Pluto's heart holds key to ocean beneath icy surface

Pluto, once dismissed as stagnant and boring, is turning out to be as flamboyant as they come.

Two new studies buttress the idea that Pluto is coyly concealing an ocean beneath its icy shell. The same research also suggests the dwarf planet has a history of rolling over — literally. After being struck by a gargantuan celestial missile billions of years ago, Pluto spun south and east, researchers argue, and geologic features once nearer Pluto’s north pole moved toward its equator.

The movement would’ve been “quite dramatic,” says the University of Arizona’s James Keane, co-author of one of the studies. He estimates Pluto swung around 60 degrees, “like swapping North and South America.”

When the New Horizons spacecraft buzzed Pluto in 2015, it sent back exquisite images of a pale region shaped exactly like a Valentine’s Day heart. The heart’s left lobe turned out to be a low area, now called Sputnik Planitia, roughly two miles deep and 540 miles wide. This emblem of love was probably gouged out eons ago by an incoming projectile of ice and rock.

Oddly enough, the depression now sits almost exactly opposite the spot on Pluto that constantly faces Pluto’s biggest moon, “a striking coincidence,” Keane says. He and his colleagues decided to investigate. So did a team headed by Francis Nimmo of the University of California, Santa Cruz.

According to the scenario proposed by both groups in this week’s Nature, Sputnik Planitia once huddled closer to Pluto’s north pole than it does today. Then the planet rolled downward, carrying the depression some 300 miles southeast toward Pluto’s midsection, according to Nimmo’s calculations. If the Earth went though the same motion, Minneapolis would move to where Havana is now. Working with data collected by New Horizons, Keane’s group found an even bigger swing.

Both groups think such a dramatic swivel required a massive extra bulk at Sputnik Planitia. Extra mass there, whatever it is, would tend to make Pluto swing downward. The most likely culprit is a combination of ice and a buried ocean laced with ammonia, Nimmo’s team argues.

Pluto’s cracked surface bolsters that explanation. Keane’s team analyzed the patterns of long chasms cutting across the planet and concluded that the best explanation for the cracks is a combination of forces exerted by a swiveling Pluto, a subsurface ocean and an ice layer.

Other scientists said the paired studies provide powerful evidence for why Pluto looks as it does.

The new studies “make a strong case” that Pluto re-oriented, says Brandon Johnson of Brown University. “It seems really strange but it’s very common for planetary bodies,” among them the moon and Mars.

“It's very likely that Pluto has an ocean today,” says Amy Barr Mlinar of the Planetary Science Institute. “These papers give me more confidence in making that statement.”

The results raise the possibility that numerous dwarf planets harbor hidden seas. Pluto occupies a distant zone in the solar system known as the Kuiper Belt, which is full of worlds similar in size and density to Pluto, Nimmo notes. “It seems quite likely that if Pluto has an ocean,” he says, “then they should have oceans too.”


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