Pluto, once considered the ninth planet, is a dwarf planet orbiting in the distant Kuiper Belt, an area of the solar system far beyond Neptune. This remote location results in extremely frigid surface temperatures, which average around -220 degrees Celsius. For decades, scientists expected this distant world to be geologically inert, an ancient, unchanging ball of ice and rock. Whether Pluto, despite its isolation and cold environment, possesses any form of volcanic activity was a major question after the 2015 reconnaissance. The answer, surprisingly, is yes, but the process is an icy variant unique to the outer solar system.
Defining Cryovolcanism
The volcanic process on Pluto, known as cryovolcanism or “ice volcanism,” is fundamentally different from the fiery eruptions seen on Earth. Terrestrial volcanism involves the eruption of silicate rock (magma) driven by intense internal heat. Cryovolcanism, in contrast, involves the eruption of volatile materials that are liquid or slushy at the interior temperatures of icy bodies. This eruptive material, often called cryolava, is composed of a mixture of water, ammonia, methane, and other ices. These compounds act as antifreezes, allowing the water to remain mobile far below its normal freezing point, and when extruded, the cold mixture quickly freezes and solidifies into ice-rich geological structures.
Identifying Pluto’s Volcanic Structures
The direct evidence for Pluto’s cryovolcanism was gathered by the New Horizons spacecraft, revealing a surprisingly young and geologically active surface. Scientists identified two enormous mountains, Wright Mons and Piccard Mons, as the strongest candidates for cryovolcanic structures. Wright Mons is a massive dome-like feature, standing 4 to 5 kilometers high and spanning 150 kilometers across its base, featuring a large, bowl-shaped depression (caldera) at its summit. Piccard Mons is even larger, reaching up to 7 kilometers in height and 225 kilometers in width. The most compelling evidence for their recent formation is the near-total absence of impact craters, suggesting the surface was recently resurfaced by icy flows, which left behind a hummocky texture interpreted as solidified remnants of viscous cryolava.
The Mechanism of Eruption
The eruption process requires a mechanism to mobilize and force the icy material through the crust. The presence of cryovolcanoes suggests a subsurface ocean, likely liquid water mixed with antifreezing agents like ammonia, exists or existed deep beneath the icy shell. This reservoir becomes pressurized, possibly due to the freezing and expansion of the ocean’s upper layers or tectonic stresses, forcing the liquid-rich material upward through fractures in the overlying ice shell. Due to the extremely low surface temperatures and low atmospheric pressure, the cryolava would likely be a thick, slushy mixture. The process appears to be primarily effusive, involving slow, massive extrusions that flow across the surface before quickly freezing into large, dome-like structures. The material’s ability to flow before solidifying points to a relatively low-viscosity, water-rich composition altered by dissolved volatiles.
What Cryovolcanoes Reveal About Pluto’s Past
The discovery of cryovolcanism has profound implications for understanding Pluto’s geological history and internal dynamics. Such recent activity suggests that Pluto retained enough internal heat to sustain geological processes long after its formation, likely generated by the decay of radioactive elements within its rocky core. The sheer scale and youth of the cryovolcanic features indicate that a subsurface ocean may have persisted for billions of years, possibly insulated by a layer of gas hydrates. This extended period of internal warmth and the presence of liquid water raises the possibility that Pluto’s interior could have provided a stable environment. The existence of recent cryovolcanism pushes scientists to reconsider how heat is retained and sustained on distant, isolated icy bodies throughout the outer solar system.