The Bennu Sample Contains Clues About Where Earth's Water Came From — Here Is What Scientists Found

Scientists studying the Bennu asteroid sample have found three chemically distinct region types with different water histories. Here is what this tells us about how Earth got its water.

The question of how Earth got its water — one of planetary science's most enduring and consequential open questions — is being addressed through the Bennu asteroid samples in ways that are both clarifying the picture and revealing its unexpected complexity. Water, which covers 71 percent of Earth's surface and is essential for all life as we understand it, could not have formed in significant quantities on early Earth's surface — the planet was too hot, and the solar radiation too intense, for water to have condensed from the primordial material.

The consensus model is that water was delivered to early Earth from the outer solar system, primarily through asteroid and comet impacts. Bennu, which orbits in the inner asteroid belt at distances accessible to early Earth-crossing trajectories, is exactly the type of body that could have been a water delivery vehicle. The specific chemical signatures in Bennu's hydrated minerals — the water-bearing clay minerals that OSIRIS-REx's analysis confirmed are abundant in Bennu's surface material — tell us about the history of that water.

The three chemically distinct region types found in the samples complicate the simple delivery story. Each region type has different hydration signatures — different isotopic ratios of hydrogen that indicate different water sources and different processing histories. This suggests that the material that assembled into Bennu — and by extension, the material that would have been delivered to early Earth from similar bodies — was not a uniform water source but a mixture of components with different water histories.

For Earth's specific water budget, the isotopic ratio of hydrogen in Earth's oceans has long been used to constrain which types of solar system bodies delivered that water. The Bennu results add specificity to this constraint analysis by showing that even a single asteroid contains material from multiple source regions — complicating the 'Earth's water came from asteroids like Bennu' narrative while enriching the underlying story.