Just seven hours after closest approach, the spacecraft looked back and captured an incredible image of the dwarf planet’s atmosphere, backlit by the Sun. The image reveals layers of haze that are several times higher than scientists predicted. The spacecraft also found that the atmosphere of Pluto has an unexpectedly low surface pressure.
New Horizons’ LORRI instrument captured this view about seven hours after the spacecraft’s closest approach, at distance of about 225,000 miles (360,000 km) from Pluto. Inset: false-color image of hazes reveals a variety of structures, including two distinct layers. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
An analysis of the stunning snapshot of Pluto’s night side shows two distinct layers of haze – one about 50 miles (80 km) above the surface and the other at an altitude of about 30 miles (50 km).
“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 Dr Michael Summers of George Mason University, a New Horizons co-investigator.
“My jaw was on the ground when I saw this first image of an alien atmosphere in the Kuiper Belt. It reminds us that exploration brings us more than just incredible discoveries – it brings incredible beauty,” said Dr Alan Stern of the Southwest Research Institute, principal investigator for New Horizons.
Models suggest that the hazes form when UV sunlight breaks apart methane. The breakdown of this gas triggers the buildup of more complex hydrocarbon gases, such as ethylene and acetylene, which were also discovered at Pluto.
As these hydrocarbons fall to the lower, colder parts of the atmosphere, they condense as ice particles, forming the hazes. UV sunlight chemically converts hazes into tholins, the dark hydrocarbons that color Pluto’s surface.
“With New Horizons detecting hazes at up to 80 miles (130 km), we’re going to need some new ideas to figure out what’s going on,” Dr Summers said.
The atmosphere of Pluto rings its silhouette like a luminous halo in this image taken by New Horizons on July 15. This global portrait of the atmosphere was captured when the spacecraft was about 1.25 million miles (2 million km) from Pluto and shows structures as small as 12 miles across. The image, delivered to Earth on July 23, is displayed with north at the top of the frame. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
Measurements with the New Horizons’ REX radio experiment, made about one hour after closest approach to Pluto on July 14, reveal that the atmospheric surface pressure is about half the value previously inferred from Earth-based observations.
One explanation for the low pressure is that about half of the dwarf planet’s atmosphere may have recently frozen onto Pluto’s surface. If confirmed, it could indicate that further decreases in pressure may soon be in store.
“For the first time we have ground truth, measuring the surface pressure at Pluto, giving us an invaluable perspective on conditions at the surface of the planet. This crucial measurement may be telling us that Pluto is undergoing long-anticipated global change,” said Dr Ivan Linscott of Stanford University, a scientist for the New Horizons mission.
