Pluto Is A More Diverse and Evolving World Than We Ever Imagined -- So What?
Over the course of several posts since New Horizons' closest approach to Pluto on 14 July, I've been trying to keep up with the avalanche of revelations about Pluto produced by the spacecraft's science team, based on just the initial trickle of data that's been returned to us. The overarching theme has been how complex and dynamic a world Pluto is; in just a little over a week we've found a thick atmospheric haze layer, nitrogen ice flowing like glaciers across its surface, towering water ice mountains the size of the Rockies, and a vast, smooth plain criss-crossed with puzzling trough features that we have yet to understand.
My excitement about all of this was probably tangible, and one of my goals has been and continues to be infecting others with my enthusiasm about these discoveries. But in the rush to comprehend and describe them, I'm worried that I haven't painted a big enough picture of Pluto, the New Horizons mission, and what it means. After all, why should someone, regardless of how closely they follow spaceflight or planetary science, care about these individual features, remarkable though they are? Without the proper context, what is the value of knowing about an atmospheric haze layer, or nitrogen ice flows, to a given resident of the planet Earth, three billion miles from Pluto?
I want to provide this context, which happens to be truly ancient in scope and staggering in its implications. I want to explain why the data coming back from New Horizons contains a piece of all of our stories as human beings, or perhaps more accurately, the very ink that those stories are written with. The explanation begins deep in our past, before we were born, before the United States existed, before the Renaissance, before Babylon, before humans wrote their history down, or learned how to plant and grow their food. It begins before Homo sapiens, before dinosaurs, before single-celled organisms or DNA; it begins before the Earth, Mars, or any of the other planets even existed. To understand Pluto's importance we must go back 4.5 billion years, to the birth of our solar system.
Image of the protoplanetary disk surrounding the star HL Tauri, taken by the Atacama Large Millimeter Array. The young star at center is surrounded by a rotating disk of dust and gas; interactions between dust particles will result in larger and larger clumps that will collide to form planetary bodies, and a new solar system. This is how our solar system was made. Credit: ALMA (ESO/NAOJ/NRAO)
Our Sun was born at the center of what's called a molecular cloud, a massive cloud of dust and gas. When material at the center of this cloud reached a critical density, gravity caused this region to collapse inward. Temperatures and pressures rocketed upwards as it did so, eventually becoming great enough to trigger the start of nuclear fusion, smashing hydrogen atoms together to create helium and releasing huge amounts of energy. This region was now a white hot ball of plasma, constantly fusing its hydrogen to create light and heat: our young Sun. The huge mass of remaining dust and gas in the cloud contracted around it, flattening out into a spinning disk surrounding the Sun, like the one pictured above.
This was called the protoplanetary disk. One component was a large amount of turbulent gas orbiting the new Sun, containing mostly hydrogen and helium, but also noble gasses like neon, xenon, and argon, as well as water vapor and carbon monoxide. Mixed into this gas were tiny solid dust particles. Closer to the Sun, these were materials that could be solid at high temperatures like silicates and metals. Further out, there was a great deal of water ice and solid hydrocarbons. Beyond that, in the coldest regions of the disk, dust grains included methane, nitrogen, and carbon monoxide ices, which are only solid at extremely low temperatures.
All of that gas and all of that dust orbited the new Sun in the form of the protoplanetary disk. The disk contained all of the materials that would be used to build the planets and other bodies that we know, including Earth. The disk contained all of the material used to create every rock you've ever seen, every component in every machine you've ever used, every pane of glass you've ever looked through, every piece of food you've ever eaten, and every molecule of air you've ever breathed. Inside that spinning disk were all of the molecules that would be used to build and sustain your body, the bodies of everyone you've ever known, and those of all of your ancestors before you. Inside the disk were you, and I, and everyone, spread out into clumps of dust and eddies of gas.
The story of you, the reader, is the story of the ways that gravity and energy conspired to organize this messy mix of dust and gas into increasingly complicated forms. It began with these dust grains sticking together to create larger and larger clumps, with the gravity of these clumps drawing them together to collide and form larger and larger solid bodies. Eventually they grew to diameters of a kilometer and higher, continually accreting other smaller clumps as they plowed through the disk. These objects were planetesimals, and their composition depended upon the materials that were available to them at the time of formation -- i.e., where in the protoplanetary disk they were born. The continuous merging of these planetesimals into still larger bodies is the process that created the planets and other bodies we know today.
This process was fairly well understood by the mid-20th century, but some of the details and theoretical obstacles were not entirely worked out (and some still aren't now). The only tools available to us for studying the process, as far as we knew, were mathematical modeling, Earth-based experiments, and observations of the small group of planetary bodies we were aware of. This changed in the late 1980s and early 1990s with the discovery of the Kuiper Belt. Rather than a lonely distant body, Pluto turned out to be the largest of a huge family of objects, numbering in the hundreds of thousands, that orbit the Sun beyond Neptune.
Diagram showing known Kuiper belt objects (blue dots) in our solar system, viewed from above. The scale at left and on the bottom are astronomical units (1 AU = distance from Sun to Earth). Jupiter, Saturn, Uranus, and Neptune are also marked (J,S,U,N) for comparison. Credit: WilyD at English Wikipedia
Our outer solar system was chock full of small bodies, and, more importantly these bodies were leftover planetesimals from the formative stages of our solar system, flung into distant orbits by the gravitational perturbations of the large outer planets. With the Kuiper belt, its king Pluto, and other bodies in our solar system, we now had a visible step-by-step guide to the process of planetary formation described above.
Earth, at one point in its formation, must have once been Pluto-sized. Pluto must have once been the size of one of its typical Kuiper belt brothers. Typical Kuiper belt objects must have once been the size of small comets like 67P/Churyumov–Gerasimenko, a tiny Kuiper belt object that made its way to the inner solar system, and is now being orbited by ESA's Rosetta mission. Each of these bodies is a snapshot-in-time of the process of collision and merging that organized all of the material in that spinning protoplanetary disk of dust and gas into a system of planetary bodies, into the Earth, into biomes and ecosystems and communities, and ultimately into you and I. Understanding the differences between them, and thus connecting the dots, is figuring out how we came to be here, right now.
More than that, Pluto and its largest moon Charon is the only other pair of bodies in the Solar System besides Earth and our Moon thought to have been formed by a giant impact. This, too, has direct consequences for you and I. Without our large moon and the massive impact that created it, the Earth would not have its tides, its relatively slow rotation, or its seasons. Who knows if life at all, let alone intelligent life, could have arisen without these things? Pluto and Charon are the only other bodies in the solar system that can help us understand how the dramatic impact that made all of those things possible could work.
To travel all the way to Pluto and find that it isn't just some battered relic, but a dynamic, evolving world, is a great leap forward in understanding the process of planetary accretion, and of giant impact systems. It's as if we sought to open a tomb to learn about ancient Egyptian culture and found not a corpse, but a living, breathing ancient Egyptian to tell us all about it. Pluto has a piece of all of our origin stories not just frozen into it, but flowing through it, like its nitrogen ices, or tumbling turbulently through its atmosphere, like the particles in its enormous haze layer. There is a hugely complicated winding thread that connects the heart-shaped Tombaugh Regio region and the heart that beats in our chests.
This is why the energy was contagious and inexhaustible at the Applied Physics Laboratory this month, and why even the most reserved scientist had an irrepressible grin on their faces. Over-evolved apes that we are, we've always tended to poke and prod things to better understand them, mostly as a way of better understanding ourselves, and why we exist at all. We've built tools, developed religions, and brought into being entire philosophical disciplines to bring to bear on this question. For Pluto, with all that it could tell us about it, the price of admission was its seemingly insurmountable distance, and all of the countless hurdles that had to be navigated to reach it. We marshaled nearly everything we've learned as a species about science and engineering, and stretched our ape brains to the absolute limit of their abilities, and paid that price.
And the reward is now flowing back to us in ones and zeroes from the high gain antenna of New Horizons, our most advanced prodding-stick to date. Alan Stern, Alice Bowman, Cathy Olkin, John Spencer, Hal Weaver, and the hundreds of people that devoted years of their lives to New Horizons,developed and flew a little robot through 3 billion miles of space, and found a great big piece of themselves, and all of us.
We often think of ourselves as residents in the universe, living in and walking through its spaces. But really we are a part of the universe, which includes every bit of space and every second of time that has ever existed in one cohesive fabric. If we can be said to have any sort of purpose as a part of our universe (and perhaps we cannot), maybe, as the only beings we know of with the ability to perceive and to think about and contextualize these perceptions, we are the Universe's introspection, a way for it to perceive itself. Maybe, as Carl Sagan said, "we are a way for the Cosmos to know itself."
Missions like New Horizons are just a very small step in that sort of charge, but in knowing Pluto we unravel not only the story of the universe, but the story of how it conjured us into being. Pluto can help fill in the temporal expanse between the cloud of gas and dust and the living humans that emerged from it. And we're not trying to infer this story from an ancient, inert artifact. Pluto is an active and vibrant world, and it's ready to tell us.