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Sulfur Cycle on Mercury: An experimental and theoretical study

Final Report Summary - SULFURONMERCURY (Sulfur Cycle on Mercury: An experimental and theoretical study)

The planet Mercury is the smallest planet from our Solar System and is also the closest to the Sun. The geology of this planet and the chemical composition of its surface were unknown until 2009 when the MESSENGER spacecraft from the NASA entered in orbit around Mercury. This spacecraft measured the chemical composition of the planet and revealed that its crust is made of lava flows originating from volcanic eruptions. A particularity that was revealed by the MESSENGER spacecraft is the very high sulfur content of lavas on Mercury. These lavas contain 2-4 wt.% of lavas, which is one order of magnitude higher than the sulfur contents observed in lavas from any other planet such as the Earth, the Moon and Mars.
The SULFURONMERCURY project (June 2014 to June 2016) had several scientific objectives all aiming at better understanding how the planet Mercury formed and evolved through time. The first objective was to understand how much sulfur can be dissolved in basalts from Mercury. Because this planet formed closer to the Sun than the Earth, the amount of oxygen that was available during the formation of the planet was smaller which may impact the distribution of sulfur in the basalts. The second objective was to understand the behaviour of sulfur when the planet cooled down after its initial formation. The third objective was to understand how the distribution of sulfur changes with depth (i.e. pressure) in Mercury’s mantle and understand how this element behaves when magmas migrate from the mantle to the surface of the planet. The last objective was to determine if sulfur degassing during volcanic eruptions may have contributed to the composition of the thin atmosphere around Mercury.
During the project we used chemical data from the MESSENGER spacecraft to calculate chemical composition maps for the Northern hemisphere of the planet. Such a calculation was impossible for the Southern hemisphere for which chemical data from MESSENGER has a too low resolution. When chemical data were calculated, we used statistical methods to discriminate different geochemical regions. We realized that old lavas (dated at 4.2 to 3.7 billion years) have a distinct chemical composition than younger lavas (younger than 3.7 billion years). For these two main groups of lavas, we calculated average compositions and reproduce synthetic lavas in the laboratory. These synthetic lavas were produced by mixing chemical components such as silica, aluminium, magnesium, sulfur... The two synthetic compositions were then used to perform low- to high-pressure (from the atmospheric pressure to 40000 times the atmospheric pressure) at high temperature (from 1000 to 1800˚C). Each experiment was performed for a few hours in a furnace or in a hydraulic press. In each experiment, we produced a magma and some crystals for which we measured the chemical composition. In details, for each experimental magma we measured the sulfur content. In total, we ran more than 200 experiments during the course of the project. Low- to medium-pressure experiments were all performed at the University of Hannover whereas high-pressure experiments were performed both at the University of Bayreuth and at the Massachusetts Institute of Technology.
The main results of our experimental program can be summarized in three points: (1) the sulfur content that can be dissolved in lavas from Mercury is much higher than the sulfur contents in lavas from the Earth. This is because there is less oxygen available on Mercury and therefore sulfur will take the position of oxygen in the chemical structure of the magma. (2) the lavas that we observe at the surface of the planets come from two distinct regions of the mantle. The oldest lavas were produced at great depth in the mantle (deeper than 200 km) while the youngest lavas were produced at shallower depth (lower than 200 km). This was interpreted as resulting from extremely fast cooling of Mercury’s mantle and we suggested that Mercury may have cooled much faster than any other planet in our Solar System; (3) the old lavas contain different minerals than the young lavas. This is also due to the temperature at which lavas were formed in the mantle of Mercury. Old lavas are dominated by magnesium-rich minerals (olivine, pyroxene) while young lavas are dominated by aluminium-rich minerals (plagioclase).
The results of this project will be extremely important to further interpret the results of the MESSENGER mission. The spacecraft measured millions of ultraviolet and infrared spectra that were extremely difficult to interpret because the nature of the minerals at the surface of Mercury was unknown. Our study clarified this issue and gives a better chance of interpreting the spectra. Some of our experimental samples have been given to other teams in Germany (e.g. DLR; Berlin) were they are used to calibrate spectrometers and better interpret MESSENGER data. Our results will also be very important to interpret the chemical data that will be obtained from the BepiColombo mission from the ESA. The spacecraft will be launched in 2018 and will arrive around Mercury in 2024. In particular, our study has shown the importance of some critical elements such as sodium and sulfur on the geological history of Mercury. It will therefore be very important for BepiColombo to measure precisely these elements.