Where the chlorine resides
What happened to water inside Ryugu’s parent body cannot be inferred from the asteroid’s dry appearance today. Evaporation of liquid and freezing followed by direct loss of ice as gas leave different chemical legacies. A new study of Hayabusa2 samples narrows this distinction by examining where chlorine resides. Its contribution goes beyond another indication that water once existed: it constrains the environment that held that water and the route by which the water may have disappeared.[1]
The crucial distinction is between total chlorine measured in rock and chlorine once dissolved in water. The study associates approximately 90 percent of chlorine with lizardite and treats about 10 percent as soluble in its equilibrium calculation. Counting all chlorine as dissolved salt can make the ancient water appear too saline. Conversely, a large mineral-bound fraction is not itself a sample of ancient water: moving from the measured solid phase to the former liquid requires a chemical model.[1]
Salinity separates two histories
The reconstructed water has a salinity of approximately 7 milliequivalents per kilogram, roughly one hundredth that of terrestrial seawater. Calculated pH lies between 10.3 and 11.5, with a redox potential between −0.5 and −0.8 volts. Low salinity, strong alkalinity and reducing conditions therefore belong to the same proposed environment. Electron-probe measurements and X-ray absorption spectra constrain the elements’ mineral associations, while the numerical water conditions arise from interpreting those measurements through equilibrium calculations.[1]
As liquid water evaporates, dissolved salts can become concentrated and leave evaporite minerals. A less saline environment combined with reducing conditions that retain hydrogen is compatible with aqueous alteration beneath an ice cover, followed by freezing and sublimation. This is the history favoured by the study. Compatibility does not eliminate every alternative: the water-to-rock ratio, soluble chlorine fraction and alteration stage represented by the samples remain conditions underlying the inference.[1]
Two asteroids with different histories
Evaporite minerals found in Bennu samples provide a comparison that cautions against fitting both asteroids into one aqueous history. Low salinity inferred for Ryugu does not establish that another asteroid experienced the same conditions. The sampled parent-body region and chemical stage may differ. Retaining strong alkalinity when the water-to-rock ratio changes from 1:1 to 3:7 is a sensitivity test, rather than proof that the entire proposed history is independent of that variable.[1]
The search space being narrowed concerns water chemistry and the physical route of water loss, rather than the presence of life. Conditions favourable to some abiotic organic-synthesis pathways do not demonstrate life in the samples. Unresolved chemistry, including ammonium retention in phosphates, may also change comparisons between asteroids. A decisive open question is how widespread the low-salinity, reducing environment was within the parent body: the alteration stage represented by these two samples cannot yet be equated with its entire aqueous history.[1]