Science | October 22, 2024 8:00 a.m.

The Seven Most Amazing Discoveries We’ve Made About Pluto

Though technically not a planet, it has as rich geology as any of its planetary siblings in the solar system

A color-enhanced photo of Pluto that was captured by the New Horizons spacecraft in 2015 NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

Shi En Kim

Reporter

Pluto is probably best known for being a demoted planet. In 2006, the International Astronomical Union stripped it of its original classification, after narrowing the definition of a planet to be one that clears its orbit with its gravitational heft. Pluto doesn’t meet this membership criterion because its orbit is peppered with other icy bodies in the Kuiper belt.

While space scientists today still argue about Pluto’s rightful place in the pantheon of planets—some say its geology is as rich as that of any planet—no one can deny its scientific intrigue. From afar, Pluto once seemed boring and dead. After all, the dwarf planet is only two -thirds the size of our moon and resides more than 30 times farther out from the sun than does Earth. Before any mission to Pluto, scientists assumed that it would be an uneventful world. “I thought we were going to see something like an icy version of our moon,” says Fran Bagenal, a planetary scientist at the University of Colorado Boulder.

But despite its frigid temperatures on the surface, which drop to minus 400 degrees Fahrenheit, Pluto has a roaring internal geologic engine. That engine fashioned Pluto’s rich tapestry of flowing glaciers, jagged mountains and other unfamiliar terrestrial features on the surface.

To date, only a single mission, the 2015 New Horizons flyby, has ever dropped by Pluto. Even in a single brief encounter, the snapshots it returned of Pluto’s surface were enough to overturn many initial assumptions about this far-flung world. Here are some of the most amazing discoveries that New Horizons and previous Earth-based observations have produced of Pluto.

Pluto has strange ices

Surface of Pluto — Pluto has a spiky texture across parts of its surface. NASA / JHUAPL / SwRI

Being so cold, Pluto boasts different kinds of ices that planets closer to the sun don’t get to experience. The first kind is water ice—because the temperature on Pluto sits so far below the freezing point of 32 degrees Fahrenheit, any water on the surface is rock solid. The more interesting kinds of ices on Pluto are made up of volatiles: methane, nitrogen and carbon monoxide. These chemicals are present only as gases on Earth, but on Pluto, they cycle between gas and solid forms. As they do, the volatiles sculpt dynamic terrains on that frigid world. Their distribution on Pluto is patchy, because they regularly sublimate—turning from solid to vapor—and then freeze back down to the surface in a seasonal fashion.

The exotic methane and nitrogen ices were discovered by Earth-bound observations in 1976 and later in the 1990s, respectively. Their distribution on the planet was mapped out in greater detail when New Horizons skimmed past the dwarf planet. The mission also confirmed speculations that carbon monoxide ice lurked there as well.

Unlike the water ice on Pluto, methane and nitrogen ice are soft and behave like Silly Putty. They form glaciers that, over the eons, are capable of flowing, oozing and flattening out into plains. Most of the variegated topographical features on Pluto—dunes, hills, broken up blocks called chaotic terrains—are made up of or are the work of frozen methane and nitrogen.

Pluto is geologically rich

Convection Cells on Pluto — Parts of Pluto’s glaciers are covered with bubble-like convection cells. NASA / JHUAPL / SwRI

Despite its diminutive size, Pluto still has some primordial heat left to give. Pluto constantly erases old blemishes and refreshes its exterior as remnant heat in the interior powers geological reshuffling. Because of this, many parts of the surface are remarkably crater-free, as past impacts are erased. Instead, as New Horizons found, other craggy features that are of Pluto’s own making adorn its face: mountains, crustal fractures and canyons that wiggle in parallel near the North Pole.

One of the most intriguing geological features is a carpet of “bubbles” that blot the glaciated basin known as Sputnik Planitia. Each of these polygons measures at least 3.7 miles across and consists of roiling ice. As Pluto’s internal heat escapes through the glaciers to the surface, it churns soft nitrogen ice in pockets of upwelling and downwelling. The patchwork of frothing ice is like a “simmering a pot of oatmeal,” says Will Grundy, planetary scientist at the Lowell Observatory in Arizona. “That whole style of glaciation is totally new to us.” The resulting glacial pattern hasn’t been observed elsewhere in the solar system.

Scientists have tried to explain why Pluto has not quite completely cooled off. Perhaps Pluto’s core has radioactive elements that impart an additional source of heat. Another theory is that the crust has a special insulation made with trapped methane gas that’s adept at slowing the dissipation of heat from the core. “There shouldn’t be enough interior heat inside Pluto,” says Kelsi Singer, a geophysicist at the Southwest Research Institute in Colorado. To explain the planet’s dynamism, “we had to come up with creative ideas.”

Puny Pluto demonstrates that tiny planetary specks have astonishing secrets. “The real lesson there was that, surprisingly, small bodies can be geologically active,” Grundy says. “There are interesting things all over the place [on Pluto]—more is going on which the eye can’t see.”

Pluto has a respectable atmosphere

Atmosphere of Pluto — Pluto’s wispy atmosphere glows softly. NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

For a planetary body so tiny and distant, no one expected Pluto to harbor a gassy sheath. Scientists thought that Pluto would be too gravitationally feeble to hold onto its gases, or it would be too cold that its atmospheric constituents would have long frozen out as snow. Although telescopes from Earth detected gases on Pluto in 1988, researchers surmised that the tiny planet was losing them too quickly to build up a permanent layer in its skies.

But when New Horizon caught Pluto crossing in front of a star, it observed an unmistakable nimbus around Pluto that was a thin but substantial atmosphere. In this nitrogen dominated shell, scientists counted up to twenty distinct layers of air.

Compare that with Earth’s Moon, a body larger than Pluto that’s bereft of an atmosphere of any kind.

“This picture of a more complex atmosphere…was not what we expected,” Bagenal says.

The layers of the atmosphere are actually blankets of haze, which allow Pluto to hold onto its air. Under the bombardment of sunlight, methane ice transforms to reactive gaseous particles that eventually fuse into larger aerosol molecules such as acetylene, ethylene and ethane. These compounds are especially adept at radiating infrared back into space, keeping the gases in Pluto’s upper atmosphere cool so that molecules no longer have enough energy to escape the dwarf planet. Without this hazy atmosphere, Pluto’s volatile glaciers would have encountered the full brunt of the sun and frittered away into oblivion.

Eventually, some of the haze particles get too hefty to stay aloft, and settle onto Pluto’s surface. They form brown patches of soot that mottle parts of Pluto’s Equator and the North Pole.

Pluto has a beating heart

This color-enhanced photo shows the tan-colored western lobe of Pluto’s heart, which is known as Tombaugh Regio. NASA / JHUAPL / SwRI

Looking at a New Horizons photo of Pluto, anyone would be quick to notice a beige, heart-shaped feature stamped on the hip of the dwarf planet. This love mark is known as Tombaugh Regio, after Pluto’s discoverer Clyde Tombaugh, and it immediately jumped out to scientists in the first few pictures that New Horizons captured.

The left lobe of the heart is called Sputnik Planitia and was formed from an oblique impact from a 400-mile-wide planetary body around ten million years ago. The resulting basin holds a lot of nitrogen ice. Researchers have had a hard time telling the exact age of the impact, because Pluto’s active geology is constantly refreshing the face of the glaciers.

The nitrogen glaciers are a million square miles wide and slowly marching north to south, constantly seeping gas as they go. Elongated pits a few hundred feet deep and about a mile across indicate where some ice has gasified and then absconded into space. During the colder nights, some of this gas condenses back elsewhere on the surface.

The daily cycle of solar-driven sublimation and condensation of Pluto’s iconic Valentine structure is like a heartbeat, because it dictates atmospheric circulation on the dwarf planet. The resulting winds etch Pluto’s face. Nitrogen winds blowing up to 20 miles per hour capture methane ice grains from Pluto’s mountains, then pile them into dunes on the plains.

A cryovolcano dimples Pluto’s Southern Hemisphere

South of Pluto’s heart is an extraordinary mountain shaped like a doughnut. This caldera-shaped construct is Wright Mons, and it stretches 93 miles wide and is about the same height as Everest from base to peak. Researchers think it could be a cryovolcano, a temperamental geologic feature that wields liquid water instead of molten lava.

No one has caught Wright Mons erupting yet, but researchers have good cause to infer that it is—or was—geologically active. The exterior lacks impact craters, indicating that the surface is still new and erasing its past. Scientists think that its towering height hints that the cryovolcano is made of rock-hard water ice. The only way for Pluto to fashion such a skyscraper would be if some internal heat were moving the water around. Perhaps internal heat is causing a groundswell or triggering eruptions that pile material on the peak. Scientists can’t say for sure that is happening because of the feature’s lumpy appearance and curious lack of flow features.

Pluto has a massive moon

Pluto has five moons—“fun-looking space potatoes,” Singer calls them—and its largest is a behemoth. Charon was discovered in 1978 when scientists noticed Pluto appeared to elongate every six days. The apparent change occurred when the blob of Charon peeked out Pluto’s line of sight.

Charon is half the size of Pluto. The satellite is the closest in size to its parent of all the moon-planet pairs in our solar system. The moon is so hefty that it influences Pluto’s spin. In fact, Pluto’s spin axis lies somewhere between the dwarf planet and Charon. Scientists occasionally refer to Charon and Pluto as a double dwarf planet system.

Up close, Charon is striking. The satellite is covered in crevices that scientists think may harbor a liquid ocean. Its North Pole is coated in organic material that wandered over from Pluto and condensed there. When the sun strikes these volatiles, they react and merge into larger compounds that give the pole a red color.

Pluto’s insides may be sopping wet

Interior of Pluto — Illustration of a cross-section of Pluto’s interior. One of the subterranean layers is of an ocean that drives surface processes. Illustration by James Tuttle Keane

For a frigid planet, Pluto may be hiding a sloshy surprise roughly 125 miles beneath its shell. Scientists suspect that a global ocean of liquid water is lurking right under the surface. Though no direct evidence on the ocean exists to date, scientists have inferred its presence in several ways. For example, Pluto also has large cracks on its surface, which can be explained by underground liquid water slowly freezing solid. As water expands when it freezes, the growing ice splits the crust to form massive fissures.

How exactly Pluto has managed to keep its ocean from freezing out has scientists scratching their heads. One possible explanation is the presence of dissolved ammonia in the ocean that acts as an antifreeze. If a liquid water ocean is truly present inside Pluto, this puts the dwarf planet among the ranks of other ocean worlds in the solar system, including Jupiter’s Europa and Saturn’s Enceladus. “Any ocean world could possibly be a habitable environment,” says Bonnie Buratti, a NASA planetary scientist. Although experts are careful to point out habitability doesn’t equate to the presence of life, any likelihood of it, however minute, is tantalizing. “It’s somewhat speculative,” says Buratti, “but it’s possible.”

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Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.