Runaway cooling piled weight on Pluto’s icy heart: study

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Just 15 minutes after its closest approach to Pluto on 14 July 2015, NASA’s New Horizons spacecraft looked back toward the sun and captured this near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. The smooth expanse of the informally named icy plain Sputnik Planitia (right) is flanked to the west (left) by rugged mountains up to 3.5 kilometres high.

How did Pluto get its heavy icy heart? A new theory explains how the vast plain Sputnik Planitia came to be – and carved out the bowl in which it sits today, much like a couch potato who’s spent too much time on the sofa.

Planetary scientists in the US suggest the smooth icy plain, being brighter than its surrounds, reflected sunlight. This meant it stayed cooler and accumulated ice, and as it stacked on the weight it pressed an indent into the dwarf planet’s crust.

The work was published in Nature.

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Since the New Horizons spacecraft flew past the rocky, icy dwarf planet in July last year, planetary scientists have debated the driver behind Sputnik Planitia, a 1,000-kilometre-wide basin filled with frozen nitrogen in the heart-shaped region informally known as Tombaugh Regio (after Pluto’s discoverer, Clyde Tombaugh).

There’s so much nitrogen ice in Sputnik Planitia that it’s tidally locked to Pluto’s largest moon, Charon.

Some think an asteroid punched a crater into Pluto’s crust, which then filled with ice from a slushy underground ocean or snow from above.

The University of Maryland’s Douglas Hamilton and his colleagues propose a different route. 

Their theory is based on the so-called runaway albedo effect. A patch of ice, brighter than its surrounds, reflects more sunlight. 

This means the local area is chillier – and accumulates more ice. More ice means a bigger area reflects sunlight, cooling it further, and attracting even more ice.

In the paper, Hamilton and colleagues write that Sputnik Planitia must have emerged in Pluto’s youth, or within a million years of Charon’s formation. 

Pluto’s coldest bands sits near latitudes of 30 degrees north and south – colder, even, than the poles

Back then, Pluto’s crust was thinner, so stacking on weight would have caused the crust to slump and create a basin. 

That ice can push a planet’s (or dwarf planet’s) crust around isn’t new. We see the phenomenon on Earth – vast ice sheets, such as those blanketing Greenland and Antarctica, force the crust below downwards. As they melt, the ground beneath “bounces” up again (albeit very, very slowly).

“It’s exciting to see this kind of knowledge and models from the Earth applied to bodies elsewhere,” University of Southern Queensland astronomer Jonathan Horner says, who was not involved in the study. 

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Hamilton and his colleagues ran computer simulations of the dwarf planet. They write that Pluto’s coldest bands sit near latitudes of 30 degrees north and south – colder, even, than the poles, when averaged over its 248-year lap around the sun. 

This means ice formed naturally around Sputnik Planitia – which sits at a latitude of 25 degrees north – which, in turn, led to the runaway albedo effect.

So why isn’t there a band of ice encircling Pluto at 30-degree latitude? 

A possibility, the researchers suggest, is that the area that would become Sputnik Planitia was splashed by an ice volcano that erupted water or methane. Being brighter or shinier than its surrounds triggered preferential ice deposition.

And while this is a “convincing argument”, Horner says, it and other theories for Sputnik Planitia’s formation may not be mutually exclusive. 

“[It] that doesn’t counter the idea that there could be a hidden ocean, or that volatiles could be freezing out in the weather.” 

We’ll know eventually, though. “Barring catastrophic death of the species, given how our knowledge is expanding and we’re getting better at getting to these places, I don’t see why we wouldn’t end up knowing the answer for sure,” Horner says. 

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