One year ago today, on July 14, 2015, the New Horizons spacecraft shot past Pluto and its system of weird moons, making space history (and the history of the mission itself is great reading). It was the first time a probe had been sent to an ice world specifically to study it in detail, and —depending on your personal viewpoint— Pluto was the first dwarf planet ever seen up close, or the last planet seen up close, or the first Kuiper Belt object seen up close.
Whatever. Pluto is what it is, and what it is is amazing, and beautiful, and most of all surprising.
Before New Horizons, we did know some things about Pluto. It has one large moon, named Charon, and four smaller moons. Pluto is very shiny and reflective, probably due to nitrogen snow and ice, while Charon is much darker. Crude maps made using Hubble observations showed that Pluto had darker and brighter regions, but Pluto is just too small and far away even for the world’s biggest telescopes to reveal surface features. So while we knew quite a bit about it, what we didn’t have was details.
That changed 366 days ago.
New Horizons launched from Earth on Jan. 19, 2006, and took more than nine years to reach Pluto, even with a gravity assist from Jupiter (which shaved two years off the mission). That ought to give you an idea of just how far away Pluto is; it took nearly a decade to get there even traveling faster than 50,000 kilometers per hour.
But on July 14, 2015 it finally flew past Pluto and its odd set of moons. And when it did, Pluto went from a blurry dot to a world unto itself, with all that implies.
One of the most important features scientists were anticipating was cratering. The solar system is filled with objects big and small, and over time that means impacts. These leave behind craters, and by counting craters on the surface you can get a decent estimate of the age of the surface. Even as far out as Pluto there are enough objects that cratering was expected.
When the first images came back from New Horizons, what we saw was… huge swaths of Pluto with almost no cratering. That wasn’t just surprising, it was shocking. Most scientists expected Pluto to look a bit like the Moon, with craters densely packed on the surface. Instead, they got places with few or even no craters at all.
A big area devoid of craters means it’s young, relatively speaking. Scientists estimate those regions on Pluto may only be a hundred million years old or less. That’s youthful, geologically speaking!
That in turn means that parts of Pluto have been resurfaced, like an old road having its potholes filled in with fresh material. But what process could possibly do that on cold, frozen Pluto?
It turns out the answer to that may be even more surprising than the lack of craters itself. It’s not as frozen as we were led to believe.
On approach to Pluto, the most obvious feature was a bright region that resolved itself to be a huge heart-shaped plain. The whole feature is called Tombaugh Regio (after Clyde Tombaugh, who first discovered Pluto), and the “left hand” side Sputnik Planum. Although every part of Pluto is interesting, Sputnik Planum turned out to be amazing.
Close up shots revealed it to be very flat, reflective, and covered in segmented plates that look almost like cells. The best guess is that what we’re seeing is a frozen nitrogen shell covering a more fluid interior. Some source of heat deep inside Pluto is causing convection under the surface, with warm fluid rising and cooler material sinking. The segmented plates are the tops of these cells of convection, with warm stuff pushing up in the center of the plates and the cooler material sinking around the edges. The material isn’t liquid as we usually think of it, but more like toothpaste in consistency.
What’s the heat source? Good question. It’s possible that a small amount of radioactive material exists deep inside Pluto, and as those elements decay they warm their environment. In fact, some scientists investigated this possibility due to several very large cracks seen in Pluto’s crust. What they found is nothing short of staggering: Pluto may have an ocean of liquid water under its surface!
The cracks would have been generated as the water closer to the surface froze and expanded, pushing up on the surface. This idea is preliminary, but terribly exciting. If true, it shows that liquid water can exist on (or under) worlds when just a short time ago we would’ve thought it impossible (or extremely unlikely).
Another strange sight on Pluto’s Sputnik Planum are huge fields of what look like pits in the surface, from tens to hundreds of meters in size. These may be sublimation pits; if a small pock or dip exists on the surface sunlight can warm it, turning the nitrogen from a solid directly into a gas (that’s called sublimation), making the dip grow. They appear to form in lines, and sometimes sweeping curves. It may be they formed that way due to stress patterns in the surface, or perhaps they align as the surface moves slowly, responding to the convection from underneath. Either way: Weird. That seems to be the operational word for Pluto.
Pluto doesn’t have plate tectonics like Earth does, so mountain building would be unexpected… and yet, there are mountains on the tiny world. And they’re not little; some tower 3,500 meters (11,000 feet) over the surface! These are likely to made of water ice; at Pluto’s surface temperature (-230° C!) water ice is as hard as rock here on Earth. They’re also coated with a layer of methane ice, which may have fallen as snow just like water does on mountains on Earth.
The thing is, it’s not at all clear what forces pushed these mountains up. They’re far higher than anyone would have guessed, so something with a lot of oomph forced them up. Perhaps it’s connected with the heat source in the core, or the potential liquid water ocean under the surface. Scientists are still scratching their heads over this.
New Horizons screamed past Pluto at high speed, but even after closest encounter it wasn’t done. Once past Pluto it turned around, taking images of it “from behind”, so to speak, with Pluto backlit by the Sun. The idea was to see if Pluto’s ethereally thin atmosphere could be seen.
The image above makes it quite clear it could! But surprises lurked there as well. The atmosphere isn’t just a big puffy shell around Pluto, but layered, with dozens of distinct layers. These are composed of haze, complex organic (carbon-based) molecules built up as ultraviolet light from the distant Sun breaks down simple molecules like methane, which then reorganize themselves into molecules like acetylene and ethylene.
The major component of the atmosphere, though, is nitrogen, just like Earth’s. Also, just like here at home, these molecules scatter sunlight, giving Pluto’s atmosphere a distinctly blue hue! If you were standing on Pluto (and adequately protected) and looked up you’d almost certainly see a blue tint to the sky, especially near sunrise and sunset, when you’re looking through more of the atmosphere toward the Sun.
Pluto has about the most alien landscape in the solar system, yet there is a part of it that is quite Earth-like. How about that?
We knew very little about the large moon Charon before the flyby, other than it was about half the diameter of Pluto, and orbits once every six days (actually, they orbit each other, doing the barycenter dance). When New Horizons flew by, what we got was… Frankenstein’s moon.
Seriously, it looks like someone grabbed a moon, ripped it into big chunks, and then slapped them back together again. Look at it! The southern hemisphere is relatively smooth, though pocked with craters, but the northern half looks, well, wounded. It’s not clear what causes the dichotomy, but it’s likely that Charon and the other smaller moons formed when Pluto was hit by a large object, splashing material into space which coalesced to form the moons. Perhaps the topography is a remnant of that event.
The north polar region of Charon is very dark, and has been nicknamed Mordor Macula (a macula is a spot or blotch, and Mordor is a place you don’t simply walk into). It’s quite red, and no one is sure what it is. It might be an impact basin, or it might be dark due to Pluto’s atmosphere leaking complex carbon molecules into space which then collect there.
Charon also sports a deep canyon called Argo Chasma. It may have the tallest cliffs in the solar system, some nine kilometers top to bottom. If you jumped off the edge, it would take four minutes to fall to the bottom!
Unlike so many other NASA missions, New Horizons didn’t go into orbit around Pluto. The outer solar system is so far away that the probe burned up nearly all its fuel to get there as rapidly as possible, and it would’ve taken a huge amount more to slow the craft down enough to enter orbit. So this was a flyby, and a very rapid one at that. What we got is what we got. Nearly all the data from the flyby have been sent back to Earth now, and will keep scientists busy for decades.
But what’s next? New Horizons sails on, and there’s still more solar system out there. Just a couple of weeks ago NASA gave the go-ahead for a mission extension, granting permission for the spacecraft to visit another object, called 2014 MU69, a Kuiper Belt object that’s probably 40 or so kilometers across and about 6.5 billion km from the Earth. On Jan. 1, 2019, New Horizons will give it a very close shave, passing less than 10,000 km from its surface (that’s less than the diameter of the Earth!), getting our first close-up views of what’s thought to be a pristine remnant left over from the formation of the solar system itself.
What will we see?
Who knows? That’s the beauty of this. If we knew what we’d find, it wouldn’t be exploration. But if we take what we learned at Pluto, then the lesson of New Horizons is clear: Expect surprises.
Ironic, isn’t it? But that’s precisely what we get every time we explore a strange, new world. And there are many, many such worlds awaiting us in just our own back yard. New Horizons may have been the first, but you can bet it won’t be the last.
Corrections, July 14, 2016: I made a couple of errors in the article, and a couple of typos. New Horizons launched on Jan. 19, 2006, not Jan. 26. It passed Pluto at more than 50,000 kph; I originally wrote 40,000. The crater picture caption said they were in Sputnik Planum, but they're north of there. I also misidentified Argo Chasma as Argo Rupes.