For the days and weeks leading up to New Horizons' closest approach of Pluto, the theme for the science returns was the unfamiliar -- with each new data point, the science team realized that Pluto isn't what they thought it was. It wasn't like predictions made from ground-based observations, it wasn't like Neptune's moon Triton (previously accepted as a reasonable analogue for Pluto), and with each kilometer that the spacecraft grew closer, previous speculation needed extensive revision.

Part of this entailed scientists' expectations for the level of activity on Pluto. For a period of time following its discovery, it was generally thought that Pluto would be an icy, ancient body -- pock-marked with craters, its surface reflecting billions of years of residence in the heavily populated Kuiper Belt. With the realization in the 1980s that Pluto somehow boasted an atmosphere, and bolstered by Hubble Space Telescope observations that showed stark differences in brightness across its surface, scientists began to understand that some amount of ongoing activity would be shaping Pluto's terrain.

The first single high resolution image returned from New Horizons' closest approach, arriving at Earth on Wednesday morning, hammered home how vastly they had underestimated that phenomenon. As with the first image returned from the Mariner 4 mission, which flew by Mars 50 years ago to the day of New Horizons', it totally upended our assumptions and predictions about a world we've long wondered about.

The first high resolution image returned from New Horizons' close fly by of Pluto, focusing on the southwest region of "The Heart," which the team is now calling Tombaugh Regio after the dwarf planet's discoverer. Credit: NASA/JUAPL

"It's baffling, in a very interesting and wonderful way," said John Spencer, Co-Investigator with a focus on geology for the New Horizons team. "I would never have believed that the very first high res image of Pluto's surface would have zero impact craters."

The absence of craters is vitally important -- they act as a sort of timepiece for judging the age of terrain for planetary bodies. The more craters present, the older the visible terrain, generally. Fewer craters indicates younger terrain, which hasn't had time to be bombarded by impactors as the body travels through the solar system. That this first image didn't show any obvious craters means that the surface is very young; Spencer offered an estimate of 100 million years or younger, which represents perhaps 2% of Pluto's 4.5 billion year history. This means that some process must be replacing that surface.

The photo was also momentous for what it did show -- mountains of ice, some up to 11,000 feet high, comparable to Earth's Rocky Mountains. They are composed of water ice, and supported by a bedrock of water ice, which, unlike the nitrogen, methane, and other ices of Pluto's surface, remains strong and solid in the Pluto environment, rather than sublimating (turning from solid to gas) into the atmosphere. That these water ice mountains can erupt through Pluto's volatile nitrogen and methane surface ice must mean that the surface volatiles are a thin veneer. If this thin layer could not regenerate itself, the process of sublimating into the atmosphere, combined with the atmosphere's continuous escape from Pluto's gravity, would mean we should see no layer of volatile ices at all; it would have evaporated very early in Pluto's history. But since such a thin layer is omnipresent on Pluto's surface, some internal process must be replenishing the volatile ices.

This is a conclusive indication that there is active, internal geology on Pluto. The significance of this can't be overstated. Unlike bodies such as Jupiter's moon Europa, or Neptune's moon Triton, Pluto has no giant body wracking it with the stress of tidal forces, generating frictional heating that drives geological processes. Charon isn't large enough to do this. So, the science team can conclude, smaller icy bodies do not require tidal energy to power active internal geology. This is an entirely new discovery, made on Wednesday, enabled only by the return of data from New Horizons.

Spencer acknowledged that this is a potential game-changer for understanding our outer solar system. We had been considering smaller Kuiper Belt objects, like Eris, Sedna, and Makemake, to essentially be "candy-coated lumps of ice," he said. This new finding raises the possibility that active geology can happen in small icy bodies anywhere around the Sun, not just in the orbit of massive gas and ice giants.

The obvious question of what, then, may be driving this active geology is left open, and may be answered by further data from New Horizons and the interpretation thereof. It could be that the heat leftover from Pluto's formation is greater than was expected, or that altogether less is required to drive geology.

Heating by radioactive materials in the interior is also a possible explanation. We know from its density that Pluto isn't just ices; it must contain some quantity of silicate rocks, like those that largely compose the Earth. Mixed into these materials are quantities of radioactive elements like thorium and uranium that generate heat as they decay. It was previously thought that, for smaller bodies like Pluto, the quantity of these would not be sufficient to produce the heat required to drive geology. This notion may now need revision. A great deal more high resolution imagery is still due to come down and be processed, along with stereo imagery, spectroscopy information, and temperature data that could help fill in the unknowns about the peculiar features of this totally unexpected terrain. On Friday, we will likely be able to see a much larger region of the terrain at that resolution.

High resolution image of Charon taken by New Horizons during the closest approach phase. Credit: NASA/JUAPL

Perhaps more surprising, if that were possible, was the high resolution imagery of Charon acquired in the downlink Wednesday morning, which Deputy Project Scientist Cathy Olkin said "just blew our socks off." Unlike Pluto, the team expected that Charon's surface would be truly ancient -- a dead, crater-ridden surface that would help them refine impact modeling in the Kuiper Belt. Instead, Charon's surface is astoundingly young, showing only a few handfuls of impact craters. Still more surprisingly, a diverse collection of large surface features -- canyons, scarps, fractures -- show up loud and clear.

In particular, Olkin highlighted a series of apparent troughs and cliffs that stretch for 600 miles along Charon's equator, a canyon possibly six miles deep on the limb of the planet (2 o'clock on the image), and areas that exhibit obvious resurfacing. Principal Investigator Alan Stern couldn't help opening the press conference with the inescapable conclusion: "Charon has been active." What's driving this geological activity, as well as what created the dark region at the northern pole, now considered a thin "painted-on" veneer of material, is another question that will have to await the avalanche of data coming down over the next days and weeks.

Stern described the mood in the Mission Operations Center, as Pluto exceeded their wildest hopes and highest expectations, as "something close to bedlam." It will likely remain that way for the forseeable future. "We are, frankly, just skimming the top of it," Stern said. The second of many press conferences presenting state of the art results from the imagers and other instruments is scheduled for Friday. But it's obvious already that Pluto is an impossibly deep well of surprises and revelations that will take years, if not decades, to plumb. Stay tuned.