Le site de vulgarisation scientifique de l’Université de Liège. ULg, Université de Liège

Spotlight on Mercury!
5/6/16

Mercury in the laboratory 

Mercury is very distant, inhospitable and there is no possibility of going there to take rock samples (like in the case of the Moon) or even to send a robot capable of carrying out soil analyses (like in the case of Mars). MESSENGER remained in orbit around the planet while various on-board instruments pick up different types of useful signals. The team from Liege were particularly interested in data gathered by an X-Ray spectrometer the objective of which was to analyse the composition of the surface of Mercury. Its detectors measured the X-Ray fluorescence emitted by the surface under the effects of solar radiation. Once they were equipped with this data, the geologists were able to begin interpreting it. “We first had to do some statistical processing”, explains Olivier Namur, an FNRS research associate in the laboratory, “to keep the most significant compositions”. Once the rock compositions were defined, the researchers synthesised them in the laboratory. This material can then be melted under different pressure and temperature conditions, in an environment with a low level of oxygen. These experiments, which were observed under microscope and analysed by means of an electron microprobe, enabled the geologists to identify the liquid, metal, sulphur and crystal equilibria.

Mercury electronic microscope

This information means that it is possible to interpret what is happening in the crust, the mantle and even the core of the planet according to the various temperatures and pressures to which the powders are subjected. These samples are like samples taken at different depths of Mercury. “The results that we are publishing today are the result of experiments conducted in two German laboratories, at the Leibniz University of Hannover, where Olivier Namur finished his post-doctorate, and the Bayerisches Geoinstitut of Bayreuth. Soon we will be able to conduct experiments here in Liege”, says Bernard Charlier. Thanks to the financial support of an FNRS loan facility and a BRAIN project financed by BELSPO, the University of Liege has in fact acquired the necessary equipment to carry out experiments of this kind. Using this equipment the rock powders will be subjected to temperatures of up to 2,000°C and pressures equivalent to that which exists at a depth of nearly 150 km below the Earth’s crust”, says Bernard Charlier.

The deep structure of Mercury

We already knew that Mercury’s mantle is very different from those of the other planets because it does not contain iron; in fact, all the iron was “pumped” into the core which is gigantic (60% of the volume of the planet compared to 15% for Earth). The silicate mantle is only 400 km thick. “This very weak thickness, allied to a rich composition of silica and magnesium is an important constraint for explaining the formation of the secondary crust. To melt the rocks of the mantle, very high temperatures are required. This is a conclusion we drew from our work: the high temperatures in the mantle and the significant degrees of fusion have produced lava that is very rich in magnesium”, explains Olivier Namur. Another important result detailed in the publications is the identification of the mineral composition of the mantle: the latter is essentially composed of two magnesium silicates, olivine (Mg2SiO4) and orthopyroxene (MgSiO3).

The researchers were also able to identify the conditions under which lava is produced in the mantle. The oldest lava is formed at the base of the mantle and the more recent lava in the upper part. Bernard Charlier explains, “We identified a rapid cooling of Mercury’s mantle during the first 500 million years. The mantle then becomes too cold and there is no longer any magmatic activity.  This is a characteristic of Mercury: after 3.8 billion years there was no longer any significant magmatic activity on the planet!"

Temperatures Mantle Mercury

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