This image from the New Horizons spacecraft was our first look at Pluto’s atmosphere in infrared wavelengths. The planet is backlit with sunlight coming from above and behind. The image was captured captured just after the flyby on 14 July, 2015, while 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. No SUVs were detected during the flyby, so this is likely a natural process. Note that Pluto is moving further from the Sun for the next century or so (it’s orbit is highly eccentric), so it is experiencing global cooling.
The whitish patches around Pluto’s limb are from sunlight bouncing off more reflective or smoother areas on its surface.
Image Credit: NASA