The parent body of near-Earth asteroid Ryugu appears to have harbored water in the form of ice for more than a billion years, Japanese researchers have found, in a discovery that could reshape current theories on how the Earth got its oceans.

Analyzing samples from Ryugu collected by the Japan Aerospace Exploration Agency’s Hayabusa2 spacecraft, a research team led by Tsuyoshi Iizuka, associate professor on cosmochemistry at the University of Tokyo, found evidence that its parent body had stored water as ice far longer than scientists thought possible.

The findings, published Thursday in the scientific journal Nature, suggest asteroids delivered water to Earth not only as hydrous minerals — in which water molecules or hydroxyl groups, containing oxygen and hydrogen, are chemically incorporated into the mineral’s crystalline structure — but also as ice.

Asteroids like Ryugu are thought to have formed about 4.56 billion years ago in the outer regions of the solar system, and were rich in carbon and water. Scientists long assumed the water would have quickly reacted to form hydrous minerals or evaporated away.

Instead, Ryugu’s parent body had retained water in the form of ice for more than a billion years, before it melted and evaporated during what is believed to have been a collision with another celestial object, based on isotope dating of the samples, the research said.

Collisions with other celestial objects had distorted dating results that initially made the samples appear 4.8 billion years old — older than the solar system, itself.

It was previously thought that carbonaceous asteroids — which became the building blocks for the Earth — carried a volume of water equivalent to a few tens of ocean masses, mostly locked within minerals. By factoring in long-stored ice, the research team estimates the asteroids may have delivered two to three times more water than previously thought — equivalent to 60 to 90 times the mass of the Earth’s oceans.

Isotope comparisons show that material from asteroids like Ryugu make up about 6% of the Earth’s matter. If those objects carried vast amounts of ice, the total water delivered to the Earth could have accounted for as much as 1.8% of the planet’s mass.

“The idea that Ryugu-like objects held on to ice for so long is remarkable,” said Iizuka. “It suggests that the building blocks of Earth were far wetter than we imagined. This forces us to rethink the starting conditions for our planet’s water system.”

The next question, the study asks, is where all that water went.

Iizuka said his team will look into how much water escaped into space during the Earth’s formation, how much was stored deep in its mantle and core, and how enough water remained on the Earth's surface to create the balance of continents and oceans that sustains life here today.