New images returned from NASA's New Horizons are spacecraft are unveiling a world that's evolving in slow motion: Glaciers made of nitrogen ice creep across its surface, hazes cycle through its puffy atmosphere, and dark organic compounds rain down.
"Pluto has a very interesting history, and there is a lot of work that we need to do to understand this very complicated place," New Horizons principal investigator Alan Stern said during a press briefing Friday.
It's been just ten days since New Horizons flew by Pluto and its five known moons on July 14. Since then, scientists have learned that the frosted dwarf planet is no stagnant husk of rock—everything they've looked at appears to be in some kind of flux.
It's surprising to see that amount of activity, they say, on such a small world in a part of the solar system where failed planetary building blocks revolve in perpetual dusk.
"We have actual evidence for recent geological activity," says team member Bill McKinnon, of Washington University in St. Louis. Such a discovery "is simply a dream come true."
Pluto's hazy skies form a halo around the dwarf planet in this backlit image taken by New Horizons on July 15. [Photograph By Nasa/Jhuapl]
Small, Wild World
Some of the images, released July 24, zoomed in on a portion of Pluto's surface informally called Tombaugh Regio. Better known as "the heart," this part of the planet hosts some of the most strikingly varied terrains. While the heart itself is relatively smooth, the terrains sculpting its edges are not.
Steep, icy mountains and eroded, pockmarked landscapes ring part of the heart's western half. In the north, at the rim of a vast ice field known as Sputnik Planum, glaciers made of flowing nitrogen ice are brushing up against surface bumps.
In the south, where mountain ranges named after Mt. Everest climbers Edmund Hillary and Tenzing Norgay have punched through the surface, non-water ices are flowing around and surrounding those mountains, and pooling in large impact craters.
"Nitrogen ice, carbon monoxide ice, methane ice—these ices are geologically soft and malleable, even at Pluto conditions, and they will flow in the same way that glaciers do on the Earth," McKinnon says. "This is really a young unit."
Near the southwestern margin of Pluto's heart, a jumble of mountains sits between bright, icy plains and old, dark, cratered terrains. The large, ice-filled crater near the left of center is about the size of the Washington, D.C., metro area. [Photograph By Nasa/Jhuapl]
Those ice flows, plus polygonal features revealed last week, suggest that Pluto's shifting terrains could be powered by heat escaping from the planet's interior. The polygons are smooth, raised areas surrounded by shallow fractures that could be formed by convection within Pluto's icy shell.
In other words, what we're seeing are Pluto's ices roiling in slow motion.
But there's more. In new, enhanced color images, Pluto looks like a world that's gotten caught in the crossfire of an interplanetary paintball fight. Splattered all over its surface are colorful blotches: some dark, some bright, some blue, others pinkish-white.
"Some parts are kind of baked, like near the equator, and other parts receive the condensation of these ices, as you can see on the north pole," says Cathy Olkin, New Horizons deputy project scientist.
A big peachy patch makes up part of Pluto's iconic heart. But, as scientists have shown, that heart is clearly broken: It has two fractured halves, and one of them is bleeding ices that are trickling down past Pluto's equator.
"Bright material, probably nitrogen snow, is being transported off the source region of the western lobe. Perhaps by winds," Stern says, "or perhaps by sublimation."
And then there's the planet's polar cap, rendered in orange bronze and dribbling down from Pluto's northern pole over an area of vastly complex terrains. More data coming back from New Horizons will help scientists figure out what kinds of compositional elements these varying colors correspond to.
But what about the stuff that's overhead? Pluto's atmosphere, which we've known about since 1988 but haven't actually seen until now, is a tenuous but puffy shroud that extends for about 1,000 miles (1,600 kilometers) above the world's surface.
Numerous theories have described how that shroud should behave as Pluto trundles along its egg-shaped orbit, which is now slinging it away from the sun and toward Plutonian winter.
But the atmosphere didn't look like it was following predictions, which suggested that as Pluto headed for winter, it should collapse.
Instead, the atmosphere has been getting puffier over these nearly two decades of observations. Now, though, new data from New Horizons' REX instrument suggests Pluto's atmosphere has actually lost about half its mass over the past two years.
"It is just one data point, but it is significant. We are going to have to figure it out," says team member Michael Summers of George Mason University.
Two of Pluto's small moons, Nix and Hydra, are coming into focus. Nix, on the left, has a bright red spot that could be its original color, revealed by an impact or other process. [Photograph By Nasa/Jhuapl]
It's too soon to know exactly what this REX data point means, but it does suggest that something dramatic has changed. Whether it's indicative of a downward trend, or is just a temporary dip is unclear.
What is clear, though, is that Pluto's skies are filled with layers of haze—small particles that waft up to 100 miles (160 kilometers) above Pluto's surface and then fall back onto it, contributing to its reddish tint.
It's too thin to see if you were standing on the surface, but much like Saturn's moon Titan, Pluto's haze is visible when the planet is backlit by the sun. New Horizons got that view as it sped away from the dwarf planet—it's now more than 7 million miles (11.2 million kilometers) away from Pluto.
"The haze is extensive," Summers says. "It's forming high in the atmosphere, where the temperatures are hot … It's a mystery. It's one of the things we're going to have to sort out in the coming days."