When Earth was born around 4.5 billion years ago, it was far from the blue planet we know today. Instead, it resembled a fiery inferno—an ocean of molten rock, intense volcanic activity, and relentless asteroid impacts. In such harsh conditions, any early water would have been instantly vaporized. And yet today, over 70% of Earth’s surface is covered in water. So, where did it all come from?

That’s the question listener Bill from the USA posed—and presenter Caroline Steel went in search of answers.

A Hostile Beginning

Assistant Professor Muhammad Abdul Latif, an early Earth physicist at UAE University, explains that Earth’s earliest environment was completely inhospitable to water. The planet’s surface temperatures were so extreme that any initial water would have evaporated. Violent asteroid collisions and intense solar radiation only made things worse.

Adding to the mystery, Professor Richard Greenwood from the Open University notes that if water had existed in Earth’s original molten mix, the planet’s iron core would have oxidized—turning into a ball of rust. Since that didn’t happen, scientists believe our water must have arrived after Earth’s violent formation began to settle.

The Cosmic Delivery Theory

If Earth didn’t start out with water, could it have been delivered?

That’s where space rocks come into play. At the Natural History Museum in London, Professor Sara Russell has been studying the isotopic signature of Earth’s water—the unique atomic “fingerprint” that tells us where it might have come from. Her team compared this with samples from asteroid Bennu, brought back by NASA’s Osiris-Rex mission.

The result? A striking match. The chemical makeup of Bennu’s water closely resembles that of Earth’s oceans. This supports a growing theory: that asteroids and comets, carrying frozen water, crashed into Earth long after its surface cooled, delivering the life-giving liquid.

When Did Water Arrive?

While it’s still difficult to pinpoint exactly when Earth received its water, new modeling by Abdul Latif suggests that it could have started accumulating earlier than we once believed—possibly during the planet’s first few hundred million years. This was a time known as the Late Heavy Bombardment, when Earth was under constant assault by space debris, including icy asteroids rich in water.

Over time, this steady bombardment may have built up the water that now fills our oceans.

Still a Mystery, But Closer to the Truth

Despite these advances, scientists still haven’t closed the case. Earth’s water could have more than one source—some from asteroids like Bennu, some from deeper space, or even from chemical reactions within the mantle that created water long after the planet cooled.

But the evidence is mounting that much of our water was brought here by cosmic delivery, not born with the Earth itself.

As science continues to reach into space and back through time, the mystery of Earth’s water may one day be solved. For now, one thing is certain: the water that sustains all life here on Earth might just be a gift from the stars.