A Treasure Trove from a Brief Encounter
The New Horizons flyby on July 14, 2015, was a fleeting event. After a journey of over nine years and five billion kilometres, the probe had only hours to frantically gather as much information as possible before speeding onward into the Kuiper Belt.
Yet, that brief encounter generated a deluge of data, so much that it took over a year to transmit it all back to Earth. Scientists expected to find a cold, dead, crater-pocked ball of ice—a relic frozen in time. Instead, the images and measurements painted a picture of a dynamic, layered, and geologically active world that defied all predictions. The story of Pluto, it turned out, was just beginning.
A Heart That's Geologically Young
Pluto's most iconic feature is a massive, heart-shaped basin of ice called Tombaugh Regio. Its western lobe, Sputnik Planitia, is a vast glacier of frozen nitrogen, methane, and carbon monoxide larger than Texas and Oklahoma combined. The most startling discovery about this region is its complete lack of impact craters. On a planetary surface, craters accumulate over time, so a smooth surface implies that it is geologically young. Scientists estimate that Sputnik Planitia is no more than 10 million years old, a mere blink of an eye in the 4.5-billion-year history of the solar system. This suggests that processes like convection—where ice churns slowly like wax in a lava lamp—are actively resurfacing the area, erasing craters and keeping it looking perpetually youthful.
The Mystery of the Ice Volcanoes
Perhaps the most dramatic evidence of Pluto's ongoing activity is the presence of enormous cryovolcanoes. Unlike volcanoes on Earth that spew molten rock, these structures erupt a slushy mix of water ice, nitrogen, and ammonia. Analysis of a region southwest of Sputnik Planitia revealed multiple large domes, some rivaling the size of Mauna Loa in Hawaii. Features like Wright Mons, a mound 150 kilometres wide and nearly five kilometres tall, are thought to have been formed by these icy eruptions. Because this area also lacks craters, scientists believe this cryovolcanism was geologically recent—and could even still be happening. This activity poses a major puzzle, as a small, distant world like Pluto should have lost most of its internal heat billions of years ago.
An Ocean Beneath the Ice?
So, where is the heat coming from? The most compelling answer points to a stunning possibility: a vast ocean of liquid water hiding beneath Pluto's thick ice shell. The existence of this ocean is inferred from multiple lines of evidence. The geology of Sputnik Planitia suggests that the massive weight of the ice-filled basin caused Pluto to reorient itself, a process that would have created massive cracks and faults in the crust—faults that are visible in New Horizons images. This reorientation is best explained if the crust is floating on a liquid layer. Models suggest this ocean could be over 100 kilometres thick and extremely salty, with a composition similar to Utah's Great Salt Lake or the Dead Sea. Such an ocean would not only provide a source of retained heat from Pluto's formation but also the material for the cryovolcanoes observed on the surface.
A World of Constant Surprise
The discoveries don't stop there. Recent analysis of New Horizons imagery, published as recently as August 2026, suggests that liquid nitrogen may be seeping up through cracks on Sputnik Planitia, evidence of recently flowing liquid on the surface. Scientists have also found evidence for massive landslides large enough to bury a city, a complex, multi-layered blue haze in its thin atmosphere, and confirmed that its bedrock is made of rock-hard water ice. Each new finding reinforces the same conclusion: Pluto is not a simple, static object. It is a complex dwarf planet with active geology, a unique climate, and a story that continues to unfold with every new look at the data.
