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Methane snow visible on Pluto’s mountaintops; clouds seen in atmosphere

Cthulhu Regio snowcaps

Cthulhu Regio snowcaps. (Click to enlarge.) Image Credit: NASA/JHU-APL/SwRI

Individual clouds in Pluto’s atmosphere and bright regions of methane snow on the small world’s mountaintops in the area known as Cthulhu Regio are visible in the latest images and data sent back by NASA’s New Horizons spacecraft.

Cthulhu Regio is a large, dark area at the bottom left of Pluto’s encounter side, beginning to the west of Sputnik Planum and stretching almost halfway around the dwarf planet’s equator.

About 1,850 miles (3,000 kilometers) long and 450 miles (750 kilometers) wide, Cthulhu Regio is composed of a variety of terrains, including craters, fractured areas, smooth regions, and mountains.

While most of this area appears dark red, the tops of its highest mountains are covered with bright material.

An enhanced color image of this area with a resolution of 2,230 feet (680 meters) per pixel was created using compositional data taken by the Ralph/Multispectral Visible Imaging Camera (MVIC) approximately 45 minutes before the spacecraft’s closest approach to Pluto, from a distance of about 21,100 miles (33,900 kilometers).

The enhanced region is approximately 280 miles (450 kilometers) long and 140 miles (225 kilometers) wide.

Two enlarged versions of the region highlighted in the whole picture of Pluto that clearly outline the bright peaks visible among the mostly dark Cthulhu Regio were also released by the mission team.

Methane Snow on Pluto's Peaks

Methane snow on Pluto’s peaks. A chain of snow-capped mountains stretches across the dark expanse informally named Cthulhu Regio. (Click to enlarge.) Image Credit: NASA/JHU-APL/SwRI

They show a 260-mile (420-kilometer) long mountain range in the southeastern section of Cthulhu Regio, where individual peaks are separated by thin valleys and craters are visible.

The bright material on the mountain peaks, which stands out in contrast to dark plains of the region, is presumed by scientists to be mostly methane snow that condensed from the atmosphere onto the mountaintops.

The false color enlarged photo that depicts methane in purple shows a direct correlation between the bright, icy mountaintops and the presence of methane ice.

Scientists think the dark red color characterizing most of Cthulhu Regio is produced by tholins—complex molecules created through the interaction of sunlight with methane.

“That this material coats only the upper slopes of the peaks suggests methane ice may act like water in Earth’s atmosphere, condensing as frost at high altitude,” said New Horizons science team member John Stansberry of the Space Telescope Institute in Baltimore, Maryland.

Pluto’s atmosphere shows a similar degree of complexity. Individual clouds have been spotted in photos showing the planet’s layered haze.

New Horizons' near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto's horizon.

Just 15 minutes after its closest approach to Pluto on July 14, 2015, NASA’s New Horizons spacecraft looked back toward the Sun and captured this near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. The smooth expanse of the informally named icy plain Sputnik Planum (right) is flanked to the west (left) by rugged mountains up to 11,000 feet (3,500 meters) high, including the informally named Norgay Montes in the foreground and Hillary Montes on the skyline. To the right, east of Sputnik Planum, the rougher terrain is cut by apparent glaciers. The backlighting highlights more than a dozen layers of haze in Pluto’s tenuous but distended atmosphere. The image was taken from a distance of 11,000 miles (18,000 kilometers) to Pluto; the scene is 780 miles (1,250 kilometers) wide. (Click to enlarge.) Image & Caption Credit: NASA/JHU-APL/SwRI

The clouds were first detected last September, both on Pluto’s limb (edge) and hovering over its surface.

Mission team member Will Grundy of the Lowell Observatory in Flagstaff, Arizona, drew arrows to what appear to be clouds in one of the haze photographs.

“There’s a few fairly localized low-altitude features just above the limb that I’ve drawn lame arrows pointing to, but also a few bright cloud-like things that seem to be above and cutting across the topography in the circled area,” Grundy said in a message to mission team members analyzing the images.

John Spencer of the Southwest Research Institute in Boulder, Colorado, also a member of the New Horizons team, noted seeing a very bright haze at a low altitude over the southeastern region of Sputnik Planum—the bright left side of Pluto’s “heart”, an area covered in nitrogen ice—in one of the haze images.

He also pointed out a “discrete fuzzy cloud” visible against the sunlit terrain of an area on the right of the same image, known as Krun Macula.

The apparent clouds did not cast shadows on the surface, leading some scientists to question whether they were real. Complicating the discussion were questions about the difference between clouds and haze. Principal Investigator Alan Stern views clouds as discrete features and hazes as those more widespread.

Pluto’s clouds are likely composed primarily of nitrogen, along with small amounts of methane, acetylene, ethane, and ethylene. Their presence indicates the atmosphere is far more rich, complex, and diverse than scientists had previously believed.


Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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