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Project Progress

The sub-permafrost zone of Mars is acknowledged as having the greatest potential for habitability because water is readily available. CH₄ leakage from the sub-permafrost zone would occur wherever the permafrost zone has been breached by recent fracturing due to impacts or by geothermal heating. Leakage and resultant brines may be associated with brine seeps and Martian “gullies” may represent a manifestation of such sub-permafrost leakage. In order to better understand the role high salinity brines may play in dissociating hydrate deposits within permafrost on Mars, a series of hydrate dissociation experiments are being conducted at Oak Ridge National Laboratory using traditional small-scale pressure vessels (0.5 to 4.5 L volumes, Figure 1), as well as the 72 L mesoscale Seafloor Process Simulator (SPS, Figure 2). The large experimental volume allows for dissociation experiments to be conducted over a period of several hours or days at thermal equilibrium. The effects of high salinity, mixed chloride-sulfate brines on hydrate stability are being determined by measuring the temperature and pressure of the system during gradual hydrate dissociation. Gas clathrates or hydrates are ice-like solids formed when guest molecules (CH₄, CO₂, H₂S, etc.) are trapped inside cage-like structures of water molecules. Hydrate is synthesized at elevated pressure conditions within high salinity brines of interest. The system will then be allowed to warm adiabatically. Due to the endothermic nature of hydrate dissociation, temperature is buffered to near isothermal conditions during dissociation, while pressure continues to increase as gas is released. The final temperature and pressure recorded during dissociation represents conditions for the aqueous system in question. Additional experiments are investigating hydrate dissociation rates as a result of brine influx. In these experiments, hydrate will be synthesized in distilled, deionized water. The pressure/temperature of the sample vessel will be held within the hydrate stability field. A known volume of high salinity brine will be injected into the vessel, thus increasing the salinity of the fluids in contact with the hydrate. Hydrate dissociation is monitored using a series of thermocouples and pressure transducers. The rate of hydrate dissociation is determined by analyzing the pressure increase in the system as gas is released from the hydrate structure.

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Modeling the release of CH₄ into the Martian atmosphere requires accurate experimental data on the stability of CH₄ hydrate in the presence of brine and altered rock. Natural gas hydrates are stable at low temperatures (<10° C) and high pressures (>3 MPa) typical of deep (>300 m) marine sediments or permafrost regions. On Earth, CH₄ is the most common gas component in natural gas hydrates that occur in permafrost and deep marine sediments lower in brines but rich in microbial life. On Mars, much of the interest in gas hydrates has focused on potential subsurface reservoirs for storing both water and greenhouse gases, CH₄ and CO₂, as well as the dissociation of hydrate as potential energy source for large-scale erosional features. Hydrate stability is determined by the effective pressure of the hydrate forming gas(es), temperature and the activity of water. Increasing temperature, increasing salinity (decreasing the activity of water in the system), or decreasing pressure conditions may destabilize gas hydrates leading to dissociation into liquid water + gas, ice+ gas, or ice + liquid CO₂ and CH₄ depending on the conditions. The removal of overlying permafrost, impact fracturing, or other processes of depressurizing confined hydrate reservoirs, therefore, may result in dissociation and release of trapped gases. In addition, injection of warmer and/or more saline fluids may also destabilize hydrates and result in CH₄ release. Whereas several studies have suggested that dissociation of hydrates may have been responsible for catastrophic erosion on Mars, mesoscale laboratory experiments suggest that hydrate dissociation due to increasing temperature or depressurization may be a relatively slow and an endothermic processes resulting in local cooling, thus providing a negative feedback or temperature buffering effect which may inhibit further hydrate dissociation (see Fig. 3 & 4). The effects of temperature and pressure changes on gas hydrate stability and dissociation rates have been well documented under terrestrial conditions. High salinity and mixed solute brines similar to those expected on Mars, however, have received little to no attention, nor has the growth or survival of microoganisms to those conditions been examined.

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