Origin of Earth's Oceans: Where the Water Came From

The origin of Earth’s oceans is one of the longest-running puzzles in Earth science. Evidence from ancient minerals suggests that liquid water had gathered on the planet’s surface by about 4.4 to 4.0 billion years ago, yet Earth formed in a hot part of the Solar System where water ice could not easily survive. Scientists now think the water came mostly from the planet’s own building blocks, with extra contributions from asteroids and, to a lesser extent, comets.

Background: a dry neighbourhood

The planets formed from a disc of gas and dust around the young Sun. Close to the Sun, where Earth grew, temperatures were too high for water to freeze into ice. Beyond a boundary called the frost line, in the region of today’s asteroid belt and beyond, ice was plentiful.

This raised an obvious question. If Earth formed from dry rock, how did it end up with oceans covering about 71 percent of its surface? The violent events of the formation of the Earth, including a giant impact that formed the Moon, would also have driven off much of any early water.

The main explanations

Water from within

One explanation is that Earth’s water was locked in its rocky building blocks as hydrogen and oxygen bound into minerals. As the planet heated and melted, volcanic activity released this water as steam, a process known as outgassing. When the surface cooled, the steam condensed and fell as rain, filling the low-lying basins.

In 2020 a team led by Laurette Piani reported that enstatite chondrites, meteorites thought to resemble the material that built the inner planets, contain enough hydrogen to have supplied several times the water in Earth’s oceans. Their hydrogen isotopes also resemble Earth’s.

Water from asteroids

Another source is water-rich asteroids from farther out in the Solar System. Carbonaceous chondrite meteorites, which come from such asteroids, can contain several percent water by mass, bound in clay minerals. Their ratio of deuterium, a heavy form of hydrogen, to ordinary hydrogen is close to that of seawater. Gravitational disturbances from Jupiter and Saturn could have scattered such bodies towards the inner Solar System while Earth was growing.

Water from comets

Comets are made largely of ice, and for decades they were a popular candidate. Measurements of comets such as Halley, Hyakutake and Hale-Bopp, however, showed about twice as much deuterium as Earth’s oceans.

In 2011 the Herschel space telescope found ocean-like water in comet 103P/Hartley 2, reviving the idea. But in 2014 and 2015 the European Space Agency’s Rosetta mission found that comet 67P/Churyumov–Gerasimenko has roughly three times the oceanic deuterium ratio. Most researchers now conclude that comets supplied only a minor share of Earth’s water.

When the oceans formed

The oldest evidence comes from the Jack Hills zircons of Western Australia. Their oxygen isotopes suggest that, around 4.4 to 4.3 billion years ago, some crust had interacted with liquid water at low temperatures.

Firmer evidence appears later. Rocks about 3.8 billion years old in the Isua region of Greenland include pillow lavas, rounded blobs of lava that form only when lava erupts under water, as well as sediments laid down in water. By this time, oceans were clearly present.

The early oceans may have been partly evaporated by large impacts, especially during the proposed Late Heavy Bombardment, but they appear to have re-formed each time.

By the numbers

The figures show why the debate is subtle. Earth’s oceans are a thin film on a large rocky planet, so even a small amount of water locked in the original rock, or a modest supply of asteroids, could have been enough. The deep mantle may also hold a significant amount of water in its minerals, although estimates vary widely.

The faint young Sun puzzle

Liquid oceans on the early Earth raise a further question. The young Sun was about 25 to 30 percent fainter than today, so the planet should have frozen over. In 1972 the astronomers Carl Sagan and George Mullen highlighted this “faint young Sun paradox”. The usual explanation is that the early atmosphere held much larger amounts of greenhouse gases, such as carbon dioxide and possibly methane, which kept the surface warm.

Timeline

• c. 4,568 million years ago: The Solar System forms; water ice is common only beyond the frost line.
• c. 4,540 million years ago: Earth assembles from mostly rocky material that contains some hydrogen.
• c. 4,400–4,300 million years ago: Zircon chemistry suggests liquid water at Earth’s surface.
• c. 3,800 million years ago: Pillow lavas in Greenland show that oceans existed.
• AD 1972: Sagan and Mullen describe the faint young Sun paradox.
• AD 2011: Herschel finds ocean-like water in comet Hartley 2.
• AD 2014–2015: Rosetta measures a high deuterium ratio in comet 67P.
• AD 2020: Study of enstatite chondrites supports an inherited, internal source of water.

Why it matters

Oceans regulate Earth’s climate, drive the water cycle and, in many scientists’ view, provided the setting for the origin of life. Knowing where the water came from also helps astronomers judge how common watery, potentially habitable planets might be around other stars.