Four billion years ago, salty oceans may have sloshed beneath the surface of asteroids in the outer reaches of our solar system. This remarkable finding comes from delicate crystals discovered in samples from asteroid Ryugu, providing new clues about how water – and potentially the ingredients for life – spread through our cosmic neighborhood.
Scientists at Kyoto University, analyzing grains returned by Japan’s Hayabusa2 mission, have found pristine deposits of sodium carbonate, halite (table salt), and other minerals that could only have formed in the presence of liquid water. These fragile crystals, preserved in the vacuum of space, tell a story of ancient seas hidden within what are now seemingly barren space rocks.
“Careful handling allowed us to identify the delicate salt minerals, providing a unique glimpse into Ryugu’s chemical history,” says lead researcher Toru Matsumoto. The findings, published in Nature Astronomy, challenge our understanding of where liquid water existed in the early solar system.
The study focuses on Ryugu, a 900-meter wide asteroid that likely broke off from a larger parent body around 4.5 billion years ago. While the asteroid is bone-dry today, the salt deposits suggest it once harbored warm, briny water heated by radioactive decay.
The research team had suspected they might find water-soluble materials that aren’t typically preserved in meteorites that land on Earth. These delicate compounds usually react with Earth’s atmosphere, erasing evidence of their existence. The pristine space samples, however, kept their secrets intact.
“These crystals tell us how liquid water disappeared from Ryugu’s parent body,” Matsumoto explains. The team believes the salty water either evaporated into space through fractures or froze as the asteroid body cooled. “The salt minerals we’ve found are the crystallized remnants of that water.”
The implications extend far beyond Ryugu itself. Similar salt deposits are expected to exist on other bodies in our solar system, including the dwarf planet Ceres in the asteroid belt, Saturn’s moon Enceladus (known for its dramatic water plumes), and Jupiter’s moons Europa and Ganymede – all suspected to harbor subsurface oceans today.
This discovery provides a new way to compare how water has shaped different worlds in our solar system. The specific mix of salts found can reveal the chemical conditions present when they formed, much like how different types of rock tell geologists about Earth’s past.
The study required extraordinary precision. The salt crystals dissolve easily in water, suggesting they could only have formed in highly concentrated brine under specific conditions. The team’s analysis suggests the parent body’s internal ocean was heated to temperatures below 100°C by radioactive elements present during the solar system’s youth.
These findings add to mounting evidence that water – and the chemical processes it enables – were more widespread in the early solar system than previously thought. Understanding where and how this water existed could help explain how Earth and other planets acquired their water, and potentially, the organic compounds necessary for life.
For researchers studying the origins of water in our solar system, these tiny salt crystals from Ryugu offer a rare window into conditions that existed billions of years ago, preserved in the cold vacuum of space until their recent return to Earth.