There Are Winds Blowing On Pluto, Driven by Frozen Nitrogen

Earth and Pluto don’t have much in common. Earth is a vibrant, living world, whereas Pluto is cold, distant and lifeless. But one thing they do have in common is nitrogen. Earth’s atmosphere is about 78% nitrogen, and Pluto’s primary atmospheric constituent is also nitrogen, although the exact percentage is unclear.

On Pluto, where the surface temperature is about 42 Kelvin (-231 Celsius) most of that nitrogen is frozen. A new study says that Pluto’s frozen nitrogen drives the planet’s winds, and shapes its feature surfaces.

Prior to NASA’s New Horizons spacecraft arriving at Pluto, we didn’t know much about the planet or its surface features. When the spacecraft arrived in July 2015, we were all surprised to find that Pluto was a much more active place than we thought. It’s also when we first saw Tombaugh Regio, a large, lightly colored region on the surface of the planet.

The New Horizons team informally named Pluto’s heart-shaped feature “Tombaugh Regio” in honor of astronomer Clyde Tombaugh, who discovered the dwarf planet. The bright expanse of the western lobe of Pluto’s “heart” is informally called Sputnik Planum. Above left: Pluto’s surface sports a remarkable range of land-forms that have their own distinct colors, telling a complex geological and climatological story. Credit: Courtesy NASA / JHUAPL / SwRI

Tombaugh Regio is a very strange place, to human eyes anyway. It has two large lobes that make it look like a heart, and astronomers sometimes call it the “Heart of Pluto.” The western lobe is called Sputnik Planitia, and it features 6200 meter (20,000 ft) high mountains (Tenzing Montes, formerly Norgay Montes) made of water ice, and a vast plain covered in nitrogen ice.

This annotated image of the southern region of Sputnik Planitia illustrates its complexity, including the polygonal shapes of Pluto’s icy plains, its two mountain ranges, and a region where it appears that ancient, heavily-cratered terrain has been invaded by much newer icy deposits. The large crater highlighted in the image is about 30 miles (50 kilometers) wide, approximately the size of the greater Washington, DC area. Credits: NASA/JHUAPL/SwRI

A new paper says that the vast nitrogen deposit in Sputnik Planitia drives Pluto’s winds, and shapes the surface of the planet. The paper is titled “Pluto’s beating heart regulates the atmospheric circulation: results from high resolution and multi-year numerical climate simulations.” It’s published in the Journal of Geophysical Research. The lead author is Tanguy Bertrand, an astrophysicist and planetary scientist at NASA’s Ames Research Center.

“Pluto has some mystery for everybody.”

Tanguy Bertrand, Lead Author, Ames Research Center

Most of Pluto’s thin atmosphere is nitrogen, and there are also small amounts of carbon dioxide and methane. A vast amount of frozen nitrogen sits in Sputnik Planitia, and during the day, the temperature rises enough to sublimate it, turning it into vapor. At night, the process reverses, and the nitrogen freezes again, falling to the surface. Each time the cycle repeats, it acts like a pump, or a “heart-beat”, pumping winds of nitrogen around the planet.

That wind flows in the opposite direction of the planet’s rotation, and it may be responsible for unusual surface features on the planet. As the thin, nitrogen-rich wind blows along the surface, it transports heat, grains of ice and haze particles to create dark wind streaks and plains across the north and northwestern regions.

“This highlights the fact that Pluto’s atmosphere and winds – even if the density of the atmosphere is very low – can impact the surface,” said Tanguy Bertrand, an astrophysicist and planetary scientist at NASA’s Ames Research Center in California and the study’s lead author.

Hi Res mosaic of ‘Tombaugh Regio’ shows the heart-shaped region on Pluto and focuses on icy mountain ranges of ‘Norgay Montes’ and ice plains of ‘Sputnik Planum.’ The new mosaic combines highest resolution imagery captured by NASA’s New Horizons LORRI imager during history making closest approach flyby on July 14, 2015, draped over a wider, lower resolution view of Tombaugh Regio. Inset at left shows possible wind streaks. Inset at right shows global view of Pluto with location of huge heart-shaped region in context. Annotated with place names. Credit: NASA/JHUAPL/SWRI/ Marco Di Lorenzo/Ken Kremer/kenkremer.com

The Sputnik Planitia region, or the left lobe of Pluto’s heart, is lower elevation than the rest of the planet, and it harbors most of the nitrogen. Sputnik Planitia is a 1,000-kilometer (620-mile) ice sheet located in a 3-kilometer (1.9-mile) deep basin. The right lobe is mostly highlands and nitrogen glaciers.

The vast nitrogen ice plains of Pluto’s informally named Sputnik Planum – the western half of Pluto’s “heart”. Image Credit: NASA/JHUAPL/SwRI

“Before New Horizons, everyone thought Pluto was going to be a netball – completely flat, almost no diversity,” Bertrand said in a press release. “But it’s completely different. It has a lot of different landscapes and we are trying to understand what’s going on there.”

To describe Pluto’s atmosphere as thin is an understatement. It’s about 100,000 times thinner than Earth’s. So how does wind in an atmosphere that thin shape the landscape?

Bertrand’s team took data from the New Horizons flyby of Pluto, and then built a weather forecast model to simulate the nitrogen winds.

The team found that winds above 4 km (2.5 miles) blow to the west, which is in the opposite direction of Pluto’s spin. When frozen nitrogen in Tombaugh Regio sublimates into vapor in the north, then becomes ice again in the south, that movement triggers the westward winds. This situation is likely unique in our Solar System, with the possible exception of Triton, Neptune’s moon.

The researchers found another wind current, too. This one is a strong, fast moving wind close to the surface. It blows along the western edge of the Sputnik Planitia basin. There are similar wind patterns on Earth, that follow the contours of the landscape.

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 a near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. The smooth expanse of the informally named 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. Image Credit: By NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Public Domain

The wind is driven by the nitrogen vapor condensing back into ice, according to the study. The high-elevation cliffs of Sputnik Planitia traps cold air inside the basin. As it circulates there, it becomes stronger.

If Pluto’s nitrogen heart-beat is driving these winds, they might explain the wind streaks and dark plains west of Sputnik Planitia. If the winds bring enough heat to warm the surface, that could cause the streaks and plains. Or it could deposit particles of haze, which can darken and erode the ice. And if the wind blew in the opposite direction—meaning in the same direction as Pluto’s spin—the landscapes could be very different.

A digital elevation model (DEM) of Sputnik Planitia. Image Credit: Bertrand et al 2020
A simple geological map showing bright N2 ice plains (red), dark N2 ice plains (blue), mountains and hills lining the western rim of Sputnik Planitia (green), and bright pitted uplands of east Tombaugh Regio (cyan). Yellow line maps the continuous boundary between the bright and dark plains, as well as the northern boundary of the bright pitted uplands. Black box indicates the location of features in Sputnik Planitia interpreted as wind streaks, as mapped in purple in the inset. Blue box and white arrows indicates the location of dark troughs, possibly filled with dark materials. Image Credit: Bertrand et al 2020
A map of diurnal mean horizontal winds in Sputnik Planitia obtained with the researchers’ Global Climate Model for July 2015 at 1000 m above the surface. Yellow line replicates the bright/dark boundary in the above geological map. Image Credit: Bertrand et al 2020.

“Sputnik Planitia may be as important for Pluto’s climate as the ocean is for Earth’s climate,” Bertrand said. “If you remove Sputnik Planitia – if you remove the heart of Pluto – you won’t have the same circulation,” he added.

The most “famous feature” on Pluto is probably the bladed terrain. The bladed terrain are fields of skyscraper-sized, jagged land-forms made of primarily methane ice. They’re found at high altitudes near the equator. Could they be an artifact of Pluto’s beating nitrogen heart?

Pluto’s bladed terrain as seen from New Horizons during its July 2015 flyby. Credits: NASA/JHUAPL/SwRI

In their paper, the researchers say “… during periods of equatorial accumulation of CH4 (methane) ice, the retro-rotation and the injection of cold N2-rich air from Sputnik Planitia could transport and push gaseous CH4 westward, so that it favors the accumulation of CH4 ice at the westernmost longitudes (that is, east of Sputnik Planitia) leading to the formation of the Bladed Terrain there.”

They also say “… the ridges (”blades“) of the Bladed Terrain deposits display a dominant N-S orientation, which could also originate in part from this peculiar atmospheric circulation regime.”

For now, it seems uncertain if these nitrogen winds could cause the bladed terrain. But the team intends to try and find out. “In the future, we plan to further explore these ideas and investigate the processes leading to these longitudinal asymmetries and peculiar geological formations, by using high resolution long-term GCM simulations.”

In their conclusion, the authors say “Our work confirms that despite a frozen surface and a tenuous atmosphere, Pluto’s climate is remarkably active.” Much more active than anyone probably thought.

New Horizons was unable to enter orbit around Pluto. That’s difficult to do, and that was never its mission. NASA is considering a Pluto orbiter in the future, but in the meantime, everything we learned about the icy dwarf planet, we learned from a single fly-by. Even so, we learned enough to be intrigued, and to want to know more about this fascinating, mysterious world.

“Pluto has some mystery for everybody,” Bertrand said.

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Evan Gough

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