FROM ROVER TO LABORATORY: EXAMINING JAROSITE DISSOLUTION AND COUPLED HEMATITE PRECIPITATION TO CONSTRAIN ANCIENT AQUEOUS ENVIRONMENTS AT MERIDIANI PLANUM

Observations of abundant jarosite and hematite throughout Meridiani Planum by the Mossbauer and Mini-TES spectrometers on MER Opportunity have led to interpretations of widespread, though ephemeral, acidic fluids that may have also affected much of the surface on Mars. Here we report the results of aqueous geochemical experiments conducted in the laboratory to 1) measure the rate of jarosite dissolution under varying environmental conditions to constrain the lifetime of jarosite and duration of aqueous diagenesis in jarosite-bearing sediments and 2) examine the effects of solution geochemistry, temperature, and drying conditions on the mineralogy and texture of nanophase iron oxides which precipitate as jarosite dissolves.

Initial K-jarosite dissolution rates and products vary as a result of changing pH, iron solubility, and temperature. Dissolution rates vary from 6 x 10^-8 to 2 x 10^-9 mol m^-2 s^-1 at 323 and 277 K, respectively, yielding an activation energy of 59 kJ/mol. Low pH resulted in slightly increased rates of dissolution (2 x 10^-8 mol m^-2 s^-1 at 295 K), while increased iron solubility in the presence of oxalic acid increased dissolution rates by another order of magnitude. Using a shrinking sphere model of congruent dissolution yields lifetimes of <1-60 years for 1mm jarosite particles in water. However, TEM imaging and diffraction of jarosite dissolution reaction products demonstrates that jarosite dissolution is incongruent at pH >4, producing maghemite after 10 hours at 295, later transforming to hematite and goethite after 14 days, except at pH 8 where ferrihydrite dominates with some maghemite. Thermal emission specta of hematite nanoparticles with similar size and morphology to those formed in our jarosite dissolution experiments contain the same 390 cm^-1 feature from spectra recorded by Opportunity when cryodesiccated. Higher temperatures produced ferrihydrite and hematite, while low temperature dissolution produced schwertmannite after 10 days. No reaction products were observed at low pH, or in the presence of oxalic acid. However, the formation of iron oxide reaction products, including hematite, at moderate to high pH may limit the rate of long-term dissolution, lengthening jarosite lifetimes by 1-2 orders of magnitude in longer term experiments.