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SYFY WIRE Bad Astronomy

A Strip of Pluto

By Phil Plait

I know, that image above doesn’t look like much, does it? But go ahead, click it. I dare you. Because I took the original 1,000 x 15,000 pixel image, rotated it, and shrank it to fit the width of the blog.

It’s a jaw-dropping close-up look at a very, very long strip of Pluto’s surface, created using the highest-resolution images from the New Horizons spacecraft as it shot by the tiny frigid world in July 2015 at a dozen times faster than a rifle bullet.

Simply scrolling down that image is like mainlining science directly into your brain. NASA kindly released a short video annotating some of the gross features:

We’ve seen most of the subimages making up this composite before, but context is everything when you’re surveying the surface of an alien world.

For example, one of the most interesting features on Pluto (and there are so many to choose from!) is the nitrogen plains making up the western half of Pluto’s heart, called Sputnik Planum (the entire heart feature is called Tombaugh Regio, after Clyde Tombaugh, the astronomer who discovered Pluto). Much of Sputnik Planum is segmented, which is something you see on Earth in frozen lakes, when warm water rises and colder water sinks (called convection) and the frozen surface forms abutting geometric plates.

On Pluto those plates are a few kilometers thick, but underneath them may be more fluid material with the consistency of toothpaste. It convects, possibly at speeds of several centimeters per year. But this raises the question: Why is Pluto’s interior warm? It’s small enough that it should have frozen solid billions of years ago! This is one of the biggest questions the New Horizons observations have generated. Maybe Pluto suffered a huge collision in the past billion years, creating the moons, and warming its interior. Maybe it’s something else entirely. But the nitrogen plains are pointing toward some huge energy input that made Pluto’s interior warm.

Also in those plains are thousands of little pits, probably sublimation features, caused when small pits in the ice grew when warmed by the Sun, the nitrogen turning directly from solid to gas in the near-vacuum conditions. I’ve been intrigued by them since we first saw them in the initial images. Why do so many of them seem to align along lines and curves?

The obvious answer is, again, the surface is moving, pushed around by forces inside the little world. As the ice flows, the pits get stretched out … or perhaps it’s the other way around: The flowing ice gets cracks or deformations in it that then form the pits, already seeded along the stretch marks.

What a weird place.

One other thing: Looking along the image strip are these features, labeled as “rugged, dark highlands.” I was struck by how the big one at the upper right appears to have a huge mountain in the middle of it. I wonder if the mountain got pushed up by some tectoniclike force, then subsided (sank), pulling down the surrounding region with it. Or perhaps it's just what's left of the surface as a series of areas collapsed. The giant pit to the lower left doesn’t have such a feature, so clearly there’s more going on here. 

These images are truly incredible, and we’ve had to wait a long time to see them; data are still coming back from the probe, and it takes time to create a mosaic like this. But as hard as it is to wait for the images, it’s even more maddening to have to wait for the papers to trickle in. I want to know what’s going on there, what expert planetary scientists think about them! I’m just thinking out loud here, trying to figure out what I’m seeing, but they have vast experience interpreting such landforms … and yet even they are baffled by what they see in the New Horizons imagery. Such an active and diverse and just plain odd little thing Pluto is!

And it’s just one of dozens of such large bodies out there, in the black past Neptune. Pluto is only the first one we’ve seen up close. I wonder what other bizarreness awaits us when, eventually, we send our probes out there? I hope we get the chance to find out.

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