When the New Horizons spacecraft sent back its first images of Pluto in July, the view was glorious and extraordinary. It’s not every day that we get to see a (dwarf) planet up close for the first time. As planetary scientists scrambled to put the pieces of their blown minds back together, we got some initial observations and hypotheses about Pluto’s surprising surface. But the bulk of the data from New Horizons’ brief encounter had yet to be transmitted back to Earth. As that data continues to stream in, more detailed science is being done.

This week in Science, a stack of five papers lays the foundation for that science by describing Pluto’s geology and atmosphere, as well as that of Charon and the smaller satellites orbiting the dwarf planet with the big heart.

That includes a basic description of the Plutonian landscape—at least the hemisphere that greeted the New Horizons spacecraft on approach. The lower quality data from the other side will eventually be analyzed as well.

Icy geology

The bright heart-shaped region of Pluto’s surface, dubbed Tombaugh Regio after the astronomer who discovered Pluto, got a lot of attention when the first images were released. The left half of the heart is a deep basin that drops several kilometers below the surrounding highlands. It appears to be a huge impact crater that has been re-shaped over Pluto’s history. The smooth, uncratered surface within the basin, on the other hand, is quite young and clearly is being actively maintained. Nitrogen ice, carbon monoxide ice, and methane ice are all present on the surface here (and to a lesser extent elsewhere). Those are volatile, able to migrate around Pluto by sublimating to gas and condensing somewhere else, so this frosty mantling likely changes with the seasons.

Within the basin that makes up the left side of the heart, the researchers say a thick layer of (mainly) nitrogen ice is responsible for the strange, bubbly appearance of the surface. The “bubbles" are separated by nearly 100-meter-deep troughs and appear to be the result of convection within the ice, with warmer ice beneath the cold surface moving upwards. This activity would help explain the basin’s youthful appearance.

At the western edge of the basin, there are remarkable mountain ranges composed of Pluto’s water ice “bedrock.” The water ice was broken into blocks that have shifted around and rotated into their current positions. They look almost like a lizard’s scales in the images, but these mountain blocks rise as much as 5 kilometers (16,000 feet) above their surroundings. Because water ice is actually less dense than the nitrogen and carbon monoxide ices in the basin, these mountains are basically floating, like gargantuan icebergs, in an also-frozen sea.

The dark region to the west of the basin, called Cthulhu Regio, is a much more ancient and densely cratered surface. The landscape there is diverse, but it lacks the volatile ices that make Tombaugh Regio appear so bright. Instead, water ice is exposed there, and a dark residue of material has accumulated as atmospheric gases react in response to ultraviolet light.

Most of Pluto’s surface has been there for 4 billion years, going back to the final round of intense collisions following the formation of the Solar System. But there has also been more than one period of tectonic activity, during which Pluto’s crust fractured as regions of it were stretched. Some of those long faults produce several-kilometer-high steps in the surface. The nature of these faults points to a pretty thick water ice crust, but the researchers say that their timing “is consistent with predicted recent extensional stresses associated with a late, possibly partial freezing of a subsurface ocean, though other explanations are also possible.”

To the moons

As for Pluto’s moon Charon, it has its own wonders and mysteries. Its surface is mainly water ice, without the volatile ices that brighten up portions of Pluto. There were, however, signs of ammonia ice in and around impact craters that may have been excavated from beneath the surface. The rusty red color of Charon’s northern pole (the region that has been named Mordor Macula) couldn’t be identified, although it could be the same sort of UV-produced schmutz found in Pluto’s Cthulhu Regio.

Even if Charon’s surface color is a little more homogeneous, its geology is still pretty wild. Its face is divided by a belt of remarkable extension faults, including a couple rifts that are 5 to 7 kilometers (up to 23,000 feet) deep. North of that line, the incredibly rough terrain is scarred by enough impact craters that the surface looks to be more than 4 billion years old.

To the south lies relatively smooth terrain interrupted by fewer craters, indicating a slightly younger age of around 4 billion years. There are, however, also some hills that the researchers believe could be cryovolcanoes—relics from Charon’s early history, when a still-warm interior could have fueled gaseous eruptions. It’s possible that a young Charon had a subsurface ocean.

About all this pleasantly unexpected complexity, the researchers write, “all three major Kuiper belt bodies (past or present) visited by spacecraft so far—Pluto, Charon, and Triton—are more different than similar and bear witness to the potential diversity awaiting the future exploration of their realm.”

And these are just a few selected highlights from the surface observations. Pluto’s slightly hazy, nitrogen-dominated atmosphere also bucked expectations. It is (at least currently) losing much less gas to space than our models predicted, and what gas it is losing is mostly methane, rather than nitrogen.

There are a number of puzzles yet to be solved as scientists back on Earth work to make sense of the reports from our robotic emissary to Pluto, and the puzzles are just as interesting as the answers we’ve already won.

Open Access at Science, 2016. DOI: 10.1126/science.aad8866, 10.1126/science.aae0030, 10.1126/science.aad9045, 10.1126/science.aad9189, 10.1126/science.aad7055 (About DOIs).