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

Abstract

[1] Chemical analyses of three Martian soil samples were performed using the Wet Chemistry Laboratories on the 2007 Phoenix Mars Scout Lander. One soil sample was obtained from the top $2 cm (Rosy Red) and two were obtained at $5 cm depth from the ice table interface (Sorceress 1 and Sorceress 2). When mixed with water in a $1:25 soil to solution ratio (by volume), a portion of the soil components solvated. Ion concentrations were measured using an array of ion selective electrodes and solution conductivity using a conductivity cell. The measured concentrations represent the minimum leachable ions in the soil and do not take into account species remaining in the soil. Described is the data processing and analysis for determining concentrations of seven ionic species directly measured in the soil/solution mixture. There were no significant differences in concentrations, pH, or conductivity, between the three samples. Using laboratory experiments, refinement of the surface calibrations, and modeling, we have determined a pH for the soil solution of 7.7(±0.3), under prevalent conditions, carbonate buffering, and P CO2 in the cell headspace. Perchlorate was the dominant anion in solution with a concentration for Rosy Red of 2.7(±1) mM. Equilibrium modeling indicates that measured [Ca 2+ ] at 0.56(±0.5) mM and [Mg 2+ ] at 2.9(±1.5) mM, are consistent with carbonate equilibrium for a saturated solution. The [Na + ] and [K + ] were 1.4(±0.6), and 0.36(±0.3) mM, respectively. Results indicate that the leached portion of soils at the Phoenix 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 ions and conductivity and another species may be present.

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

“…One of the instruments used in the analysis was the Thermal and Evolved Gas Analyzer (TEGA), which consisted of a high temperature furnace paired with a magnetic sector mass spectrometer. Results from the TEGA analyses were consistent with the detection of carbonate minerals within the soils near the lander, and this was corroborated by analyses from the Wet Chemistry Laboratory (WCL) on the same lander Kounaves et al 2010a).…”

Concentrated perchlorate at the Mars Phoenix landing site: Evidence for thin film liquid water on Mars

“…WCL could detect and measure Cl ions, and did not detect bischofite [Kounaves et al, 2010a[Kounaves et al, , 2010b. Our geochemical modeling indicates that zeolites are unlikely candidates for the patches described here, because they are unlikely to form under Phoenix site conditions (see Section 3.1 above).…”

“…[23] Previous studies have proposed that perchlorate on Mars might form from atmospheric interactions between ozone and volatile chlorine compounds as aerosols or at the surface, as in the Atacama Desert of Chile [Catling et al, 2010] and the Antarctic Dry Valleys [Kounaves et al, 2010a[Kounaves et al, , 2010b. Dissolution and redistribution of perchlorate could occur when summer mid-day temperatures exceed the perchlorate eutectic point.…”

Perchlorate Radiolysis on Mars and the Origin of Martian Soil Reactivity

“…Absorbance measurements were made with an Ocean Optics HR2000 UV-vis-NIR spectrometer, an Ocean Optics HD2000 UV-vis-NIR light source, a 1 cm cuvette, and associated optical fibers. Since the pH of Phoenix and Viking soils is slightly basic (Quinn and Orenberg, 1993;Kounaves et al, 2010), the irradiated perchlorate products were measured in a slightly basic aqueous solution (pH 9 borax buffer).…”