Scientists stitched together images from the Long Range Reconnaissance Imager (LORRI) from NASA's New Horizons spacecraft when it was about 120,000 miles (200,000 kilometers) away from Pluto. The resulting image of Pluto's receding crescent shows a spectacular blue "haze" in the dwarf planet's atmosphere:
Scientists believe the haze is a photochemical smog resulting from the action of sunlight on methane and other molecules in Pluto's atmosphere, producing a complex mixture of hydrocarbons such as acetylene and ethylene. These hydrocarbons accumulate into small haze particles, a fraction of a micrometer in size, which preferentially scatter blue sunlight – the same process that can make haze appear bluish on Earth.
As they settle down through the atmosphere, the haze particles form numerous intricate, horizontal layers, some extending for hundreds of miles around large portions of the limb of Pluto. The haze layers extend to altitudes of over 120 miles (200 kilometers). Pluto's circumference is 4,667 miles (7,466 kilometers).