Sputnik Planum forms the "left ventricle" of Pluto's "heart"
The spectacular, flat landscape that dominates the left side of Pluto's icy "heart" can now be explained, say scientists.
Sputnik Planum is the most prominent feature on the diminutive world, covering 900,000 square km.
Broken into an array of polygons, it is devoid of any impact craters.
Reporting in the journal Nature, the researchers say that roiling cells of nitrogen ice remove any blemishes, maintaining a super-smooth appearance.
They argue that a competing idea, that the polygons are a consequence of cooling and contraction akin to giant "mud cracks", does not fit with the observations.
"It's a vigorously - from a geological point of view - churning layer of solid nitrogen, and it's [as if] the heart of Pluto is truly beating," said Prof Bill McKinnon from Washington University in St Louis, Missouri.
The assessment is based on the data acquired by the US space agency's New Horizons probe, which made the historic first flyby of Pluto last July.
Two separate teams have looked at the information from that encounter and have come to broadly the same conclusion - that only overturning ice, driven by the dwarf planet's internal heat, can produce the cellular terrain.
The polygons are typically about 10-40km across, tiling a deep basin that is surrounded by high mountains.
Each cell is domed, standing some 50m above its edges. Those edges then give way to troughs that can reach 100m in depth.
New Horizons found the planum ices to contain mostly nitrogen, with limited amounts of methane and carbon monoxide.
At the temperatures that persist on Pluto's surface (an extremely frigid -235C), this material is still capable of flowing.
Modelling work suggests just a few centimetres per year of horizontal movement in the tops of the domes would be sufficient to re-surface them in short order - significantly faster than the likely rate of impacts from bodies falling on to Pluto.
"From the calculated average velocity of convection, about 1.5cm per year, we compute the time needed for the ice surface to renew itself, and therefore the maximum age of the surface of Sputnik Planum, to be about one million years," report Alex Trowbridge, from Purdue University, Indiana, and colleagues in the second of the two Nature papers.
Convection requires energy, of course. And the scientists say this would come from radioactive elements incorporated into Pluto at its formation, and which should still be producing the necessary heat to promote the rise and fall of ice within the cells.
There are, however, scaling laws for convection that describe the relationship between the width and depth of convective cells. Taken at face value, these might imply the basin in which the polygons are sitting to be 10-20km deep.
This is somewhat deeper than expected for the underlying basin, but Bill McKinnon and colleagues invoke an idea they call "sluggish lid convection". This sees the dome surface move at a much slower pace than the deeper, warmer subsurface. If this operates at Pluto, the convection system could be much shallower; perhaps 3-6km deep, says Prof McKinnon.
"It all depends on the rheology - how nitrogen ice responds to pressure, temperature and other factors. The stuff at the top moves much more slowly because it's colder. It's 37 degrees kelvin (-236C). It's only modestly warmer down below but it's enough to make the nitrogen ice move," he told BBC News.
The polygons are typically about 10-40km across
Sputnik Planum was without doubt one of the standout discoveries of the New Horizons flyby.
It is the place where geologically recent activity on Pluto is most evident.
Glaciers of nitrogen ice are observed to move away from the water-ice mountains, into the plain.
Giant boulders are carried by some of these ice streams. And because water-ice floats on nitrogen ice, the mountain fragments tend to collect in the polygons' troughs, unable to sink with the downward flow of convection. This gives the appearance of chains of hills.
Although no impact craters are seen on Sputnik Planum, there are fields of kilometre-scale pits, particularly in its eastern and southern regions. These are sectors where there is no convection.
The stagnant ice here likely vaporises over time to produce the pits. It is all part of the process that cycles nitrogen between the plain, the atmosphere, the mountains, and back into the plain via the glaciers.
Pluto’s Heart: Like a Cosmic ‘Lava Lamp’
Like a cosmic lava lamp, a large section of Pluto’s icy surface is being constantly renewed by a process called convection that replace older surface ices with fresher material.
Scientists from NASA’s New Horizons mission used state-of-the-art computer simulations to show that the surface of Pluto’s informally named Sputnik Planum is covered with churning ice "cells" that are geologically young and turning over due to a process called convection. The scene above, which is about 250 miles (400 kilometers) across, uses data from the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC), gathered July 14, 2015.
Combining computer models with topographic and compositional data gathered by NASA’s New Horizons spacecraft last summer, New Horizons team members have determined the depth of this layer of solid nitrogen ice within Pluto's distinctive "heart" feature – a large plain informally known as Sputnik Planum – and how fast that ice is flowing. The study is published in the June 2 issue of the journal Nature.
“For the first time, we can determine what these strange welts on the icy surface of Pluto really are.” William McKinnon, Washington University in St. Louis
Mission scientists used state-of-the-art computer simulations to show that the surface of Sputnik Planum is covered with icy, churning, convective "cells" 10 to 30 miles (16 to 48 kilometers) across, and less than one million years old. The findings offer additional insight into the unusual and highly active geology on Pluto and, perhaps, other bodies like it on the outskirts of the solar system.
“For the first time, we can determine what these strange welts on the icy surface of Pluto really are,” said William B. McKinnon, from Washington University in St. Louis, who led the study and is a co-investigator on the New Horizons science team. “We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff,’ meaning something as soft and pliable as solid nitrogen.”
McKinnon and colleagues believe the pattern of these cells stems from the slow thermal convection of the nitrogen-dominated ices that fill Sputnik Planum. A reservoir that’s likely several miles deep in some places, the solid nitrogen is warmed by Pluto’s modest internal heat, becomes buoyant and rises up in great blobs – like a lava lamp – before cooling off and sinking again to renew the cycle.
The computer models show that ice need only be a few miles deep for this process to occur, and that the convection cells are very broad. The models also show that these blobs of overturning solid nitrogen can slowly evolve and merge over millions of years. Ridges that mark where cooled nitrogen ice sinks back down can be pinched off and abandoned, resulting in Y- or X-shaped features in junctions where three or four convection cells once met.
“Sputnik Planum is one of the most amazing geological discoveries in 50-plus years of planetary exploration, and the finding by McKinnon and others on our science team that this vast area—bigger than Texas and Oklahoma combined – is created by current day ice convection is among the most spectacular of the New Horizons mission,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado.
These convective surface motions average only a few centimeters a year – about as fast as your fingernails grow – which means cells recycle their surfaces every 500,000 years or so. While slow on human clocks, it’s a fast clip on geological timescales.
“This activity probably helps support Pluto’s atmosphere by continually refreshing the surface of ‘the heart,’” McKinnon said. “It wouldn’t surprise us to see this process on other dwarf planets in the Kuiper Belt. Hopefully, we’ll get a chance to find out someday with future exploration missions there.”
New Horizons could also potentially take a close-up look at a smaller, more ancient object much farther out in the Kuiper Belt – the disk-shaped region beyond the orbit of Neptune believed to contain comets, asteroids and other small, icy bodies. New Horizons flew through the Pluto system on July 14, 2015, making the first close observations of Pluto and its family of five moons. The spacecraft is on course for an ultra-close flyby of another Kuiper Belt object, 2014 MU69, on Jan. 1, 2019, pending NASA approval of funding for an extended mission.
Secrets Revealed from Pluto’s ‘Twilight Zone’
NASA’s New Horizons spacecraft took this stunning image of Pluto only a few minutes after closest approach on July 14, 2015. The image was obtained at a high phase angle –that is, with the sun on the other side of Pluto, as viewed by New Horizons. Seen here, sunlight filters through and illuminates Pluto’s complex atmospheric haze layers. The southern portions of the nitrogen ice plains informally named Sputnik Planum, as well as mountains of the informally named Norgay Montes, can also be seen across Pluto’s crescent at the top of the image.
Looking back at Pluto with images like this gives New Horizons scientists information about Pluto’s hazes and surface properties that they can’t get from images taken on approach. The image was obtained by New Horizons’ Ralph/Multispectral Visual Imaging Camera (MVIC) approximately 13,400 miles (21,550 kilometers) from Pluto, about 19 minutes after New Horizons’ closest approach. The image has a resolution of 1,400 feet (430 meters) per pixel. Pluto’s diameter is 1,475 miles (2,374 kilometers).
The inset at top right shows a detail of Pluto’s crescent, including an intriguing bright wisp (near the center) measuring tens of miles across that may be a discreet, low-lying cloud in Pluto’s atmosphere; if so, it would be the only one yet identified in New Horizons imagery. This cloud – if that’s what it is – is visible for the same reason the haze layers are so bright: illumination from the sunlight grazing Pluto’s surface at a low angle. Atmospheric models suggest that methane clouds can occasionally form in Pluto’s atmosphere. The scene in this inset is 140 miles (230 kilometers) across.
The inset at bottom right shows more detail on the night side of Pluto. This terrain can be seen because it is illuminated from behind by hazes that silhouette the limb. The topography here appears quite rugged, and broad valleys and sharp peaks with relief totaling 3 miles (5 kilometers) are apparent. This image, made from closer range, is much better than the lower-resolution images of this same terrain taken several days before closest approach. These silhouetted terrains therefore act as a useful “anchor point,” giving New Horizons scientists a rare, detailed glimpse at the lay of the land in this mysterious part of Pluto seen at high resolution only in twilight. The scene in this inset is 460 miles (750 kilometers) wide.
A full-resolution, unannotated view of Pluto’s ‘Twilight Zone’
Processing Pluto’s Pictures
This week’s blog comes from Tod Lauer, a research astrophysicist at the National Optical Astronomy Observatory in Tucson, Arizona.
New Horizons Principal Investigator: “Lauer! We’ve got to have full resolution! Now!”
Me: “I’m pushing the images as hard as I can – any more and the pixels will blow apart for sure!”
Okay, the New Horizons Pluto encounter didn’t quite play out that way, but the science team really did want to get the most out of all the images of Pluto and its moons that we could, and often, as quickly as we could. I’m Tod Lauer, an astrophysicist who mainly works on stuff far beyond our galaxy. But I also love tough imaging challenges, and I enjoyed working with New Horizons as a sort-of utility image-processing engineer.
A few summers ago I wrote to New Horizons Project Scientist Hal Weaver on a whim to ask about the search for hazards to the spacecraft as it entered Plutonian space. Hal kindly replied with a note describing the capabilities of the New Horizons spacecraft and a report describing the search in detail. New Horizons co-investigator John Spencer, who was leading the hazard detection effort, also joined in. I was incredibly intrigued by the task: Search for unknown faint sources close to Pluto, which was embedded in an incredibly crowded field of stars (the heart of the Milky Way!), using heavily compressed images with the optical blur-pattern of the camera varying significantly from exposure to exposure – all on a critical timeline. I offered one approach, which led to me joining the “Crow’s Nest” crew that John and Hal assembled to search for hazards in the distant-encounter images. This work in turn led to an opportunity for me to help out with the encounter images as well.
n these simulated images from New Horizons’ Long-Range Reconnaissance Imager (LORRI), I demonstrated an approach to the hazard search. The image on the left (prepared by New Horizons’ John Spencer) shows Pluto and Charon greatly over-exposed to capture faint moons hiding among the heavily crowded background of Milky Way stars. The image at right shows a model of the fixed stars subtracted to reveal the four known small satellites of Pluto. This approach worked extremely well for the actual search. Credits: NASA/JHUAPL/SwRI
One task was getting the best resolution out of the images. Starting in April 2015, I worked to get the first glimpses of detail on Pluto and Charon as New Horizons’ long cruise across interplanetary space transitioned into the flyby itself. This continued up to closest approach and beyond as the images came back to Earth after the flyby. This work started with weaving a set of images of an object into a master image that preserved all the fine structure scattered about the image set.
At left, a LORRI image of Pluto taken July 12, 2015, two days before closest approach. The image at right and others taken at the same time were combined as single image with a 2x-finer pixel scale and corrected for blurring to reveal many more details. Credits: NASA/JHUAPL/SwRI
The next step was to correct for the blurring due to the New Horizons optics. The final step required the greatest care – satisfying my fellow scientists on the team that they could trust the results for their research! The main objective was not to leave anything on the plate: It took hundreds of people working for two decades to get to Pluto, and it may be a while before we get back there. Every drop of information we can squeeze out of the images is immensely valuable.
The process in action, from left: Picture A is one of a set of four LORRI images of Pluto’s small moon Kerberos; in B, the four images have been combined to produce a 2x-finer finer pixel scale; C is the combined image corrected for blurring; and D has been interpolated to remove the blocky appearance and reveal new details about Pluto’s moon Kerberos. Credits: NASA/JHUAPL/SwRI
Another problem, which might seem surprising for a mission to Pluto, was dealing with the brilliant glare of the distant sun. On the way out to Pluto we had the sun to our back, while after close approach, we turned around to look at the night sides of Charon (and later Pluto), which had New Horizons’ cameras looking almost right back into the sun. Sunlight scattered into the camera strongly washed out the darkened hemisphere. The trick was to use a technique to capture how the scattered sunlight varied over a large collection of images, providing a way to build a perfect model of it for any image. With the sun canceled out we could see the night side of Charon softly lit up by “Plutoshine.”
At left is one of more than 200 LORRI images obtained to image the dark side of Charon by “Plutoshine;” the bright striations are sunlight scattered into the camera. At right, after all of the images are combined and corrected for the scattered light—Charon’s crescent and nightside are revealed! Credits: NASA/JHUAPL/SwRI
The best part of my experience with the New Horizons team was watching everyone work together to make the encounter a fantastic success. The hazard search concluded two weeks before the flyby, and having found nothing in our way, we stayed on our original, planned course to the Pluto system. From then on the tempo and energy level steadily rose as we flew ever closer to Pluto. For this astrophysicist, it was a treat to see the immense and diverse skills of the New Horizons team for planetary exploration brought to bear. If New Horizons were a ship, the team was its crew, with everyone smartly working at their stations but always keeping an eye on the big picture. Each of us used our talents in a unique way. No one wanted us to miss anything.
The Jagged Shores of Pluto’s Highlands
This enhanced color view from NASA’s New Horizons spacecraft zooms in on the southeastern portion of Pluto’s great ice plains, where at lower right the plains border rugged, dark highlands informally named Krun Macula. (Krun is the lord of the underworld in the Mandaean religion, and a ‘macula’ is a dark feature on a planetary surface.)
Pluto is believed to get its dark red color from tholins, complex molecules found across much of the surface. Krun Macula rises 1.5 miles (2.5 kilometers) above the surrounding plain – informally named Sputnik Planum – and is scarred by clusters of connected, roughly circular pits that typically reach between 5 and 8 miles (8 and 13 kilometers) across, and up to 1.5 miles (2.5 kilometers) deep.
At the boundary with Sputnik Planum, these pits form deep valleys reaching more than 25 miles (40 kilometers) long, 12.5 miles (20 kilometers) wide and almost 2 miles (3 kilometers) deep – almost twice as deep as the Grand Canyon in Arizona – and have floors covered with nitrogen ice. New Horizons scientists think these pits may have formed through surface collapse, although what may have prompted such a collapse is a mystery.
This scene was created using three separate observations made by New Horizons in July 2015. The right half of the image is composed of 260 feet- (80 meter-) per-pixel data from the Long Range Reconnaissance Imager (LORRI), obtained at 9,850 miles (15,850 kilometers) from Pluto, about 23 minutes before New Horizons’ closest approach. The left half is composed of 410 feet- (125 meter-) per-pixel LORRI data, obtained about six minutes earlier, with New Horizons 15,470 miles (24,900 kilometers) from Pluto.
These data respectively represent portions of the highest- and second-highest-resolution observations obtained by New Horizons in the Pluto system. The entire scene was then colorized using 2,230 feet- (680 meter-) per-pixel data from New Horizons’ Ralph/Multispectral Visual Imaging Camera (MVIC), obtained at 21,100 miles (33,900 kilometers) from Pluto, about 45 minutes before closest approach.
This dramatic image from NASA’s New Horizons spacecraft shows the dark, rugged highlands known as Krun Macula (lower right), which border a section of Pluto’s icy plains. Click on the image and zoom in for maximum detail.
New Data Compare, Contrast Pluto’s Icy Moons
Pluto’s moons Charon, Nix and Hydra. Charon and Nix were imaged in color by NASA’s New Horizons spacecraft, but Hydra was not.
A newly downlinked spectral observation of Pluto’s moon Nix from NASA’s New Horizons spacecraft provides compelling evidence that its surface is covered in water ice, similar to what the New Horizons team discovered recently for another of Pluto’s small satellites, Hydra. This new result provides further clues about the formation of Pluto’s satellite system.
With this observation by New Horizons’ LEISA – the compositional spectral imager aboard the spacecraft – mission scientists also are piecing together a more detailed picture of Pluto's system of four small, outer moons (Styx, Nix, Kerberos and Hydra).
A comparison of the compositional spectra of Pluto’s moons Charon, Nix and Hydra to pure water ice. Nix’s surface displays the deepest water-ice spectral features seen among Pluto’s three satellites – Charon, Nix and Hydra – for which New Horizons obtained surface spectra.
The deeper spectral features on Nix seen in the graph above are a signature of water ice that is relatively coarse-grained and pure, because the shape and depth of water-ice absorption depends on the size and purity of the icy grains on the surface. Scattering from smaller, or less pure, icy grains tends to wash out spectral absorption features, making them shallower.
“Pluto’s small satellites probably all formed out of the cloud of debris created by the impact of a small planet onto a young Pluto,” said New Horizons Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “So we would expect them all to be made of similar material. The strong signature of water-ice absorption on the surfaces of all three satellites adds weight to this scenario. Although we didn’t collect spectra of Pluto’s two tiniest satellites, Styx and Kerberos, their high reflectivity argues that they are also likely to have water-ice surfaces.”
The difference in the depths of the water ice absorption features in the Nix and Hydra spectra raises new questions. Specifically, the science team is puzzling over why Nix and Hydra apparently have different ice textures on their surfaces, despite their similar sizes. Another mystery resulting from the Pluto flyby data is why Hydra’s surface reflectivity at visible wavelengths is higher than Nix’s – a New Horizons result published in March in the journal Science – even though Nix’s surface appears to be icier, implying higher reflectivity at visible wavelengths.
The LEISA Nix observation was captured on July 14, 2015, from a range of 37,000 miles (60,000 kilometers), resulting in a spatial resolution of about 2.3 miles per pixel (3.7 kilometers per pixel).
Pluto’s Methane Snowcaps on the Edge of Darkness
The southernmost part of Pluto that NASA’s New Horizons spacecraft could “see” during closest approach in July 2015 contains a range of fascinating geological features, and offers clues into what might lurk in the regions shrouded in darkness during the flyby.
The area shown above is south of Pluto’s dark equatorial band informally named Cthulhu Regio, and southwest of the vast nitrogen ice plains informally named Sputnik Planum or Sputnik Planitia, as the mission team recently redesignated the area to more accurately reflect the low elevation of the plains. North is at the top; in the western portion of the image, a chain of bright mountains extends north into Cthulhu Regio. The mountains reveal themselves as snowcapped—something hauntingly familiar from our Earthbased experience. But New Horizons compositional data indicate the bright snowcap material covering these mountains isn’t water, but atmospheric methane that has condensed as frost onto these surfaces at high elevation. Between some mountains are sharply cut valleys – indicated by the white arrows below. These valleys are each a few miles across and tens of miles long.
A similar valley system in the expansive plains to the east (blue arrows) appears to be branched, with smaller valleys leading into it. New Horizons scientists think flowing nitrogen ice that once covered this area -- perhaps when the ice in Sputnik was at a higher elevation -- may have formed these valleys. The area is also marked by irregularly shaped, flat-floored depressions (green arrows) that can reach more than 50 miles (80 kilometers) across and almost 2 miles (3 kilometers) deep. The great widths and depths of these depressions suggest that they may have formed when the surface collapsed, rather than through the sublimation of ice into the atmosphere.
Mystery of Charon's Red Cap Solved
The reddish hue above the north pole of Pluto's largest moon may be caused by trapped gas.
Image: Mosiac of New Horizons images of Charon. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
When NASA's New Horizons spacecraft flew through the Pluto system last year, scientists were surprised to find that Charon, Pluto's largest moon, has a dark red polar cap.
A new study may have figured out the reason why: trapped gas.
Lowell Observatory astronomer Will Grundy and colleagues combined analysis of New Horizons imagery with computer models to show that Charon's north pole grew cold enough during its century-long winter to trap methane escaping from Pluto. As sunlight returned to the pole, the methane was then converted into red-colored chemicals, known as tholins, the study shows.
"Who would have thought that Pluto is a graffiti artist, spray-painting its companion with a reddish stain that covers an area the size of New Mexico?" Grundy said in a press release . "Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes."
WATCH VIDEO: Where Is New Horizons Now?
Surface temperatures during Charon's long winters dip to -430 Fahrenheit, cold enough to freeze methane gas into a solid.
"The methane molecules bounce around on Charon's surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring," Grundy said.
At point, the methane ice quickly vaporizes, leaving heavier hydrocarbons that were created from it on the surface.
Sunlight further irradiates the hydrocarbons and turns them red.
"New Horizons' observations of Charon's other pole, currently in winter darkness -- and seen by New Horizons only by light reflecting from Pluto -- confirmed that the same activity was occurring at both poles," Johns Hopkins University Applied Physics Lab wrote in a press release about the study.
"The distribution of dark, reddish material around Charon's northern pole is notable for its generally symmetric distribution across longitudes and its gradual increase with latitude, although there are local irregularities associated with craters, topographic features and perhaps subsurface variations in thermal properties," Grundy and colleagues write in this week's Nature.
"These characteristics … are consistent with our hypothesis that the combination of Pluto's escaping atmosphere and Charon's long, cold winters enables methane to be seasonally cold-trapped at high latitudes, where some is photolytically processed into heavier molecules that are subsequently converted to reddish tholin-like materials," the study shows.
That left the team wondering if the process could be happening elsewhere. Nix, one of Pluto's four small moons, has a reddish spot, but it orbits farther away form Pluto and is much smaller, which would make the process less efficient, the scientists note.
How Pluto got its heart of ice
New climate simulations help explain how Pluto’s heart-shaped Sputnik Planum (lower right) got its thick glaciers of nitrogen and carbon monoxide. Some of the glaciers, first seen by the New Horizons spacecraft in July, hold ice that circulates like the material in a lava lamp. Now, to find out how the glaciers formed in the first place, scientists created models that simulated atmospheric circulation on the dwarf planet for the last 50,000 years (a mere 200 orbits around the sun for Pluto). At the beginning of the simulations, the researchers gave Pluto a planet-wide veneer of nitrogen, carbon monoxide, and methane ices a few millimeters thick; then, the planet’s surface and atmosphere evolved as the icy orb passed through orbit after orbit. If Pluto were a completely smooth sphere, it would have either a permanent swath of nitrogen ice at the equator or seasonal snow caps at its poles. But that’s not what the planet looks like today. When researchers added realistic topography to the model, including the 4-kilometer-deep Sputnik Planum and two other large craters, the basin gradually trapped Pluto’s nitrogen, carbon monoxide, and much of its methane, the researchers report online today in Nature. That’s because the dwarf planet’s sparse atmosphere is thickest at lowest elevation, making condensation of the ices most effective there. Besides helping explain the current pattern of ices on Pluto’s surface, the new simulations also shed light on substantial changes in Pluto’s atmospheric pressure observed in recent years. The team’s models suggest that in the coming decade, the dwarf planet’s atmospheric pressure will decrease and the frosts now seen in Pluto’s northern hemisphere will disappear. If it happens, this will be a crucial verification of their model, the researchers say.
How Pluto's big frozen heart grew and evolved
Numerical simulations of the dwarf planet's chemistry and atmosphere show its famous glacier growing, as well as seasonal frosts expanding and shrinking.
The vast glacier in Pluto's heart could have grown in just 10,000 Earth years as nitrogen ice froze out of its atmosphere and was sequestered in the deep basin, according to new modelling.
Tanguy Bertrand and François Forget from Sorbonne University in France simulated the climate and topography of the dwarf planet and found nitrogen ice, along with some carbon monoxide and methane, naturally congregated in the lower elevations.
Since New Horizons beamed back images of Pluto's big, pale heart – called Tombaugh Regio after Pluto's discovered Clyde Tombaugh – planetary scientists have been trying to get to the bottom of the vast, craterless region within.
Informally named Sputnik Planum, the glacier is mostly nitrogen ice. But how did the basin, which is more than 1,000 kilometres wide and thought to be four kilometres deep, fill up?
Computer models, like those used by climate scientists to recreate out planet's ancient climate history, have tried to track how the Plutonian atmosphere changed and mixed, but have been too slow to run over multiple seasons.
So Bertrand and Forget developed a new model – one that could run through Earth 50,000 years and see what happened to nitrogen (and other volatiles) on the icy little dwarf planet.
They found, when they added a basin and a couple of craters to their modelled Pluto, all the nitrogen ice was sequestered in the centre of the basin – just as it does on Sputnik Planum – after just 10,000 Earth years.
This is because the basin floor has higher surface pressure than the surrounds. Higher surface pressure means nitrogen is more likely to condense and freeze. This "cold trap" sets the ball rolling for more nitrogen ice to pile on.
They note this phenomenon is also seen on Mars, where carbon dioxide frost preferentially forms at low elevations, such as at the Hellas Basin.
Their model also ended up with carbon monoxide and methane frozen into the glacier ice.
Indeed, data from New Horizons indicated carbon monoxide in Sputnik Planum, while methane tended to spread all the way around Pluto's equatorial region.
Finally, the model accounted for seasonal methane frosts that have been seen on Pluto's north pole in the 1980s, 1994 and 2002.
They predict these frosts should, for the most part, disappear in the next decade.