Life › Origin of life › Formation of Earth
Water & the first oceans
by c. 4.4 billion years ago (zircon evidence)· Event· Earth, climate & extinctions
Strong evidenceGood evidence; dates or details still being refined.
Water gathered on Earth's surface remarkably early, forming oceans by about 4.4 billion years ago.
See it on the Life map →What happened
Most of Earth's water probably came from water-bearing rock in its building blocks, released as steam by volcanoes, with a smaller share from later asteroids and comets. Once the surface cooled, rain filled the basins, and there was probably little dry land. The young Sun was about 25–30% dimmer than today, so the oceans should have frozen. Thick carbon dioxide, perhaps with methane, likely kept them liquid; this puzzle is called the faint young Sun paradox.
Why it matters
Liquid water is essential to all known life, so early oceans mean the basic conditions for life were in place within about 150 million years of Earth forming.
The evidence (fossils, genes, rocks)
Jack Hills zircons, 4.4 billion years old, with oxygen isotope ratios pointing to liquid water; 3.8-billion-year-old pillow lavas in Greenland; seawater's hydrogen isotopes, which match water-rich asteroids.
Still debated
How deep and warm the first oceans were, how much land stood above them, and exactly how the faint young Sun paradox was resolved.
Connections (4)
▸Formation of Earth cooled enough to hold Water & the first oceansStrong evidence
The connection
As impacts slowed and the molten surface cooled and hardened, water vapor from volcanoes and from water locked in Earth's building blocks condensed. Rain fell and pooled into oceans. A thick carbon dioxide atmosphere probably kept them liquid even though the young Sun was about 25–30% dimmer than today.
When and how
Within roughly 150 million years of Earth forming; key zircon evidence from Jack Hills, Western Australia, published in 2001 by separate teams led by Simon Wilde and Stephen Mojzsis.
How sure are we?
Zircons 4.4 billion years old carry oxygen isotope ratios suggesting contact with liquid water; 3.8-billion-year-old pillow lavas in Greenland formed underwater.
Debates and open questions
How much water came from Earth's own rock versus later asteroids, and whether big impacts boiled early oceans away before they re-formed.
▸Late Heavy Bombardment may have topped up Water & the first oceansDebated
The connection
Comets and water-rich asteroids striking Earth would have added some water. But oceans already existed by about 4.4 billion years ago, and seawater's hydrogen 'fingerprint' matches certain asteroids better than most comets. So late impacts probably added only a small share, and the biggest may have partly boiled the seas.
When and how
About 4.1–3.8 billion years ago; the key comet test came from the European Rosetta probe at comet 67P in 2014.
How sure are we?
Ratios of heavy hydrogen (deuterium) in seawater, meteorites and comets; Rosetta found comet 67P's water about three times richer in deuterium than Earth's.
Debates and open questions
What share of Earth's water arrived late versus with its original building blocks; some recent studies argue Earth's own rock held most of it.
▸Water & the first oceans provided the setting for Origin of lifeStrong evidence
The connection
All known life is built of watery cells, so liquid water was the stage. Oceans dissolved minerals, carried chemicals from volcanoes and vents, and shielded molecules from harsh ultraviolet light. Whether life began in the open sea, at the seafloor or in pools on land fed by rain is unresolved.
When and how
Oceans existed by about 4.4 billion years ago; the earliest likely traces of life follow by about 3.7–3.5 billion years ago.
How sure are we?
The link rests on biochemistry: every known cell needs liquid water. The exact setting is inferred, not observed.
Debates and open questions
Water also breaks long molecules apart, which is why some scientists favor drying pools on land over the open ocean.
▸Water & the first oceans hosted the vents imagined in Deep-sea hydrothermal vent originStrong evidence
The connection
Once oceans covered young crust, seawater seeped into hot rock, picked up minerals and gushed back out as vents. Some, like today's alkaline vents, also make hydrogen gas as water reacts with mantle rock. These are the chemical factories the vent hypothesis imagines early life tapping.
When and how
Vents were discovered in 1977 on the Galápagos Rift by the submersible Alvin; the Lost City alkaline vent field in the Atlantic was found in 2000.
How sure are we?
Vents form wherever seawater meets hot crust, so early oceans must have had them; Australian rocks about 3.2 billion years old preserve ancient vent deposits.
Debates and open questions
Whether alkaline vents like Lost City were common on early Earth, when crust and ocean chemistry differed from today.