First synthesised in 1837 by Carl Julius Fritzsche, magnesium sulfate undecahydrate – MgSO4·11H2O, is a long known, but little studied material. Suggestions that it could be a major rock-forming mineral on the icy satellites of Jupiter has re-awakened interest.

A combination of solar system formation models and near-IR spectroscopic evidence acquired by the Galileo space-craft supports the presence of highly hydrated magnesium sulfates on Jupiter's large icy moons.

It is possible that two of these moons Ganymede and Callisto have outermost layers rich in MgSO4·11H2O and ice, which may be 500 – 800 km deep. Within these layers the temperature is likely to increase from 100 K – 300 K, the pressure at the bottom of the layer being   approx. 1 – 1.5 GPa. The dehydration reaction forming MgSO4·7H2O (epsomite) + MgSO4-brine or ice from MgSO4·11H2O may be responsible for significant rifting of Jupiter's moon Ganymede.

Although it is very well known that water ice has many phase transitions in these pressure and temperature ranges, the behaviour of other ‘icy minerals’, including MgSO4·11H2O, are very poorly known.

To create accurate geophysical models of the icy satellite's interior, planetary scientists need to build a picture of the constituent material's behaviour at high pressures and low temperatures. High resolution powder neutron diffraction measurements at ISIS [Fortes et al., Phys Chem (2008) 35, 201] have established that when compressed to 1 GPa at 240 K the crystal gives up some hydration water to form a high-pressure polymorph of epsomite (MgSO4.7D2O) and the high-pressure phase VI of ice.

Dominic Fortes, an STFC Advanced Research Fellow at University College London and the lead scientist on the study says that the result has implications for the geology of large icy satellites.

“It has been speculated that dehydration of MgSO4·11H2O inside Ganymede might result in a net volume increase of the satellite, and consequently extensional fracturing of the surface, which is indeed what we observe,” he said. “High-pressure neutron powder diffraction offers a window into the possible internal structure and dynamics of icy satellites that is difficult to obtain in any other way.”