Pluto's Icy Heart Controls Its Wind Circulation: NASA study

Pluto's famous heart-shaped structure containing frozen nitrogen controls its wind circulation, and may give rise to certain features on its surface, according to a NASA-led study. The structure, named Tombaugh Regio, became famous after NASA's New Horizons mission captured footage of the dwarf planet in 2015, and revealed it isn't the barren world scientists thought it was.

The study, published in the Journal of Geophysical Research: Planets, shows Pluto's renowned nitrogen heart rules its atmospheric circulation. Such findings can pinpoint both similar and distinctive features between Earth and a dwarf planet billions of miles away, the researchers said.

Nitrogen gas -- an element also found in air on the Earth -- comprises most of Pluto's thin atmosphere, along with small amounts of carbon monoxide, and the greenhouse gas methane. Frozen nitrogen also covers part of Pluto's surface in the shape of a heart. During the day, a thin layer of this nitrogen ice warms and turns into vapour. At night, the vapour condenses and once again forms ice.

Each sequence is like a heartbeat, pumping nitrogen winds around the dwarf planet, the researchers said. The latest study suggests this cycle pushes Pluto's atmosphere to circulate in the opposite direction of its spin -- a unique phenomenon called retro-rotation. As air whips close to 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 at NASA's Ames Research Center in the US. Most of Pluto's nitrogen ice is confined to Tombaugh Regio, according to the researchers. Its left "lobe" is a 1,000-kilometre ice sheet located in a 3-kilometer deep basin named Sputnik Planitia -- an area that holds most of the dwarf planet's nitrogen ice because of its low elevation, they said.

The heart's right "lobe" is comprised of highlands and nitrogen-rich glaciers that extend into the basin, the researchers said. "Before New Horizons, everyone thought Pluto was going to be a netball -- completely flat, almost no diversity," Bertrand said. "But it's completely different. It has a lot of different landscapes and we are trying to understand what's going on there," he said.

The researchers set out to determine how circulating air -- which is 100,000 times thinner than that of the Earth's -- might shape features on the surface. They pulled data from New Horizons' 2015 flyby to depict Pluto's topography and its blankets of nitrogen ice. The team simulated the nitrogen cycle with a weather forecast model, and assessed how winds blew across the surface.

The group discovered Pluto's winds above 4 kilometers blow to the west -- the opposite direction from the dwarf planet's eastern spin -- in a retro-rotation during most of its year. As nitrogen within Tombaugh Regio vaporises in the north and becomes ice in the south, its movement triggers westward winds, according to the study. The researchers also found a strong current of fast-moving, near-surface air along the western boundary of the Sputnik Planitia basin.

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The airflow is like wind patterns on the Earth, such as the Kuroshio along the eastern edge of Asia, they said. Atmospheric nitrogen condensing into ice drives this wind pattern, according to the new findings. The researchers said Sputnik Planitia's high cliffs trap the cold air inside the basin, where it circulates and becomes stronger as it passes through the western region.

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