The Heart of the Matter
At the centre of this discovery is Sputnik Planitia, the massive, continent-sized glacier that makes up the western lobe of Pluto's famous heart-shaped feature, Tombaugh Regio. This isn't a glacier of water ice like we have on Earth, but a deep basin
filled with frozen nitrogen, methane, and carbon monoxide. When NASA's New Horizons spacecraft flew by in 2015, it revealed a surprisingly young and active surface, free of craters and marked by strange polygonal patterns suggesting slow convection, like a cosmic lava lamp. But recent analysis of those images has uncovered something even more peculiar: dark streaks and patches that look remarkably similar to features on Earth's ice sheets where meltwater has wetted the surface.
The Paradox of Melting
The surface temperature on Pluto can plummet to a staggering minus 233 degrees Celsius, so liquid on the surface is out of the question. The new study, published in The Planetary Science Journal, proposes an ingenious solution to this puzzle: basal melting. The idea is that deep beneath the kilometers-thick nitrogen ice of Sputnik Planitia, the immense pressure, combined with modest heat from Pluto's interior, is enough to melt the nitrogen ice at its base. According to computer models from the Southwest Research Institute and the SETI Institute, this creates liquid nitrogen. Being less dense than the solid ice above it, this liquid would become buoyant, collecting in reservoirs before rising to the surface through fractures and conduits, much like magma on Earth.
A Surprisingly Warm Core?
This leads to the biggest question of all: where is this heat coming from on a world so far from the Sun? The answer likely lies deep inside Pluto. While not 'hot' by Earth's standards, Pluto's core is believed to have retained some residual heat from its formation billions of years ago. Furthermore, the slow radioactive decay of elements within its rocky core could provide a steady, long-term source of warmth. This internal heat, while not enough to warm the surface, appears sufficient to create the specific conditions needed for nitrogen to melt deep below the ice. This finding supports other theories that Pluto may still be geologically active and could even harbour a vast liquid water ocean deep beneath its entire icy shell, insulated by the same internal heat.
What This Means for Pluto
The discovery of potential recent liquid flows is a game-changer. It transforms our view of Pluto from a static, frozen ball of ice at the edge of the solar system into a dynamic, complex world that is still active today. Alan Stern, the principal investigator for the New Horizons mission, noted that "Pluto never stops surprising us." This finding is the first evidence of recently flowing liquid on Pluto's surface and suggests that similar processes could be happening on other distant, icy bodies in the Kuiper Belt, like Neptune's moon Triton, which is known for its nitrogen geysers. The dark streaks seen by New Horizons may be the telltale signs of liquid nitrogen that has briefly flowed and wetted the surface before refreezing, a process that could be ongoing or episodic.
