Pluto’s frozen heart of nitrogen makes its atmosphere go backwards

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Pluto has a heart-shaped patch of frozen nitrogen, responsible for gusty winds that push in the opposite direction to the dwarf planet’s rotation, say NASA scientists.

As nitrogen within the heart-shaped ice region, called Tombaugh Regio, vaporises in the north and becomes ice in the south, its movement triggers western winds.

Pluto’s winds above 2.5 miles blow to the west – opposite to the dwarf planet’s spin – in a ‘retro-rotation’ during most of its year, which lasts a hefty 248 Earth years.

The team used data from NASA’s New Horizons spacecraft, which captured observations of Pluto back in 2015, including Tombaugh Regio, which lies just north of the equator.

No planet in our Solar System has this unusual retro-rotation, and if winds on Pluto pulled in a different direction, its dramatic landscape of icy valleys and mountains would look very different.

‘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. 

The findings pinpoint distinctions between Earth and this dwarf planet 3.7 billion miles away from the sun, whose status as a planet was downgraded in 2006.

Nitrogen gas comprises most of Pluto’s thin atmosphere, along with small amounts of carbon dioxide and methane.

During the plutonian day – which lasts six days, nine hours and 36 minutes in Earth time – a thin layer of this nitrogen warms and turns into vapour, and at night, the vapour condenses and once again forms ice.

Each sequence is like a ‘heartbeat’, pumping nitrogen winds around the planet.

As air whips close to the surface, it transports heat, ice grains and haze particles to create dark wind streaks and plains across the north and north-western regions.

Tombaugh Regio’s eastern ‘lobe’ is comprised of highlands and nitrogen-rich glaciers, while its western side is a 620-mile ice sheet located in a 1.9-mile-deep basin called Sputnik Planitia – which is key to this unusual retro-rotation.

‘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.’

The researchers found a strong current of fast-moving near-surface air along the western boundary of the Sputnik Planta basin was driven by atmospheric nitrogen condensing into ice.

Sputnik Plantitia’s high cliffs trap the cold air inside the basin, where it circulates and becomes stronger as it passes through the western region.

Wind patterns that come from Tombaugh Regio may explain why it hosts dark plains and wind streaks to the west of Sputnik Plantitia.

Winds could transport heat or could erode and darken the ice by transporting and depositing haze particles.

The team pulled data from New Horizons’ 2015 flyby to depict Pluto’s topography and its blankets of nitrogen ice.

They then simulated the nitrogen cycle with a weather forecast model to assess how winds blew across the surface.

‘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.’

The legendary New Horizons interplanetary space probe was launched in 2006 and flew 7,800 miles above the surface of Pluto, capturing detailed images of its surface.

The study has been published in Journal of Geophysical Research: Planets.

 

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