Laboratory Studies of Heterogeneous Processes Relevant to Mars

摘要

Heterogeneous (gas-surface) processes may play an important role in both the atmospheric and surface chemistry of Mars. Atmospheric species may be affected by the chemistry and physical properties of the planetary surface and the surface material may be affected by the components and properties of the atmosphere. In this thesis, several laboratory studies are described which experimentally investigate two types of atmosphere-surface systems likely to exist on Mars.First, experiments were performed to better understand the spatial and temporal variability of atmospheric methane (CH4) on Mars. Reported CH4 plumes in the atmosphere of Mars are difficult to explain using known chemical or physical processes. The observations imply a strong, present-day source and also a rapid yet unknown CH4 sink. We have investigated the potential role of mineral dust in CH4 variability. First, using a Knudsen cell capable of simulating Martian temperature and pressure conditions, we have studied the adsorption of CH4 to a Martian mineral analog as a function of temperature. An uptake coefficient was determined and then applied to the Martian surface-atmosphere system. Our results suggest that adsorption to soil grains could possibly affect the CH4 mixing ratio on a seasonal time scale especially at mid-latitude regions.Additionally, chemical oxidation of CH4 by oxidants thought to exist in the Martian regolith was studied. The Viking mission in the 1970’s found Martian soil was able to oxidize complex organic compounds to CO2. The identity of the oxidant is unknown, but has been proposed to be either hydrogen peroxide or perchlorate salts. We used a gas chromatograph to determine if simulated Mars soil containing these oxidants was able to oxidize CH4 to CO2. However, no CH4 was oxidized within the detection limit of the instrument and only an upper limit reaction coefficient could be reported. Even these upper limit values suggest CH4 could not be removed from the Martian atmosphere rapidly enough to cause variability.We have also studied the interactions of another important trace gas on Mars, water vapor, with perchlorate, a highly deliquescent salt recently discovered in polar soil. A Raman microscope equipped with an environmental cell was used to study phase transitions of the salts. The relative humidity (RH) at which deliquescence (absorption of water vapor by the solid to become an aqueous solution) and efflorescence (crystallization of the aqueous solution) occur were determined as a function of temperature, hydration state and associated cation. We show that the deliquescence RH for perchlorate salts can be low (~40% RH for anhydrous sodium perchlorate, for example). Thermodynamics can predict deliquescence; however, the kinetic inhibition of crystallization causes efflorescence to occur at much lower RH values than deliquescence which allows supersaturated salt solutions to exist in a metastable state. Based on the diurnal RH and temperature cycles on Mars, aqueous solutions could be stable or metastable for several hours a day at the Phoenix landing site. The astrobiological implications of potential liquid H2O on Mars are significant.