Until 14 July 2015, every picture of Pluto was a fuzzy blob. Even the Hubble Space Telescope — the most powerful telescope humans had ever built — could only show Pluto as a few coloured dots. Astronomers had been studying it for 85 years without really knowing what its surface looked like.
Then NASA's New Horizons spacecraft swept past Pluto at 14 kilometres a second and started taking photographs. What came back was breathtaking. Pluto wasn't a featureless ice ball at all. It was a world of mountains, ice volcanoes, frozen plains and red-painted dust — and right in the middle of it, a heart.
Pluto's surface has a giant heart-shaped plain of frozen nitrogen called Tombaugh Regio, mountains made of water-ice that rise 3 km high, ice volcanoes that erupt slushy water, and a dark red dust called tholin that paints parts of it brown. Scientists think there may even be a slushy ocean hidden under the surface.
The Heart of Pluto — Tombaugh Regio
The most famous feature on Pluto is a huge bright region shaped like a heart. It is about 1,600 km across — bigger than the entire country of France. Scientists named it Tombaugh Regio, after Clyde Tombaugh, the man who discovered Pluto in 1930.
The heart isn't just there for show. It is doing something incredibly strange. The left half of the heart is the lowest, flattest, smoothest place on Pluto. It is called Sputnik Planitia, and it is a giant basin filled with frozen nitrogen. The right half is older and rougher.
Sputnik Planitia — A Frozen Lake the Size of Texas
Sputnik Planitia is a flat plain of frozen nitrogen the size of the US state of Texas. If you stood on it, you would see a smooth pale surface stretching all the way to the horizon, broken into giant polygon-shaped patches like cracked dried mud, except each patch is dozens of kilometres across.
Scientists think the patches are slowly moving. The frozen nitrogen flows like very slow glacier ice, churning and turning over once every million years or so. The whole plain is a giant slow-motion conveyor belt.
The Mountains Are Made of Water
Some of Pluto's mountains are 3,000 metres tall — taller than the Rocky Mountains in America. But here's the strange part. They aren't made of rock. They are made of water-ice.
At Pluto's temperature — about −229 °C — water-ice is hard as granite. So at the edge of the heart there are two ranges of huge frozen mountains: Norgay Montes and Hillary Montes, both named after the climbers who first reached the summit of Mount Everest.
Earth's mountains are made of rock. Pluto's are made of frozen water. If you could pick up a Pluto mountain and bring it to Earth, it would melt into a giant puddle and flood several cities. On Pluto, that same water is so cold it has the strength of granite.
Ice Volcanoes — Yes, Really
Two huge mountains on Pluto called Wright Mons and Piccard Mons look like volcanoes. They have wide bases and deep central craters — just like Earth volcanoes. But they don't erupt molten rock. Scientists think they erupt slushy water and ammonia.
These are called cryovolcanoes, or "ice volcanoes." Wright Mons is enormous — about 150 km wide and 4 km tall. As far as we know, Pluto is the only place in the outer solar system where you can see them up close.
The Dark Patches — Tholins and Cthulhu Macula
If you look at a photo of Pluto, you'll see bright white regions, pinkish-tan zones, and dark red patches. The dark patches come from a sticky chemical called tholin, which forms high up in Pluto's thin atmosphere when sunlight breaks apart methane gas. The tholins are reddish-brown, and they slowly drift down onto the surface like fine snow, painting Pluto in slow motion over billions of years.
The darkest patch on Pluto's surface has its own creepy name: Cthulhu Macula. It is named after a fictional underworld monster from a famous horror story. It is roughly the length of the United States.
The same tholins that paint Pluto's surface red also make Pluto's sky blue. The particles scatter blue light the same way Earth's air does, even though Pluto's atmosphere is barely there. Pluto's blue sky explained.
The Bladed Terrain — A Forest of Frozen Teeth
Of all the strange landscapes on Pluto, one is so weird it doesn't exist anywhere else in the solar system. It is called the bladed terrain, found in a region named Tartarus Dorsa. From space, it looks like a frozen forest of razor-sharp teeth standing in long parallel rows.
Each blade is about 500 metres tall. They stretch for hundreds of kilometres. Scientists think they are made of frozen methane, sharpened over billions of years by exposure to the very faint sunlight at the edge of the solar system.
The Hidden Ocean
Underneath the heart of Pluto, scientists believe something extraordinary may be hiding: a slushy ocean of liquid water.
The ocean would be deep beneath the surface, kept warm by heat slowly escaping from Pluto's rocky core. Scientists can't see it directly, but the way Sputnik Planitia formed, and the shape of certain features on Pluto, both make the most sense if there is liquid water hidden inside the planet. If the ocean really exists, Pluto becomes one of the few places in the solar system where life could possibly survive — deep below, sheltered from the cold.
Read about the New Horizons mission that took these photos, the blue sky above the surface, or jump back to the main Pluto facts page.
Frequently Asked Questions
What is the heart on Pluto?
The heart on Pluto is a giant heart-shaped region called Tombaugh Regio, named after Pluto's discoverer Clyde Tombaugh. The left half is a plain of frozen nitrogen called Sputnik Planitia. The heart is about 1,600 km across — bigger than France.
Why is Pluto red?
Pluto's reddish-brown colour comes from tholins — sticky chemical particles that form when sunlight hits the methane in Pluto's thin atmosphere. The tholins drift down onto the surface as a fine dust, painting Pluto red over billions of years.
Are there mountains on Pluto?
Yes — and they are made of water-ice, not rock. At Pluto's temperature (about −229 °C), water-ice is as hard as granite. The biggest mountains, Norgay Montes and Hillary Montes, are up to 3,000 metres tall, taller than the Rocky Mountains.
Are there volcanoes on Pluto?
Yes, but not the kind that erupt hot lava. Pluto has two huge ice volcanoes (cryovolcanoes) called Wright Mons and Piccard Mons. They erupt slushy water and ammonia. Wright Mons is about 4 km tall and 150 km wide.
Is there water on Pluto?
Yes, lots of it — mostly as ice. Pluto's mountains are made of frozen water, and the bedrock under its nitrogen plains is water-ice. Scientists also believe a slushy ocean of liquid water may exist deep beneath the surface, kept warm by heat from Pluto's rocky core.