What we’ve learned about Pluto since the flyby

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An enhanced color view of Pluto's surface diversity. At lower right, ancient, heavily cratered terrain is coated with dark, reddish tholins -- organic compounds responsible for Pluto's tones. At upper right, volatile ices filling the informally named Sputnik Planum have modified the surface, creating a chaos-like array of blocky mountains. Volatile ice also occupies a few nearby deep craters, and in some areas the volatile ice is pocked with arrays of small sublimation pits. At left, and across the bottom of the scene, gray-white methane ice deposits modify tectonic ridges, the rims of craters, and north-facing slopes. Photo and caption by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

An enhanced color view of Pluto’s surface diversity. At lower right, ancient, heavily cratered terrain is coated with dark, reddish tholins — organic compounds responsible for Pluto’s tones. At upper right, volatile ices filling the informally named Sputnik Planum have modified the surface, creating a chaos-like array of blocky mountains. Volatile ice also occupies a few nearby deep craters, and in some areas the volatile ice is pocked with arrays of small sublimation pits. At left, and across the bottom of the scene, gray-white methane ice deposits modify tectonic ridges, the rims of craters, and north-facing slopes. Photo and caption by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

It’s been 248 days since the New Horizons space probe whizzed by Pluto, recording measurements and snapping more hi-resolution photos than the paparazzi. Yet the spacecraft’s data continues to amaze. A new set of five studies published today in the journal, Science detail what NASA and its many collaborators across the country have learned since the flyby. Though NASA has leaked many of its findings since July’s encounter, today’s package revises some of those ideas, along with a handful of fresh perspectives on the dwarf planet and its moons.

There’s much still to learn — New Horizons has only beamed back 40 percent of its data — but here’s a highlight reel of what scientists have discovered so far.

Pluto’s geology is two-faced in time

Frozen methane, nitrogen and carbon monoxide fill Sputnik Planum, in the western lobe of Pluto's heart, otherwise called Tombaugh Regio. This pattern differs from Pluto's northern pole, which is primarily methane and nitrogen. The finding suggests Sputnik Planum might contain a local reservoir of carbon monoxide, which may drive its strange shape.   Photo by NASA/JHUAPL/SwRI.

Frozen methane, nitrogen and carbon monoxide fill Sputnik Planum, in the western lobe of Pluto’s heart, otherwise called Tombaugh Regio. This pattern differs from Pluto’s northern pole, which is primarily methane and nitrogen. The finding suggests Sputnik Planum might contain a local reservoir of carbon monoxide, which may drive its strange shape.
Photo by NASA/JHUAPL/SwRI.

Tombaugh Regio, Pluto’s bright, heart-shaped region, captivated our curiosity immediately following the flyby. Upon closer inspection, this planetary feature and its surrounding areas are a scientific goldmine, according to an analysis of the geological features on Pluto and its largest moon, Charon.

Take Sputnik Planum, the left side of the ‘heart,’ as an example. This smooth plain is actually a giant, 2.5 mile deep basin that’s been filled by ice. Its origins have been tied to a massive impact between Pluto and another space object. That’s an anomaly when compared to Pluto’s other craters, which range from 0.3 to 155 miles in diameter. By comparison, Sputnik Planum stretches 650 miles north to south, and in terms of total area, is bigger than Texas.

Pluto from NASA’s New Horizons spacecraft is a large region of jumbled, broken terrain on the northwestern edge of the vast, icy plain informally called Sputnik Planum. Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Pluto from NASA’s New Horizons spacecraft is a large region of jumbled, broken terrain on the northwestern edge of the vast, icy plain informally called Sputnik Planum. Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Sputnik Planum’s age and smooth surface are puzzling too. The basin is young, at most 10 million years old. Meanwhile, many of the heavily cratered regions on Pluto and all its five moons are ancient, dating back 4 billion years to the early era of our solar system. Scientists suspect the plain’s smoothness may be due to a hot radioactive core that is constantly melting and molding the surface ice. This regular flow of ice may erode any craters that previously dotted landscape.

Huge mountains — as large as 25 miles across and three miles high — pepper the southern and western sides of Sputnik Planum. Pitted uplands, inexplicable bladed terrain and glaciers sit to the east, completing a picture of a dynamic geologic landscape that remains in flux to this day. In contrast, Charon’s landscape is complex, but appears stuck in time. Canyons, craters and plains sketch across this moon, but its surface hasn’t changed significantly in 2 billion years. Plus, the team suspects a once-liquid underground ocean on Charon is now frozen solid.

 Sputnik Planum on Pluto (top) and the informally named Vulcan Planum on Charon (bottom). The Sputnik Planum strip measures 228 miles (367 kilometers) long, and the Vulcan Planum strip measures 194 miles (312 kilometers) long.  The bright, nitrogen-ice plains are defined by a network of crisscrossing troughs. The Vulcan Planum view in the bottom panel includes the “moated mountain” Clarke Mons just above the center of the image. The water ice-rich plains display a range of surface textures, from smooth and grooved at left, to pitted and hummocky at right. Photo and caption by NASA/JHUAPL/SwRI

Sputnik Planum on Pluto (top) and the informally named Vulcan Planum on Charon (bottom). The Sputnik Planum strip measures 228 miles (367 kilometers) long, and the Vulcan Planum strip measures 194 miles (312 kilometers) long. The bright, nitrogen-ice plains are defined by a network of crisscrossing troughs. The Vulcan Planum view in the bottom panel includes the “moated mountain” Clarke Mons just above the center of the image. The water ice-rich plains display a range of surface textures, from smooth and grooved at left, to pitted and hummocky at right.
Photo and caption by NASA/JHUAPL/SwRI

A second study found crater densities on Pluto’s other four moons — Styx, Nix, Kerberos, and Hydra — that suggest these formations are 4 billion years old too. Unlike the spherical Charon, these four moons are highly elongated, like an index fingernail. Their shiny surfaces suggest the presence of abundant water ice. Also, these four moons are tiny and boast strange rotations. If Nix and Hydra were spherical, their diameters would be 24 miles long — or about as long as two Manhattans stacked tip to tip. Styx and Kerberos are a quarter the size of Nix and Hydra.

Twirling far above Pluto, are Charon plus four tiny moons -- Styx, Nix, Kerberos and Hydra (in order of distance from Pluto). The smaller moons have highly irregular rotations, with Hydra spinning around its axis about 89 times during a loop around Pluto and Charon. Weaver HA et al., Science, 2016

Twirling far above Pluto, are Charon plus four tiny moons — Styx, Nix, Kerberos and Hydra (in order of distance from Pluto). The smaller moons have highly irregular rotations, with Hydra spinning around its axis about 89 times during a loop around Pluto and Charon. Weaver HA et al., Science, 2016

(N)ice chemistry

Pluto’s ice is extremely cold, based on an investigation into the dwarf planet’s chemical composition. Surface temperatures on the dwarf planet range from -396 to -369.67 degrees Fahrenheit — and yet these ices are moving, much like glaciers on Earth. Their mobile nature comes from their composition. Rather than water, Pluto’s ice is primarily made of nitrogen, methane and carbon monoxide — all of which are still volatile, despite the planet’s low temperatures.

Giant masses of these ices move across Pluto, shaping surface features like its smooth plains. But while these three chemicals reign supreme in Pluto’s ices, their distribution is far from uniform. Researchers suspect factors like seasonal temperature changes, solar wind and the thermal properties of the individual compounds influence where each substance lands.

Maps of Pluto’s volatile ices methane (CH4, left), nitrogen (N2, middle) and carbon monoxide (CO, right), as measured by New Horizons' Linear Etalon Imaging Spectral Array. Photo by Grundy WM et al., Science, (2016)

Maps of Pluto’s volatile ices methane (CH4, left), nitrogen (N2, middle) and carbon monoxide (CO, right), as measured by New Horizons’ Linear Etalon Imaging Spectral Array. Photo by Grundy WM et al., Science, (2016)

Water ice of the H2O variety exists on Pluto too. It’s less volatile due to the cold temperatures, and at times collects into three-mile-high glaciers.

Want the purest water on Pluto? This map shows Pluto’s water (H2O) ice, namely places where it doesn't appear to be contaminated by other chemicals. Photo by Grundy WM et al., Science, (2016)

Want the purest water on Pluto? This map shows Pluto’s water (H2O) ice, namely places where it doesn’t appear to be contaminated by other chemicals. Photo by Grundy WM et al., Science, (2016)

Pluto’s atmosphere looks like an onion

A fourth study found that Pluto’s atmosphere is more compact than researchers initially thought. But it still extends 124 miles off the planet’s surface thanks to 20 layers of milky haze. Earth’s atmosphere, by comparison, stretches 6,200 miles.

Haze layers above Pluto taken by the Ralph/Multispectral Visible Imaging Camera (MVIC) on NASA’s New Horizons spacecraft. About 20 haze layers are seen; the layers have been found to typically extend horizontally over hundreds of kilometers, but are not strictly parallel to the surface.” Photo by G.R. Gladstone et al., Science (2016)

Haze layers above Pluto taken by the Ralph/Multispectral Visible Imaging Camera (MVIC) on NASA’s New Horizons spacecraft. About 20 haze layers are seen; the layers have been found to typically extend horizontally over hundreds of kilometers, but are not strictly parallel to the surface.” Photo by G.R. Gladstone et al., Science (2016)

Early observations suggested that solar wind tears at this atmosphere, dragging away loads of nitrogen gas like the tail of a comet. But this new atmospheric study has issued a correction. The rate of nitrogen escape is four orders of magnitude lower than initially presumed. Plus, the many layers of thick haze cool and condense the atmosphere, so it stays put.

Last July, the New Horizons team estimated Pluto had lost 1,000 to 9,000 feet of nitrogen over the course of its lifespan, but the new data argues for closer to two inches. The researchers write that “the New Horizons observations have revolutionized our understanding of Pluto’s atmosphere.”

Protons and electrons streaming from the Sun at ~250 miles per second are slowed and deflected around Pluto because of a combination of ionization of Pluto’s atmosphere and electrical currents induced in Pluto’s ionosphere. Photo by  Bagenal F et al., Science, (2016)

Protons and electrons streaming from the Sun
at ~250 miles per second are slowed and deflected around Pluto because of a combination of ionization of Pluto’s atmosphere
and electrical currents induced in Pluto’s ionosphere. Photo by Bagenal F et al., Science, (2016)

This observation is backed by the final study, which directly measured how much solar wind interacts with Pluto. The answer is surprisingly little, due to the planet’s compact atmosphere. This report also provides findings from an interplanetary dust counter built by students at the University of Colorado Boulder. The space around Pluto and its moons is surprisingly “clean,” as the experiment only detected 6 particles of dust per cubic mile.

“The bottom line is that space is mostly empty,” CU Boulder atmospheric and space physicist Fran Bagenal said in a statement. “Any debris created when Pluto’s moons were captured or created during impacts has long since been removed by planetary processes.”

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