What we've learned about Pluto since New Horizons flew past

On Earth, the only ice is frozen water. On Pluto, nitrogen, methane and carbon monoxide also freeze solid.

The most striking feature that NASA's New Horizons spacecraft saw when it flew past Pluto in July was a heart-shaped region now named Tombaugh Regio after Clyde Tombaugh, the discoverer of Pluto.

The left half is covered by mostly nitrogen snow; the right side is more methane ice.

PHOTO GALLERY: These may be the sharpest images of Pluto humans will ever see

Eight months after NASA's New Horizons spacecraft had its quick, close-up look at Pluto, scientists are reaping the scientific rewards from a bounty of data the spacecraft collected. Mission scientists reported their findings in five articles published Thursday in the journal Science.

Not a planet, but not boring

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This enhanced color view of Pluto's surface diversity shows, at lower right, ancient, heavily cratered terrain coated with dark, reddish tholins. At upper right, volatile ices filling the informally named Sputnik Planum have modified the surface, creating a chaos-like array of blocky mountains.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This enhanced color view of Pluto's surface diversity shows, at lower right, ancient, heavily cratered terrain coated with dark, reddish tholins. At upper right, volatile ices filling the informally named Sputnik Planum have modified the surface, creating a chaos-like array of blocky mountains.

From earlier observations from Earth and the Hubble Space Telescope, scientists knew Pluto was blotchy. Still, they wouldn't have been surprised if the landscape turned out to be geologically bland.

That's because the sun, 3 billion miles away, provides little energy, and Pluto is so small, smaller than the Earth's moon, that its interior could have cooled down long ago.

"You'd expect to see a boring cratered ball," said William M. Grundy of Lowell Observatory in Arizona, who leads the team analyzing the composition of Pluto's surface.

Others expected Pluto to look somewhat like Triton, a Pluto-size moon captured into orbit around Neptune.

Instead, New Horizons photographed a dazzling variety of landscapes, from soaring mountains to flat plains. Pluto is proving to be far more diverse and quite different from Triton.

"The big surprise is that Pluto turned out so surprising," said Jeffrey M. Moore of NASA's Ames Research Center in California, who heads the mission's geophysics and imaging team.

An ice volcano?

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Wright Mons was informally named by the New Horizons team in honor of the Wright brothers. At about 90 miles across and 2.5 miles high, this feature is enormous. If it is in fact an ice volcano, as suspected, it would be the largest such feature discovered in the outer solar system.

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NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Wright Mons was informally named by the New Horizons team in honor of the Wright brothers. At about 90 miles across and 2.5 miles high, this feature is enormous. If it is in fact an ice volcano, as suspected, it would be the largest such feature discovered in the outer solar system.

Nitrogen might also flow deep enough to be warmed by the interior and then erupt back at the surface — producing what scientists are surmising might be an ice volcano. They are studying a mountain named Wright Mons that rises two miles, spans 90 miles across and has a hole at the center.

"It's not like any feature we've seen anywhere else in the solar system," said John R. Spencer, a planetary scientist at the Southwest Research Institute in Boulder, Colo.

The varying mix of ices could form different alloys with very different properties, similar to how adding carbon transforms iron into steel, and that could help explain the wide range of topography.

"That's the new physics that needs to be learned," Grundy said.

A big, fractured moon

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

New Horizons captured this high-resolution enhanced color view of Charon just before closest approach on July 14, 2015. The image combines blue, red and infrared images taken by the spacecraft's Ralph/Multispectral Visual Imaging Camera; the colors are processed to best highlight the variation of surface properties across Charon.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

New Horizons captured this high-resolution enhanced color view of Charon just before closest approach on July 14, 2015. The image combines blue, red and infrared images taken by the spacecraft's Ralph/Multispectral Visual Imaging Camera; the colors are processed to best highlight the variation of surface properties across Charon.

The flyby spotted an enormous gash in Pluto's largest moon, Charon, that differs from Pluto in the makeup of its surface.

Charon appears to made of just water ice without the other ices seen on Pluto. That matched expectations, because Charon, with less gravity, would have not have been able to hold on to methane, nitrogen and carbon monoxide.

The most striking feature on Charon is the 600-mile long gash, longer than the Grand Canyon. Harold A. Weaver Jr., the mission's project scientist, said the gash was probably formed early in Charon's history when the surface cracked and material from the still-warm interior oozed out.

"Charon burst at the seams," he said.

Smaller Moons Spin Rapidly, While Tipped Over

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This image of Pluto's small satellite Nix taken by the Multispectral Visible Imaging Camera aboard New Horizons is one of the best images of Pluto's third-largest moon generated by the NASA mission.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This image of Pluto's small satellite Nix taken by the Multispectral Visible Imaging Camera aboard New Horizons is one of the best images of Pluto's third-largest moon generated by the NASA mission.

The four small moons of Pluto turned out to be brighter and smaller than expected and spin quickly. Their axes are also tipped sideways, a configuration that defies easy explanation.

Pluto and its miniature planetary system are believed to have coalesced out of a cataclysmic collision earlier in the history of the solar system. Over time, the rotation of moons tend to become gravitationally locked so that the same side of the moon is always facing the planet. That occurred with Charon.

But the four smaller moons — Nix, Hydra, Styx and Kerberos — are tiny and farther away. A month before the flyby, two astronomers suggested, based on years of Hubble photographs, that Nix and Hydra, appeared to be rotating chaotically, jostled by the competing gravitational pulls of Pluto and Charon. They also said that Kerberos was markedly darker than the other three.

The New Horizons photographs showed otherwise. None of the moons appear to be rotating chaotically, and their spin is faster than expected, not at all locked to their orbital periods, which range from 20 to 38 days. Hydra spins fastest, at once every 10 hours. Kerberos turns out not to be dark; the four small moons are all brighter and smaller than previously estimated, ranging in reflectivity between fresh concrete and fresh snow.

The rotation of the small moons are also tipped over, almost at 90-degree angles from what would be expected.

"We have no idea what that means yet," Spencer said.

One of the astronomers who reported the chaotic rotations, Mark R. Showalter of the SETI Institute in Mountain View, Calif., said that the New Horizons' observations surprised him, and that he was working on reconciling them with what Hubble had seen.

Surprise in Pluto's Atmosphere

Another finding indicates that the upper atmosphere of Pluto is much colder, meaning that nitrogen escapes at a rate of about a hundredth of what had been expected.

Frances Bagenal of the University of Colorado, head of the team that performed that analysis, said the calculation runs counter to what mission scientists were saying a week before the flyby, when New Horizons had already detected nitrogen escaping from Pluto.

"We were being fooled by something else," Bagenal said.

Up Next

The New Horizons spacecraft is now headed toward a New Year's encounter with a smaller body in the outer solar system called 2014 MU69, which will provide more data about a neighborhood that has been little studied.

So Pluto has now been left behind, and the chances of scientists getting another close-up look in the next few decades are slim.