The Mystery of Pluto's Beating Heart

Paul Sutter is an astrophysicist at The Ohio State University and the chief scientist at COSI science center. Sutter is also host of Ask a Spaceman, RealSpace and COSI Science Now. He contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

I don't know about you, but I was shocked when NASA's New Horizons mission sent back its first batch of high-resolution images of the surface of Pluto. 

That shock came not just because the achievement was so monumental — the mission did, after all, manage to fling a spacecraft to the distant edge of the solar system at tens of thousands of miles per hour — but also because of the incredible complexity and subtlety on display in that cold little world. [Amazing Pluto Shines in Best Close-Up Views Yet (Photo, Video)]

I heart Pluto

Deep chasms, overlapping craters, mountain chains, nearly featureless plains and shades of pink, red and black cross Pluto's surface. It is a world of surprises, with a hundred questions answered and a thousand more asked.

And of course, the dwarf planet hosts the now-iconic heart-shaped feature: Tombaugh Regio. Pluto's heart spans a thousand kilometers (620 miles), the eastern half crumbled with uplifted terrain and scraggily mountains, the western half smooth and plain. 

Before I continue describing this incredible feature, take a moment to pause and reflect on the fact that Pluto, a world unknown to humans for millennia, and identifiable only as a tiny speck of light after it was discovered, is now revealed in all its wonder and glory. Humans are giving names to landmarks forty times farther from the sun than the Earth is. That's kind of amazing.

But I digress. What's going on with Pluto's heart? How, and when, did it form?

Motion of the (frozen) ocean

Since that flyby in June 2015, the New Horizons spacecraft has sent its hard-won data back to Earth. And eager astronomers and planetary geologists have hungrily consumed the information and images, searching for clues to the origins and operations of Pluto's now-famous feature.

And what a treat they got.

Those mountains in the eastern half of Tombaugh Regio? They're made of ice — water ice. Good old H20, chilled to a temperature of 40 kelvins (minus 230 degrees Celsius, or minus 390 degrees Fahrenheit), is literally as strong as rock. It's strong enough to build mountain ranges as tall as the Himalayas. 

Planetary geologists think that underneath Pluto's frozen nitrogen crust is a mantle of water ice, and it appears that in this region of the heart feature, the mantle has burst through, creating a jumbled mess of mountains, boulders and chaotic terrain.

This eastern complex stands in stark contrast to the almost featureless plains of frozen nitrogen that dominate the western half of the Tombaugh Regio, dubbed the Sputnik Planitia. But detailed imagery of the plain reveals a suspicious hexagonal pattern: Crisscrossing the frozen nitrogen like a fantastically cold beehive, the ice is segmented, with deep crevasses separating blocks of glacial nitrogen. 

The hexagons reveal the secret of the heart; they're the signatures of the fluid transfer of heat from one side to another, called convection processes. Such processes appear everywhere a fluid is warmed unevenly, from the sun to a tea kettle. This convection mean that something underneath the Sputnik Planitia is warmer than 40 kelvins, and that's causing the great plain of frozen nitrogen to slowly churn over the course of millions of years.

Researchers see more evidence for motion in that glacial mass than just the convection cells. Remember those water-ice mountains? Sometimes through geologic activity they break apart and fall into the plains. Though hard as rock, the water ice is less dense than the nitrogen ice that dominates the plain, so giant boulders half the size of mountains literally float on the vast pool of Sputnik Planitia.

Floating mountains of ice — let the awesomeness of that impress you for a moment.

Over the course of thousands of years, as the frozen nitrogen heaves and flows on glacial timescales, the water-ice boulders get wedged in the cracks among the convection cells. This produces a junkyard of abandoned, unwanted ice — a sign of a living, "beating" heart.

Under the Ice

This constant churning means that the Sputnik Planitia is young compared to the rest of Pluto's surface. And indeed, researchers have not spotted a single crater on Sputnik Planitia's surface. Elsewhere on Pluto and its moon Charon, craters dot the landscape. These are signs of meteoric impacts throughout these worlds' billions of years of not-so-lonely existence at the edge of the solar system.

Just how young is the heart? Scientists don't know. The surface could be refreshing itself, and erasing the occasional crater, every hundred thousand years or every hundred million years. That's certainly "old" in human terms but young for astronomical timescales, and it matches the geologic timescales of plate tectonics here on Earth.

The mysteries of Pluto aren't just skin-deep, though. This icy world is smaller than Earth's moon, but something is keeping Pluto warm through the eons. There must be some decay processes or some sort of residual heat from the dwarf planet's formation that's activating the Sputnik Planitia and keeping its nitrogen ice churning. Why didn't the Planitia simply freeze solid long ago, like so many other objects in the solar system? 

At this stage of understanding, it's simply a mystery, and that mystery gets even more mysterious the deeper researchers look. Scientists are not 100 percent sure — like for most things in astronomy — how Tombaugh Regio formed in the first place, but the most likely scenario is that a massive impactor slammed into the newly formed dwarf planet in the early days of the solar system.

According to this scenario, that impact spread debris into the vicinity of Pluto, with the material eventually coalescing into Charon and the other moons. And the same impact left a massive wound in the side of young Pluto, cracking the nascent nitrogen shell like an egg and leaving uplifted ice mountains on one side and a permanent basin on the other. [Watch: Pluto's Secret Ocean]

That feature never quite healed, according to this scenario. The weight of additional nitrogen snow depressed the basin, and gravitational interactions with Charon rotated Pluto so that the wound lay in the most-shaded part of the world. As long as Pluto's core stayed warm, Sputnik Planitia would continue its slow, cold roil.

I've saved the best part for last. In order for the above scenario to work, in Pluto's early days, the dwarf planet had to have possessed a liquid-water ocean underneath its icy crust. And careful observations of Pluto's rotation, together with the stark fact that the dwarf planet is obviously still somewhat warm, has led many planetary geologists to conclude that there might still be a layer of liquid water just under the surface. 

That's right. Today. Right now. There's a very real possibility of a subsurface ocean within Pluto.

How can this liquid ocean be possible? Unfortunately, it may take another probe to find out.

Learn more by listening to the episode "What Has New Horizons Learned About Pluto?" on the Ask a Spaceman podcast, available on iTunes and on the web at http://www.askaspaceman.com. Thanks to romerobryan83 for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

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