How did the oceans form?

-Karthik Gurumurthy

The story of how Earth’s oceans formed goes all the way back to when the planet itself was taking shape, about 4.6 billion years ago. Back then, the Earth was growing by pulling in a bunch of smaller objects called planetesimals. There are a few different ideas about where the water came from. It could have separated out from the rocks that make up most of the Earth’s mass, or it might have been delivered by meteorites rich in water. Another possibility is that it came from icy bodies — basically comets — that slammed into the Earth later on.

When scientists look at the makeup of ocean water, they can pick up some clues about where it came from. For example, if comets were the main source, we’d expect the water to match what we see in comets like Halley and Hyakutake — the only ones we’ve been able to study closely. But the problem is, the ice in comets has way more deuterium (a heavy form of hydrogen) compared to the water in Earth’s oceans. That suggests comets alone couldn’t have brought all the water we have today.

Meteorites don’t fully explain it either. If all the water had come from meteorites, Earth’s atmosphere would have way more xenon gas than it actually does — almost 10 times more. Meteorites are loaded with xenon, and if they had delivered all the water, we’d see the excess xenon in the air.

Scientists also tried combining the two ideas — a mix of comet and meteorite water — but that didn’t fit either. Even that combination would leave too much deuterium compared to what we find in ocean water.

Right now, the best idea is that Earth’s water came from a mix: some from water-rich comets and some that was already trapped inside the rocky material that built the Earth. This model explains both the deuterium and xenon levels we see today. There’s even some evidence suggesting that the rocky material near Earth’s orbit picked up extra local water from the gas and dust cloud (the solar nebula) that surrounded the young Sun — before the Earth fully formed. Recent lab experiments also show that water vapor could have exchanged with the rocky material in space, tweaking the deuterium levels before the Earth even finished forming.