We've seen a lot of worlds in our solar system, but Pluto may be the strangest.

It is much smaller than the Moon and made mostly of rock and ice, giving it a low density and weak gravity. It has an atmosphere, but not much of one. It is cold, with a surface temperature hovering around -270. The surface is covered with ice, and it's like being buried.

It was a surprise to see what looked like volcanoes on the surface.

On bodies like Earth, Jupiter, and Venus, the power for volcanoes comes from below, heat stored deep inside that causes warmer material to rise up and, sometimes, burst forth. The standard idea was that Pluto was frozen solid long ago, but it is so small and far from the Sun that finding volcanic features is unexpected.

The changes that were made were due to New Horizons. The surface of the planet was mapped out when the spaceship flew past it in 2015. Tombaugh Regio was one of the most striking features. The western part of the heart is covered in nitrogen ice, but not craters. It is a young feature that has been repaved recently, wiping out the craters that may have been there. Nitrogen ice is soft and can flow like glaciers. Sputnik may have been a huge impact site that flooded with nitrogen, and features seen there indicate it is still warmer than expected. That means that Pluto may have a source of heat underground.

There are two large mountains just to the southwest of Sputnik Planitia. They look like calderas, volcanoes, and scientists wondered if they were cold volcanoes that spit out water and other liquids instead of molten rock.

A team of planetary scientists have come up with a somewhat different idea after looking at high-resolution images and mineral maps. They think that the entire region has undergone a lot of cryovolcanism in the past. The mounds that can be up to 20 kilometers wide and several high are formed when the liquids that quickly freeze into ice are flung out. As the mounds grew, they would collide against each other to form a hummocky, undulating surface. The resemblance is actually a coincidence, as some of them merged together to create ring-shaped mounds with central depressions that look like singular volcanoes.

If they are correct, the Wright and Piccard Montes are just a bunch of individual hills that form ring-shaped structures.

There are three main lines of evidence for this. The area is younger than the terrain around it because there are no impacts craters. You would expect to see impact over time if the area is old. Liquid water upwelling can fill in the craters. The region to the southwest of Sputnik Planitia is young. That is young to a planetary scientist.

The undulating hummocky hills are hard to explain as mountains on Earth. The scale is wrong because they don't form coherent patterns like ridges. They are not erosional, as if a flat area had some sort of erosion that carved depression and left mounds behind. There is no known mechanism for that.

The central depressions in Wright and Piccard don't match what calderas look like. There are huge chambers of liquid underneath the volcano. After an eruption, the chamber empties and the solid material above it collapses, leaving behind the volcano shape. The depressions in the two features are too steep and have hummocks in them. The depression in Wright Mons isn't deep enough.

Maybe they aren't monolith volcanoes if this is put together. The hills are made up of multiple domes merging from different vents. They are big. Wright Mons is about 150 kilometers across and three high, while Piccard Mons is about 200 km across and seven high. That is more volume than Mt. The summit of Everest.

The amount of material needed to account for all this terrain is a lot. It is enough to fill a cube over 20 km on a side. A lot. The scientists think this happened over multiple episodes.

It is believed that Pluto is better able to retain internal heat than was thought prior to the visit.

I would be surprised if that weren't a surprise. The entire close encounter with New Horizons only lasted a few hours. Imagine if we had a satellite there. If we had years of observations, what would we learn?

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