Using the SANDALS diffractometer at ISIS Neutron and Muon Source, which was specially built for investigating the structure of liquids and amorphous materials, the scientists’ conclusions could explain how Martian channels formed, shedding light on a phenomenon that has mystified scientists for centuries.
The search for extra-terrestrial life is often regarded as one of the most exciting inquiries of the present day. Owing to its requirement for a range of biophysical and biochemical processes, water is widely regarded as a prerequisite for the development and survival of life and it is for this reason that the search for extra-terrestrial life generally starts as a search for water.
Over the years, there has been considerable interest in the growing evidence for the presence of flowing water on the surface of our neighbouring planet, Mars, despite the knowledge that the Martian atmosphere does not provide a suitable climate for pure water to remain in liquid form. In recent times, NASA’s Phoenix Mars Lander, a robotic spacecraft launched in 2007, discovered perchlorate in the regolith of the Martian northern plains, hinting at the plausibility that liquid water could exist below the Martian surface. What had yet to be defined, however, were further details regarding the structure of this perchlorate salt and how it could provide an explanation for the observed channelling and weathering in the Martian surface.
In order to unravel this mystery, Dr Dougan and her team, Drs Samuel Lenton, Natasha H. Rhys, and James J. Towey, and ISIS Neutron and Muon Source senior scientist Dr Alan Soper studied the structure of water in magnesium perchlorate solutions, at its eutectic composition, using SANDALS neutron diffraction, in combination with hydrogen isotope substitution. Data were then interpreted using a computational modelling technique known as Empirical Structure Refinement (EPSR).
Perchlorate solutions are widely used as mimetics in studies of Martian “briny" water, especially magnesium perchlorate, as it has a lower eutectic point, the temperature at which the mixture freezes or melts, than many other solutions, making it suitable for investigating salts discovered on Mars where the average temperature is -81 degrees Fahrenheit.
In this experiment, neutron total scattering techniques were chosen as they provided one of the most direct ways of studying the perchlorate structure in aqueous solutions through the investigation of hydrogen bonding. Neutrons were used because they are strongly scattered by hydrogen atoms, whose contribution to the scattering pattern can be markedly altered by substituting deuterium for hydrogen.
Upon investigation, the researchers found that Magnesium perchlorate had a major impact on water structure in solution. The tetrahedral structure of water was heavily perturbed and the Mg2+ and CIO4- ions appeared charge-ordered, confining the water on length scales of order 9 Å. The paper, published in the journal Nature Communications concluded that this highly compressed structure of water was most likely caused by the replacement of water-water hydrogen bonds in the pure liquid with cation-water-anion bonds in solution, thus contributing to it's very deep eutectic point and consequently, preventing ice formation at a low temperature. This could therefore explain how these salt solutions remain liquid within the low and widely varying humidity environment of the Martian surface, because of their low evaporation rates and high deliquescence.
Dougan and her team have therefore shown that aqueous perchlorate solution could provide the means for water to have the flow needed for generating geological channels at the sub-zero temperatures found on Mars. As water in the form of a briny liquid is likely to occur in other parts of the Solar System, this research represents a very exciting step forward in the search for extra-terrestrial liquid water – and with it, life.
“We found these observations quite intriguing. It gives a different perspective of how salts dissolve in water. The magnesium perchlorate is clearly a major contributing factor on the freezing point of this solution and paves the way for understanding how a fluid might exist under the sub-freezing conditions of Mars.
This highlights the importance of studying life in extreme environments in both terrestrial and non-terrestrial environments so that we can fully understand the natural limits of life."
Dr Lorna Dougan, the University of Leeds
The project was supported by a grant from the Engineering and Physics Sciences Research Council (EP/H020616/1). Dr L. Dougan is supported by a fellowship from the European Research Council (258259-EXTREME BIOPHYSICS). Experiments at the ISIS Neutron and Muon Source were supported by a beam time allocation from the Science and Technology Facilities Council under proposal number RB1300009.
The full publication is available to view in Nature Communications here.
For further information about the research, please contact Dr Lorna Dougan (L.Dougan@leeds.ac.uk) and Dr Alan Soper (alan.soper@stfc.ac.uk).
Further reading
S. Lenton, N. Rhys, J. Towey, A. Soper and L. Dougan, "Highly compressed water structure observed in a perchlorate aqueous solution," Nature Communications, In Press, 2017. DOI:10.1038/s41467-017-01039-9
