Pluto may be a planet no longer, but it still has an atmosphere (unlike Mercury). This image, taken by infrared cameras aboard the New Horizons spacecraft on 14 July 2015, shows Pluto's atmosphere at a distance of 180,000 km. It is a true-colour image captured while Pluto was backlit by the Sun, with New Horizons in its shadow (known as occultation). The blue colour comes from sunlight scattering off particles in the haze — a smog consisting of mostly nitrogen gas and particles of mixed hydrocarbons such as acetylene and ethylene.

Given Pluto's distance from Earth, 7.5 billion km at its greatest, the atmosphere was not detected, even indirectly, until 1976. Its surface is mainly nitrogen ice, with frozen methane and carbon monoxide. Incoming cosmic rays vaporize the surface ices to replenish the atmosphere. As the gravity (0.66 m s−2) and atmospheric pressure (1 Pa) are weak, the gases escape at a rate of 1027–1028 molecules of nitrogen per second, or at least several hundred metres of surface material over the lifetime of the Solar System. New Horizons will be able to quantify the surface loss so we can gain a better understanding of Pluto's surface evolution.

Credit: NASA/JHUAPL/SWRI