Rotational Movies of Pluto and Charon: It’s Show Time!

Today’s blog post is written by Constantine Tsang, a senior research scientist at the Southwest Research Institute (SwRI) in Boulder, Colorado. Con was a member of the New Horizons’ Geology, Geophysics Investigations (GGI) and Composition teams during the Pluto flyby, creating approach and photometric stereo movies of Pluto’s terrain.

It’s amazing that we’ve come such a long way in our exploration of the Pluto system, and it’s only been five months since the close flyby of New Horizons. From the exceptionally young ice-covered plain informally named Tombaugh Regio on Pluto to the deep canyons cut into Charon, the terrains we’re seeing are just amazing. Everyone, from the mission scientists to the general public, seems to be having a field day coming up with pet theories and comparisons with other places in our solar system to explain the alien Pluto system worlds we’re seeing.

New Horizons Team
The New Horizons Science Team Meeting in Boulder, Colorado, in November 2015. Mission scientists enjoyed the fruits of their labor by viewing stereographic projections of geology 3-D terrain maps of Pluto and Charon. Credit: Constantine Tsang

Approaching the Pluto System

To put this into context, I’d like to take us back a few weeks just prior to the July 14 flyby, before we got the exquisitely detailed images we routinely downlink now. During that phase, we were getting longer range, low-resolution views of Pluto and Charon, and my job was to create approach movies showing New Horizons rushing up to meet the pair in space.

In workrooms at the Johns Hopkins Applied Physics Lab (JHUAPL) in Laurel, Maryland, I and many of the science team members had arrived for the flyby, and were working seven days a week to keep on top of the data flowing in. The majority of the science data being received at this time were in the form of panchromatic (black and white) images from the telescopic LORRI imager on New Horizons. These Optical Navigation images, or “OpNavs,” came in different flavors, in part based on their exposure times. These images are used to refine the approach trajectory of New Horizons and to search for hazards on approach. Sequencing these images together had the added benefit of allowing us to make movies of Charon and Pluto rotating on their axes and orbiting one another at closer and closer range.

The Pluto System in the Barycentric Reference Frame

The procedure I used to make these movies was not trivial, mind you. Each movie imager, or “frame,” from LORRI was, in fact, a stack of four separate images, taken at slightly different times. This allowed us to “sub-sample” Pluto’s (and eventually Charon’s) disk to get the best possible spatial resolution out of the LORRI telescope. This work was mainly done by science team member and image processing expert Tod Lauer at the National Astronomical Optical Observatories.

Because we wanted to get the best resolution out of the data, Tod and I enhanced the images using a technique called image deconvolution, which sharpens them. The practical problem with this was we didn’t sometimes know what features were real on Pluto and Charon (because we’ve never been there before!), and what were potentially introduced as artifacts by the deconvolution process. So we deconvolved the images separately, using multiple techniques, and then compared our results to see what features at the edge of resolution were common to differing image processing techniques—we knew we could trust those. Needless to say, I was gratified to see such features pop out from the LORRI images consistently.

Because Pluto was in a slightly different place in each frame, I then co-registered and centered Pluto to create a single movie Plutocentric frame that gave the approach movies the appearance of a motionless Pluto at the center of each movie. Every few days, another set of images was taken and I repeated the procedure. But I wasn’t quite done. To remove the barycentric “wobble” caused by Pluto and Charon tugging on each other, I then took each of the frames and co-registered them against a background star that appeared in the field of view of all the frames. Every 6.4 days, Charon would make a full rotation around Pluto, and I could compile a new rotation movie of Charon going around Pluto (see Figure 1). In these movies the features on Pluto would rotate, getting bigger in the field of view with each image, and we could finally begin to see the surface details that are so obvious now (see Figure 2).

On each movie frame, I also printed in ancillary data such as distance to Pluto and time to closest approach. I was humbled to be part of the process of giving the world its first look at the Pluto system up close.

As time went on, and we got data from the MVIC color camera aboard New Horizons, I could also overlay the color information on the monochromatic images to colorize the movie.

Approach Movie
Figure 2: An approach movie I made comprised of LORRI images of a number of Charon rotations about Pluto, in the Plutocentric reference frame. The regions informally named Cthulhu Regio (dark region at the “bottom” of the Pluto image) and Tombaugh Regio (bright feature next to Cthulhu) could already be made out weeks before closest approach. Aside from one or two MVIC color frames, these images and movies were all we had before the July 14 flyby. Credit: NASA/JHUAPL/SwRI
Approach movie
Figure 2: Same as figure 1, but presented in the barycentric reference frame. Credit: NASA/JHUAPL/SwRI


You may wonder why this process is at all relevant now, given the incredible high resolution images we got later. Well, here are a couple of reasons. First, the approach movies contain data on parts of Pluto and Charon that were not imaged at closest approach. These images will be used to get as much information out as possible about the Pluto system, its global geography, its surface properties, and potential temporal variations as we approached. Second, the barycentric movies are a great visualization of the two-body binary system and provide an invaluable teaching tool for educators and the general public. Finally, I think it just looks cool! It puts into perspective how we on New Horizons and NASA are always exploring, and how far we have come to explore the Pluto system.

Constantine Tsang
Constantine Tsang
Credit: Rayna Tedford

Pluto, Closer to Home

Today’s post is from Veronica Bray, a planetary scientist at the Lunar and Planetary Laboratory in Tucson. She specializes in comparing the surfaces of planetary bodies across the solar system.

I love looking at New Horizons’ images of Pluto! But I spend most of my time looking elsewhere. Why? Because comparing Pluto with other planetary bodies helps me to understand what processes could be operating on Pluto’s surface and beneath its icy crust. Although a full understanding of planetary processes is a more complicated matter, the initial steps that I take as a comparative planetologist are simple: compare what the features look like on the different bodies.

Pluto’s surface is comprised of water ice and other exotic types of ice (e.g. methane, carbon monoxide, nitrogen). This makes comparison to the icy moons of the outer solar system a logical place for me to look for analogous landforms. However, as the close-up images of Pluto came back from New Horizons, I was reminded of places closer to home. This blog post presents two examples of features on Pluto that remind me of landforms on Earth and Mars.

Polygons: Polygons on a planetary surface typically have five or more sides and can form in several different ways. When the New Horizons team first saw the polygons of the icy plain on Pluto that we informally call Sputnik Planum (Figure 1B), a number of questions arose: Were these patterns due to the heating up and cooling of the surface, leading to expansion and contraction cracking like the polygons seen on Mars (Figure 1A)? Was sub-surface convection of warmer ices creating a cracked surface above the convection ‘cells’ as can be seen in the surface ice of frozen lakes on Earth (Figure 1C)? Or were they similar to ‘dessication’ mud cracks (Figure 1D) formed by the drying out of the surface material? We each had our own theory and the team buzzed with discussion about what might be going on. We showed one another pictures taken by spacecraft in other places it the solar system; we even shared photos that we ourselves had taken from business trips and fieldwork! All so that we could take the next step in understanding these features: to compare the morphology.

The polygons on Pluto’s Sputnik Planum have edges that are smoother and more curved than the linear sides of the mud cracks or Martian freeze-thaw polygons. Instead, the polygons of Sputnik Planum most closely resemble those formed by sub-surface convection (although on a MUCH larger scale than the example used in Figure 1C.) Our current understanding of the polygons of Sputnik Planum is that they mark the top of convection cells within a slowly churning mass of nitrogen and carbon monoxide ices. So, although I was initially reminded of processes on the Earth and Mars, this process on Pluto is far more exotic and has never been seen anywhere else in the solar system!

Examples of polygons on the Earth, Mars and Pluto.
Figure 1: Examples of polygons on the Earth, Mars and Pluto. A) Freeze-thaw polygons in the periglacial terrain of Mars. In this image from NASA’s High Resolution Imaging Science Experiment, frost is highlighting the cracks at the edges of each polygon. Looking closely we can see smaller polygons within the larger polygons. The image is approximately 12 miles (20 kilometers) across. Credit: NASA B) New Horizons’ image of a ~ 120 mile (200 kilometer) wide section of ‘Sputnik Planum’ on Pluto. Credit: NASA/JHUAPL/SwRI C) Fracture polygons on a frozen lake on Earth. The image is approximately 13 feet (4 meters) across. Credit: Paul Schenk D) Mud crack polygons on Earth. This image shows large polygons (~ 33 feet or 10 meters across), and the smaller mud cracks within them, created as a muddy surface dries out. See it for yourself at Red Lake Playa in Death Valley. Credit: Jason Barnes

Impact Craters:

I can usually count on impact craters to be present to assist me with comparing planetary surfaces. All but a few surfaces in the solar system are scarred by hypervelocity impacts that penetrate down into the crust. I compare the shapes of impact craters on Pluto and other bodies to investigate the crustal and sub-surface properties.

Figure 2 shows examples of ‘central pit’ craters – large complex craters with a pit at or near their center. The diameter of the pit compared to the diameter of the crater for the Pluto example (Figure 2B) is similar to the Martian example (Figure 2A), which might suggest a similar formation mechanism. Although various types of pits or pitted-peaks can be found in craters across the solar system, this type of central pit crater has a particularly large pit relative to the crater size and is only found on ice-rich bodies. Their formation has consequently been linked to the presence of water ice in the crust. It is not surprising then that we found this central pit on Pluto in the informally-named Cthulu Regio, in an area of noted water-ice content. But of course, the story is never that simple. The presence of sub-surface layering has also been suggested as a reason for the formation of central pits.

I am currently measuring and comparing the central pit craters across the solar system to determine why and how central pits form, and consequently, whether their presence on a planetary surface can be used as a prospecting tool for water ice or target layering. The new observation of a central pit crater on Pluto from New Horizons provides another important data point to add to my quest to understand how these craters form.

: Examples of central floor pit craters on Mars and Pluto.
Figure 2: Examples of central floor pit craters on A) Mars and B) Pluto. Scale bars are approximately 12 miles (20 kilometers) across. The Mars image is from ESA’s High Resolution Stereo Camera and features a crater in the Thaumasia Planum region. Credit: ESA. The Pluto example shows an as yet unnamed crater in the dark terrain of Cthulu Regio. Credit: NASA/JHUAPL/SwRI
Veronica Bray
Veronica Bray
Credit: Ken Sterns

Radio Signals from Earth Probe Pluto’s Atmosphere

Today’s post is from William Woods, a doctoral candidate in Electrical Engineering at Stanford University. He investigates radioscience and remote sensing, with a focus on signal processing for the radioscience experiment (REX) onboard New Horizons. He studies both engineering and science under Dr. Ivan Linscott and Dr. Howard Zebker at Stanford.

Pluto's atmosphereI study remote sensing and signal processing for the radioscience experiment (REX) onboard New Horizons. REX performs three experiments at the Pluto system. The first is radio occultation, in which NASA’s Deep Space Network (DSN) and REX form a radar system that probes the atmosphere of the occulting body. REX performs radio occultation on both Pluto and Charon that yields estimates of atmospheric temperature and pressure as a function of altitude. REX also performs thermal scans of Pluto and Charon. Thermal scans passively measure blackbody radiation emitted by the target in order to estimate surface temperature. Finally, REX data estimates mass of the entire Pluto system by measuring curvature in New Horizons trajectory due to local gravity conditions.

The spreading of energy as it travels through the atmosphere to REX severely limits spatial resolution of raw radio occultation data. Without extensive post-processing this resolution can be thousands of times worse than the experiment’s design specifications. I develop linear and non-linear techniques that focus energy in REX occultation data down to a resolution near the fundamental limit of the instrument itself.


Figure 1 above shows the geometry of New Horizons’ radio occultation of Pluto. Historic occultation techniques exchange the locations of transmitter and receiver, but experiment fundamentals remain the same. Earth DSN transmits a monochromatic plane-wave of 7.2 GHz to New Horizons as it flies behind Pluto. The signal passes through Pluto’s atmosphere and refracts, resulting in a small change of direction, exaggerated for clarity in Figure 1. The direction change manifests as a Doppler shift in frequency, therefore REX measures a signal rate slightly different from the original 7.2 GHz. The direction change, or bending angle, is proportional to dielectric constant in the atmosphere. Given prior knowledge of Pluto’s atmospheric composition, we use dielectric constant to calculate temperature and pressure in the atmosphere as a function of altitude.

Energy in the plane wave spreads as it passes Pluto

Energy in the plane wave also spreads as it passes Pluto’s limb, shown above in Figure 2. This energy spreading, called diffraction, causes each single data point measured at REX to be a weighted sum of energy from many points in Pluto’s atmosphere. Without focusing the energy back into its original point locations in the atmosphere we can only achieve spatial resolution on the order of a few kilometers. But once focused during post-processing the REX radio occultation at Pluto measures atmospheric temperature and pressure at absolute accuracies of 3 K and 0.1 Pa, respectively, with spatial resolution of a few meters. Preliminary results from REX data processing show surface pressure on Pluto of approximately 10 uBars.

My final contribution to New Horizons is a short video that explains the significance of the mission to the broader public. I attempt to put the science and engineering of New Horizons into the human context of Dr. Ivan Linscott, my PhD advisor at Stanford University. Enjoy.

Ten years ago, NASA launched the first ever mission to Pluto. On July 14, 2015 New Horizons reached its destination. And what we found may change our understanding of life on Earth. Credit: Red Dwarf LLC

Will Woods
Will Woods, Stanford University. Credit: Kyle Cassidy


In the Shadows of Pluto and Charon

Today’s post is written by Josh Kammer, a New Horizons postdoctoral researcher at the Southwest Research Institute (SwRI) in Boulder, Colorado. Josh came to SwRI directly after his PhD in planetary science from Caltech; his undergrad work in chemistry was at Texas A&M. Josh’s work on New Horizons focuses on analysis of ultraviolet spectra acquired by the Alice instrument.

occultations of Pluto and Charon
The geometry of the occultations of Pluto and Charon, as observed by New Horizons. Credit: NASA/JHUAPL/SwRI

As someone who primarily studies atmospheres, I think the most exciting observations made by New Horizons this past summer were the solar occultations by Pluto and Charon. Achieving the required alignment of spacecraft, planet and the sun during an occultation was a difficult challenge – especially when there was a significant amount of uncertainty in the exact position of Pluto at the moment of New Horizons’ flyby. However, the mission planners and our navigation team were able to take this uncertainty into account, and not only managed to thread the eye of the needle by flying New Horizons through Pluto’s shadow, but through a portion of Charon’s shadow as well!

Chasing After Shadows

So why is this so exciting from an atmospheric science point of view? Well, scientists had previously used stellar occultations by Pluto and Charon as seen from Earth to probe for possible atmospheres. In those cases, they looked at more distant stars as they passed behind Pluto and Charon. From the dips in starlight, they found plenty of evidence for an atmosphere around Pluto, but nothing to indicate an atmosphere around Charon.

There are limitations to ground-based observations of stellar occultations, however – the Earth’s own atmosphere absorbs starlight as well, especially at many scientifically valuable wavelengths. Also, the brightest star in the sky, the sun, can’t be used for such observations from Earth, since the observer needs to be on the other side of Pluto to find the sun’s shadow. New Horizons was, therefore, the first spacecraft in history to be able to observe such a solar occultation in the Pluto system – and its proximity meant that it had the highest sensitivity by far to detect the individual constituents of Pluto’s atmosphere, and to probe even closer to find a possible atmosphere around Charon. Being outside the atmosphere of Earth also meant that the whole solar spectrum was available for study, including a key region of the ultraviolet where many molecules interact with sunlight.

Scanning for ‘Fingerprints’ in the UV

spectrum showing the absorption features of several atmospheric molecules in the far-ultraviolet
Example spectrum showing the absorption features of several atmospheric molecules in the far-ultraviolet. For a given line of sight abundance, each component contributes differently to the overall transmission of sunlight in the UV. Credit: SwRI/Josh Kammer

So how do we detect different components of atmospheres using occultations? Depending on composition, atmospheres will absorb sunlight more strongly or weakly at different wavelengths. Each individual component of the atmosphere has its own ‘fingerprint’ that varies with wavelength – and one of the most revealing parts of the solar spectrum is in ultraviolet (UV) wavelengths, where even very thin atmospheres absorb strongly. Mission scientists designed the Alice UV spectrograph onboard New Horizons to detect the ‘fingerprints’ of a wide range of possible molecules that we expected to find on Pluto, including nitrogen (the most prevalent molecule in our own atmosphere), as well as hydrocarbons like methane.

These particular molecules are interesting for many reasons. For one, Pluto is so cold at its distance from the sun that these components of its atmosphere can freeze onto the surface and form nitrogen and methane ice. These same molecules can also react in the atmosphere to form heavier hydrocarbons, nitriles, and even hazes, which can clearly be seen in some spectacular images of Pluto already sent back by New Horizons.

Analyzing the Data

Alice Occulations
This figure shows how the Alice instrument count rate changed over time during the sunset and sunrise observations. The count rate is largest when the line of sight to the sun is outside of the atmosphere at the start and end times. Molecular nitrogen starts absorbing sunlight in the upper reaches of Pluto’s atmosphere, decreasing as the spacecraft approaches the planet’s shadow. As the occultation progresses, atmospheric methane and hydrocarbons can also absorb the sunlight and further decrease the count rate. When the spacecraft is totally in Pluto’s shadow the count rate goes to zero. As the spacecraft emerges from Pluto’s shadow into sunrise, the process is reversed. By plotting the observed count rate in the reverse time direction, it is seen that the atmospheres on opposite sides of Pluto are nearly identical. Credit: NASA/JHUAPL/SwRI

Immediately following the flyby, initial results from Alice and New Horizons included count rates of the combined amount of sunlight in the UV, which was consistent with our expectations of a nitrogen atmosphere with other hydrocarbons present like methane. These absorption profiles revealed for the first time just how far the atmosphere of Pluto extends, and also showed that it is very symmetric during both sunrise and sunset on Pluto. There was also tantalizing evidence for some kind of haze that could be seen over 100 miles (160 kilometers) above the surface.

It wasn’t until just last month that New Horizons sent back the spectra to Earth with information about the transmission of sunlight as a function of wavelength in the UV. This exciting new data allows us to analyze the individual chemical components of Pluto’s atmosphere even further, determining how composition changes with altitude. We are also using similar data for Charon to probe even deeper for a possible atmosphere there, too – these results and a lot more are coming soon, so stay tuned!

Josh Kammer
Josh Kammer, Credit: Michael Wong

A Planet for All Seasons

Today’s post is written by one of the early career members of the New Horizons Science Team. Alissa Earle is a graduate student in Planetary Science at the Massachusetts Institute of Technology. Her work focuses on the long-term seasonal variations that may be affecting what we see on Pluto’s surface.

Pluto’s diverse surface, typified by the smooth bright plains of Sputnik Planum being adjacent to the dark terrains of Cthulhu, defies any easy explanation for how it got that way. In my work, I am trying to explore how “seasons” on Pluto might be part of the explanation.

There are two reasons for Pluto having seasons. The first is for the same reason that Earth has seasons. The second is unique to objects with elongated orbits, like Pluto.

Both Earth and Pluto have seasons because their spin axes are “tipped over.” Earth has a rather modest tilt, only about a 23-degree slant compared with being straight up-and-down in its orbit. The consequence of this tilt is that over the course of one year, the North and South Poles take turns being tipped toward the sun. Earth’s North Pole is tipped more toward the sun in June, and six months later with the Earth being on the opposite side of the sun, Earth’s South Pole is tipped more toward the sun in December. This gives us alternate seasons of summer and winter, even though the Earth is in a nearly perfect circular orbit.

Earth diagram
Earth’s seasons are caused by its axis being tilted about 23 degrees. Credits: MIT/Alissa Earle

While Earth’s 23-degree axis tilt is mild, Pluto’s axis tilt is extreme: 119.5 degrees! What this means is that Pluto’s north pole is nearly upside down compared with Earth. What’s more, the extreme tilt gives very extreme seasons. On Earth, the midnight sun in summer and arctic darkness in winter are limited to the highest latitudes on Earth (the Arctic Circle at 67 degrees north and Antarctic Circle at 67 degrees south). On Pluto, the midnight sun and long arctic darkness affect nearly the entire planet. The equivalent for Earth would mean all of North America and Europe would experience months of midnight summer sun and then months of darkness in winter. Because Pluto takes 248 Earth years to go around the sun, these long seasons last more than a century.

But wait, there’s more. The second factor driving Pluto’s seasons is that its orbit is not circular. Instead, it is very stretched out into an ellipse. (To imagine an ellipse, think of the silhouette of an egg tipped over on its side.) Earth’s orbit is also elliptical, but to a much smaller degree than Pluto’s.

Pluto diagram
Since Pluto has a much higher axial tilt of roughly 119.5 degrees, it has much larger arctic and antarctic circles, each stretching from roughly 29.5 degrees latitude to their respective poles. This means that at the peak of Pluto’s summer the area receiving the most direct sunlight is actually in the arctic circle. Credits: MIT/Alissa Earle

During its orbit, Pluto passes as close to the sun as about 30 times the distance between the Earth and the sun (bringing it even closer to the sun than Neptune) and travels as far away as about 50 times the Earth-sun distance. This means during some seasons on Pluto, the effect is even more dramatic: for example, “hotter summers” for whichever part of Pluto is tilted toward the sun at the closest point in its orbit. This effect is not unique to Pluto, although Pluto is one of the most extreme cases of it. For example, Mars experiences a less dramatic version of this effect because it has an orbital tilt a little greater than 25 degrees and moves between 1.4 and 1.7 Earth-sun distances over the course of its orbit. Keep in mind, that on Pluto, “hotter” is a relative term. A summertime high might only be “in the 70’s” as measured in Kelvins—about 330 degrees below zero Fahrenheit (minus 200 degrees Celsius).

It’s refreshing to see at least one example that makes our Boston winters seem mild.

Alissa Earle
Author: Alissa Earle Credits: MIT/Richard Binzel

The Impact of Craters

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.

This portrait of Pluto is in enhanced color, to illustrate differences in the composition and texture of Pluto’s surface. Credit: NASA/JHUAPL/SwRI

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.

Sunlight on Pluto
These close-up images of Pluto and Charon illustrate the effect of lighting geometry on the appearance of craters.
Credit: SwRI/Kelsi Singer

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.

Kelsi Singer
Kelsi Singer

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’s Small Moons Nix and Hydra

Today’s post is written by Simon Porter, a New Horizons postdoctoral researcher at the Southwest Research Institute in Boulder, Colorado. Simon’s work focuses on the small satellites of Pluto.

This week’s beautiful Charon images remind us that Pluto is not just one body; it’s a whole system of 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.

Pluto schematic
The orbits of Pluto and its moons Charon, Styx, Nix, Kerberos and Hydra are illustrated around their common center of mass. Credit: SwRI/S. Porter

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.

Three faces of Nix
Pluto’s moon Nix is viewed at three different times during the New Horizons July 2015 flyby. Credit: NASA/JHUAPL/SwRI

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.

Pluto's Moon Nix as seen by New Horizons
Pluto’s moon Nix is shown in high-resolution black-and-white and lower resolution color. Credit: NASA/JHUAPL/SwRI

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!

Pluto's Moon Hydra as seen by New Horizons
Pluto’s moon Hydra as seen from NASA’s New Horizons spacecraft, July 14, 2015. Credit: NASA/JHUAPL/SwRI

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.

Simon Porter
Simon Porter Credit: Isaac Smith/SwRI

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.

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!