Where did Earth’s water come from?

CrowdScience

BBC

Science, Technology

4.8 • 985 Ratings

🗓️ 11 July 2025

⏱️ 32 minutes

🔗️ Recording | iTunes | RSS

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Summary

Here's a conundrum that has captivated scientists: when Earth formed 4.5 billion years ago, our planet was essentially a ball of molten rock. Any water that might have been present during the planet's formation would surely have boiled away immediately. Yet today, water covers about 70% of Earth's surface.

So where did all this water come from? And more intriguingly, when did it arrive? Listener Bill in the USA wants to know, and Presenter Caroline Steel is after answers.

Assistant Professor Muhammad Abdul Latif is an early earth physicist at United Arab Emirates University. He explains how his modelling has helped us to understand when water first appeared in our universe.

The early earth was not a water-friendly place - a hellscape of molten rock, volcanic eruptions and constant bombardments from comets and asteroids, with high levels of solar radiation. These conditions would have evaporated the water. And according to Professor Richard Greenwood at Open University, our earth’s molten iron core would have been a ball of rust if there had been water in the proto-earth mix.

So if the water hasn’t always been here, where did it come from?

At the Natural History Museum in London, Professor Sara Russell has been comparing the isotopic "fingerprint" of Earth's water with water found in the asteroid Bennu, captured and brought back by the recent Osiris Rex NASA mission. It’s a good match for earth’s water, but could it really be the answer to our question?

Presenter: Caroline Steel Producer: Marnie Chesterton Editor: Ben Motley

(Image: Man overlooking the sea from cliff top. Credit: Gary Yeowell via Getty Images)

Transcript

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0:00.0	I'm Mariana Spring, the BBC's social media investigations correspondent.
0:06.0	In my podcast, I've been investigating what happened to the daughter of a conspiracy theorist who died having rejected chemotherapy.
0:13.0	It would mean the world to me if I could make it that she wasn't just another in the long line of people that die in this way.
0:19.0	How does this reflect the rise of health conspiracy theories
0:22.0	on social media and beyond? The new series of Mariana in Conspiracy Land, listen on BBC Sounds.
0:30.9	You're listening to Crowd Science from the BBC World Service. I'm Caroline Steele and I'm
0:36.4	stood on the banks of the watery heart of London, the River Thames.
0:40.3	It's fast flowing, it's really busy, there's a lot of boats, a lot of people, it's very wide and long.
0:47.3	And I imagine pretty deep. There's a lot of water.
0:51.3	And this episode's listener lives near an even bigger body of water, an ocean.
0:56.5	And that's got him marvelling too.
0:59.0	I've thought about this a lot because I'm not sure where we call Earth. Earth, we should call it water,
1:04.5	because there's so much water.
1:06.8	Hello, crowd science. This is Bill from Los Angeles. I was wondering, where did all the water come from?
1:11.6	There's plenty of it here.
1:13.6	I live in Los Angeles on the west side near the ocean,
1:16.6	and every time I drive down the hill in the neighborhood
1:18.6	and I look at the ocean, I go, where did all that water come from?
1:22.6	I mean, I've thought about this a lot.
1:24.6	I mean, 70% of the surface is water, and water is, you know, maybe an average of
1:31.4	five or six hundred feet. That's a lot of water. Yeah, it is vast. So what are you expecting
1:38.5	us to find? Have you got any sort of inklings about where all the water might have come from?
	...

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