The first oceans

The story

The blue planet has not always been blue

From space the Earth looks blue: more than seven tenths of its surface is covered with water, about 1.3 billion cubic kilometres of ocean. No other planet in the Solar System, as far as we know, has liquid seas on its surface. But where did all this water come from? And since when has it been here? Scientists are still working on these questions, and the answers are partial.

How an ocean forms

In the earliest times the Earth was so hot that water could exist only as vapour, mixed with the gases of the air. As millions of years went by, the surface cooled. When the temperature fell below 100 °C, at that pressure, the vapour could condense into drops. It began to rain. Scientists imagine very long rains, perhaps lasting thousands of years, which filled the lowest parts of the crust and formed the first oceans. Endless rain, but with nobody to see it.

How early did this happen? The oldest evidence comes from the zircon crystals of Jack Hills, which you read about in the previous card. Some of them, dated to around 4.4 billion years ago, have in their composition a signature typical of rocks that interact with liquid water. For this reason many scholars think that oceans, or at least large bodies of water, already existed in that period, only 150 million years after the Earth was born. Other researchers are more cautious, because the evidence is indirect. That is why the date on the timeline is approximate: around 4.3 billion years ago.

Where does the water come from?

Here scientists have two main hypotheses, and probably both are partly true. The first says that the water was already in the materials from which the Earth formed. Many rocks contain water chemically bound in their minerals. When the planet was molten, some of this water came out through volcanoes as vapour, a process called degassing. Even today eruptions release large amounts of vapour.

The second hypothesis says that some of the water arrived from space. Water-rich asteroids and other bodies from the outer parts of the Solar System rained down on the young Earth, in enormous numbers. In favour of this idea there is chemical evidence: water contains a heavier kind of hydrogen, deuterium, in a proportion that can be measured. The proportion in the water of our seas is very like that of a group of meteorites called carbonaceous chondrites, and less like that of many comets. In 2014 the European probe Rosetta analysed comet 67P and found that its water was different from ours, and this made the idea of comets as the source less popular.

Between 2022 and 2025 more evidence arrived: the samples brought back from the asteroids Ryugu and Bennu contain minerals with water, clays and salts, and strengthen the idea that asteroids of this kind are a good source. Probably the Earth's water has both origins; the share of each is still being studied.

A sea under a weak Sun

There is a puzzle. At the beginning the Sun was about 30 per cent less bright than today, like every young star. With so little light the oceans should have frozen, yet geologists find signs of liquid water. This problem is called the “faint young Sun paradox”. The most accepted explanation is that the early atmosphere contained a lot of carbon dioxide and perhaps methane, gases that trap heat, and that they kept the Earth warm enough.

Those first oceans were different from ours. There was no oxygen in the air, the water was rich in dissolved iron and probably had a greenish colour. The continents were few and small. But they were real oceans, with waves, currents and hot springs on the seabed, which are the places where many scientists think life began.