Any groundwater rising towards the surface would begin to evaporate as it encountered the low-pressure Martian atmosphere, leading to the crystallization of any dissolved species. The process of groundwater upwelling and evaporation would leave a distinctive pattern of crystallization that varies with depth and location in the central Hebes mound. At the low-pressure surface, evaporation will cause the groundwater to become supersaturated, and thus precipitation of primary salts occurs through efflorescence, a process common in building stones on Earth [e.g., Scherer, 2004]. The first salts to crystallize through efflorescence at the low relative humidity surface will be anhydrous, such as anhydrite (CaSO4), or low hydrates, such as monohydrates like kieserite (MgSO4 · H2O) [e.g., Chou and Seal, 2007]. The higher relative humidity inside the central mound will result in the crystallization of polyhydrated minerals, such as epsomite (MgSO4 · 7H2O) and mirabilite (Na2SO4 · 10H2O), through subflorescence, or crystallization below the surface [e.g., Rodriguez-Navarro and Doehne, 1999]. However, this zoning might be offset to a certain extent by any temperature gradient between the surface and interior of the mound, as polyhydrates tend to form at lower temperatures [e.g., Peterson and Wang, 2006]. But as long as evaporation dominates over any temperature effect, as would likely be the case in an atmosphere similar to that of present-day Mars, then groundwater-driven crystallization would result in distinct zones of hydration: anhydrates and monohydrates at the surface through efflorescence, and polyhydrates in the interior through subflorescence.
The crystallization of salt can lead to significant stress damage in porous rocks [e.g., Scherer, 2004; Steiger et al., 2008]. A crystal growing in a supersaturated solution within a confined pore space will exert a pressure on the surrounding matrix. These stresses are often sufficient to cause failure in tension of the host rock, especially if there are repeated cycles of evaporation [e.g., Steiger and Asmussen, 2008]. For example, in the sodium sulfate-water system, crystallization pressures are higher at lower temperatures, and at 0°C can be as high as 37 or 27 MPa in mirabilite or the metastable heptahydrate (Na2SO4 · 7H2O) respectively [Steiger and Asmussen, 2008]. Although this pressure is exerted at the scale of the pores, if a large fraction of the crystals are in contact with their pore walls, and if the matrix is saturated with groundwater, then the crystallization pressures can be directly compared with the tensile strength of the rock matrix [Steiger and Asmussen, 2008]. The tensile strength of rocks is of the order of one-tenth of the uniaxial compressive strength (UCS), and so has an upper limit of about 30 MPa, but can vary due to a number of factors, including composition, porosity, alteration, and stress history [Paterson and Wong, 2005]. For example, the tensile strength of sandstone can vary from less than 1 MPa up to 20 MPa [e.g., Bell, 1999]. The tensile strength of Martian sulfate hydrates is unknown, but analog materials have UCS values of up to about 70 MPa, depending on porosity [Grindrod et al., 2010]. Regardless of the exact tensile strength of any particular rock mass that makes up Hebes Montes, it is clear that under the right conditions, the stress generated through the crystallization of salts is capable of causing rock damage, and subsequent fracture and collapse. In fact, as higher crystallization pressures are generally the result of precipitation of the highest hydrates, then it is possible that failure in coherent planes of polyhydrated subflorescence layers in the interior of Hebes Montes could be more important than at the surface, causing large-scale collapse.