Abstract
Salinity, pH, and redox states are fundamental properties that characterize natural waters. These properties of surface waters on early Mars reflect palaeoenvironments, and thus provide clues on the palaeoclimate and habitability. Here we constrain these properties of pore water within lacustrine sediments of Gale Crater, Mars, using smectite interlayer compositions. Regardless of formation conditions of smectite, the pore water that last interacted with the sediments was of Na-Cl type with mild salinity (~0.1–0.5 mol/kg) and circumneutral pH. To interpret this, multiple scenarios for post-depositional alterations are considered. The estimated Na-Cl concentrations would reflect hyposaline, early lakes developed in 104–106-year-long semiarid climates. Assuming that post-depositional sulfate-rich fluids interacted with the sediments, the redox disequilibria in secondary minerals suggest infiltration of oxidizing fluids into reducing sediments. Assuming no interactions, the redox disequilibria could have been generated by interactions of upwelling groundwater with oxidized sediments in early post-depositional stages.
Highlights
Salinity, pH, and redox states are fundamental properties that characterize natural waters
There have been a number of attempts to estimate water chemistry on early Mars by means of geochemical reactiontransport modeling[1,2,3]
Regardless of the chemical compositions and formation conditions of smectite, the top-down approach can provide a robust snapshot of the chemical properties of the pore water that interacts with the clay-bearing rocks[4] if the clay-bearing rocks contain smectite
Summary
PH, and redox states are fundamental properties that characterize natural waters. To quantitatively estimate the water chemistry of the pore water that interacted with the sediments, we use the exchangeable cation compositions of smectite’s interlayer together with the presence of secondary minerals commonly found in Yellowknife Bay (see Methods).
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