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Asteroids like Itokawa might help solve mystery of Earth's water source

Scientists believe Earth’s abundant H2O was supplied by space rocks that slammed into the planet during its infancy 4.6 billion years ago.

But the isotope-ratio fingerprint of Earth's ocean water only partly matches the pattern found in meteorites. So where did the rest of the water come from?

New  research published in the journal Nature Astronomy offers a possible solution: weathering of silicate space rocks by charged particles from the sun.

"You can actually detect the water on the asteroid that presumably wasn't originally there. But it formed from the solar wind hitting the surface — the hydrogen hitting the surface — reacting and making water," said coauthor and NAU astronomer Mark Loeffler.

On Earth, one molecule of water in 10,000 is made of deuterium — hydrogen with a neutron in its nucleus.

That "heavy water" ratio varies throughout the solar system, including in the carbon-rich meteorites thought to be Earth's main water-bearers. 

But, surprisingly, supposedly dry silicate asteroids can make water, too, when charged hydrogen atoms from solar wind combine with oxygen atoms in their surface rock.

"You can make water from the hydrogen implanting into this surface, which has a lot of oxygen in it in the form of some rocky material, or silicates," said Loeffler, who helped test the hypothesis. 

Adding water from silicate meteorites and dust grains could offset the heavier isotope ratio of water from carbon meteorites, resulting in a cocktail that matches isotope fingerprint of Earth's oceans.

The findings were based on studies of an asteroid sample from the silicate asteroid Itokawa, returned to Earth by the Japanese Hayabusa space probe mission a decade ago. 

"This is the first detection of water in any asteroid material that's been returned from space," said Loeffler.

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Nicholas Gerbis joined KJZZ’s Arizona Science Desk in 2016. A longtime science, health and technology journalist and editor, his extensive background in related nonprofit and science communications inform his reporting on Earth and space sciences, neuroscience and behavioral health, and bioscience/biotechnology.Apart from travel and three years in Delaware spent earning his master’s degree in physical geography (climatology), Gerbis has spent most of his life in Arizona. He also holds a master’s degree in journalism and mass communication from Arizona State University’s Cronkite School and a bachelor’s degree in geography (climatology/meteorology), also from ASU.Gerbis briefly “retired in reverse” and moved from Arizona to Wisconsin, where he taught science history and science-fiction film courses at University of Wisconsin-Eau Claire. He is glad to be back in the Valley and enjoys contributing to KJZZ’s Untold Arizona series.During the COVID-19 pandemic, Gerbis focused almost solely on coronavirus-related stories and analysis. In addition to reporting on the course of the disease and related research, he delved into deeper questions, such as the impact of shutdowns on science and medicine, the roots of vaccine reluctance and the policies that exacerbated the virus’s impact, particularly on vulnerable populations.