One year ago today, humanity got its first close look at a world that has managed to command the passions of Earthlings from billions of miles away.
On July 14, 2015, NASA’s New Horizons spacecraft swept up close to Pluto, giving us our first close look at the tiny icy world since its discovery in 1930.
It took all of three minutes for the spacecraft to cross the dwarf planet’s heart-stained face, but it spent much longer sailing through the entire system and staring at Pluto, its huge moon Charon, and the four small moons Styx, Nix, Hydra, and Kerberos.
The data it gathered during that fleeting encounter continue to surprise and mystify scientists, just as Pluto continues to be as much a public favorite as it ever was, even inspiring the U.S. Postal Service to issue a stamp revising its 1991 “Pluto—Not Yet Explored” release.
“There's something about the underdog Pluto that inspires and interests people, more than I've seen for any of the targets I've worked on,” says New Horizons team member Carly Howett of the Southwest Research Institute in Colorado.
As data from the spacecraft continue to stream back to eager scientists, here are a few of the surprises the New Horizons mission has revealed.
In 1930, Clyde Tombaugh discovered Pluto when he compared this image (with Pluto circled) with another taken six days earlier and noticed the bright speck had moved.
Pluto Is Geologically Alive
Some of Pluto’s features are among the most alien in the solar system, including a vast field of bladed methane ice now known as snakeskin terrain. Yet at it its core, Pluto is more than a mere planetary oddity: It is fundamentally revising ideas about how icy worlds work.
Before New Horizons flew by, scientists thought there wouldn’t be much in the way of geological activity happening out there on the fringe, where temperatures are decidedly abyssal and materials tend to freeze in place.
Sure, Earth-based observations hinted that ices may seasonally slide across Pluto’s face, but the team was unprepared for what New Horizons revealed: Pluto is, or has been in the very recent past, geologically alive. Materials on the surface are being replenished in a process powered by the heat in the planet’s interior.
“I was quite surprised to see activity on Pluto, because of its large distance from the sun,” says team member Cathy Olkin of the Southwest Research Institute, pointing to the vast, 745-mile-wide (1,200-kilometer-wide) ice field known as Sputnik Planum.
The smooth, frosted field is divided into polygonal cells that suggest convection, or a sort of slow-motion bubbling, is taking place below. And it’s surprisingly young, getting a fresh coat of ice every 500,000 to a million years or so.
“We were expecting some activity related to interactions between Pluto's surface and atmosphere as it orbits the sun, but nothing like a convecting mass of nitrogen ice the size of several states!” says team member Oliver White of NASA’s Ames Research Center in California.
Pluto Has Nitrogen Glaciers and Floating Mountains
Flowing into Sputnik Planum are glaciers made of nitrogen ice that, somewhat oddly, behave just as glaciers do on Earth. As they move, these exotic glaciers flow near enormous, floating mountains made of water ice, carry chunks of icy debris with them, and carve channels into Pluto’s surface.
Whether liquids also chiseled channels into Pluto’s face is still unknown, says team member Will Grundy of Lowell Observatory. And the subsurface plumbing, which could include a buried ocean, remains mysterious, he says.
Another persisting enigma is the two large mountains with suggestive pits at their summits. If scientists are right, these mountains—called Wright and Piccard Mons—are ice volcanoes, erupting lava made of icy mixtures rather than fiery magma onto Pluto’s face.
“Any time you get a first close-up look at something, it generates way more mysteries than answers,” Grundy says. “Or at least it seems that way.”
Charon Is Red But Not Exactly Dead
It would be easy to stop with Pluto and call the mission a success. But scene-stealing Pluto wasn’t the only world in New Horizons’ eyes.
Its large moon Charon, for instance, has also turned out to be fantastically weird. Large enough to be considered a binary planet in addition to a moon, Charon is not just the cratered relic many scientists had imagined.
“Perhaps naively I expected Charon to look more like [Saturn’s moon] Rhea, boringly just covered in craters and gray,” Howett says.
Yes, there are craters, but while Charon definitely has more of them than Pluto, there aren’t nearly as many pockmarks as expected. Overall, the world is much darker than Pluto and has a perplexingly dark red pole named Mordor Macula, which is thought to be stained by molecules wafting over from Pluto.
Charon is also riven with chasms that scientists still can’t quite explain, but that could have formed when a buried ocean froze, expanded, and fractured its surface. And like Pluto, it also has some surprisingly smooth, seemingly young surfaces.
“Perhaps it took longer to cool down from formation than we expected,” Howett says. “Or maybe our understanding of things available to hit Charon is wrong, so it's young but perhaps not quite as young as we first thought.”
The Small Moons Are Bizarrely Bright
As scientists ponder Pluto’s dark grey companion, they’re also searching for hints about how the binary planet system formed. For now, they suspect it grew out of a giant collision similar to the one that formed Earth’s moon. Clues about that ancient catastrophic event are hidden not only in Pluto and Charon but also in the smaller moons Styx, Nix, Kerberos, and Hydra.
According to the prevailing story, those four small moons were once shrapnel hurled into space by the giant, Charon-forming collision, says team member Simon Porter of the Southwest Research Institute.
Trouble is, the four little moons are so far refusing to play well with that version of events.
“The small moons’ orbits are even odder than we thought,” says team member Kelsi Singer of the Southwest Research Institute.
As unruly children could be prone to do, some of the moons rotate rapidly and asynchronously and are on tilted orbits. They’re also not quite the right colors, being uniformly brighter than Charon. Nix has a reddish crater, while Hydra looks like it grew from two pieces of debris that collided and stuck together, forming a two-lobed body not unlike the duck-shaped comet 67P/Churyumov–Gerasimenko.
It all makes for a rather motley collection of small satellites.
“They were a challenge to anyone trying to understand their origin even before the flyby, and all the observations we collected have in many ways only served to deepen the mystery,” says team member Alex Parker of the Southwest Research Institute.
Pluto Gets Puffed Up
After New Horizons zipped through the Pluto system, it turned around to capture both Pluto and Charon in silhouette. And when the image of Pluto in profile came back to the ground, it stunned scientists. Backlit by the sun, the planet was ringed in an eerie, glowing blue.
What’s more, that sheath is divided into many layers of haze that scientists are just beginning to understand.
Pluto’s puffy, nitrogen atmosphere was discovered in 1988, when a distant star’s light shone through its layers. Many times more voluminous than Earth’s, Pluto’s atmosphere has been a puzzle for at least two decades. Scientists had assumed that as Pluto moved farther from the sun, its atmosphere would freeze and collapse, raining onto the planet’s surface.
That didn’t happen. Instead, it got puffier—at least as seen from Earth.
Until New Horizons flew by, decades of Earth-based observations suggested that large amounts of molecules were escaping Pluto’s gravity and wafting into space.
But as the mission revealed, not only was the atmosphere less puffy than expected, it was also mostly staying put. Now, scientists suspect something “unknown and unpredicted” is helping keep the atmosphere cool and relatively stable, Singer says.
Though it might sound frustrating to have decades of data questioned in just a few minutes, it’s exactly the kind of thing the scientists relish.
“The flyby has really highlighted how dynamic Pluto must be, for example, the structured hazes in the atmosphere or the smooth, cellular structure in the plains of Sputnik Planum,” Parker says, adding that the time frames on which Pluto changes are still unknown. “We really only have this single snapshot in time from which to infer how Pluto changes in detail.”
As New Horizons sails on to its next target, a body in the Kuiper belt provisionally called 2014 MU69, it is continuing to beam data back to Earth.
Perhaps those data will answer some of lingering questions over the next years, but there are enough Plutonian mysteries to keep scientists wishing for another visit to the dwarf world, perhaps by a spacecraft that pulls into orbit.
“Pluto has revealed itself to be a fascinating enigma in the solar system, some aspects of which we think we have a good handle on explaining, whereas other aspects are still pretty baffling,” White says.
For starters, while the spacecraft got a tantalizing glimpse of some spectacular scenery as it approached Pluto, it could only record one hemisphere in high resolution, so half of the planet is still shrouded in mystery.
“What really lies on the non-encounter hemisphere of Pluto, much less the southern hemisphere wrapped in polar darkness?” wonders team member Bill McKinnon of Washington University in St. Louis. Future missions might also help us understand Pluto’s activity over time, which would speak to the planet’s past, including the early days of the solar system when the small world likely orbited closer to the sun.
“What was Pluto like, back when it orbited closer to the sun?” McKinnon asks. “Is there any preserved record of these 'before times' on its surface? And if not, what accounts for the evidence of an extraordinarily geologically active past?”
Now, alone again but no longer anonymous, Pluto continues to excite controversy, though this time it’s less pedantic than the debate over planethood. That’s because, regardless of what you call it, Pluto has turned out to be even better than expected.
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