The origin of water on Earth is a subject of large interest and debate in planetary science. A leading model of Earth’s formation assumes that the water and other volatile species on Earth originate from small, ice-rich carbonaceous asteroids that formed in the Jupiter–Saturn region of the solar system. According to this model, after Jupiter and Saturn formed, they migrated inward toward the Sun, scattering the small carbonaceous asteroids toward Earth. One problem with the hypothesis that carbonaceous asteroids were the main source of water delivered to Earth is that Earth’s water has a lower deuterium-to-hydrogen ratio (D/H) than carbonaceous asteroids. This suggests that Earth’s water may not be derived principally, or entirely, from carbonaceous asteroids, and additional isotopically light sources are needed.

A team led by Luke Daly of the University of Glasgow suggests that solar wind could provide an answer to the origin of Earth’s water. The research team examined the sample of the Itokawa asteroid that was collected by the Japanese spacecraft Hayabusa in 2010. They conducted atom probe tomography and directly observed a significant amount of water on the solar-wind-irradiated surface of an olivine grain. The composition of solar wind is about 96% H, and laboratory experiments confirmed that solar wind irradiation of rocky materials results in reaction between H ions and silicate minerals to produce OH and water. Such solar-wind-derived water has isotopically light (low D/H ratio) compositions that could balance the isotopic composition of Earth’s water. Thus, various types of solar-wind-irradiated dust particles delivered to Earth could represent the missing reservoir that contributed to Earth’s water. READ MORE