Pluto, a dwarf planet orbiting in the distant Kuiper Belt, was long a mystery due to its extreme distance from Earth. The question of whether Pluto contains water was definitively answered following the 2015 flyby of NASA’s New Horizons spacecraft. Data showed that water is a major component of this icy world, existing primarily as rock-hard ice that forms the planet’s crust. This discovery transformed our understanding of Pluto, revealing a geologically active body that likely harbors a liquid water ocean deep beneath its frigid surface.
Surface Composition: A World of Water Ice
Pluto’s crust is composed of water ice, which acts as the stable bedrock for the entire planet. At surface temperatures of approximately -400 degrees Fahrenheit, this water ice is as rigid as rock on Earth. Water ice is considered non-volatile because it does not easily sublimate or evaporate into the thin atmosphere, unlike other surface materials.
Water ice is often obscured by a blanket of more volatile, or easily evaporated, ices. These lighter ices include frozen nitrogen, methane, and carbon monoxide, forming a thin, seasonally changing layer across the landscape. Spectroscopy data from the New Horizons’ Ralph instrument, specifically the Linear Etalon Imaging Spectral Array (LEISA), allowed scientists to identify the unique spectral signature of water ice.
The instrument revealed that while volatile ices cover the vast plains, water ice is exposed in numerous small regions, particularly in the mountains where the volatile blanket is thin. This non-volatile water ice provides the structural integrity necessary to support Pluto’s impressive topographic features, such as its towering mountain ranges. Its distribution confirms that water ice is the most abundant ice on Pluto, forming the planet’s solid, underlying shell.
Evidence for a Subsurface Ocean
The most compelling evidence for a liquid water ocean beneath the icy crust comes from analyzing Pluto’s internal structure and its famous “heart” feature. The western lobe of the heart, a massive, low-lying plain called Sputnik Planitia, is aligned almost directly opposite Pluto’s moon, Charon. This alignment suggests Sputnik Planitia contains an excess of mass, causing the dwarf planet to reorient itself over time into its current configuration.
A deep basin like Sputnik Planitia, thought to be an ancient impact site, should ordinarily represent a deficit of mass. To explain the reorientation, the region must possess a “positive gravity anomaly,” meaning it has more mass than the surrounding areas despite its negative topography. Models show that without a subsurface ocean, the nitrogen ice deposit in the basin would need to be over 25 miles thick to create this anomaly, which is an implausible depth.
The presence of a liquid water ocean below the crust provides a natural explanation for the extra mass. If the impact thinned the icy crust over the ocean, the denser liquid water would have pushed upward, elevating the material below the basin floor. This uplifted layer of dense water, combined with a modest layer of nitrogen ice, creates the positive gravity anomaly necessary to pull the entire planet into tidal lock with Charon. For this ocean to remain liquid today, it must be sustained by residual heat from Pluto’s formation or the slow decay of radioactive elements within its rocky core. Additionally, the water is likely mixed with antifreeze agents, such as ammonia, which lowers the freezing point and helps maintain a liquid state beneath the insulating ice shell.
Geological Features Linked to Water Activity
The dynamic internal processes driven by a potential subsurface ocean have left striking, visible marks on Pluto’s surface. Among the most dramatic features are two large mounds, Wright Mons and Piccard Mons, interpreted as massive cryovolcanoes, or “ice volcanoes.” These features are unlike conventional volcanoes, as they do not erupt molten rock.
Wright Mons is an enormous construct, standing approximately 2.5 to 3 miles high and spanning about 90 miles wide, with a large central depression. The materials erupted are not lava but a slushy mixture of water ice and other volatiles like nitrogen, methane, and ammonia, which is extruded onto the surface. Cryovolcanism represents a way for subsurface water or water-rich slurries to be transported from the interior to the surface.
Beyond the cryovolcanoes, vast networks of tectonic faults and cracks crisscross Pluto’s icy surface. These features are evidence of expansion and contraction in the water ice crust over geological time. As a subsurface ocean freezes, the resulting volume increase puts tremendous stress on the surrounding ice shell, causing it to crack and form the observed extensional fault lines. The existence of these landforms confirms that water has been a geologically active agent on Pluto, shaping its complex surface.