Multiple planets and moons in our solar system contain water in some form, from buried oceans to atmospheric vapor to massive ice deposits. Mars, Jupiter, Venus, Uranus, and Neptune all hold water, and several moons, particularly Jupiter’s moon Europa, may contain more liquid water than Earth does. Here’s where that water exists and what form it takes.
Mars: Ice Caps and Salty Surface Flows
Mars is the most well-known example. Its south polar ice cap alone contains enough frozen water to cover the entire planet in a liquid layer about 11 meters (36 feet) deep. That ice deposit covers an area bigger than Texas and reaches as deep as 3.7 kilometers (2.3 miles) below the surface. Radar measurements show the layered deposits are at least 90 percent frozen water.
Mars also shows signs of liquid water activity today, though not the kind you could scoop up in a cup. Dark streaks called recurring slope lineae appear on Martian hillsides during warmer seasons and fade during colder ones, suggesting seasonal flows of extremely salty water, or brine. Scientists have identified hydrated salts at four of these locations, reinforcing the idea that brine rather than pure water is responsible. Pure water can’t stay liquid under Mars’s thin atmosphere and low pressure, but salty water can remain liquid at temperatures as low as minus 63°C (minus 81°F) and at vapor pressures far below what pure water requires.
Europa: A Hidden Ocean Deeper Than Earth’s
Jupiter’s moon Europa is arguably the most exciting water world in the solar system. Beneath a shell of ice lies a global saltwater ocean estimated to be about 100 kilometers (62 miles) deep. For comparison, Earth’s oceans average just 3.7 kilometers deep. Despite being far deeper, Europa’s ocean sits on a rocky seafloor, giving it a rock-to-water ratio surprisingly similar to Earth’s. The moon is roughly 90 percent rock by composition.
That rocky seafloor matters because it opens the possibility of chemical reactions between water and minerals, the same kind of interactions that support life around hydrothermal vents on Earth. NASA’s Europa Clipper spacecraft, launched in 2024, will arrive at Jupiter in 2030 and conduct nearly 50 flybys of Europa to study the ice shell, ocean composition, and potential for habitability.
Jupiter: Water in the Atmosphere
Jupiter itself contains water, though not in the form of lakes or ice caps. Data from NASA’s Juno spacecraft showed that water makes up about 0.25 percent of the molecules in Jupiter’s atmosphere at the equator. That’s almost three times the proportion of water found in the Sun. While a fraction of a percent sounds tiny, Jupiter is so massive that even a small percentage translates to an enormous quantity of water vapor mixed into its thick atmosphere of hydrogen and helium.
Uranus and Neptune: Water-Rich Interiors
Uranus and Neptune are sometimes called “ice giants” rather than gas giants, and the name is well earned. Their interiors are dominated by a mixture of water, methane, and ammonia in exotic high-pressure states. Water is by far the most abundant of these ices, delivered in a ratio of roughly seven parts water to four parts methane to one part ammonia during the planets’ formation.
Computer simulations show that under the extreme pressure and temperature inside these planets, the mixture separates into layers. A water-dominated fluid forms in the upper mantle, while methane and ammonia settle below. The water-rich layer alone makes up approximately 60 percent of each planet’s total mass. This isn’t water in any form you’d recognize. At those pressures, it behaves more like a hot, dense fluid that can conduct electricity.
Venus: Trace Vapor in the Clouds
Venus is the driest planet in the inner solar system, but it isn’t completely waterless. Spacecraft measurements have detected water vapor in the Venusian cloud layer at concentrations of about 5 to 7 parts per million, at altitudes around 60 to 62 kilometers. The amount varies by latitude: equatorial regions average about 6.1 parts per million, mid-latitudes dip slightly to 5.4, and polar regions climb to about 7.2. Higher up, between 70 and 120 kilometers, the concentration drops to 0 to 5 parts per million.
These are vanishingly small amounts. Venus likely had far more water billions of years ago, but a runaway greenhouse effect boiled it away. The little that remains is suspended in a thick, scorching atmosphere dominated by carbon dioxide and sulfuric acid clouds.
The Moon: Ice in Permanent Shadow
Earth’s own Moon holds water ice in craters near its poles that never receive sunlight. These permanently shadowed regions stay cold enough for ice to persist indefinitely. NASA’s Lunar Prospector mission found in 1998 that the highest concentrations of hydrogen on the lunar surface occur in these lightless zones, pointing to water ice. Later missions confirmed the finding: when a spacecraft was deliberately crashed into a shadowed crater, instruments detected grains of water ice in the ejected material.
The total amount of lunar water ice remains uncertain, but its existence at the south pole is one reason NASA chose that region as the target for its Artemis crewed landing program.
Other Moons With Water
Europa isn’t the only moon hiding an ocean. Jupiter’s moon Ganymede is thought to have an even deeper subsurface ocean, though it may be sandwiched between layers of ice rather than sitting on rock. Saturn’s moon Enceladus actively sprays water ice and vapor into space through cracks near its south pole, direct evidence of a subsurface liquid water reservoir. Saturn’s largest moon, Titan, has a thick atmosphere and lakes of liquid methane on its surface, but also shows signs of a buried water ocean beneath its icy crust.
Water turns out to be one of the most common molecules in the solar system. The real question scientists are chasing isn’t which worlds have water, but which ones have the right combination of liquid water, chemistry, and energy to support life.