A Heart of Surprises
When NASA's New Horizons spacecraft flew past Pluto in 2015, it transformed our understanding of the dwarf planet. Instead of a cratered, inactive world, the probe revealed towering mountains of water ice, vast plains, and flowing glaciers of frozen nitrogen.
The most iconic of these features is Tombaugh Regio, a bright, heart-shaped region on Pluto's surface. The western lobe of this heart, a massive basin named Sputnik Planitia, has become the focus of intense scientific study. This feature, larger than Texas and Oklahoma combined, is a deep basin filled with nitrogen ice that shows signs of geological activity, including city-sized convection cells that churn the ice.
The Clues in the Ice
A recent analysis of high-resolution images from New Horizons has uncovered peculiar dark streaks and patches along the boundaries of these convection cells in northern Sputnik Planitia. These features bear a striking resemblance to patterns seen on Earth's glaciers when they are wetted by rain or by liquid emerging from below. However, Pluto's incredibly thin atmosphere and frigid temperatures make nitrogen rain impossible. This led scientists to a fascinating hypothesis: the liquid must be coming from underneath the ice sheet. According to a study published in the Planetary Science Journal, the best explanation for these darkened features is that they have been temporarily wetted by liquid nitrogen seeping up from below.
How a Subsurface Flow Is Possible
But how could liquid exist on a world where surface temperatures hover around minus 230 degrees Celsius? The answer may lie deep beneath Sputnik Planitia's several-kilometre-thick ice sheet. Computer models suggest that heat radiating from Pluto's rocky core, combined with the immense pressure from the overlying ice, could be just enough to melt the nitrogen at the very bottom of the glacier. This liquid nitrogen, being slightly less dense than the solid ice above it, would become buoyant. Scientists theorise that it collects in reservoirs before erupting upwards through cracks and fractures in the ice, a process compared to volcanic dike systems on Earth. These eruptions would likely be short, intense pulses, releasing large volumes of liquid nitrogen onto the surface before it quickly refreezes, darkening the ice as it spreads.
A New Understanding of a Distant World
This discovery marks the first evidence of recently flowing liquid on Pluto's surface. "Pluto never stops surprising us," said Alan Stern, the principal investigator for the New Horizons mission. The findings suggest that Pluto is not a geologically dead world but remains active, with ongoing processes that continue to shape its landscape. This dynamic activity, including giant landslides recently discovered elsewhere on the dwarf planet, paints a picture of a complex and evolving world. The idea of subsurface liquids isn't entirely new for Pluto; previous studies have long pointed to the possibility of a deep ocean of liquid water mixed with ammonia, which would act as an antifreeze, far beneath the entire ice crust. This new evidence, however, points to a separate, shallower reservoir of liquid nitrogen directly linked to surface features.
