Effect of hydrogen on the melting temperature of FeS at high pressure: Implications for the core of Ganymede

We have carried out in situ X-ray diffraction experiments on the FeS-H system up to 16.5GPa and 1723K using a Kawai-type multianvil high-pressure apparatus employing synchrotron X-ray radiation. Hydrogen was supplied to FeS from the thermal decomposition of LiAlH₄, and FeSH_x was formed at high pressures and temperatures. The melting temperature and phase relationships of FeSH_x were determined based on in situ powder X-ray diffraction data. The melting temperature of FeSH_x was reduced by 150-250K comparing with that of pure FeS. The hydrogen concentration in FeSH_x was determined to be x=0.2-0.4 just before melting occurred between 3.0 and 16.5GPa. It is considered that sulfur is the major light element in the core of Ganymede, one of the Galilean satellites of Jupiter. Although the interior of Ganymede is differentiated today, the silicate rock and the iron alloy mixed with H₂O, and the iron alloy could react with H₂O (as ice or water) or the hydrous silicate before the differentiation occurred in an early period, resulting in a formation of iron hydride. Therefore, Ganymede's core may be composed of an Fe-S-H system. According to our results, hydrogen dissolved in Ganymede's core lowers the melting temperature of the core composition, and so today, the core could have solid FeSH_x inner core and liquid FeH_x-FeSH_x outer core and the present core temperature is considered to be relatively low.