Pluto’s surface is a patchwork of striking contrasts: bright white plains of nitrogen ice, rust-red highlands stained by organic compounds, towering water-ice mountains, and blade-like ridges of frozen methane. Far from the dull, featureless rock many expected before NASA’s New Horizons spacecraft flew past in July 2015, Pluto turned out to be one of the most geologically diverse worlds in the solar system, with an average surface temperature of roughly -375°F (-225°C).
Colors and Surface Materials
From a distance, Pluto looks like a mottled sphere of pale tan, reddish-brown, and brilliant white. The reddish and dark brown tones come from complex organic molecules called tholins, which form when ultraviolet light from the Sun breaks apart simple gases like nitrogen and methane in Pluto’s atmosphere. The fragments recombine into large, sticky molecules that drift down and coat the surface. Tholins range in color from yellow and orange to dark red and nearly black, depending on their composition, which includes compounds like urea, carboxylic acids, and nitriles.
The brightest areas are dominated by nitrogen ice, with smaller amounts of carbon monoxide ice and methane ice mixed in. These ices are volatile at Pluto’s conditions, meaning they slowly sublimate, move through the thin atmosphere, and refreeze elsewhere, constantly refreshing some surfaces while leaving others dark and weathered.
The Giant Heart: Tombaugh Regio
The most recognizable feature on Pluto is a massive heart-shaped bright region roughly 1,000 miles across, officially named Tombaugh Regio. Its left lobe, called Sputnik Planitia, is a vast basin filled with nitrogen ice that sits 2 to 3.5 kilometers below the surrounding rim. The basin acts as a powerful cold trap, drawing in volatile ices from across Pluto’s surface. Nitrogen is the dominant ice there, contaminated by smaller amounts of methane and carbon monoxide that dissolve into it at Pluto’s surface temperature of about 40 Kelvin (-387°F).
What makes Sputnik Planitia visually striking is its texture. The ice sheet is broken into polygonal cells, each spanning a few hundred to a thousand square kilometers. These are the tops of slow convection cells: warmer nitrogen ice rises from below, spreads across the surface, cools, and sinks back down at the edges, moving at roughly 10 centimeters per year. The result looks like a massive tiled floor, with subtle troughs marking the boundaries between cells.
Perhaps most remarkable is what’s missing. Sputnik Planitia has no detectable impact craters, even at imaging resolutions fine enough to spot features as small as 625 meters. That absence means the surface is less than about 10 million years old, an eyeblink in solar system terms. Some process, likely the convective churning of the nitrogen ice, continuously erases craters and renews the surface.
Mountains of Water Ice
Along the western edge of Sputnik Planitia, jagged mountain ranges rise abruptly from the plains. The tallest, Tenzing Montes, reach peaks of about 6.2 kilometers (roughly 20,000 feet), with an average slope of over 19 degrees. Nearby, the Hillary Montes stand at roughly half that height. These mountains are made of water ice, which at Pluto’s temperatures is rock-hard and behaves like stone on Earth. The softer nitrogen and methane ices cannot support such steep terrain, so water ice is the only material strong enough to build mountains this tall.
The Dark Equatorial Belt
Stretching along Pluto’s equator, west of the heart, lies Cthulhu Macula, a vast dark region roughly the size of Alaska. Its surface has an extremely low reflectivity, about 10%, giving it a deep reddish-brown to nearly black appearance. This darkness comes from a thick mantle of tholins that have settled out of the atmosphere over long periods.
Despite its dark, ancient-looking terrain, Cthulhu holds surprises. Bright methane frost caps the tops of mountains and coats the rims and walls of craters throughout the region, creating a look that closely resembles snow-capped mountain ranges on Earth. The frost is concentrated on elevated surfaces and north-facing slopes, where atmospheric conditions favor methane condensation. The contrast between the dark tholin-covered lowlands and the bright methane-frosted peaks is one of the most visually dramatic features on Pluto.
Blade-Like Ice Ridges
In the Tartarus Dorsa region near Pluto’s eastern limb, the terrain looks unlike anything else in the solar system. Towering blade-shaped ridges of methane ice rise more than 500 meters (roughly 1,600 feet) high, spaced two to three miles apart. They resemble a formation found on Earth called penitentes, which are spiky ice structures that form in dry, cold mountain environments through sublimation. On Earth, penitentes are typically only a few feet tall. Pluto’s versions are hundreds of times larger, likely because methane ice has been sublimating and reshaping for millions of years under weak but persistent sunlight. The ridges align in a consistent direction, matching what models predict for features carved by solar-driven erosion.
Possible Cryovolcanoes
South of Sputnik Planitia, two enormous mound-shaped features hint at volcanic activity powered not by molten rock but by slushy mixtures of water, ammonia, or other ices. Wright Mons is about 150 kilometers (90 miles) across and 4 kilometers (2.5 miles) high, with a large central depression that resembles a volcanic caldera. Only one impact crater has been identified on its surface, suggesting it formed or was resurfaced relatively recently in Pluto’s history. If these features are indeed cryovolcanoes, they would point to residual internal heat driving geological activity on a world that was long assumed to be frozen solid and inert.
A Thin Blue Atmosphere
Pluto’s surface appearance is also shaped by its thin nitrogen atmosphere. When New Horizons looked back at Pluto after flying past, it captured a stunning backlit view showing roughly 20 distinct haze layers extending up to at least 200 kilometers above the surface. These hazes scatter blue light preferentially, giving Pluto a blue-tinted atmospheric glow at visible wavelengths. The haze is brightest near the surface and fades with altitude, with a brightness scale height of about 50 kilometers over the lowest 150 kilometers. The particles in these haze layers eventually settle onto the surface, contributing to the tholin coating that colors so much of Pluto’s terrain. In this way, the atmosphere and surface are locked in a continuous cycle: ices sublimate upward, sunlight transforms atmospheric gases into tholins, and those tholins snow back down, gradually painting the surface in shades of red and brown.