Sulphur and more sulphur

Another very exceptional characteristic highlighted by the Liege publications is the important presence of sulphur in the mantle around the core and on the surface of the planet. In general, the rocks on the surfaces of the planets of the solar system contain between 500 and 2000 ppm of sulphur. On Mercury, it is 10 to 40 times more, between 2% and 4%! On Earth, such quantities are only found in extractable deposits. On Mercury, they are the norm! What is the explanation for this? “We studied how the sulphur is distributed (speciation) and behaves in the magmas”, explains Camille Cartier, a post-doctoral student in the laboratory. “The phenomenon is linked to the availability of oxygen in silicate liquids. There is very little oxygen Mercury’s magmas which strongly favours the solubility of sulphur. It will take the place left vacant by the oxygen, so to speak. The S₂ molecules substitute for the O₂ molecules. On Earth, given the abundance of oxygen, this leads inevitably to the formation of oxides; in the conditions on Mercury, sulphides are produced”.

The high level of sulphur present on Mercury has been a subject of intense debate ever since the first data was supplied by MESSENGER: Is the external section of the core of the planet a layer of sulphur?  “What we have shown”, explains Olivier Namur, “is that if such a layer exists, it is thin, undoubtedly less than 80 km thick, a far cry from the 200 km that some individuals imagine. We have set a limit, so to speak”.

Mercure has undoubtedly not revealed all its secrets and the new experimental petrology laboratory of the University of Liege intends to make this one of its foremost research projects. The researchers impatiently await the launch of the European BepiColombo space mission which is intended to continue the work of MESSENGER, even though it will not reach the planet until 2024!