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Asteroid Ryugu samples suggest presence of salty water in outer solar system

Asteroids that orbit close to the Earth inevitably cause us some anxiety due to the even remote possibility of a collision. But their proximity also offers ample opportunities to learn more about the universe. Ryugu, a 900-meter diameter asteroid in the Apollo belt, has recently proven useful in our search for signs of life's precursors elsewhere in our solar system.

A team of researchers at Kyoto University have found evidence of salt minerals in samples recovered from Ryugu during the initial phase of Japan's Hayabusa2 mission. The discovery of these deposits, containing sodium carbonate, halite, and sodium sulfates, suggest that liquid saline water once existed within a parent body of Ryugu.

Before examining the samples, the team expected that sample grains returned from the asteroid might contain substances not generally found in meteorites. They anticipated that these could be highly water-soluble materials, which readily react with moisture in Earth's atmosphere and are difficult to detect unless examined in their pristine state as preserved in the vacuum of space.

"Careful handling allowed us to identify the delicate salt minerals, providing a unique glimpse into Ryugu's chemical history," says corresponding researcher Toru Matsumoto.

Experts believe the asteroid was once part of a larger parent body that existed about 4.5 billion years ago, shortly after the formation of the solar system. This parent body would have been heated by radioactive decay, creating an environment of hot water below 100°C. While Ryugu and its grains did not contain any moisture, questions remain about how the liquid water was lost.

"These crystals tell us how liquid water disappeared from Ryugu's parent body," says Matsumoto. The salt crystals dissolve easily in water, suggesting that they could only have precipitated within highly saline water and in conditions with a limited amount of liquid.

"We hypothesized that as fractures exposed the saltwater to space or as the parent body cooled, this liquid could have either evaporated or frozen," Matsumoto explains. "The salt minerals we've found are the crystallized remnants of that water."

The deposits could prove crucial in comparing the evolved water in the dwarf planet Ceres—located in the Asteroid Belt—and the moons of Jupiter and Saturn, since researchers believe these icy bodies harbor subsurface oceans or liquid reservoirs. They expect sodium carbonate and halite will be found in surface deposits on Ceres, in water plumes from Saturn's satellite Enceladus, and on the surfaces of Jupiter's satellites Europa and Ganymede.

Since salt production is closely linked to the geological settings and brine chemistry in these aqueous bodies, the discovery of sodium salts in the Ryugu samples provides new insights for comparing the role that water has played in the development of planets and moons in the outer solar system.