Kieserite properties relevant to Mars and icy satellites

The occurrence of the mineral kieserite (magnesium sulfate monohydrate) and other hydrous sulfates is one of the surprising findings on the recent scientific endeavor to retrieve detailed knowledge about the planetary composition of extraterrestrial bodies, such as the surface of Mars or the icy moons of Jupiter and Saturn, e.g. Ganymede and Europa. While the relevance of kieserite in geochemical processes on Earth is limited, it plays a vital role for the water budget of Mars and very probably also for the interior of large icy moons, where it could be a crucial factor for the existence of subsurface oceans and phenomena such as cryovolcanism. These subsurface oceans could, in the most exciting scenario, support extraterrestrial life within our solar system. Numerous studies on these planetary objects brought a wealth of new insights but, naturally, countless important questions remain, several of which concern kieserite and related sulfates. Apart from very little knowledge on the stability of the kieserite structure and its variation with composition, temperature and pressure, it is the still unsolved enigma on a presumable second variant of kieserite with unknown crystal structure perhaps prevailing on the surface of Mars that deserves appropriate attention within the scope of an experimental study. Likewise, very little is known about properties of kieserite mixed crystals where Mg is partially replaced by cosmochemically highly relevant and abundant cations, notably Fe, Mn, or Ni. At intermediate chemical compositions as well as with varying temperature and pressure conditions relevant for extraterrestrial bodies, the crystal structure will change continuously or even abruptly, and with it its spectral pattern. As such spectroscopic data, usually measured from reconnaissance orbiters (partly also by rovers on the surface of Mars), represent the primary source of information about extraterrestrial bodies, a comparison with artificially prepared material is crucial for a deeper understanding of these observations. Therefore we will conduct a complex crystallographic and spectroscopic study on synthetic kieserite crystals of variable chemical composition, applying infrared and Raman spectroscopic as well as X-ray diffraction methods. Furthermore, to understand the behavior of kieserite in the interior of large icy moons, we will also study crystals at extreme pressures using diamond anvil cells. The laboratory data and results emerging from the planned investigations of these cosmochemically relevant sulfate monohydrates shall allow us and the scientific community to achieve a more detailed interpretation and understanding of respective reconnaissance data from objects in our solar system and, especially, to gain deeper insight into astromineralogical and cosmochemical processes relevant for Mars and icy moons of Jupiter and Saturn.

The presence of abundant amounts of hydrated sulfate minerals in our Solar system, notably (but not only) on Mars, has been studied in the course of various orbiter and rover space missions over the last four decades, and thus has become something like 'common knowledge'. These minerals fulfill many important roles, e.g. governing the water budget on Mars in its equatorial regions, or causing the formation of brines on the icy moons of Jupiter and Saturn, in turn enabling the evolution of subsurface oceans, which could potentially even sustain extraterrestrial life. Sulfates are also responsible for the occurrence of cryovolcanic phenomena: Geysers spewing particulate matter from the south-polar region of Saturn's moon Enceladus ensure the persistence of Saturn's beautiful rings by constantly replenishing them with new material. Remotely sensed spectra are, at present, the only chance to deduce global information about sulfate deposits on extraterrestrial bodies, if based on reliable data of synthetic or mineral analogues on Earth. Nevertheless, our knowledge of the detailed spectroscopic behavior, compositional boundaries and crystal chemistry of many of these sulfates is still rather limited, especially if cosmochemically abundant transition metal ions are incorporated into their structures. In our project we concentrated on one such important mineral, namely kieserite (magnesium sulfate monohydrate), found on Mars in abundance, and potentially present in the sulfate assemblages on the icy moons of Jupiter and Saturn. Preparing synthetic samples, we thoroughly studied how the gradual replacement of magnesium by the transition metals iron, nickel, manganese and cobalt influences the kieserite structure and the position of spectral bands in its infrared and Raman spectra, and revealed tight linear relationships with the chemical composition. We also documented the influence of low temperatures, relevant to the surface of Mars, on the spectra and crystal structures of these kieserite samples. Furthermore, by exposing selected samples to high pressures, corresponding to those prevailing in the interior of the Jovian icy moons, we discovered and characterized two structural phase transitions in kieserite, entailing important implications for any further thermodynamic considerations. The findings and detailed data obtained in the scope of the project represent important resources and benchmarks, enabling astronomers to evaluate present and future as well as to re-evaluate past spectral data related to kieserite not only on Mars, but also on other extraterrestrial bodies. With an at least rough estimate of surface temperature during an orbiter remote measurement, it is possible to infer the chemical composition of the measured material in a truly quantitative way. This enables chemical mapping of kieserite-containing sulfate deposits on our neighbor planet and elsewhere in our Solar system, offering a much more detailed insight into their origins and formation.