How is it that Pluto’s smog looks prettier than our own? On both planets, smog is caused by a photochemical reaction. Sunlight acts on the chemicals in the atmosphere and creates by-products which we find in smog. On Earth, the sunlight reacts with nitrogen oxide and creates aldehydes, while on Pluto, sunlight reacts with methane to create acetylene and ethylene. These form hydrocarbon groups and scatter blue light, which is why we see a blue haze! 

Image credit: @colchrishadfield (Tumblr)