Asteroid dust brought back to Earth may explain where our water came from with hydrogen clues

Oceans cover more than 70% of the Earth’s surface, and scientists argue that the planet’s interior also contains a lot of water. But where did all this water come from?

I and my postdoc Ziliang Jin analyzed grains of the mineral pyroxene from an asteroid called Itokawa, which is the first asteroid that humankind ever sampled. The Japanese probe Hayabusa brought back about 1,500 particles from the asteroid’s surface in 2010, and our recent measurements show that this asteroid is wetter than we imagined.

Samples of meteorites that came from Itokawa-like asteroids that have been analyzed for water revealed barely detectable quantities. This led scientists to speculate that rubble-pile asteroids like Itokawa would be bone dry. But we found lots of water in these particles. To be clear, the specific amount of water that Itokawa particles contain is still low with respect to anything in our human experience. But the discovery of even these amounts of water with the correct isotope signature means that asteroids like it that struck the Earth could have provided more than half of Earth’s oceans.

I am a cosmochemist at Arizona State University who is interested in the small bodies in our solar system like asteroids and comets, which are the building blocks of the planets. Studying the chemistry of these types of seed materials can tell us a lot about how the planets formed and the conditions in the early stages of planet growth.

We were interested in studying samples from the asteroid Itokawa because we had speculated that Itokawa particles should have some water, based on some back-of-the-envelope calculations Ziliang had done. Then I wrote a proposal to the Japanese Space Agency and received the samples that we ended up studying.

Asteroid Itokawa is a battered remnant of a larger parent body. ASU scientists have discovered that this rubble-pile asteroid still contains significant amounts of water in its minerals. Japan Aerospace Exploration Agency, CC BY-SA

Water on Itokawa

The source of water on Earth and other planets like Mars is hotly debated in the planetary science community. It is thought that our oceans were delivered by water-rich asteroids from the outer solar system. This happened during different periods of planetary formation – early, when proto-Earth was not as big as it is now, or late, after the Earth had completely formed.

The planetary science community argues that asteroids were the first objects to form in our solar system and that terrestrial planets – Mercury, Venus, Earth and Mars – grew bigger with the accretion of small asteroids and comets.