New Horizons team finds haze, flowing ice on Pluto

Just 10 days after closest approach, the distant world is showing a diversity of planetary geology that has mission scientists thrilled.
By and | Published: July 24, 2015 | Last updated on May 18, 2023
Backlit by the Sun, atmospheric haze rings Pluto’s silhouette like a luminous halo in this image taken by NASA’s New Horizons spacecraft around midnight EDT on July 15. This global portrait of the atmosphere was captured when the spacecraft was about 1.25 million miles (2 million kilometers) from Pluto and shows structures as small as 12 miles (19km) across. The image was delivered to Earth on July 23.

Flowing ice and a surprising extended haze are among the newest discoveries from NASA’s New Horizons mission, which reveal distant Pluto to be an icy world of wonders.

“We knew that a mission to Pluto would bring some surprises, and now — 10 days after closest approach — we can say that our expectation has been more than surpassed,” said John Grunsfeld, NASA’s associate administrator for the Science Mission Directorate. “With flowing ices, exotic surface chemistry, mountain ranges, and vast haze, Pluto is showing a diversity of planetary geology that is truly thrilling.”

Related gallery: Pluto up close

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 (80km) above the surface and the other at an altitude of about 30 miles (50km).

“My jaw was on the ground when I saw this first image of an alien atmosphere in the Kuiper Belt,” said Alan Stern, principal investigator for New Horizons at the Southwest Research Institute (SwRI) in 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 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.

“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, New Horizons co-investigator at George Mason University in Fairfax, Virginia.

Models suggest the hazes form when ultraviolet sunlight breaks up methane gas particles — a simple hydrocarbon in Pluto’s atmosphere. The breakdown of methane triggers the buildup of more complex hydrocarbon gases, such as ethylene and acetylene, which also were discovered in Pluto’s atmosphere by New Horizons. As these hydrocarbons fall to the lower, colder parts of the atmosphere, they condense into ice particles that create the hazes. Ultraviolent sunlight chemically converts hazes into tholins, the dark hydrocarbons that color Pluto’s surface.

Scientists previously had calculated temperatures would be too warm for hazes to form at altitudes higher than 20 miles (30km) above Pluto’s surface.

“We’re going to need some new ideas to figure out what’s going on,” said Summers.

In the northwestern region of Pluto’s Sputnik Planum, swirl-shaped patterns of light and dark suggest that a surface layer of exotic ices has flowed around obstacles and into depressions, much like glaciers on Earth. Click here for unannotated version.
The New Horizons mission also found in LORRI images evidence of exotic ices flowing across Pluto’s surface and revealing signs of recent geologic activity, something scientists hoped to find but didn’t expect.   

The new images show fascinating details within the Texas-sized plain, informally named Sputnik Planum, which lies within the western half of Pluto’s heart-shaped feature, known as Tombaugh Regio. There, a sheet of ice clearly appears to have flowed — and may still be flowing — in a manner similar to glaciers on Earth.

“We’ve only seen surfaces like this on active worlds like Earth and Mars,” said mission co-investigator John Spencer of SwRI. “I’m really smiling.”

The southern region of Pluto’s Sputnik Planum contains newly discovered ranges of mountains that have been informally named Hillary Montes (Hillary Mountains) and Norgay Montes (Norgay Montes) for Sir Edmund Hillary and Tenzing Norgay, the first two humans to reach the summit of Mount Everest in 1953. The large crater highlighted in the image is about 30 miles (50 kilometers) wide, approximately the size of the greater Washington, D.C. area. Click here for unannotated version.
Additionally, new compositional data from New Horizons’ Ralph instrument indicate the center of Sputnik Planum is rich in nitrogen, carbon monoxide, and methane ices.

“At Pluto’s temperatures of –390° Fahrenheit, these ices can flow like a glacier,” said Bill McKinnon, deputy leader of the New Horizons Geology, Geophysics and Imaging team at Washington University in St. Louis. “In the southernmost region of the heart, adjacent to the dark equatorial region, it appears that ancient, heavily cratered terrain [informally named Cthulhu Regio] has been invaded by much newer icy deposits.”

Simulated flyover of Pluto’s Sputnik Planum and Hillary Montes, created from New Horizons close approach images.

The newly discovered range of mountains rises one mile (1.6 kilometers) above the surrounding plains, similar to the height of the Appalachian Mountains in the United States. These peaks have been informally named Hillary Montes (Hillary Mountains) for Sir Edmund Hillary, who first summited Mount Everest with Tenzing Norgay in 1953.

“For many years, we referred to Pluto as the Everest of planetary exploration,” said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado. “It’s fitting that the two climbers who first summited Earth’s highest mountain, Edmund Hillary and Tenzing Norgay, now have their names on this new Everest.”

Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this enhanced color global view of Pluto. (The lower right edge of Pluto in this view currently lacks high-resolution color coverage.)
Meanwhile, New Horizons scientists are using enhanced color images to detect differences in the composition and texture of Pluto’s surface. When close-up images are combined with color data from the Ralph instrument, they paint a new and surprising portrait of Pluto in which a global pattern of zones vary by latitude. The darkest terrains appear at the equator, mid-tones are the norm at mid-latitudes, and a brighter icy expanse dominates the north polar region. The New Horizons science team is interpreting this pattern to be the result of seasonal transport of ices from equator to pole.

This pattern is dramatically interrupted by the bright “beating heart” of Pluto.

The “heart of the heart,” Sputnik Planum, is suggestive of a reservoir of ices. The two bluish-white “lobes” that extend to the southwest and northeast of the “heart” may represent exotic ices being transported away from Sputnik Planum.

The New Horizons mission will continue to send data stored in its onboard recorders back to Earth through late 2016. The spacecraft currently is 7.6 million miles (12.2 million km) beyond Pluto, healthy and flying deeper into the Kuiper Belt.