The question of which planet is mostly covered in water has a definitive answer within our own star system. While many celestial bodies contain frozen or hidden water, Earth stands alone among the planets of the Solar System as the one visibly blanketed by liquid water. Our planet’s familiar blue appearance from space is a direct result of this massive water coverage, distinguishing it from every other world orbiting the Sun. This abundance of stable, surface-level water defines Earth as the solar system’s true water world.
Earth: The Solar System’s Water World
Earth’s surface is famously covered by water, with oceans, seas, and lakes masking approximately 71% of the planet’s total area. This extensive coverage earned Earth the nickname “the blue planet.” The vast majority of this water, an estimated 96.5%, is saline water held within the oceans.
The remaining fraction is freshwater, distributed across the planet in various forms. Most of this freshwater, about 68%, is locked away in ice caps, glaciers, and permanent snow cover. A significant amount also exists as groundwater, deep beneath the surface in aquifers and soil moisture.
Only a tiny percentage of the planet’s total water is readily available as surface freshwater in rivers and lakes, or as vapor in the atmosphere. The presence of water in all three physical states—liquid, solid ice, and gaseous vapor—is unique among the major planets.
The Mechanisms That Retain Earth’s Oceans
Earth’s ability to maintain its vast liquid oceans is a consequence of a delicate balance of physical factors. The planet resides within the Sun’s circumstellar habitable zone, often called the “Goldilocks Zone,” where temperatures are just right for liquid water to exist stably on the surface. This orbital position is a prerequisite for maintaining a water-rich surface.
A strong, globally encompassing magnetic field, generated by the motion of molten iron in the outer core, provides an additional layer of protection. This magnetosphere deflects the constant stream of charged particles known as the solar wind, preventing it from stripping away Earth’s atmosphere. Without this magnetic shield, the solar wind would have gradually eroded the atmospheric pressure necessary to keep water in its liquid form, similar to what is thought to have happened on Mars.
The origin of Earth’s water is attributed to a combination of internal and external sources. Volcanic outgassing from the planet’s interior released water vapor, which condensed to form the early oceans as the surface cooled. This internal source was supplemented by the impact of water-rich asteroids, specifically carbonaceous chondrites, which delivered additional volatile materials to the young planet.
Water Reserves on Other Solar System Bodies
While Earth is the only planet with vast liquid oceans on its surface, other bodies in the Solar System hold considerable water reserves, mostly in a frozen or subterranean state. Mars, for example, shows extensive geological evidence of large, ancient oceans that covered its surface billions of years ago. Today, nearly all Martian water is locked up as ice, primarily in the polar caps and beneath the surface, due to the planet’s thin atmosphere and cold temperatures.
In stark contrast, Venus, which is closer to the Sun, is an extremely dry world, having only trace amounts of water vapor in its atmosphere. Scientists believe Venus suffered a runaway greenhouse effect early in its history, causing all its surface water to boil off and escape into space. This loss of water highlights the importance of Earth’s specific location in the habitable zone.
The greatest volumes of liquid water outside of Earth are found hidden beneath the icy crusts of the outer Solar System’s moons. Jupiter’s moon Europa and Saturn’s moon Enceladus are believed to harbor deep, salty, subsurface oceans, kept liquid by tidal heating from their massive host planets. Europa’s ocean is estimated to contain more than twice the volume of all the water on Earth. This water is not on the surface, which is the key distinction that makes Earth the planet mostly covered in water.