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Pluto's edge
NASA’s New Horizons probe captured this backlit image of Pluto as it flew past the dwarf planet on July 14. Scattered sunlight reveals numerous haze layers within Pluto’s thin atmosphere, while the surprisingly diverse surface landscape indicates ongoing geological activity. (Credit: NASA / JHUAPL / SwRI)

The first peer-reviewed scientific paper about the New Horizons probe’s July flyby past Pluto lays out puzzling evidence that suggests the dwarf planet isn’t frozen in time. Rather, its smooth plains, high mountains and nitrogen glaciers are leading the NASA mission’s researchers to suspect that it’s geologically active even now.

“Pluto’s still got an engine, and it’s still running,” principal investigator Alan Stern of Southwest Research Institute told journalists in advance of the paper’s publication today by the journal Science.

The study provides an overview on the amazing discoveries made by the New Horizons team in the wake of the spacecraft’s July 14 encounter with Pluto and its moons, 3 billion miles from Earth. Stern told GeekWire that the geological diversity seen on the dwarf planet is “just unparalleled in the solar system.”

“We have said things like this before, but getting it through peer review is a different matter,” Stern said.

Among the top findings:

  • Although some areas of Pluto are heavily cratered, a stretch of bright terrain that’s been nicknamed Sputnik Planum has no large craters at all. That implies the region is being reshaped by tectonic changes that could still be under way. The New Horizons team hasn’t determined what’s driving those changes, but it could be heat generated by the decay of radioactive minerals in Pluto’s core.
  • Pluto’s snowcapped mountains rise as high as 11,000 feet (3 kilometers) above the surrounding terrain – so high that researchers say they have to consist mostly of water ice, with a topping of frozen nitrogen, carbon monoxide and methane. The observations suggest that frozen H2O forms a layer of solid “bedrock” that’s only occasionally exposed at the surface.
  • Some areas of the surface are markedly yellower or redder than others. That’s thought to be due to the presence of tholins, organic residues that are created when mixtures of nitrogen and methane are irradiated by the sun’s ultraviolet light.
  • Pluto is covered by a thin, multilayered haze of nitrogen and other gases that extends about 90 miles (150 kilometers) out from the surface. The atmospheric pressure is lower than researchers expected, but it’s too early to say whether that means Pluto’s air is rapidly freezing out. Some pictures show features that look like wind streaks or dunes.
  • Pluto’s biggest moon, Charon, looks dramatically different: Its most dramatic features include a canyon system that’s more than twice as deep, wide and long as the Grand Canyon on Earth … and a dark spot at the north pole that’s been nicknamed Mordor Macula. The spot may be trapping volatile materials and turning them into tholins, which would explain Mordor Macula’s reddish color.
  • Like Charon, the smaller Plutonian moons Nix and Hydra are made mostly of water ice. But the ice on Nix and Hydra looks cleaner and brighter than Charon’s. Detailed pictures of two other moons, Styx and Kerberos, haven’t yet come down from the spacecraft – and New Horizons’ searches have turned up no evidence of additional moons or rings.
Mordor Macula on Charon
NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto’s big moon Charon just before closest approach on July 14. The colors are processed to best highlight the variation of surface properties across Charon. Charon’s color palette is not as diverse as Pluto’s; most striking is the reddish north polar region, informally named Mordor Macula. (Credit: NASA / JHUAPL / SwRI)

Stern emphasized that this was just a first look at the Plutonian system. The New Horizons spacecraft stored up tens of gigabits’ worth of data during the July flyby, but can send it back to Earth at a rate of only 2,000 bits per second. Only 15 percent of the data has been received so far, Stern said. It will take until late 2016 to get it all down.

“The very best data is still on the spacecraft,” Stern said.

The findings so far have been “unexpectedly exciting,” Mark Sykes, director of the Arizona-based Planetary Science Institute, told GeekWire. Sykes reviewed the research paper in advance but is not part of the New Horizons team. He said the Plutonian system turned out to be more diverse and dynamic than he would have thought.

“It raises the question, well, what about Eris and the other dwarf planets as well?” Sykes said.

Sykes said the paper sheds new light on yet another question: Is Pluto a planet? In 2006, the International Astronomical Union created the category “dwarf planet” and ruled that dwarf planets like Pluto were not planets per se. But in the Science paper, New Horizons’ scientists refer to Pluto and its kin as “small planets” – which runs counter to the IAU’s preferred usage.

“They have done a far more imaginative, engaging, and even respectful job than I would expect the IAU to do,” Sykes, who has previously criticized the 2006 ruling, said in an email. “The IAU should defer to the discovery team.”

The Science paper is titled ‘The Pluto System: Initial Results From Its Exploration by New Horizons” and has 151 co-authors, including Stern. This report incorporates comments that Stern made at last week’s ScienceWriters2015 meeting in Cambridge, Mass. Alan Boyle is the president of the Council for the Advancement of Science Writing, one of the organizers of the New Horizons in Science briefings at ScienceWriters2015.

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