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.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.

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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.

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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.

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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 Blue Sky: Pluto’s haze layer shows its blue color in this picture taken by the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). The high-altitude haze is thought to be similar in nature to that seen at Saturn’s moon Titan. The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles (called tholins) that grow as they settle toward the surface. This image was generated by software that combines information from blue, red and near-infrared images to replicate the color a human eye would perceive as closely as possible.

Credits: NASA/JHUAPL/SwRI

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The first color images of Pluto’s atmospheric hazes, returned by NASA’s New Horizons spacecraft last week, reveal that the hazes are blue.

“Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous,” said Alan Stern, New Horizons principal investigator from Southwest Research Institute (SwRI), Boulder, Colorado.

The haze particles themselves are likely gray or red, but the way they scatter blue light has gotten the attention of the New Horizons science team. “That striking blue tint tells us about the size and composition of the haze particles,” said science team researcher Carly Howett, also of SwRI. “A blue sky often results from scattering of sunlight by very small particles. On Earth, those particles are very tiny nitrogen molecules. On Pluto they appear to be larger — but still relatively small — soot-like particles we call tholins.”

Scientists believe the tholin particles form high in the atmosphere, where ultraviolet sunlight breaks apart and ionizes nitrogen and methane molecules and allows them to react with one another to form more and more complex negatively and positively charged ions. When they recombine, they form very complex macromolecules, a process first found to occur in the upper atmosphere of Saturn’s moon Titan. The more complex molecules continue to combine and grow until they become small particles; volatile gases condense and coat their surfaces with ice frost before they have time to fall through the atmosphere to the surface, where they add to Pluto’s red coloring.

In a second significant finding, New Horizons has detected numerous small, exposed regions of water ice on Pluto. The discovery was made from data collected by the Ralph spectral composition mapper on New Horizons. 

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From Pluto’s unusual heart-shaped region to its extended atmosphere and intriguing moons, New Horizons has revealed a degree of diversity and complexity in the Pluto system that few expected in the frigid outer reaches of the solar system.

The New Horizons team describes a wide range of findings about the Pluto system in its first science paper, released today. “The Pluto System: Initial Results from its Exploration by New Horizons,” led by mission Principal Investigator Alan Stern, appears as the cover story in the Oct. 16 issue of Science, just three months after NASA’s historic first exploration of the Pluto system in July.

“The New Horizons mission completes our initial reconnaissance of the solar system, giving humanity our first look at this fascinating world and its system of moons,” said Jim Green, director of planetary science at NASA Headquarters in Washington. “New Horizons is not only writing the textbook on the Pluto system, it’s serving to inspire current and future generations to keep exploring — to keep searching for what’s beyond the next hill.”

NASA’s New Horizons spacecraft reached a distance of 7,750 miles (12,500 kilometers) from Pluto’s surface during its July 14 closest approach, gathering so much data it will take almost another year to return to Earth. The data returned so far show a surprisingly wide variety of landforms and terrain ages on Pluto, as well as variations in color, composition and albedo (surface reflectivity). Team members also discovered evidence for a water-ice rich crust, multiple haze layers above the surface in Pluto’s atmosphere, and that Pluto is somewhat larger and a bit more ice rich than expected.

“The Pluto system surprised us in many ways, most notably teaching us that small planets can remain active billions of years after their formation,” said Stern, with the Southwest Research Institute (SwRI) in Boulder, Colorado. “We were also taught important lessons by the degree of geological complexity that both Pluto and its large moon Charon display.”

Some of the processes on Pluto appear to have occurred geologically recently, including those that involve the water-ice rich bedrock as well as the more volatile, and presumably more mobile, ices of the western lobe of Pluto’s “heart.” The diverse geology and apparent recent activity raise fundamental questions about how small planetary bodies remain active many billions of years after formation. The research suggests that other large worlds in the Kuiper belt — such as Eris, Makemake and Haumea — could also have similarly complex histories that rival those of terrestrial planets such as Mars and Earth.

The New Horizons team notes that Triton, likely a former Kuiper Belt planet captured by Neptune, was considered the best analog for Pluto prior to the July 14 flyby. The team now believes that the geologies of Triton and Pluto are more different than similar, but will know more as additional data are returned.

New Horizons is part of NASA’s New Frontiers Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, designed, built and operates the New Horizons spacecraft and manages the mission for NASA’s Science Mission Directorate. SwRI leads the science mission, payload operations and encounter science planning.

Quelle: The Johns Hopkins University

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This image was taken by the Long Range Reconnaissance Imager (LORRI) on NASA's New Horizons spacecraft shortly before closest approach to Pluto on July 14, 2015; it resolves details as small as 270 yards (250 meters). The scene shown is about 130 miles (210 kilometers) across. The sun illuminates the scene from the left, and north is to the upper left.

Credits: NASA/JHUAPL/SwRI

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It seems that the more we see of Pluto, the more fascinating it gets. With its prominent heart-shaped feature, icy mountains, and “snakeskin” terrain, Pluto has already surprised New Horizons scientists with the variety and complexity of its surface features.

Now this latest image, from the heart of Pluto’s heart feature, show the plains’ enigmatic cellular pattern (at left) as well as unusual clusters of small pits and troughs (from lower left to upper right).

Scientists believe that this area, informally known as Sputnik Planum, is composed of volatile ices such as solid nitrogen. They theorize that the pits and troughs – typically hundreds of meters across and tens of meters deep – are possibly formed by sublimation or evaporation of these ices. However, the reasons for the striking shapes and alignments of these features are a mystery. Adding to the intrigue is that even at this resolution, no impact craters are seen, testifying to the extreme geologic youth of Sputnik Planum.  

“Pluto is weird, in a good way,” said Hal Weaver, New Horizons project scientist with Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “The pits, and the way they’re aligned, provide clues about the ice flow and the exchange of volatiles between the surface and atmosphere, and the science team is working hard to understand what physical processes are at play here.”

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Quelle: NASA