The dwarf planet Pluto is coming into focus as a complex, geologically diverse world that has been actively reshaping its surface for billions of years, thanks to data gathered by the New Horizons spacecraft. An eclectic mixture of mountains, valleys, and plains cover Pluto's surface. And the composition of the surface is just as wide-ranging as the world's geography. Because of these findings, scientists think Pluto may be one of the most unique objects in the outer Solar System.

Pluto may be one of the most unique objects in the outer Solar System

The New Horizons data is also giving researchers a better understanding of Pluto's space environment. The dwarf planet's atmosphere was found to be much colder than originally thought, and the air doesn't extend as far out into space as expected. That means the atmosphere's gases are escaping out into space at a relatively slow rate.

NASA has hinted at these details before, but they've now been fleshed out and published in five studies in the journal Science. The NASA's New Horizons spacecraft flew by the dwarf planet in July 2015. During the flyby, the probe's cameras and instruments gathered details about Pluto's surface, atmosphere, and unexpectedly cold environment. NASA has been downloading this data since July, and as more information reaches Earth, scientists are able to piece together a comprehensive picture of how the space rock formed.

Geology of Pluto

One of the studies published today reveals that Pluto is home to a healthy mixture of geological landforms, including large pits, craters, and valleys that cover most of the dwarf planet's surface. But a very smooth plain called Sputnik Planum also extends more than 620 miles (1,000 kilometers) across Pluto's northern hemisphere. This region is surrounded by icy mountains standing several miles high, and large glaciers flowing into the plains toward the north. The glacial flows have helped to carve out many of the valleys seen on Pluto's surface.

Pluto has been actively reshaping its surface since the Solar System first formed

Because the plains region is so smooth, the researchers believe that Pluto has been actively reshaping its surface since the Solar System first formed. "This plain on Sputnik Planum has no impact craters on it," says Jeffrey Moore, a New Horizons co-investigator at NASA Ames Research Center. "It can’t possibly be more than 10 million years old; it could also be a day old." The leading explanation is that Pluto has a big, rocky core that's made up of radioactive minerals. These minerals are heating up the surface above, causing the icy materials inside Sputnik Planum to stir and smooth out any craters that may have been there before. This internal heating could also mean that a subsurface ocean lies underneath Pluto's crust, according to the studies.

The icy materials inside of Sputnik Planum are thought to be made mostly of frozen nitrogen, which is found in abundance across Pluto. That's in addition to many other frozen surface materials, like methane, carbon monoxide, and water ice that are distributed across the surface. This diverse surface composition is unique for objects in the outer Solar System, according to study author Anne Verbiscer, an astronomer at the University of Virginia. Normally, frozen materials like methane and nitrogen would turn into gases near Saturn or Neptune, thanks to heating from the Sun. But Pluto seems to be at a far enough distance for these materials to survive in frozen form. "The distribution of different materials on the surface is amazing," Verbiscer says. "We haven’t seen anything like this anywhere else in the Solar System."

A cold atmosphere

Researchers have also discovered that Pluto's atmosphere is colder than they expected. Close to the dwarf planet's surface, the temperature hovers around -387 degrees Fahrenheit (40 Kelvin). As the atmosphere extends farther away from Pluto, the temperature warms up slightly to about -261 degrees Fahrenheit (110 Kelvin) thanks to rising methane that's heating up the air. But then the gases cool back down to about -333 Fahrenheit (70 Kelvin) in the outer atmosphere, possibly due to the presence of a coolant molecule called hydrogen cyanide.

Pluto's atmosphere is colder than they expected

Because of these lower temperatures, less gases are escaping out into space than expected. Higher temperatures would cause gases to become more energized and move around quickly, allowing them to break free of Pluto's gravity. But since the upper atmosphere is so cool, Pluto's gases are sticking close to the tiny world. "It doesn’t take much to escape from [Pluto], but because it’s so cold, we think it's a lot better at holding onto these gases," says Randy Gladstone, an astronomer at the Southwest Research Institute.

This also means that less gases are being carried away by solar activity. Solar winds coming from the Sun are constantly slamming into Pluto's atmosphere and pushing escaping gases deeper into space. The area where this mingling occurs is called the solar interaction region. But since Pluto's atmosphere is so compact, this interaction region is also much smaller than expected. "It shows Pluto is much more like Mars than like a comet," said study author Fran Bagenal, a planetary scientist at the University of Colorado, in regards to its solar interaction.

The unique findings of these studies all serve as a jumping off point for researchers to study Pluto in greater detail, as there are still many questions left to answer about the dwarf planet. For instance, New Horizons spotted a few mounds several miles high, with deep holes in their centers. Moore and his colleagues think these may be cryovolcanoes, which erupt freezing water or other materials instead of lava. But the researchers can't say for sure until they run computer models to see if such a landform is even possible on Pluto. Studying Pluto has also made some of the researchers eager to visit other objects in the outer Solar System. "You just wonder what you are going to see on those bodies too," Verbiscer says.

Correction: This article originally stated that the Pluto flyby happened in July 2014. It occurred in July 2015, and the text has been updated.