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Pluto’s smooth surface is probably a sea of slow-flowing nitrogen ice

It took the New Horizons probe almost a decade to make its way to the solar system’s former ninth planet. When we finally got our first up-close look at Pluto, scientists were surprised by how much it differed from our expectations. A large section of the dwarf planet’s surface is almost completely smooth and devoid of impact craters, which indicates it could be very young. How is that possible on such an old, inactive planet? A pair of studies have just been published that could offer an explanation — convecting nitrogen ice.

The region in question is called Sputnik Planum, and you’ve probably seen it in almost every image of Pluto since New Horizons sent back its first shots. It’s the left half of the “heart” of Pluto, which you might notice is much smoother than the right half. NASA estimates that the surface of Sputnik Planum might be just millions of years old. The planet formed billions of years ago and isn’t large enough to support most geophysical processes. Two teams ran parallel studies to determine how such a feature could be created, and they came to very similar conclusions.

Pluto

On Earth, inert nitrogen gas makes up most of our atmosphere. On Pluto, it’s well below the freezing point of nitrogen, so you’d naturally expect to find some nitrogen ice. From a distance, the nitrogen ice sheet of Sputnik Planum appears to be completely smooth, but up close there are some interesting geometric projections that give its origins away. The surface of Sputnik Planum is covered in shallow polygonal bulges a few miles across. The borders between these bulges are home to short, irregular peaks, possibly composed of water ice floating on top of the nitrogen.

In order to test a theory about the nature of Sputnik Planum, the researchers of both teams focused on its internal structure. If we assume its rocky core is composed of similar elements as other planetoids, it would have some radioactive material that gives off a small amount of heat over time. It wouldn’t be much, just three milliwatts per square meter of surface. However, nitrogen ice has some unique properties that could allow this small amount of energy to power convection. It’s much denser than water ice, and more importantly, the molecules aren’t held together by strong interactions. These properties make it easy to deform.

Sputnik Planum

The teams created models of Pluto based on the nitrogen ice hypothesis in order to see if similar irregular bulges would appear. The simulated convection produced the same pattern we see in Sputnik Planum in less than a million simulated years. This supports the idea that constant turnover of nitrogen ice on Pluto’s surface erases impact craters in the region.

One big question about Sputnik Planum remains unanswered, though. Based on the size of Pluto, Sputnik Planum seems to have nearly all of the nitrogen on the planet. Scientists don’t currently have an explanation as to why all the dwarf planet’s nitrogen would have collected in what is probably an ancient impact crater. There’s still a lot of science to do at the edge of the solar system.

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