New Horizons has sent back another set of images from its Pluto flyby — and they're just as fascinating as what we saw earlier this week.
The video up top is a simulated flyover of the 11,000-foot-tall mountains we saw in photos Wednesday, as well as icy plains revealed in new photos of Pluto's surface:
The icy plains make up part of the large, heart-shaped feature seen in previous Pluto photos, and border a mountainous region.
Close examination of the photos reveals features that scientists think may be evidence of wind erosion. It's also possible they could be volcanic plumes. Right now, we still don't know all that much about Pluto — and have lots of questions.
Scientists also released the first-ever photo of Pluto's moon Nix:
They think we're looking at the end of a long, skinny moon, just 25 miles across.
More than anything else, all this new data confirms something pretty remarkable about Pluto. "Pluto is every bit as active any other geological body we have in the solar system," Jeffrey Moore, one of the mission scientists, said during the press conference. This sort of ongoing geologic activity is something no one expected before New Horizons arrived.
Why it's taking so long to get all the Pluto photos
New Horizons began sending back photos and other scientific data shortly after its flyby. But even traveling at the speed of light, it takes about 4.5 hours for those signals to reach Earth. On top of that, the huge distance means the signal is extremely faint and must be transmitted very slowly: An image that's 1024 pixels wide takes about 42 minutes to come through.
The spacecraft also collected lots of data on Pluto's temperature, atmosphere, and interactions with the solar wind (the charged plasma released by the sun), as well as the five moons. It's sent back several photos and many scientific measurements, but it'll take 16 months for it to relay everything.
What New Horizons has taught us about Pluto so far
We've only received about 1 to 2 percent of New Horizons' data — but it's already revealed all sorts of surprises about Pluto, and created a number of mysteries.
The biggest surprise is the fact that parts of Pluto's surface seem relatively young, with little or no craters. It also has ice mountains that are 11,000 feet tall — as high as the Rockies. Together, these observations tell us that some sort of underlying geologic process is going on, generating fresh terrain and features over time, perhaps even volcanic plumes.
But geologic activity requires some internal source of energy — and previously, scientists assumed Pluto wouldn't have any. Models suggest it's too small to still have large amount of radioactive materials left over from its creation (these materials decay over time, releasing heat).
And it's not orbiting a large planet, which can lead to tectonic activity in places such as Jupiter's moon Europa. That works through a phenomenon called tidal heating, in which the moon is squeezed by the gravity of the planet that it orbits, generating energy. Ultimately, Pluto's activity remains a big mystery at the moment.
New Horizons data has also shown that Pluto is slightly larger than previously thought — 1,473 miles in diameter, making it the largest dwarf planet. Its reddish-brown color is the result of ultraviolet light hitting methane gas and creating complex molecules called tholins. And its ice caps, made of methane and nitrogen, might generate snow during some parts of the Plutonian year, as the world cools down while drifting farther away from the sun.
The latest data download, meanwhile, revealed that the large, heart-shaped feature on Pluto's surface — nicknamed Tombaugh Regio, after Pluto's discoverer Clyde Tombaugh — has an extremely high concentration of carbon monoxide.
Understanding Pluto will help us better understand the whole solar system
What makes all these findings especially intriguing is that Pluto is our first taste of a much broader class of objects — the thousands of chunks of rock and ice that orbit the sun in a region called the Kuiper belt, beyond Neptune.
That means that if Pluto has, say, an ocean hiding underneath its ice — one possible explanation for its mysterious energy source — lots of other worlds at the outer edge of the solar system could, too.
Additionally, scientists believe that Pluto was created at the same time as the rest of our solar system, from the same materials. It likely formed much closer in to the sun — going through the same early stages of growth as Earth and the other rocky planets — before being flung outward billions of years ago.
This means that all the data collected on Pluto's geology, atmosphere, and moons will help scientists refine their ideas about this early era in our planet's history. "We know that the Earth went through the stage of growth that Pluto stopped at," Alan Stern, New Horizons' principal investigator, told me in April. "This will help us connect the dots."