Researchers have now found evidence that points to the fact that asteroids may have delivered a significant portion of water while the Earth was forming. Dust grains from asteroid Itokawa not only carry a surprising amount of water, the chemistry of that water very closely matches the water on Earth.

In 2010, a Japanese mission called Hayabusa returned to Earth from a seven-year space journey, bringing back with it images, data and samples from the asteroid Itokawa. Hayabusa’s samples have been studied by many teams across the world, including the latest analysis by researchers from Arizona State University who were the ones who found the water.

Itokawa comes from a population of asteroids that orbit between one-third and three times Earth’s orbit, meaning they’re local. There are many asteroids like Itokawa that could have impacted Earth over the aeons, so the researchers believed that this analysis would provide a better idea of the type of effect they could have had.

Accounting for the heating, weathering, and collisions that Itokawa would have endured since the early days of the solar system, the researchers focused on an iron and magnesium-bearing silicate known as a pyroxene, which is almost entirely free of calcium. Using an ion microprobe, which bombarded a sample with a beam of ions in order to probe the composition of its surface, they found that the grains being analysed had up to 1000 parts per million of water.

Knowing the composition of Itokawa, the researchers could then estimate the water content of the entire asteroid. This came to between 160 and 510 parts per million of water, a lot more than they had thought they would find. Previous measurements of similar asteroids found between 30 and 300 parts per million of water.

Pyroxene is not usually associated with water. It is considered a Nominally Anhydrous Mineral (NAM), meaning it doesn’t contain vacant sites for water molecules in the same way hydrous minerals would. However, the sensitivity of the technique that the researchers used allowed them to detect and measure tiny quantities of water.

The researchers have speculated that asteroids like Itokawa got the water from the cloud of gas and dust, also known as a protoplanetary disk, that made up the Earth while it was forming. That would indicate that even though most of our original water was lost while it had a hot and molten surface, it was added again during collisions by numerous S-type asteroids like Itokawa.

Whether their suppositions that Itokawa and asteroids like it could have delivered half of Earth’s water reservoirs are correct or not may be established over the next few years. Hayabusa’s successor, Hayabusa2, is currently in orbit around another asteroid, Ryugu, and a NASA mission called OSIRIS-REx is exploring asteroid Bennu. Both missions will bring home their own asteroid samples and add to the conflicting but always growing mound of evidence about the origin of Earth’s oceans.