Hello! It’s Kelsi Singer again from the New Horizons science team to talk about one of my favorite planetary geologic features –impact craters. They may just look like holes in the ground, but amazingly, craters can give us all sorts of useful clues to a planet’s history.
There are many ways scientists investigate a planet they’re seeing for the first time, and this is one example. With a flyby mission you can’t probe the ice on the surface or analyze samples, so you have other methods to determine a planet’s makeup and age – and gain insight into how it evolved into the world it is today. Analyzing craters can help us understand the age of a planet’s surface.
A surface with more craters indicates that it’s older, geologically, than a less-cratered surface. Pluto displays many good examples of this concept. Any guesses as to which parts of Pluto mission scientists think are younger?
If you went with the informally named Sputnik Planum – the left half of Pluto’s “heart” feature – as a young geologic unit, then you are on the right track. So far we have not identified any obvious craters on Sputnik Planum. We can also try to put an actual date on a surface (e.g., that a given surface is 1 billion years old), but this is more difficult because it requires some knowledge of how often impactors (chunks of space debris) of a certain size hit the surface to make craters.
Age-dating based on craters is complicated by a number of other factors as well. I will highlight one big issue here: How one “sees” craters is affected by lighting over the planet’s surface. Just like on Earth, the lighting on Pluto and Charon changes both with latitude and over the course of the day. When the sun is directly overhead, there are very few shadows cast and it is hard to see topography, but that overhead lighting makes it much easier to see dark or bright markings. The opposite is true when the sun hits the surface at a shallow angle near sunrise or sunset: topography is easy to see, but bright and dark colorations are often washed out.
Because New Horizons flew by Pluto so quickly (in just a few hours out of Pluto’s day, which is 6.4 Earth days), our highest-resolution pictures were all taken under the same lighting conditions. The northern latitudes of Pluto and Charon have the sun mostly overhead, while near the equator the sun hits the surface at an angle. This gives the effect that craters in the north look flatter and have stronger dark/bright contrasts, while craters near the equator look more 3-D.
There are many other complicating factors, such as variable image resolutions and variable surface erosion or degradation. Even fragments of surface material ejected while creating a “primary” crater can pose a problem because they may litter the surface and make smaller, so-called “secondary craters” that influence crater statistics. We are taking all of these factors into account as we map Pluto and Charon’s craters and mine this data to learn more about the history and evolution of these amazing, mysterious worlds.
Pluto and its largest moon Charon dance around each other, making circles around their common center of mass, which lies in an empty space between them. Around the dancing couple are four small moons. In order of increasing distance, their names are Styx (just beyond Charon), then Nix, Kerberos and Hydra. These tiny moons also orbit around the system’s center of mass. The orbits line up like a miniature solar system, except with a binary system at the center, similar to the planetary system around the star Kepler 47. All four of the small moons are less than about 30 miles (50 kilometers) in their longest dimension. Each has a lumpy shape because, unlike Pluto and Charon, they aren’t big enough for gravity to squish them into a ball.
Nix and Hydra were discovered in 2005, shortly before New Horizons launched in 2006, and their initials were a subtle nod to the New Horizons mission that started the search for them, just as the P and L in Pluto are a subtle nod to astronomer Percival Lowell, who began the search for Pluto.
Styx and Kerberos weren’t discovered until 2011 and 2012, well after the New Horizons spacecraft was on its way to Pluto. Although the mission’s observing plans were pretty well set by then, the New Horizons science team anticipated that new discoveries from other facilities might be made during the long cruise to Pluto and had left room for a handful of “TBD” observations, which became the only ones specifically devoted to Kerberos and Styx. That’s why New Horizons took many more pictures of Nix and Hydra than of Styx and Kerberos.
Nix is the second-largest of Pluto’s small moons and was the closest to New Horizons during the flyby, so we got better imaging of it than any of the other small moons. So far, we’ve been able to download close-up pictures of Nix taken at three different times by the Long Range Reconnaissance Imager (LORRI) high-resolution camera, but the best image is still on the spacecraft’s digital recorders waiting to come to Earth.
From looking at Nix as point of light with the Hubble Space Telescope and with New Horizons on approach, we knew that Nix’s brightness regularly changed over time, and therefore it was probably elongated. However, the first image (on the left) really surprised us, because Nix appeared to be round—not at all elongated.
Mark Showalter – who discovered Styx and Kerberos — pointed out that we were probably just looking down the long axis, and that the next images would look more “potato-ish.” Sure enough, the next image showed Nix looking far more elongated, but with one great surprise in it: a big crater! Nix isn’t very large, and there is a very fine line between an impact that will make a crater that big and one that will break Nix apart. So either Nix was very lucky in surviving that collision, or it’s a fragment of an older moon that was somehow destroyed.
The last Nix image we have so far was taken right after the spacecraft passed Pluto and started to look back on its crescent. Because Nix has no atmosphere, it isn’t as spectacular as the images looking back at Pluto, but measuring the brightness of that little crescent of light can help tell us about what the surface of Nix is made of, and whether its surface is smooth or covered in boulders.
What we do know about the big crater on Nix is that it appears to be a different color than the rest of the moon. The color image below was taken by New Horizons’ Ralph-Multispectral Visible Imaging Camera (MVIC) three minutes before the LORRI picture; MVIC has one-fourth of LORRI’s resolution, but it can see in four colors: blue, red, near-infrared, and methane. In this image, the RGB colors are mapped to near-IR, red and blue, just like the enhanced-color images of Pluto and Charon. While most of Nix is a neutral white, the crater and its ejecta blanket (the material thrown out by the crater) appear to be a much redder material. Craters excavate material from below and throw it on the surface. This tells us that under its white surface, Nix is probably made of much darker material. We don’t actually know what either the dark or the light material is, nor will we be able to tell until we download the Nix data from the Ralph-Linear Etalon Imaging Spectral Array (LEISA) composition mapping spectrometer.
New Horizons also imaged Hydra and has sent some of these images to Earth. Below is the best LORRI image of Hydra taken by New Horizons. Unfortunately, Hydra was on the opposite side of Pluto from New Horizons at closest approach, so the images of Hydra are from farther away and therefore are at lower resolution than the Nix images we have. Because Hydra’s orbit was still somewhat uncertain, the mission planners designed this observation to be a mosaic of six slightly-overlapping shots. As it turns out, we hit the jackpot and Hydra fell right at the intersection of four of those six shots, meaning we got two full and two half-views of Hydra for the price of one!
The composite of these images shows that Hydra has a much more complicated shape than Nix and looks a bit like a much bigger version of comet 67P/Churyumov-Gerasimenko, which is currently being orbited by the Rosetta spacecraft. As some have proposed for 67P, it is possible that Hydra is the result of a low-speed collision of two older moons. We haven’t yet had a chance to download the LORRI images of Styx and Kerberos, but they are coming soon, and will be of similar resolution to this image of Hydra.
Finally, I want to tell you how I processed these images of Nix and Hydra. I created them in Python with AstroPy to read the images and translate from pixels to on-sky coordinates, and Scikit-Image to process and sharpen the images. LORRI’s pixels are smaller than the resolution limit of the imager, which makes the images appear a little bit blurry. But since the optics of LORRI are very stable, we can use a process called “deconvolution” to back out what the image would have been if the camera were perfectly sampled. This makes the images sharper, but also adds “noise.” We can minimize the noise by taking several deconvolved images and finding the median value at each pixel location; this keeps the real detail while throwing out much of the deconvolution noise. Both Astropy and Scikit-Image are free and open sources, and make astronomical image processing fun (and a bit addicting) for anyone with a basic knowledge of Python.
Today’s post is written by Alex Parker, a research scientist at the Southwest Research Institute in Boulder, Colorado, working on NASA’s New Horizons mission.
It’s approaching dusk on an alien world, and the only eyes to witness the scene belong to a machine that has traveled billions of miles to be here at just this moment.
Sunlight is filtered through an atmosphere filigreed with layers of haze, and even areas that should be cast into total darkness by the shadows of vast mountains are illuminated by a diffuse glow. Light streaming through gaps between those mountains falls obliquely on a low-lying haze bank, revealing itself as luminous beams in the sky, like those of a dramatic Earthly sunset.
The world is Pluto, the far-from-home machine is New Horizons, the atmosphere is a tenuous skin of nitrogen, carbon monoxide, and methane gases, and the hazes permeating that atmosphere are suspended organic particulates.
Few — if any — of us expected such an alien, remote, and hostile place to look so familiar in twilight.
Hazes by day
Just a few days after New Horizons’ flyby of Pluto, a new set of images came down that surprised us all: a simple look-back at the edge of Pluto, backlit by the sun. Very little of Pluto’s surface was to be illuminated in these images—just a tiny sliver of a crescent. When the images appeared, several of us who routinely process New Horizons images to enhance their detail opened them to see what we could do with them.
And our jaws dropped.
Pluto was surrounded by a brilliant halo, extending far above its surface. The halo was not smooth, either; it was cut through by sharply-defined brighter layers. This halo was our first glimpse of Pluto’s stunningly complex atmospheric haze. We had known for decades that Pluto had an atmosphere and that it might be hazy. What we didn’t expect was just how bright and structured those hazes might be.
We set to work trying to understand what these images might tell us about the nature of the hazes and the dynamics of Pluto’s atmosphere. One of the ways we examine the structure within these images is to “unwrap” them—to peel the planet’s circular horizon back into a straight line. In these unwrapped images, structures that extend radially within and beyond the edge of the planet stand vertically, while concentric layers within the atmosphere appear as stacked horizontal features.
In these unwrapped images, the hazes appear brighter in the evening sky than in the morning sky, possibly suggesting that the hazes and their distribution are controlled by diurnal processes, becoming more concentrated over the course of Pluto’s long day and depleting during Pluto’s long night. Perhaps the haze particles gently rain down onto the surface through the night, staining Pluto with a distinctive reddish cast, or perhaps other atmospheric processes act to move and concentrate the haze.
Sunbeams on the Horizon
Another feature in the unwrapped images struck us: subtle bright traces running inward along the planet’s edge, well beyond where the sunlight should be touching its surface. These had the characteristics of “crepuscular rays” — beams of sunlight illuminating particulate in the atmosphere — cast between gaps in topography near Pluto’s terminator. Simply put, we thought we might be seeing sunbeams in Pluto’s twilight sky.
The most spectacular and unequivocal example of these came as their counterpart: long, narrow rays of shadow cast in an otherwise bright haze. These appeared in the recent stunning Ralph image of Pluto’s landscapes in twilight. Their presence indicated that Pluto’s hazes are not limited to high altitudes— some must hug the surface, similar to a low cloud deck or a fog bank. This same MVIC image revealed many more fine, high-altitude haze layers, including discontinuous, quasi-periodic structures suggestive of shaping by atmospheric waves.
More images of Pluto’s skies are coming back from New Horizons. With each one we learn more about this distant world’s delicate atmosphere with its tracery of hazes, but we still have many questions left to answer. How does Pluto’s atmosphere interact with and shape Pluto’s surface? How is the atmosphere evolving as Pluto’s orbit caries it farther from the Sun? How do Pluto’s hazes relate to those of other worlds — including the hazes likely present in the Earth’s early atmosphere?
Earth, again. There is much about Pluto that is exotic and superficially incomprehensible, but beneath that there is a constant theme of familiarity. We can imagine that a far-flung human explorer, standing on the cracked and pitted ice of Tombaugh Regio, might gaze upward at the twilight sky and think it looked a bit like home.
I’m Stuart Robbins, a research scientist at the Southwest Research Institute in Boulder, Colorado. NASA’s New Horizons spacecraft made hundreds of individual observations during its flyby of the Pluto system in mid-July. The spacecraft is now sending back lots of image and composition data; over the past two weeks, New Horizons has returned to Earth dozens of images at up to 400 meters per pixel (m/px) of the flyby hemisphere, and this has given scientists and the public an unprecedented view of this mysterious world.
I primarily use these images to map craters across the surfaces of Pluto and its largest moon, Charon, to understand the population of impactors from the Kuiper Belt striking Pluto and Charon. While this is my research focus, another interest of mine is figuring out how to make visualizations that convey some of the sheer beauty and power of the features New Horizons is revealing. With that in mind, I’ve created a new animation/flyover of Pluto using images returned this month by New Horizons.
The latest images (as of Sept. 11, 2015) downloaded from NASA’s New Horizons spacecraft were stitched together and rendered on a sphere to make this flyover. This animation, made with the LORRI (Long Range Reconnaissance Imager) images, begins with a low-altitude look at the informally named Norgay Montes, flies northward over the boundary between informally named Sputnik Planum and Cthulhu Regio, turns, and drifts slowly east. During the animation, the altitude of the observer rises until it is about 10 times higher to show about 80% of the hemisphere New Horizons flew closest to on July 14, 2015. Credit: NASA/JHUAPL/SwRI, Stuart Robbins
The mosaic used in this animation was carefully constructed by New Horizons science team members with some of the latest images from the spacecraft to provide an incredibly accurate portrayal of Pluto’s surface. The mosaic starts with images of the “heart” of Pluto – informally named Tombaugh Regio – and the immediate surrounding area that are up to 400 m/px. The mosaic then includes other images of the hemisphere New Horizons flew over that are up to 800 m/px and were released last week. The rest of the mosaic that’s shown uses images at up to 2.1 km/px.
Our tour starts low over the informally named Norgay Montes at a height of about 120 miles (200 kilometers). These jagged mountains rise almost 2 miles (3 kilometers) from the surrounding surface. We head north over Sputnik Planum (bright area to the left) and Cthulhu Regio (dark area to the right). While Sputnik Planum is smooth at this pixel scale, it’s in marked contrast to Cthulhu Regio which has many large impact craters that indicate the Regio is much older. The differences in brightness are some of the largest natural brightness variations of any object in the solar system.
Our view steadily rises to a height of about 150 miles (240 kilometers) and turns to look east. From this point, we drift slowly to the east, with Pluto’s north pole to the left, Tombaugh Regio filling much of the middle of the view, and older, more cratered areas standing out in marked contrast to the younger glaciers of the “heart’s” left lobe, Sputnik Planum. As we continue to fly, our flight path rises to more than 1,500 miles (2,500 kilometers) with the final view of most of the disk that New Horizons saw on July 14.
The concept of this animation arose from a desire to showcase the most recent imagery received from the spacecraft and the huge variety of terrain types that we see on Pluto. I can hardly wait until we get even better imagery – up to seven times better pixel scale – that’s still to come of select areas of the surface and to see what new surprises Pluto has in store.
Hi, I’m Carly Howett, a senior research scientist at the Southwest Research Institute in Boulder, Colorado. I’ve been working on NASA’s New Horizons mission since 2012, focusing on an instrument named Ralph, which among other things provides the color “eyes” for the spacecraft.
When I started looking at Ralph images of Pluto and its largest moon, Charon, back in 2012, the bodies were so far away they appeared as just a speck of light, too close together to see separately. So you can imagine how excited I was to see Pluto and Charon not only as separate worlds this year, but with clear and different features across them. It is these differences, specifically across Charon, which have since been the focus of my work.
Surfaces vary in color when something about them changes; this difference could be due to composition (what the surface is made of) or physical state (changes between solid and liquid, or changes in their structure – for example at high-pressure carbon changes from graphite to diamond). We see this every day on Earth. For example, water looks different compared to sand and they both look different than ice. Another example of these differences is that carbon forms both the dark-colored graphite we use in pencils and clear sparkly diamonds. Looking at Charon, it’s very clear that the northern polar region is much redder than the rest of the moon. But what’s causing this color difference and why does it occur at the pole?
To answer the first part of this question we consider what we know about Charon. We know that Charon’s surface is too cold for anything other than solids to exist, and the surface isn’t subject to extreme changes in temperature and/or pressure, so it is unlikely significant phase transitions are occurring. Instead, we think that the color variation is due to a change in surface composition, which leads to the conclusion that the surface of Charon’s northern polar region is made up of different material than the rest of Charon.
One theory is that small amounts of Pluto’s atmosphere can escape and eventually reach Charon, where it would be temporarily trapped by Charon’s gravity before escaping to space. Charon’s polar regions are very cold, and I mean VERY cold! In fact, over the course of Charon’s year the polar temperature varies somewhere between -433 and -351 °F (-258 and -213 °C), which is only tens of degrees warmer than absolute zero. These temperatures (especially with Charon’s extremely thin atmosphere) are too cold to support surface liquid: gases are deposited straight to solids, and solids sublimate directly to gases. So — unlike at Charon’s warmer equator — any gases that arrive on the winter pole would freeze solid instead of escaping, a process scientists refer to as “cold trapping.” The basic principle that binary systems can share material is not new, but it took New Horizons to visit Charon to see its effect firsthand!
We know Pluto’s atmosphere is mainly nitrogen, with some methane and carbon monoxide, so we expect that these same constituents are slowly coating Charon’s winter pole. The frozen ices would sublimate away again as soon as Charon’s winter pole emerges back into sunlight, except for one important detail: solar radiation modifies these ices to produce a new substance, which has a higher sublimation temperature and can’t sublimate and then escape from Charon.
This new substance is called a tholin, and has been made in similar conditions in laboratories here on Earth. The color of the tholin produced depends on the ratios of the different molecules and the amount and type of radiation you expose them to: tholins colored from yellow to red to black have been made this way. An example of this (pictured above) shows various red tholins made in a laboratory by Sarah Hörst at Johns Hopkins University.
Charon likely has gradually built up a polar deposit over millions of years as Pluto’s atmosphere slowly escapes, during which time the surface is being irradiated by the sun. It appears the conditions on Charon are right to form red tholins similar to those shown, although we have yet to figure out exactly why. This is one of the many things I am looking forward to better understanding as we receive more New Horizons data over the next year and analyze it in conjunction with continued laboratory work.
An exhilarating, pioneering journey came to fruition on July 14, 2015, as NASA’s New Horizons spacecraft made its successful flight through the Pluto system, recording 60 gigabits of data that it is beginning to send to Earth. I’m Stuart Robbins, a research scientist at the Southwest Research Institute in Boulder, Colorado. While I only came onto the project relatively late – in 2012 – I’ve been able to interface with many different groups and people on New Horizons because my primary role was in planning.
The New Horizons mission is one of opportunity, not just in exploring a world in a region to which we’ve never been, but also for people who have a variety of backgrounds, interests and skills. One of my hobbies has been exploring computer-generated images and animations, and I volunteered to create the fly-through animation on this page.
The Pluto system as NASA’s New Horizons spacecraft saw it in July 2015. This animation, made with real images taken by New Horizons, begins with Pluto flying in for its close-up on July 14; we then pass behind Pluto and see the atmosphere glow in sunlight before the sun passes behind Charon. The movie ends with New Horizons’ departure, looking back on each body as thin crescents.Credit: NASA/JHUAPL/SwRI, Stuart Robbins
I strive for realism, so my first step was to build an accurate Pluto system within a 3-D environment. I used the latest data on Pluto’s orbit, its obliquity (how its pole is tilted relative to its orbit), and the orbits of all the known moons to create the system in software. I then “attached” a camera to the latest trajectory information so it would be as if you had a seat on New Horizons, watching Pluto as you zoomed past. I also worked on the lighting so that even the shadows as the spacecraft passes are at the correct angles, and the crescents during departure are at the correct positions.
In my original version, each frame (1/30th of a second) represented one minute of real time, and the field of view was that of the Long Range Reconnaissance Imager (LORRI), New Horizons’ eagle-eyed, black-and-white camera that gives us our closest views.
Unfortunately, the result was cinematically questionable, at best, because of the very brief time that the spacecraft gets its best images and the extreme change in distance between the spacecraft and planetary system over the course of July. I needed an alternative.
The final result was made differently: First, the timescale had to be variable. The final product goes from one second of movie time equaling 30 hours at the beginning and end, to one second of movie time equaling 30 minutes for the closest-approach section.
Second, the field of view could not remain as LORRI if the trajectory were to be realistic. I varied the field of view so that you can see the whole system at the beginning and end, and you can still see Pluto almost as a whole disk during the closest approach.
Third, the camera’s target – what’s in the center of the field of view – had to also vary. The movie starts and ends with the camera targeting the barycenter, the mutual point around which Pluto, Charon and the other four moons orbit. As the movie appears to zoom in for the Pluto flyby, the focus shifts to Pluto itself, and then it moves off Pluto so that it does not appear as though you are about to crash into the surface nor fly through the planet. The camera target remains on Pluto for the solar occultation – when the sun passes behind it – and then moves back to the barycenter for the solar occultation by Charon.
Fourth, the small moons – Styx, Nix, Kerberos and Hydra – were simply too small and faint to be seen to-scale. So I enlarged them by a factor of 5 and brightened them so you can at least see the two larger ones (Nix and Hydra), and I drew in their orbital paths.
Beyond that, everything about the movie is accurate: The Pluto hemisphere we see on closest approach, the lighting and shadows, the atmosphere’s size (though its brightness has been increased), the orbits of the satellites, the colors are our best estimate for what your eye would see, and so on. In addition, this movie retains Celestial North as “up” so that there are no twists, turns nor odd reorientation during the flyby.
The final result is the system as New Horizons saw it at the beginning of July 2015, flying to Pluto for its close-up on July 14, complete with the best maps we have to-date. It’s an incredible look at system we are unlikely to revisit in our lifetimes – though we have the potential to visit other bodies farther still from the sun with the craft as it continues to reveal new horizons in our solar system.
Hi, I’m Kelsi Singer, a postdoctoral researcher at the Southwest Research Institute, working on NASA’s New Horizons mission and specializing in geology and geophysics. One of my areas of expertise is impact cratering. That subject may not seem related to Pluto’s atmosphere or nitrogen at first, but let me tell you about research that New Horizons principal investigator Alan Stern and I conducted and published as a prediction paper before the flyby of Pluto.
New Horizons has returned striking images of both Pluto’s surface and its atmosphere. Pluto’s atmosphere is similar to Earth’s in that it is predominantly composed of nitrogen (N). But Pluto’s atmosphere is ~98% N, while Earth’s is only ~78% N. Pluto’s atmosphere is also considerably thinner than Earth’s with ~10,000 times lower pressure at the surface.
The nitrogen in Pluto’s atmosphere (in the form of N2 gas) is actually flowing away and escaping the planet at an estimated rate of hundreds of tons per hour. We also see what looks like flowing ice on Pluto’s surface in high resolution images made by New Horizons. The water ice (H2O) that we are familiar with on Earth would be completely rigid and stiff at Pluto’s surface temperatures, but ice made out of N2 would be able to flow like a glacier. So where does all of this nitrogen come from?
One possibility we tested was that cometary impactors could be delivering the necessary material. We explored several different ways that impacts from comets could bring nitrogen to the surface and atmosphere of Pluto and resupply the escaping nitrogen:
1) Could comets hitting Pluto directly deliver enough N to Pluto’s surface and atmosphere?
2) Could these comets excavate or expose enough N2 ice from the near-surface layers on Pluto by forming impact craters?
—> The short answer is that none of these cratering effects seem like they could supply enough nitrogen.
In our prediction paper, we suggested the next most likely suspect for supplying this N is heat and geologic activity inside Pluto itself. This activity could process nitrogen out of Pluto’s rocky interior and get it to the surface. We currently have only a tiny fraction of the data back from the New Horizons flyby, but the fact that there are young-looking areas on Pluto hints at relatively recent geologic activity.
Stay tuned as we get more data back from the New Horizons spacecraft over the coming months, which will refine our estimates of Pluto’s atmospheric escape and provide more images of Pluto’s surface to assess the types and timing of geologic activity.