Since New Horizons flew past Pluto in July 2015, we’ve been treated to countless images and theories about what has proved to be a fascinating world, from evidence for iceberg-like features to suspected cryovolcanoes on the surface.

Now, in a series of five papers published in Science this week, scientists have presented their research so far on the Plutonian system, and what it means for our understanding of the Solar System. Here, we’ll be covering the geology and surface composition of Pluto and its moon Charon. You can read our other post on the other papers for information on Pluto’s atmosphere, and its interaction with the solar wind. 

Before New Horizons arrived, scientists had thought Pluto would be a dead, barren world strewn with craters. This is because it is a largely isolated world at the edge of the Solar System, free from the gravitational effects from large planets such as Jupiter and Saturn. But that is proving to be anything but the case.

In the first of the five papers, co-authored by Oliver White from NASA’s Ames Research Center in California and his colleagues, the scientists explain how there is a surprising amount of active geology taking place on Pluto. By that, we mean that the dwarf planet’s surface has changed within the last few million years, a blink of an eye compared to its overall age of 4 billion years.

“New Horizons has reassessed what we thought was possible on these small icy bodies out at the edge of the Solar System,” White tells IFLScience. “We were thinking they would be relatively dead worlds, but we’ve seen tremendous evidence for geological activity that shows you can sustain such activity at colder, less energetic conditions than we thought was possible.”

This geological activity comes in the form of a number of things. Most notably is the heart-shaped region on the hemisphere of Pluto seen by New Horizons, called Tombaugh Regio, specifically its smooth western region of Sputnik Planum. This region shows zero evidence of craters, but we know worlds like this must get pummelled throughout their lifetime.

^{Above, the smooth Sputnik Planum heavily hints at recent geological activity. NASA/JHUAPL/SwRI}

This has allowed scientists to suggest this surface has changed in the past 10 million years, possibly even less than 100,000 years, via processes including glacial flow of the various ices on the surface, including nitrogen and methane. This is very, very surprising. “It’s a drop in the ocean compared to the age of Pluto,” said White. “Sputnik Planum is perhaps the most unexpected feature that we’ve encountered in the flyby.”

On Charon, an almost equally interesting region is Vulcan Planum. Most of Charon, unlike Pluto, is relatively old, up to 4 billion years. But Vulcan Planum’s smooth surface, like Sputnik Planum, points to recent geological activity from an unknown source of heat. “We’re wondering if maybe there was a vast outpouring of ice,” said White.

This geological activity on both worlds suggests there must also be some internal source of heat. Owing to the small sizes of Pluto and Charon (the former is slightly smaller than Earth's Moon, and the latter half that), tidal forces from their gravity are not thought to be enough. Instead, scientists think the decay of radioactive elements inside the bodies is providing the heat.

In a second paper, with Dr. William Grundy from Lowell Observatory in Arizona as the lead author, scientists looked at the surface composition of Pluto and Charon. Before New Horizons arrived, we knew the ingredients of Pluto’s surface, which includes volatile ices such as methane, nitrogen, and carbon monoxide. But what has been surprising is how these ingredients interact on the surface.

“This interaction really surprised me,” Grundy tells IFLScience. It allows for the formation of various structures on Pluto, most notably huge mountains of water ice, and also the smooth plains referenced earlier. In fact, the spread of material across the surface of Pluto even provides indirect evidence for winds on the surface, something that had really not been expected at all.

“The fact you see different patterns is a pretty strong hint of prevailing winds now, or once upon a time, flowing from west to east,” said Grundy.

This constantly shifting material may also explain the strange red pole seen on Charon (right). Scientists had been somewhat baffled by this feature when it first appeared in images. But the answer may be due to Pluto’s atmosphere slowly escaping, bathing Charon in what are called tholins, small soot-like particles that have been generated from the reaction of methane and nitrogen in the atmosphere.

The third paper, which we’ll only mention briefly, looked at the other moons of Pluto: Stix, Nix, Kerberos, and Hydra. The moons were found to not be tidally locked to either Pluto or Charon, suggesting they were formed from the collision that is thought to have produced Pluto and Charon.

For now, though, these papers serve to highlight how fascinating Pluto and its surrounding system have turned out to be. A dead world this is certainly not, and it suggests other objects in the Kuiper Belt – one of which New Horizons will be visiting in 2019 – could be equally fascinating.

“There’s a lot more places to explore in the Solar System, just when we thought we had finished the job,” said Grundy.

^{Image in text: Vulcan Planum, towards the bottom of the highlighted region, points to lesser geological activity on Charon. NASA/JHUAPL/SwRI}