Two years ago this coming July, the long journey of NASA’s New Horizons spacecraft to Pluto was approaching its end. Years earlier we had used New Horizons’ long-range camera to spot Pluto as a faint point of light off the bow of the spacecraft, but it took until April of 2015 for Pluto to begin to slowly, slowly reveal itself as an incredible new world. The best images would come on the day of closest approach, July 14, 2015, but three months out we cranked up our cameras to document our steady approach. And so we watched as, week by week, and then day by day, New Horizons beamed back ever more detailed images of Pluto from the edge of the solar system.
The distant pictures, however, were more enigmatic than anything. The dot of light became a fuzzy blob, which became a little disk with bright and dark markings, which became a bigger disk with finer bright and dark markings. We couldn’t help but to speculate on what we were seeing, but the geologists and geophysicists on the team weren’t really saying that much beyond “It’s not geology, yet.” Consider an open book resting on a table across the room. From where you’re standing you see a gray mass of printed text. Maybe if you squint you can see that the mass is organized into lines and words, but that’s about it. To read it, to learn from it, you have to walk over and pick it up.
So it is with planets. We explore the worlds of our solar system by sending spacecraft across immense distances on multiyear missions to get close enough to read their stories directly. We’ve sent rovers across the dry lakebeds of Mars. We’ve watched immense thunderstorms hurl lighting bolts across the skies of Jupiter. We’ve plumbed the methane seas of Titan, and dived inside the rings of Saturn. Our journeys have made these places real. We’ve always been rewarded with startling surprises – strange new worlds with hard won new knowledge.
Three days out from Pluto the fog suddenly lifted, a shock of suddenly recognizing what’s really in that fuzzy photograph you’ve been turning over and over in your hands. The disk was now a globe. Those smudges of a week ago were now clearly craters, or maybe basins. Those funny streaks turned into mountain ranges. Over here, look! Are those canyons, channels, grabens, maybe? Those on the team trained to read the landscapes of geology, were now bursting with insights, ideas, hypotheses, guesses. What do you do exactly when you’re on the landing party tasked to explore an unknown planet?
With them were those trained in chemistry to gauge the composition of Pluto, and those trained in atmospheric physics to marvel at the role played by Pluto’s hazy atmosphere, and the geophysicists to read the surface for traces of the deep interior of Pluto, and the cartographers to make the maps and draw the vivid ruggedness of Pluto out of hundreds of flat images, and those trained to scour the surface of Pluto for story told by ancient impacts on it by other bodies in the Kuiper belt, and on and on a team of a hundred scientists drew on the their unique talents, skills, artistry really, to read the story of Pluto.
Pluto was discovered as a dot of light in 1930, but again as a new planet in 2015. A planet? That’s how the New Horizons team saw it. Take a look at the treasures returned from the far reaches of the solar system by New Horizons and see what you think. Pluto is a world with a retinue of five moons. Charon, the biggest of the lot is planet in its own right. Pluto cloaks itself with a surprisingly substantial atmosphere; hazy with an organic brew cooked by the ultraviolet radiation of the distant sun, it bathes the night side of Pluto in a prolonged icy blue twilight.
The surface of Pluto is jagged with towering mountain ranges of blocks of water-ice kilometers in height, frosted on their peaks with methane snow, interlaced with plunging valleys. Start at the Pluto’s north pole and fly to the south pole over eroded craters and systems of rifts. Suddenly, the mountains give way to an immense sea of frozen nitrogen, its surface slowly convecting, forming an immense quilted pattern. Glaciers of nitrogen flow into it from the mountain highlands rising out of its eastern shore. You fly over the sea for hundreds of kilometers, the quilt giving way to a pitted surface that looks like nothing less than a colony of immense bacteria, or to others, huge fingerprints. Finally, the sea ends, the terrain rising again into a land of what looks to be immense cryo-volcanoes. And then into twilight and darkness, the south pole remaining as terra incognita for another mission.
Mysteries abound among the images and data we got back. It will take us years to put all the pieces of the puzzle together. A subtle analysis argues that deep beneath the crust of Pluto is a world-spanning ocean of water. There is a modest crater not far from the north pole that has a gap in its rim, out of which flows a broad and sinuous channel. What liquid flowed on Pluto, and how and when?
Is Pluto a planet? It’s a word we used simply and directly to encompass a complex world rich with novelties, surprises, and lessons on how the other planets in the solar system formed and evolved. We answered this in the affirmative by taking the book of Pluto off the shelf, opening it for everyone to see, and reading its story aloud so that everyone could share in its wonders and learn from it. How do you settle a debate about an unknown world? You go explore it.