The question of which planet in the solar system holds the most water is complex, as water exists in various forms beyond the familiar liquid state on Earth. While our world is often called the “Blue Planet,” its water content is dwarfed by massive reservoirs found elsewhere. The true answer depends on whether one counts only liquid water or includes the vast quantities of water ice and super-pressurized fluids integrated deep within planetary structures. When considering the total volume of H₂O, including its icy and exotic states, the greatest concentrations are found far in the outer solar system, not on terrestrial worlds.
The Vast Water Reservoirs of the Ice Giants
The largest single-body water reservoirs in the solar system are the ice giants, Uranus and Neptune, which formed beyond the “snow line.” These planets are misnamed, as their interiors are not primarily frozen water ice but rather a dense, hot fluid mix of water, ammonia, and methane. This massive mantle is often described as a superionic water layer, where water molecules break down into a plasma-like state under immense pressure and heat.
Models suggest that Uranus and Neptune may each contain a water volume equivalent to tens of thousands of times the total amount of water in Earth’s oceans. This layer can extend for approximately 8,000 kilometers (5,000 miles) beneath the planets’ thick hydrogen and helium atmospheres. The sheer scale of these deep, pressurized fluid layers makes the ice giants the holders of the solar system’s greatest bulk water supply. Although the water is in an exotic, high-pressure form, its chemical composition confirms it as H₂O.
Subsurface Oceans: The Greatest Source of Liquid Water
While the ice giants hold the most total water, the largest volumes of liquid water are likely contained in the subsurface oceans of the icy moons orbiting Jupiter and Saturn. These oceans remain liquid despite the frigid surface temperatures because of tidal heating. This mechanism involves the giant planet’s gravity stretching and flexing the moon’s interior.
Jupiter’s moon Ganymede, the largest moon in the solar system, is thought to harbor an ocean up to 100 kilometers (60 miles) thick, buried beneath a 150-kilometer ice crust. This single ocean is estimated to contain more water than all of Earth’s surface water combined. Europa, another of Jupiter’s moons, is believed to conceal a global ocean about 100 kilometers deep beneath an ice shell that may be between 15 and 25 kilometers thick. Europa’s induced magnetic field provides strong evidence for a large, salty, liquid layer interacting with Jupiter’s magnetic environment.
Saturn’s small moon Enceladus also hosts a confirmed subsurface ocean. It is thought to be about 10 kilometers thick beneath the south polar region. Water-rich plumes erupting from the moon’s surface, often called “tiger stripes,” confirm that this liquid reservoir is actively venting into space. These icy moons are of particular interest because their liquid water is in contact with a rocky core, which could provide the chemical energy to potentially support life.
Water Ice on Terrestrial and Dwarf Worlds
Smaller bodies like terrestrial planets and dwarf worlds also contribute to the solar system’s water inventory, although on a much reduced scale compared to the ice giants or icy moons. Mars holds significant reserves of water ice primarily at its poles and beneath its surface. The total volume of confirmed Martian ice, including massive glacier-like forms, is estimated to be equivalent to a global layer of water covering the planet to a depth of around 1.5 to 2.7 meters (5 to 9 feet).
The dwarf planet Ceres, located in the main asteroid belt, has evidence suggesting that up to a quarter of its total mass is composed of water ice. In some models, the crust of Ceres may be over 90% water ice. Pluto, far out in the Kuiper Belt, is composed of a vast water ice “bedrock” that forms its mountains and crust, covered by more volatile ices like nitrogen and methane. Even this distant dwarf planet may harbor a subsurface ocean of liquid water, estimated to be 40 to 80 kilometers deep beneath its insulating ice shell.