New research explores Pluto's 'beating heart'

NASA's New Horizons spacecraft flew by Pluto in July of 2015, but data it gathered will be analyzed for years to come. Recently, the New Horizons team published new work in Nature describing how Pluto's famous "heart" is actually beating. CBC Radio science columnist Torah Kachur explains.

Lava lamp-like convection accounts for smooth surface feature on dwarf planet, according to new studies

A false colour image tweeted by NASA in July 2015 shows Pluto's heart-shaped feature. (NASA)

NASA's New Horizons spacecraft flew by Pluto in July of 2015.

But data it gathered has not stopped coming in, and will be analyzed for years to come.

Recently, the New Horizons team published new work in Nature describing how Pluto's famous "heart" is actually beating. CBC Radio science columnist Torah Kachur explains the latest findings.

What is Pluto's 'heart'?

When the New Horizons space mission started beaming data back to Earth, the most incredible thing was seen — what looked like a perfect heart-shaped structure on Pluto's surface.

A different angle on the heart-shaped Sputnik Planum from the New Horizons spacecraft in July 2015. (NASA)

It covers 900,000 square kilometres, and also has polygonal surface structures dotted across it. Needless to say, the quest begun to try to figure out what it was. 

It turns out this region, informally called the Sputnik Planum, is a glacier of sorts, made up of frozen nitrogen gas with a little bit of methane and carbon monoxide thrown in.

The first indication that this heart-shaped feature was more than just some old rocks was that it didn't show any of the impact craters that dot the rest of Pluto's surface. That suggested it was a relatively young geological feature on the tiny dwarf planet.

Does this 'heart' beat?

In a way, it does. That's one of the interesting ideas published in the latest issue of Nature. There were actually two competing research teams studying this — one from Washington University at St. Louis, and one from Purdue University.  

And they both found the same thing. The researchers combined the images and patterns seen on Pluto's surface with computer modelling of ice dynamics. The goal was to see if they could simulate the conditions of Pluto, and get results that look like Pluto's heart.

The surface of the heart is a mere –235 C, cold enough to freeze nitrogen. According to the Washington University research, there's a reservoir of nitrogen-filled ice that's likely several kilometres deep in some places. The solid nitrogen is warmed by Pluto's internal heat, and then it rises up in blobs — which has been described as a "lava lamp" sort of effect. So through this convection process, the nitrogen bubbles up to the surface and refreezes.  

In other words, the heart is pumping nitrogen up to the surface where it hardens again into the heart-shaped pattern that we see.

This is something NASA researchers had previously theorized, but the latest research seems to confirm the theory.

Bill McKinnon is a planetary scientist at Washington University at St. Louis. He says the convection forces at work on Pluto's 'heart' are important because they help replenish the dwarf planet's atmosphere. (James Byard/WUSTL Photos)

Washington University's Bill McKinnon is one of the primary authors of their report. He explained why the beating heart is really important.

"It actually keeps Pluto alive in a way, so it is almost like a beating heart," he said. "When you bring this fresh nitrogen ice to the surface ... it can basically replenish the atmosphere." 

The fresh ice releases some nitrogen gas through the chemical process of sublimation. And the extra gas maintains Pluto's small atmosphere.

So Pluto needs that continual influx of more nitrogen to sustain the atmosphere, and thus sustain its ice and temperatures.

How did the researchers discover that the heart was 'beating'?

They did a computer model to simulate the temperature and the composition of the ice on Pluto. When they did that, they found the exact same pattern we see on Pluto — including the geometric surface structures, some of the movements seen on Pluto and the idea that the Sputnik Planum is much smoother and more dynamic than the rest of the surface.  

New Horizons’ views of the Sputnik Planum (top), contrasted with the Vulcan Planum on Charon (bottom). The Sputnik Planum's bright, nitrogen-ice plains are defined by a network of crisscrossing troughs. (NASA/JHUAPL/SwRI)

And it's important to note that the two groups of scientists, who were independent, found the exact same thing — which suggests convection does explain the unusual surface of Pluto.

What are the geometric structures that cover the 'heart'?

They're key to the recent findings. When the high-resolution images of the heart came back, there were these weird, polygonal indentations.

And it turns out that what we're seeing here are the same dynamics as you'd find in that oatmeal — just happening on a much bigger scale on Pluto.

You know when you leave your oatmeal on the stove a little too long without stirring, and the top gets a bit crusty?

Look closer and you'll see small polygonal ridges. That's because there are cooler areas of the oatmeal on the surface compared to the stuff getting heated below. And the cooler areas sink back downwards.

A NASA closeup of the Sputnik Planum, showing the polygonal structures that cover the area. (NASA)
That's what creates those surface indentations. Then if you turn the stove off, the ridges collapse because there's less of a temperature difference now.

So on Pluto, when the nitrogen warms a bit near the rocky surface it pushes upwards, just slightly, against the coldest crust. That creates the polygons.

How is Pluto warm enough at the surface to melt the nitrogen ice?

There's no lava or molten core like Earth has, that's for sure. It's a hard ball of rock that hurtles through the Kuiper Belt at the edge of our solar system.

So it gets heat not so much from the sun or geologic processes, but from radioactive decay.

"A typical pile of rock in the solar system has all sorts of different elements in it," said Washington University's Bill McKinnon.

"There's a little bit of uranium, a little bit of thorium, a little bit of the isotope potassium-40, which is radioactive. And it's the slow decay of these naturally occurring, radioactive elements that gives Pluto its internal heat. It's not a lot compared to the Earth, but it's enough to make these very volatile and mobilizable ices go."

How quickly does all this happen?

Very, very slowly. In fact, we're only talking about centimetres of change every Earth year. That means that the best estimate is that the surface of Pluto will be refreshed every 500,000 to one million years.  

When you consider this is geological time, that's actually quite quick. And it means if we ever had the sophistication to visit Pluto in a few hundred thousand years, the heart may very well have changed shape.


Torah Kachur

Science Columnist

Torah Kachur is the syndicated science columnist for CBC Radio One. Torah received her PhD in molecular genetics from the University of Alberta and now teaches at the University of Alberta and MacEwan University. She's the co-creator of