Phase relations between the water-rich sulfuric acid hydrates, potential markers of thermal history on Jupiter’s icy moons

•Solutions of H2SO4/H2O were studied with high resolution powder diffraction.•Showed that four sulfuric acid hydrates can form in solutions <50 wt.% H2SO4.•Discovered that sulfuric acid hydrates crystallize as a mixture of forms.•Revealed details of a transformation between hydrate forms.

Synchrotron X-ray powder diffraction has been used to explore the water-rich (<50 wt.% H2SO4) region of the sulfuric acid and water binary phase diagram at temperatures between 80 and 285 K. The phase relations that are determined demonstrate that, on laboratory timescales, sulfuric acid hydrates crystallize as mixtures of phases. Four forms of sulfuric acid hydrates were observed along with ice Ih, with their proportions dependent on temperature and sample H2SO4 wt.%. The charting of these phase relations has revealed a transformation between hydrate forms, which could be utilized as a marker for areas of higher heat flow on the surfaces of the Galilean ice moons.