Perchlorate neutron data

Although there is no general consensus on the nature of water on Mars the discovery by the Phoenix Lander of calcium and magnesium perchlorates in Martian soil samples has fueled much speculation that flows of perchlorate brines might be the cause of the observed channeling and weathering in the surface. Magnesium perchlorate forms a eutectic mixture with water with a minimum freezing point around 206 K and this matches the observed ground temperatures, which, in summer at the equator, typically range between 195K and 280K. Here we study the structure of water in a mimetic of Martian water, namely magnesium perchlorate aqueous solution at its eutectic composition, using neutron diffraction in combination with hydrogen isotope labelling and empirical potential structure refinement. We find that the tetrahedral structure of water is heavily perturbed under these conditions, the effect being equivalent to pressurizing pure water to pressures of order 2 GPa or more. In addition, the Mg2+ and ClO4- ions appear charge-ordered, confining the water on length scales of order 9 Å, so preventing ice formation at low temperature. These effects may explain the low evaporation rates and high deliquescence of these salt solutions which are essential for stability within the low relative humidity environment of the Martian atmosphere.