The hazes in Pluto’s atmosphere, observed by NASA’s New Horizons spacecraft on July 14, provide a crucial link between the sunlight-driven chemistry in the upper atmosphere and the reddish-brown hydrocarbons called tholins that rain down and darken the surface. The animation shows several steps: 1) Ultraviolet sunlight breaks apart methane in Pluto’s upper atmosphere. 2) This leads to the buildup of complex hydrocarbons, such as ethylene and acetylene. 3) Clumps of these hydrocarbons condense as ice particles to form the hazes. 4) The hazes are chemically converted to tholins, which fall to the surface and darken Pluto.
Models suggest that the hazes form when ultraviolet sunlight breaks apart methane gas, a simple hydrocarbon known to reside throughout Pluto’s atmosphere. The breakdown of methane triggers the buildup of more complex hydrocarbon gases, such as ethylene and acetylene, which were also discovered at Pluto by New Horizons. As these hydrocarbons fall to the lower, colder parts of the atmosphere, they condense as ice particles, forming the hazes. Ultraviolent sunlight chemically converts hazes into tholins, the dark hydrocarbons that color Pluto’s surface.
Scientists had previously calculated that temperatures would be too warm for hazes to form at altitudes higher than 20 miles (30 kilometers) above Pluto’s surface. With New Horizons detecting hazes at up to 80 miles (130 kilometers), “We’re going to need some new ideas to figure out what’s going on,” said Michael Summers, a New Horizons co-investigator from George Mason University, Fairfax, Virginia.
