July 24, 2015 – Speeding away from Pluto after its July 14 closest approach, the New Horizons spacecraft looked back and captured this spectacular image of Pluto’s atmosphere, backlit by the sun. The image reveals layers of haze that are several times higher than scientists predicted.
Just seven hours after closest approach, New Horizons aimed its Long Range Reconnaissance Imager (LORRI) back at Pluto, capturing sunlight streaming through the atmosphere and revealing hazes as high as 80 miles (130 kilometers) above Pluto’s surface. A preliminary analysis of the image shows two distinct layers of haze – one about 50 miles (80 kilometers) above the surface and the other at an altitude of about 30 miles (50 kilometers).
“My jaw was on the ground when I saw this first image of an alien atmosphere in the Kuiper Belt,” said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute (SwRI), Boulder, Colorado. “It reminds us that exploration brings us more than just incredible discoveries — it brings incredible beauty.”
Studying Pluto’s atmosphere provides clues as to what’s happening below.
“The hazes detected in this image are a key element in creating the complex hydrocarbon compounds that give Pluto’s surface its reddish hue,” said Michael Summers, a New Horizons co-investigator from George Mason University, Fairfax, Virginia.
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, but New Horizons detected hazes at up to 80 miles (130 kilometers).
“We’re going to need some new ideas to figure out what’s going on,” said Summers.
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. Credits: NASA/JHUAPL/SwRI