A Postcard That Keeps on Giving
When NASA’s New Horizons spacecraft flew past Pluto in July 2015, it sent back images that fundamentally changed our view of the dwarf planet. Instead of a cold, dead world, the pictures revealed towering mountains of water ice, vast plains of frozen
nitrogen, and a surprisingly complex and youthful surface. This treasure trove of data was so rich that scientists are still unpacking its mysteries. A recent study has done just that, providing the strongest evidence yet that Pluto is geologically active today, thanks to the behavior of its most abundant surface ice: nitrogen.
The Heart of the Matter
The most iconic feature on Pluto is Tombaugh Regio, a large, heart-shaped region on its surface. The western lobe of this heart, a vast basin named Sputnik Planitia, is a colossal glacier of frozen nitrogen larger than Texas and Oklahoma combined. Initial images showed that the surface of this glacier was divided into city-sized polygonal cells, indicating that the solid nitrogen was slowly churning like a thick soup over millions of years—a process called convection. This was a major sign of activity, suggesting heat was emanating from Pluto's interior. But the latest discovery takes this story of a dynamic Pluto a step further.
Following the Flow
A new analysis of high-resolution images from New Horizons focuses on dark streaks and diffuse patches found on the northern edge of Sputnik Planitia. A team of scientists, led by New Horizons principal investigator Alan Stern, noticed these features looked remarkably similar to patterns on Earth’s own glaciers that have been wetted by liquid water. But on Pluto, where surface temperatures are incredibly low, water would be as hard as rock and it’s far too cold for liquid nitrogen rain to form in the thin atmosphere. The inescapable conclusion, supported by computer models, is that the liquid must be coming from below.
Geology in a Deep Freeze
This is how the new theory works: the sheer weight of the kilometers-thick nitrogen ice sheet creates immense pressure at its base. This pressure, possibly aided by a small amount of internal heat from Pluto's rocky core, is enough to melt the bottom layer of the nitrogen ice, creating pockets of liquid nitrogen. This liquid, which is slightly more buoyant than the solid ice above it, can then be forced upward through cracks and fissures, briefly flowing onto the surface before freezing again. This process, known as basal melting, is the first direct evidence of recently flowing liquid on Pluto. It proves that complex geological processes aren't just for warm, rocky planets like Earth; they can happen on icy worlds in the most extreme cold.
Why This Changes Everything
Finding active geology on a world nearly six billion kilometers from the sun forces scientists to rethink their models of how planets work. It was widely assumed that a small body like Pluto would have lost all its internal heat billions of years ago, becoming geologically inert. But the evidence for both slow convection and now subsurface liquid flow suggests Pluto has found a way to stay active. As Alan Stern noted, "Pluto never stops surprising us." This discovery not only rewrites our understanding of Pluto but also has implications for other icy bodies in the outer solar system and beyond, suggesting that the conditions for dynamic geology—and perhaps even for subsurface liquid oceans—may be more common in the universe than we once thought.
