It’s been a year since NASA’s New Horizons spacecraft whizzed past Pluto on 14 July 2015, providing our first detailed look at the icy dwarf planet and its moons. Data is still coming in from the craft — the last batch won’t arrive on Earth until later this year — but initial information shows that the planet is surprisingly different to what we expected, from the base of its smooth, icy expanses to the tips of its frozen mountains. What have we learned in the past year? Here are New Scientist’s top five most intriguing New Horizons discoveries:
1. Layers of atmospheric haze
After the spacecraft flew by Pluto, it took a look backward. From that vantage point, sunlight illuminated the dwarf planet’s hazy, surprisingly tall atmosphere.
“The huge vertical extent of the haze, and the layering, were two things that I wasn’t expecting,” says Randy Gladstone at the Southwest Research Institute in San Antonio, Texas. The haze stretches hundreds of kilometres into the sky and contains more than a dozen separate bands. There are even hints of distinct clouds.
Those layers are relatively stagnant, as observations taken a few hours apart indicate they last for at least that long, says Gladstone. That stability might be due to the structure of the particles that make up the haze. They are thought to be clumps of dust, shaped like a bunch of grapes. Fluffier than single flecks, this could be why they stay floating for longer.
2. Jumbled terrain
Before the fly-by, researchers weren’t sure what the surface of Pluto looked like — some thought it might be as dull and bland as a billiard ball, says Hal Weaver at Johns Hopkins University in Baltimore, Maryland.
“The fundamental thing that I wanted to do was transform Pluto from this little pixelated blob into a real world, with complexity and diversity,” says Weaver. “What we saw just went way beyond our expectations.”
Pluto’s chaotic terrain is far more varied than anticipated. One region sports what appear to be volcanoes that spew icy water instead of lava, while the informally named Sputnik Planum is a vast, relatively smooth plain flanked by mountains made of ice.
3. Youthful face
That smooth surface of Sputnik Planum — the left lobe of Pluto’s famed heart, a region known as Tombaugh Regio — suggests it’s relatively young and active, at least on planetary timescales. Researchers estimate that the area is less than 100 million years old, and perhaps even much younger than that.
Sputnik Planum is dominated by slowly churning polygons of nitrogen ice, each about 20 kilometres across, driven by heat from below. That’s a result of Pluto’s surface clocking in at about 40 kelvin, an almost perfect match for the temperature at which nitrogen can exist as a solid, liquid and a gas, says Weaver.
4. Charon’s grander canyon
The New Horizons mission didn’t leave Pluto’s smaller companions out in the cold, as its moons also got their moments in the spotlight. Images taken by the spacecraft of Charon, Pluto’s largest moon, show a giant gash sprawling across its equator.
The probe’s readings suggest it is at least 1800 kilometres long and 7.5 kilometres deep. “It dwarfs the Grand Canyon,” says Weaver. Researchers think this canyon ripped open when an ocean below Charon’s surface froze and expanded, soon after the moon’s formation.
5. Born in a collision
We didn’t know much about Pluto’s other moons — Nix, Hydra, Styx and Kerberos — before New Horizons sailed by the system. Images of the small satellites taken by the Hubble Space Telescope looked like little pinpoints of light, says Weaver.
“You had no idea how big they are, how reflective their surfaces are,” he says. “But we got all of that from the New Horizons fly-by.”
The data showed that the moons are much brighter than typical objects in the Kuiper Belt, the outer region of the solar system where Pluto lives. That suggests they are made mostly of water ice and were probably created in the same collision that produced Pluto and Charon, rather than captured after Pluto and Charon were already formed.
Where to next?
New Horizons is more than 400 million kilometres past Pluto now, but the spacecraft isn’t retiring just yet. NASA announced earlier this month that the spacecraft will fly past another object in the Kuiper Belt on 1 January 2019.
News of the official approval of the mission extension was a nice birthday present, says Gladstone. For now, the team will continue working to understand the data the spacecraft has already gathered — a task that may well last longer than remainder of the mission, he says: “I bet there’ll be students doing their theses on this for decades.”