Wet Chemistry experiments on the 2007 Phoenix Mars Scout Lander mission: Data analysis and results

摘要

Chemical analyses of three Martian soil samples were performed using the WetChemistry Laboratories on the 2007 Phoenix Mars Scout Lander. One soil sample wasobtained from the top —2 cm (Rosy Red) and two were obtained at N5 cm depth from theice table interface (Sorceress 1 and Sorceress 2). When mixed with water in a 1:25 soilto solution ratio (by volume), a portion of the soil components solvated. Ionconcentrations were measured using an array of ion selective electrodes and solutionconductivity using a conductivity cell. The measured concentrations represent theminimum leachable ions in the soil and do not take into account species remaining in thesoil. Described is the data processing and analysis for determining concentrations of sevenionic species directly measured in the soil/solution mixture. There were no significantdifferences in concentrations, pH, or conductivity, between the three samples. Usinglaboratory experiments, refinement of the surface calibrations, and modeling, we havedetermined a pH for the soil solution of 7.7(±0.3), under prevalent conditions, carbonatebuffering, and P_(CO2_in the cell headspace. Perchlorate was the dominant anion in solutionwith a concentration for Rosy Red of 2.7(±1) mM. Equilibrium modeling indicates thatmeasured [Ca~(2+)at 0.56(±0.5) mM and[Mg +]at 2.9(±1.5) mM, are consistent withcarbonate equilibrium for a saturated solution. The [Nat] and [K+] were 1.4(±0.6), and0.36(±0.3) mM, respectively. Results indicate that the leached portion of soils at thePhoenix landing site are slightly alkaline and dominated by carbonate and perchlorate.However, it should be noted that there is a 5-15 mM discrepancy between measured ionsand conductivity and another species may be present.