Pluto at Twilight

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.

Pluto landscape in twilight
Plutonian landscapes in twilight, under a hazy sky. Credit: NASA/JHU APL/SwRI

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 at high phase
Pluto at high phase, showing its brilliant backlit hazy atmosphere. Above: The original image after processing to enhance detail. Below: The same image, “unwrapped” such that Pluto’s horizon has been stretched into a straight line. Radial features in the original image appear vertical here; several of the many prominent bright radial rays are marked, candidate sunbeams cast by Pluto’s topography in the twilight sun. Red and cyan plus symbols mark same location in both versions of the image. Many other features are visible, including concentric haze layers in the atmosphere which appear as stacked horizontal bands. Credit: NASA/JHUAPL/SwRI/Alex Parker

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

Pluto's shadows and haze
Top: Shadows cast by mountains upon a low-lying haze at the edge of twilight on Pluto. Bottom: Detail of Pluto’s limb showing discontinuous haze layers just above the horizon. Credit: NASA/JHUAPL/SwRI

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.

Alex Parker
Alex Parker

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.

Art Meets Science in New Pluto Aerial Tour

pluto-flybyI’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.

Since creating the Pluto flyby movie released Aug. 28, I have used the latest images to produce an animation that shows what it might be like to take an aerial tour through Pluto’s thin atmosphere and soar above the surface that New Horizons explored.[[/embedyt]

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.

Stuart Robbins
Stuart Robbins

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.

New Horizons Probes the Mystery of Charon’s Red Pole

Pluto's moon Charon
Details of Pluto’s largest moon, Charon, are revealed in this image from New Horizons’ Long Range Reconnaissance Imager (LORRI), taken July 13, 2015, from a distance of 289,000 miles (466,000 kilometers), combined with color information obtained by New Horizons’ Ralph instrument on the same day. The marking in Charon’s north polar region appears to be a thin deposit of dark material over a distinct, sharply bounded, angular feature; scientists expect to learn more by studying higher-resolution images still to come. (Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

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?

chemical compounds called tholins
Scientists at Johns Hopkins University’s Hörst Laboratory have produced complex chemical compounds called tholins, which may give Pluto its reddish hue. (Image credit: Chao He, Xinting Yu, Sydney Riemer, and Sarah Hörst, Johns Hopkins University)

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.

Carly with a model of Pluto
Carly Howett (Image credit: JHUAPL/SwRI)

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.

Such an exciting time!



To Pluto and Beyond: Animating New Horizons’ Flight Through the Pluto System

Stuart Robbins

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.

Atmospheric Escape and Flowing N2 Ice Glaciers – What Resupplies Pluto’s Nitrogen?


Backlit by the sun, Pluto’s atmosphere rings its silhouette like a luminous halo in this image taken by NASA’s New Horizons spacecraft. Image Credit: NASA/JHUAPL/SwRI

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.

Kelsi Singer
SwRI Researcher Kelsi Singer

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.