A universal salt model based on under-ground precipitation of solid salts due to supercritical water `out-salting'

One of the common characteristics of planets Earth and Mars is that both host water (H2O) and large accumulations of salt. Whereas Earth’s surface-environment can be regarded as ‘water-friendly’ and ‘salt hostile’, the reverse can be said for the surface of Mars. This is because liquid water is stable on Earth, and the atmosphere transports humidity around the globe, whereas on planet Mars, liquid water is unstable, rendering the atmosphere dry and, therefore, ‘salt-friendly’. The riddle as to how the salt accumulated in various locations on those two planets, is one of long-lasting and great debate. The salt accumulations on Earth are traditionally termed ‘evaporites’, meaning that they formed as a consequence of the evaporation of large masses of seawater. How the accumulations on Mars formed is much harder to explain, as an ocean only existed briefly. Although water molecules and OH-groups may exist in abundance in bound form (crystal water, adsorbed water, etc.), the only place where free water is expected to be stable on Mars is within underground faults, fractures, and crevices. Here it likely occurs as brine or in the form of ice. Based on these conditions, a key to understanding the accumulation of large deposits of salt on both planets is linked to how brines behave in the subsurface when pressurized and heated beyond their supercritical point. At depths greater than about 3 km (P>300 bars) water will no longer boil in a steam phase. Rather, it becomes supercritical and will attain the phase of supercritical water vapor (SCRIW) with a specific gravity of typically 0.3 g/cm3. An important characteristic of SCRIW is its inability to dissolve the common sea salts. The salt dissolved in the brines will therefore precipitate as solid particles when brines (seawater on the Earth) move into the supercritical P&T-domain (T>400°C, P>300 bars). Numerical modeling of a hydrothermal system in the Atlantis II Deep of the Red Sea indicates that a shallow magma-chamber causes a sufficiently high heat-flow to drive a convection cell of seawater. The model shows that salt precipitates along the flow lines within the supercritical region (Hovland et al., 2006). During the various stages of planet Mars’ development, it must be inferred that zones with very high heat-flow also existed there. This meant that water (brine) confined in the crust of Mars was mobilized in a convective manner and would pass into the supercritical water zone during the down-going leg (the recharge leg) of the convective cell. The zones with supercritical out-salting would require accommodation space for large masses of solid salt, as modeled in the Red Sea analogy. However, as the accommodation space for the solid salt fills up, it will pile up and force its way upwards to form large, perhaps layered anticlines, as seen in the Hebes Mensa area of Mars and at numerous locations on Earth, including the Red Sea. Thus, we offer a universal ‘hydrothermal salt model’, which would be viable on all planets with free water in their interiors or on their surfaces, including Mars and Earth. Hovland, et al., 2006. Salt formation by supercritical seawater and submerged boiling. Marine and Petrol. Geol. 23, 855-69