Pluto’s atmosphere, such that it is, is predominantly comprised of methane, nitrogen and carbon monoxide which likely come from ice on the surface of the dwarf planet. The atmosphere changes in size as Pluto moves closer and further from the Sun. When Pluto is closer to the Sun in its elliptical orbit, the Sun’s heat causes the ice to evaporate and gases subsequently escape into space. This continues until Pluto moves far enough away from the Sun for the heat to fade; the ice begins to build up. Pluto’s orbit around the Sun takes 248 Earth years; its last approach to the Sun was in 1989.
NASA will be sending the New Horizons probe to Pluto in 2015. Researchers are trying to pinpoint the escape route of the gases in Pluto’s atmosphere to determine where the spacecraft should be looking. It has been difficult for scientists to work out the size of Pluto’s atmosphere due to debate about the best way to measure it. Its atmosphere is heated by infrared and ultraviolet light from the Sun; ultraviolet light is absorbed into the atmosphere closer to the planet. Farther away from the planet however the atmosphere is so thin that the ultraviolet light affects the molecules. For these reasons, researchers use ultraviolet heating models for the upper atmosphere.
The molecules that escape from Pluto’s atmosphere move through the thermosphere, which is where a lot of the ultraviolet light is absorbed in the atmosphere. This heating drives the escape process for the molecules. The exosphere is the top of Pluto’s atmosphere; this is where the atmosphere is so thin that collisions between particles do not happen as frequently as other sections of the atmosphere. The boundary between the thermosphere and the exosphere is called the exobase. Scientists are not sure where this boundary is for Pluto, and the mathematical models for each part of the atmosphere are different; this makes calculating the size of Pluto’s atmosphere problematic.
This new model includes the solar maximum (when Pluto is warmest) and the solar medium. The researchers also assumed a diameter for Pluto of 2,300 kilometres (1,429 miles), though the accepted measurements range by 100 km (62 miles).
The image is an artist’s impression of how the surface of Pluto might look. The image shows patches of pure methane on the surface.
Image credit: ESO/L. Calçada