Goodby Pluto. Thanks For The Memories

Behold Pluto’s Gorgeous, Spooky Halo!

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 160 kilometres above the world’s surface.

Here is New Horizons’ farewell shot of Pluto (above), taken as the spacecraft finished its flyby. It reveals the dwarf planet’s wispy atmosphere, visible because New Horizons was on the far side of Pluto when it captured the image, and the world is backlit. Mission leader Alan Stern said, “this is our Earthrise“—the shot that confirmed New Horizons really reached Pluto and succeeded in its mission.

New Horizons was 1.25 million miles past Pluto when it got the shot. More images from Pluto are set to come out during today’s press conference. 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 instrumentsuggests 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.

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.” [ Popular Mechanics.]

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