As NASA's New Horizons spacecraft departed Pluto last week, it turned its Long Range Reconnaissance Imager (LORRI) back at the dwarf planet.
The result is an image that may be the most breathtaking one taken during the entire pioneering mission so far.
Backlit by the sun, Pluto's thin atmosphere shines around the dwarf planet's edges, "like a luminous halo," as NASA put it. The image is displayed with north at the top of the frame.
"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. "It reminds us that exploration brings us more than just incredible discoveries -- it brings incredible beauty."
On a conference call with news media, Stern compared the images to the images of Earth taken by the Apollo Mission.
"You could only get this image by going to Pluto, crossing to the far side and looking back," Stern said.
This goodbye photo was taken around midnight ET on July 15, seven hours after the spacecraft made its closest approach to the dwarf planet. At the time the photo was taken, the spacecraft was about 1.25 million miles from Pluto. The image shows structures as small as 12 miles across.
The image, which was delivered on July 23, provided scientists with a surprise. NASA said it shows layers of haze that are as high as 80 miles above the planet's surface, which is "several times higher" than scientists initially predicted based on earlier information about Pluto's atmosphere.
According to a NASA press release, a preliminary analysis of the image shows two distinct layers of haze: one about 50 miles, or 80 kilometers, above the surface, and the other at an altitude of about 30 miles. The closeup of the hazes was taken by the LORRI camera when the spacecraft was about 225,000 miles from Pluto.
Scientists think the hazes discovered in Pluto's atmosphere help explain the reddish hues seen in images released last week.
“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.
According to NASA, the hazes may form when ultraviolet light from the sun breaks apart methane gas, which is known to reside in Pluto's atmosphere. "The breakdown of methane triggers the buildup of more complex hydrocarbon gases, such as ethylene and acetylene," the NASA press release states. Both of those hydrocarbons were 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. Ultraviolet sunlight chemically converts hazes into tholins, the dark hydrocarbons that color Pluto’s surface," NASA said.
Previously, scientists had thought that temperatures on Pluto would be too warm for hazes to form at altitudes higher than 20 miles above the dwarf planet's surface.