Pluto looks much different than scientists expected it to. Rather than being a barel little snowball on the outskirts of the solar system, it has a complex and fascinating geology that appears to be unique among all the planets and planet-like objects we’ve studied up close. As data is retrieved from the New Horizons probe, which completed a flyby of Pluto over the summer, NASA has more information about the dwarf planet’s composition and evolution to share. So what is Pluto? It’s a little bit of everything, according to NASA’s first official publications.
A large part of Pluto’s surface is much smoother than anyone expected — it’s the lighter region in the image above. It’s been dubbed “Sputnik Planum” in honor of the first satellite in orbit of Earth. As soon as the first images came back, scientists noted how odd it was to see virtually no craters in region. We now have good evidence that Sputnik Planum is covered in nitrogen glaciers that grow and shrink over time. This is an important aspect of Pluto’s geology.
Pluto was once referred to as the ninth planet, but there are times in its 248-year orbit that it’s closer to the sun than Neptune. The last time that was the case was in the late 1990s. As it gets closer to the sun, the nitrogen ice glaciers start to sublimate, then they refreeze when the dwarf planet moved farther away again. This is similar to the process of water and carbon dioxide ice on Mars. On Pluto, the result is a slushy nitrogen mess that constantly reflows on the surface every few centuries. In geological time, that’s really fast. Certainly fast enough to erase most impact craters.
Right next to the strikingly bright Sputnik Planum is a rust colored swath of terrain that has been given called the “Cthulhu region.” I know, that’s awesome, right? This is the other distinctive terrain type on Pluto’s surface. Unlike Sputnik Planum, this area shows every bump and scar the planet has picked up in the last few billion years. Some of the craters in this region are believed to be 4 billion years old. Researchers analyzing the New Horizons data now believe the dark coloration is the result of methane from the atmosphere that has been chemically altered to become soot-like particles called tholins before settling out like dust on the surface.
Then there are features on Pluto that are still largely mysterious. There are blocky structures punctuation the landscape in Sputnik Planum, odd ridges bordering the Cthulhu region, and the overall planetoid is surprisingly round for its size. Scientists thought it would be a bit more lumpy than it is, but it’s as close to spherical as New Horizons can measure. That indicates is was much warmer than expected in its early years, allowing it to avoid deforming (like Charon).
New Horizons still needs months to send all of its Pluto back to Earth. After that, it’s going to travel on into the Kuiper belt in search of more objects to study. Maybe they’ll have similar compositions to Pluto.