A ‘beating heart’ of frozen nitrogen on the surface of Pluto controls its winds and gives rise to the dwarf planet’s features.
New research indicates that Pluto’s nitrogen heart may control its atmospheric circulation and shape its landscape. The structure — known as Tombaugh Regio — nicknamed the ‘heart of Pluto’ due to its distinctive shape, was observed by NASA’s New Horizons mission in 2015, proving that the dwarf-planet is nor as barren as researchers had previously believed.
“Before New Horizons, everyone thought Pluto was going to be a netball — completely flat, almost no diversity,” Tanguy Bertrand, an astrophysicist and planetary scientist at NASA’s Ames Research Center in California. “But it’s completely different. It has a lot of different landscapes and we are trying to understand what’s going on there.”
Deciphering how Pluto’s atmosphere is regulated gives scientists more data to compare to Earth, and as such allows us to highlight similarities and differences between our atmosphere and those of other planets — even a dwarf planet much further away from the Sun.
Nitrogen gas comprises the vast majority of Pluto’s atmosphere, with trace elements of carbon monoxide and methane, whilst a heart-shaped patch of frozen nitrogen coats part of the dwarf planet's surface. The majority of this frozen nitrogen is found in the ‘left-lobe’ — a 1,000-kilometre ice sheet sat in a 3 km deep basin — Sputnik Planitia — as a result of this area’s low-elevation. The ‘right-lobe’ of Tombaugh Region has a higher elevation and contains nitrogen-rich glaciers extending into the basin.
During Pluto’s day, a thin layer of this nitrogen is heated and forms vapour. Conversely, at night, this vapour condenses, forming ice. Thus this day-night cycle of freezing and warming can be compared to a ‘heartbeat’ — but rather than pumping blood around a biological system, this heart pumps nitrogen winds across pluto.
The question of how this pumping of nitrogen across the dwarf planet affects its atmosphere is addressed by Bertrand and his co-authors in a new paper published in the Journal of Geophysical Research: Planets. The research suggests that the nitrogen cycle pushes Pluto’s atmosphere in the opposite direction to the dwarf planet’s spin. During this unique phenomenon — known as ‘retro-rotation’ — winds skimming Pluto’s surface transport heat, grains of ice and other particles creating dark wind streaks and plains across the dwarf planet’s north and northwestern areas.
“This highlights the fact that Pluto’s atmosphere and winds — even if the density of the atmosphere is very low — can impact the surface,” says Bertrand.
A Truly Icy Wind
The team’s aim was to determine just how the circulating atmosphere — thinner than Earth’s by a factor of 100,000 — could shape Pluto’s landscape. Extracting data from the New Horizon probe generated by its 2015 flyby of Pluto allowed them to model the dwarf planet’s topography and its blankets of nitrogen ice. Using this model, they could then simulate the nitrogen cycle, a weather model and infer how winds blow across the surface.
The movement of nitrogen as a result of its vapourisation in the north and freezing in the south creates a westerly wind 4 km above the surface. As Pluto’s wind blows westerly, it opposes the dwarf planet’s eastwardly rotation. A retro-rotation throughout most of its year. This makes its atmosphere unique in the solar system, with only Neptune’s moon Triton possibly sharing similar characteristics.
The team also discovered a strong current of rapidly moving air near to Pluto’s surface at the western boundary of the Sputnik Planitia basin. They believe this wind pattern is driven by nitrogen condensing in the atmosphere due to Sputnik Planitia’s high cliffs trap the cold air inside the basin. Here the wind becomes circulates and gains strength as it passes through the wer=stern region.
“It’s very much the kind of thing that’s due to the topography or specifics of the setting,” says Candice Hansen-Koharcheck, a planetary scientist with the Planetary Science Institute in Tucson, Arizona who wasn’t involved with the new study. “I’m impressed that Pluto’s models have advanced to the point that you can talk about regional weather.”
Bertrand believes that Sputnik Planitia could be as vital in regulating Pluto’s atmosphere as the oceans to climate on Earth: “If you remove Sputnik Planitia — if you remove the heart of Pluto — you won’t have the same circulation.
“Pluto has some mystery for everybody.”
Original research: T. Bertrand, F. Forget, O. White, et al, ‘Pluto’s beating heart regulates the atmospheric circulation: results from high resolution and multi-year numerical climate simulations,’ Journal of Geophysical Research: Planets, (2020), https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JE006120
Rob is freelance science journalist from the UK, specialising in physics, astronomy, cosmology, quantum mechanics and obscure comic books.
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