Experimental Study of the Kinetics of Co2 Hydrate Dissociation under Simulated Martian

Introduction: The composition of the martian polar caps has long been debated with a consensus opinion that the north is composed predominately of water ice and the south a combination of solid CO2 and water ice [1]. Smythe and Miller [2] suggested that the polar caps may be composed of at least some CO2 hydrate, aka. clathrate, or compound gas hydrates [3, 4, 5, 6, 7]. Durham [8] predicted that a martian ice cap composed of hydrate will melt less readily than a cap of pure water ice, as the thermal conductivity of hydrate is lower. Gas hydrate has not yet been detected on Mars [1], but this can be explained by the infrared spectral similaries between it and pure water ice [9]. With the likelihood of CO2 hydrate in the polar caps of Mars, knowledge of the kinetics of its phase changes becomes important because it will affect the mass balance of CO2 and H2O. Kinetics of hydrates under martian conditions is a relatively unexplored field [3, 4, 9], and the lack of sufficient laboratory data in this area motivated this work. Procedure: CO2 hydrate was formed for the purpose of measuring its kinetics under simulated Martian conditions. Hydrate Production. CO2 hydrate was formed following the technique of Stern et al [10]. Ultra-pure water was frozen and crushed using a food processor within a freezer at 253 K to prevent melting. Approximately 100 g of crushed ice was placed within a precooled Parr pressure vessel and pressurized to 4100 kPa with CO2 gas. The pressure vessel was allowed to warm slowly over the course of several hours through the ice melting point. As the ice melts, it converts to CO2 hydrate, buffering the temperature at 275 K. Following conversion to hydrate the temperature continues to rise. Once the vessel reached 280K, it was placed within a freezer and cooled to <263K. The vessel was then allowed to warm through another cycle to 280K. This process was repeated 3-4 times until no buffering effect was discernible in the temperature profile as the vessel warmed, indicating the ice had completely transformed into hydrate. The vessel was then cooled to 253K prior to harvesting the gas hydrate. Hydrate samples were removed from the vessel immediately after depressurization and placed in liquid nitrogen to minimize decomposition. Samples were observed to be homogenous and fizzed and popped during transfer from the vessel, indicating they were composed of CO2 hydrate. Experimental. The hydrate samples were removed from the liquid nitrogen storage vessel an hour before the run and placed in a freezer set to 263K, which is in the anomalous hydrate stability zone at atmospheric pressure (Fig 4). Once the samples warmed to this new temperature, they were then placed in an open polystyrene cylinder on a mass balance in our chamber simulating the Martian environment (6 mbar CO2 atmosphere and 263K) and allowed to dissociate. The martian enviroment was produced using the same planetary simulation chamber described in Sears and Moore [11]. Water humidity and mass loss rates were recorded, using a hygrometer and a precision mass balance, respectively. From the humidity and mass loss rates, the loss rate for water and carbon dioxide were derived. Three thermocouples recorded temperatures, two of the hydrate sample and one the atmosphere inside the chamber. Results: The dissociation rates of five hydrate samples have been measured to date. Figures 1, 2, and 3 give the results from one example in which the temperature of the chamber and atmosphere were set to 263 K.