and our own is nitrogen. For the Earth’s atmosphere, the dominant process affecting the color of the atmosphere is a process called Rayleigh scattering. This occurs when light scatters off of atoms and molecules in an atmosphere, and this process affects blue light a lot, but barely affects red light. This is why when you look at the Earth from space the atmosphere around the edge looks red. The blue light gets deflected out of the way, while the red light passes through unimpeded (this is also why the sky is blue and sunsets/sunrises look red).

A near-sunset view of Pluto's surface. Credits: NASA/JHUAPL/SwRI, Source

However in the case of Pluto, the atmosphere isn’t dense enough for Rayleigh scattering to really work. In the case of Pluto, the blue color comes from particles in the atmosphere called tholins. These particles make up the haze layers you may have heard about from the New Horizons images (see this New Horizons gallery for a really cool picture of these haze layers). We do not completely understand the nature of these particles or why they look blue, but stay tuned for new results from New Horizons! Here is a recent news article talking more about this.

I hope this answers your question. Please do not hesitate to contact me or the Ask and Astronomer page with follow up questions!

Dr. Adam McKay
UT Austin