This image from the New Horizons spacecraft is our first look at Pluto’s atmosphere in infrared wavelengths. It was taken after the spacecraft was beyond Pluto’s orbit, so the planet is backlit with sunlight coming from above and behind. New Horizons was about 180,000 km beyond Pluto. The false color image codes wavelengths around 1.25 µm as blue, 2.5 µm as red, and intermediate wavelengths as green. North in this image is at roughly 10 o’clock.
The blue band is the result of sunlight being scattered by haze particles in the planet’s atmosphere, haze which is suspected of being photochemical smog caused by the action of sunlight on methane and other molecules. These form hydrocarbons such as acetylene and ethylene which accumulate into small particles. The µm-sized scatter sunlight giving the haze its blue tint. It looks blue in visible light too.
The whitish patches around Pluto’s limb are from sunlight bouncing off more reflective or smoother areas on its surface.
Image Credit: NASA