Studying mineral precipitation in cold aqueous planetary environments in the laboratory
What is it about?
Liquid oceans and ice caps have long been considered defining features of the Earth, but space missions and observations have shown that, along with ice crusts, they are in fact common features among many of the solar system's outer planets and their satellites. Interactions of liquid water with rocky materials contained within these objects, often as a core, have resulted in saline oceans not dissimilar in many respects to those on Earth, where mineral precipitation within frozen seawater plays a significant role in both determining global properties and regulating the environment in which a complex ecosystem of extremophiles exists. Since water is considered an essential ingredient for life, the presence of oceans and ice on other solar system bodies is of great astrobiological interest. However, the processes of mineral precipitation in freezing environments are still poorly understood, due largely to the difficulties of access and sampling, and in the case of ice drill cores collected on Earth, their stable preservation during transportation for later laboratory analysis. To study the formation of low temperature mineral precipitates from oceanic water we have designed and built a sample cell for use in our long-duration experiments facility where the progress of mineral formation is followed using synchrotron X-ray powder diffraction. The cell is capable of very slow cooling rates (e.g. 0.3°C per day or less), and as a commissioning experiment, its performance was tested by a year-long study of the precipitation of the hydrated magnesium sulfate phase meridianiite (MgSO4·11H2O) from the MgSO4–H2O system. Evidence from the Mars Rover mission suggests that this hydrated phase is widespread on the present-day surface of Mars. As well as the expected hexagonal ice and meridianiite phases, an additional meta-stable epsomite (MgSO4·7H2O) phase and a disordered ice phase were observed to form. Even though meridianiite is thermodynamically favoured and initially forms in greater quantities, the faster growth rate for epsomite means that over time, the ratio of meridianiite to epsomite reduces. However, after ~340 days, there was a drop in the total observed sulfate, accompanied by a rise in the background signal. In X-ray diffraction, the background originates from X-ray scattering from disorder in the atomic structure, which in this case is a disordered ice phase. It seems likely that the sulfate phases reach a point where they become unstable with respect to the remaining liquid brine and begin re-dissolving and releasing their waters of hydration which, owing to the low temperature, rapidly freeze, thus increasing the amount of disordered ice. As the cold cell is cooled and the the first freezing of the liquid into solid ice occurs, the dissolved salts are excluded from the ice, increasing the concentration of the dissolved sulfate, which acts like antifreeze and keeps the remaining brine liquid. As the temperature drops further, solid meridianiite and epsomite eventually start precipitating out, themselves, which effectively dilutes the brine concentration allowing it to freeze, which then raises the concentration etc. etc.. Thus, somewhat counter intuitively, at low temperature there is an ongoing complex dynamic interaction between liquid, ice and sulfate which, due to the low temperatures involved, operates over a long timescale.
Why is it important?
Early in its history, Mars was wet and habitable long before similar conditions prevailed on Earth and during which life began. Furthermore, the absence of plate tectonics on Mars means that this phase in its history, unlike on Earth, is preserved within its present day surface. Mars no longer has freestanding water, having lost its ocean long ago, however, the type of hydrated minerals left behind can not only tell us much about how its ocean was lost, but also represent repositories for the storage and preservation of at least some of the ancient water. Changes in environmental conditions can lead to the release of those waters and the transient formation of liquid water on the present day surface.