Earth is rightfully called a water-world: far more than half of our planet is covered in water. There’s also water underground, in rivers, and in the atmosphere. Collectively, all of this water forms Earth’s hydrosphere. Water is critical to life as we know it, which is why researchers are studying how water cycles through the hydrosphere and how this precious resource can best be conserved.

Viewed from space, our planet resembles a blue marble. That’s because the ocean covers 71 percent of Earth’s surface. The ocean is accordingly a major component of the hydrosphere, and it plays an important role in Earth’s water cycle. Over 96 percent of Earth’s water is in the ocean. As water evaporates from the ocean, it is transported into the atmosphere, where it falls back to Earth as rainfall. Most of this precipitation falls over the ocean, but some occurs over land. The water that falls over land in the form of rain and snow has many fates: Some is absorbed into the ground and taken up by plants, like trees, and some flows into streams and rivers that eventually empty back into the ocean. Water moves through Earth’s ecosystems in many ways, and some of it is also frozen in polar ice caps, snow packs, and glaciers. Thank the hydrosphere next time you do just any anything: eat food, take a shower, enjoy a glass of water, or go skiing.

Other planets and moons in the solar system also have their own hydrospheres. For instance, there’s scientific evidence that Europa, a moon of Jupiter, contains an ocean of liquid water underneath its icy surface. And recent observations have suggested that some of Europa’s subsurface water might be jetting into the moon’s thin atmosphere. Water has also been spotted in frozen form on Mars and Earth’s moon.

Astronomers peering farther into space have also found evidence of water. It shows up in clouds of gas and dust peppered throughout the Milky Way. That’s perhaps not surprising because the elements that make up water—hydrogen and oxygen—are among the most common in the universe.

Water-Worlds

With 71 percent of its surface covered by the stuff, Earth is sometimes called a water-world. This fact is apparent when Earth is viewed from space—so-called "blue marble" pictures.

Noun

layers of gases surrounding a planet or other celestial body.

Noun

layers of gases surrounding a planet or other celestial body.

Europa
Noun

moon of Jupiter.

Noun

mass of ice that moves slowly over land.

Noun

all the Earth's water in the ground, on the surface, and in the air.

Noun

area of fewer than 50,000 square kilometers (19,000 square miles) covered by ice.

Jupiter
Noun

largest planet in the solar system, the fifth planet from the Sun.

Milky Way
Noun

galaxy in which the Earth and sun are located.

Noun

large body of salt water that covers most of the Earth.

Noun

movement of water between atmosphere, land, and ocean.