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On Monday May 9, we will be able to observe a transit of Mercury in front of the Sun, that is to say, an observable passage of the planet between the Earth and our star. While this is a very rare event, two recent publications shed new light on the history of the planet closest to the Sun.
Using data gathered by the American probe MESSENGER, geologists at the University of Liege have recreated “samples” of Mercury in the laboratory in order to better understand the formation and evolution of the rocks that make up the planet. From extraction from the core to the eruption of very ancient lavas covering its surface, the magmatic history of Mercury is revealed by experiments conducted in extreme conditions.
“When the MESSENGER probe orbited Mercury, I was engaged in post-doctoral studies at MIT”, recalls Bernard Charlier, who is currently developing an experimental petrology laboratory at the University of Liege. “I therefore had access to the first data available on the composition of the planet’s surface. In 2013 we published a first analysis based on this preliminary data but we needed more in-depth analysis which we completed in Liege. This resulted in the publication of two recent articles” (1).
MESSENGER did an outstanding job in providing a chemical mapping of the planet thereby supplying data on many tens of thousands of analysis points on Mercury’s crust. Bernard Charlier continues, “In 2013, we only had 11 analysis points on which to base our conclusions, today, our conclusions are based on 49,000 points”!
The rocks on the surface of Mercury, in the form of lava, were deposited 3.7 to 4.2 billion years ago. By way of comparison, the surface of the Earth is very recent, 200,000 years maximum even though there are rock outcrops dating back to 4 billion years ago. The existence of very old rocks on the surface evidently provide information on the initial stages of formation of the planet. At the moment of its formation, a planet is composed of two parts, a core (essentially of iron) surrounded by a silicate mantle. The silicated part partially remelts producing the lavas that will go on to form the crust on the surface.
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.
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.
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!"
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!
(1) Namur O, Collinet M, Charlier B, Grove TL, Holtz F, McCammon C (2016) Melting processes and mantle sources of lavas on Mercury. Earth and Planetary Science Letters 439: 117-128. http://www.sciencedirect.com/science/article/pii/S0012821X16000522
Namur O, Charlier B, Holtz F, Cartier C, McCammon C (sous presse) Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury. Earth and Planetary Science Letters.
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