But today NASA unveiled an animation composed of still images taken by the spacecraft's EPIC (Earth Polychromatic Imaging Camera) instrument that shows something never before seen with such a wide field of view -- the daylit far side of the Moon passing in front of the Earth. Click here for full size:

The Moon is tidally locked to the Earth. That is, it takes the same amount of time to rotate once around its axis as it does to complete one orbit around the Earth (29.5 days). This is the result of many millenia of gravitational interactions raising tides on the Earth and Moon, slowing the Moon's rotation and sending it into further and further orbits of Earth. So we only ever see one face of the moon -- what we call the near side. The other side, the far side (or, inaccurately as you can see above, the "dark side"), was totally hidden to humanity for thousands of years; we had no idea what it looked like. The Soviet Union spacecraft Luna 3 changed this in 1959, when it recorded and sent back images of the far side.

First ever image of the Moon's far side, taken by the Luna 3 spacecraft in 1959. Credit: Roscosmos

Since then, modern spacecraft from many countries have imaged the far side, revealing a much more rough and cratered terrain than the near side, for reasons we don't yet fully understand. DSCOVR, though, is the first spacecraft to record the Moon transiting the Earth from the wide field of view offered by the Earth-Sun L1 point. The Earth's axis is tilted 21 degrees relative to its orbit around the Sun, and the Moon orbits slightly out of alignment with that tilt; the Moon's orbit is tilted about 5 degrees relative to the Earth's equator. Both of these facts have been understood for centuries, but to actually see it in real images, not to mention the gorgeous faces of the two bodies, is breathtaking.

Filtering Light To Create Images

Like most spacecraft, DSCOVR has a series of filters through which it collects incoming light to generate images. These are selected to accept only light from certain regions of the electromagnetic spectrum that are scientifically interesting to the mission. DSCOVR's 905 nanometer filter, for example, only accepts light from the infrared wavelength in which water vapor in Earth's atmosphere is prominently visible, since climate monitoring is one of its main goals. Similarly but on the other end of the spectrum, its 317 and 325 nanometer filters only accept light from the ultraviolet wavelengths associated with ozone (O3) molecules in our atmosphere. Three of the filters correspond to light that our human eyes can see -- red, blue, and green visible light.

Each of the 10 filters is arranged on a wheel device so that they can be rotated in front of the Charge Coupled Device -- the imager's "eye" (for more on how CCDs collect light to make images, check out this post) When EPIC makes an observation, it rotates each of the 10 filters in front of the CCD and records an image. These 10 images can later be combined in various ways to show the team what's happening in Earth's atmosphere and on its surface.

By combining just the red, blue, and green visible light filter images, the team can generate images that show the natural color that our naked eye would see, like the moon transit animation above. But because the moon moves slightly between when the red filter image is recorded and when the green filter image is recorded, a careful look will show a slight green offset on the right edge of the moon, and a slight red offset on the left edge.

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.

The visual jewel of these new morsels of data is a LORRI image taken by the spacecraft after it had passed Pluto, when it turned itself around to look at the night side of Pluto eclipsing our Sun. The result is incredible:

Pluto's night side, imaged as it eclipses the Sun from New Horizons' perspective. The amount of visible haze stunned the atmospheric team. Credit: NASA/JHUAPL/SwRI

Michael Summers, Co-Investigator, said that this image "stunned the encounter team," and brought tears to the eyes of some of the atmospheric scientists. It's not just gorgeous, it's also a crucial scientific find: the image obviously shows a thick haze layer in Pluto's atmosphere, created by dry particles that are absorbing passing sunlight. On Pluto, these particles are suspected to be aerosolized hydrocarbons like ethylene, acetylene, and hydrogen cyanide, created from atmospheric nitrogen and methane by interactions with ultraviolet energy from the Sun (more on that here).

It was speculated before the fly-by that Pluto may have haze layers, but it wasn't known for sure, or whether they would be substantial. It turns out that Pluto's haze layers extend at least five times higher than they had predicted -- at least 100 miles high. Summers called this "a huge surprise," and conceded that the reason the layers are so much larger is still a mystery.

Yet another atmospheric surprise was the pressure of the atmosphere through which these haze-generating particles travel. Pluto's atmosphere is thought to be in constant flux. Pluto's orbit around the Sun is very eccentric, meaning there is a large difference between its perihelion (point closest to the Sun) and aphelion (point furthest from the Sun). Over the 248 years that it takes Pluto to make one orbit around the Sun, it's expected that this great difference in distances from the Sun, and hence in the amount of solar energy received, would cause dramatic changes to the atmosphere. Pluto reached perihelion in 1989, and since then has been getting further from the Sun in its orbit, causing surface temperatures to drop.

When New Horizons was being developed, it was feared that, by the time it arrived, Pluto's atmosphere would've gotten so cold that it would condense, and "freeze out," falling to the surface as nitrogen snow. There were real concerns that the spacecraft would arrive too late to observe any atmosphere at all. Strangely, and for reasons that aren't yet understood, surface-based observations in the mid and late 2000s were showing the opposite of what was expected -- Pluto's atmospheric pressure at the surface was rising:

Graph presented by the team showing estimates from Earth-based measurements of Pluto's atmospheric pressure at the surface over the time since perihelion (1989). The pressure was counter-intuitively rising, when it was expected to be falling as the atmosphere "froze out." The units on the left are in nanobars. Credit: NASA/JHUAPL/SwRI

It's clear now that New Horizons didn't miss out on Pluto's atmosphere. But a new data point from the spacecraft's REX instrument seems to indicate it was cutting things a bit closely. The REX instrument uses a radio uplink signal, sent from Earth, scheduled to arrive at the spacecraft when the edge of the planet is just about obscuring Earth, from the spacecraft's perspective. The radio signal passes through the thicker atmosphere just above Pluto's surface before being received by the spacecraft. Analyzing how the signal was altered by that passage allows scientists to measure the thickness of the atmosphere at the surface. This is what is referred to as the radio occulation experiment.

The first result from the radio occultation experiment indicates a surface atmospheric pressure much, much lower than expected, and lower than any of the Earth-based measurements above. It suggests that Pluto's surface pressure has decreased by a factor of two in just the last two years. This was another big surprise. Further investigation and contextualization of that measurement is required, and will be conducted when more data comes down from the spacecraft. But it could suggest that the theorized "collapse" or "freeze out" of Pluto's atmosphere as it distances itself from the Sun may have just begun. New Horizons may have gotten to Pluto in the nick of time.

Down on the surface, the focus continued to be on the western side of Tombaugh Regio, including Sputnik Planum and the border region with Cthulhu Regio, which I discussed in detail in this post on Wednesday. In close analysis of the edges of Sputnik Planum, the very large, very smooth region of polygonal ice in Tombaugh Regio, the geology team has identified clear signs of flowing ice, which is invading neighboring territory and surrounding other surface features.

LORRI image of the edge of Sputnik Planum, annotated by the geology team to highlight signs of flowing nitrogen ices. Credit: NASA/JHUAPL/SwRI

At the temperatures on Pluto's surface, as low as -387 degrees Fahrenheit, water ice is as hard and strong as granite rocks are on Earth. Nitrogen, methane, and carbon monoxide, which have a much lower freezing point than water, create much more malleable and volatile ices at these temperatures. Unlike water ice, these ices can flow across the surface, in the same manner that massive ice water glaciers can flow on Earth. At the borders of Sputnik Planum, there are obvious spots where nitrogen ice has filled in old craters, flowed around hills and mountains (like Hillary Montes, shown in the below image), and otherwise intruded upon nearby terrain.

Image of the region where Sputnik Planum meets Cthulhu Regio, buffered by Hillary Montes. Nitrogen ice flow has trickled around Hillary Montes and filled in at least one crater in its path. Credit: NASA/JHUAPL/SwRI

This is another stunner of a result. Like other early finds, it points to sustained, active geology on Pluto, and a very recently evolved surface. How recent?

"There's no reason why this stuff can't be going on today," based on what the team now knows, said mission scientist Bill McKinnon.

Pluto continues to grow more fascinating by the day. Potential massive tectonic features, an atmosphere in flux, and the vast volatile reservoir of Tombaugh Regio have combined to make what Principal Investigator Alan Stern calls "a scientific wonderland." With the first phase of data downlink finished, but much more still to come, Stern reiterated his confidence that the scientific payload selected for New Horizons was the proper one to unravel all of these complicated tales of Pluto, past and presesnt.

"We're going to be able to tell this story very well over the next year," Stern said. With data continuing to rain down from New Horizons, we may be in for a new plot twist each week.