Ryugu mineral chemistry points to a low-salt ancient water environment
Returned minerals from Ryugu suggest that water inside its parent body contained far less dissolved salt than Earth’s seawater. Chemical measurements distinguish elements held on minerals from those formerly dissolved in water. Equilibrium calculations favour an alkaline, reducing environment and a possible history of freezing followed by ice loss into space, with the reconstruction limited to the sampled altered rock.
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Salt attached to minerals changes the reconstruction
The distinction begins with where the elements sit. In material returned by Hayabusa2, sodium and potassium were largely associated with layered silicate minerals. Most chlorine was linked to lizardite. Counting all those elements as formerly dissolved salts would therefore give a different picture of Ryugu’s ancient water from one that separates mineral-bound and soluble components.[1], [2]
The analysis centred on the A0477 and C0120 samples and compared their chemical signatures with mineral standards. More than a hundred electron-probe locations and X-ray absorption measurements helped identify the relevant chemical forms. This combination examined what the minerals retained after alteration, rather than sampling liquid water still present inside the asteroid.[1]
Equilibrium calculations favour dilute, alkaline water
The researchers assigned approximately 90 per cent of chlorine to lizardite and treated approximately 10 per cent as soluble. Their Ryugu equilibrium model then yielded salinity near 7 milliequivalents per kilogram, around one hundredth of terrestrial seawater. The reconstructed pH lay between 10.3 and 11.5, alongside reducing conditions associated with a hydrogen-rich environment. These values describe an inferred alteration setting.[1]
Changing the assumed balance between water and rock still produced strongly alkaline solutions. Yet both that balance and the soluble chlorine fraction affect the reconstruction. The sampled, altered rock constrains a particular chemical history; it cannot establish identical water conditions everywhere in the parent body or throughout every stage of its evolution.[1]
Freezing offers a different route to losing water
Low dissolved salt and trapped hydrogen fit an interpretation involving alteration beneath surface ice, followed by freezing and sublimation into space. For comparison, Bennu’s evaporite minerals offer a contrasting water-loss history, while unresolved chemistry and model assumptions keep the comparison provisional. The study reconstructs an environment that could host chemical reactions, without reporting evidence of life.[1], [2]
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