Dynamic climate and redox interactions on early Mars inferred from water chemistry at Gale

Salinity, pH, and redox states are fundamental properties that characterize natural waters. These properties of surface waters on early Mars reflect ancient environments, and thus provide clues on the palaeoclimate and habitability. In present study, we constrain these properties of pore water within lacustrine sediments of Gale Crater, Mars, based on interlayer compositions of smectite and salt mineralogy. The pore water that last interacted with the sediments was of Na-Cl type with relatively high salinity and circumneutral pH. The Na-Cl concentrations suggest that the early Gale lakes were saline and developed in semiarid to arid climates. To achieve this salinity, hydrological cycles would have persisted for 10⁴-10⁶ years, suggesting that the Gale lakes developed as part of a global hydrological cycle during warmer episodes in the early Hesperian. The circumneutral pH and observed redox disequilibria in the pore water also suggest the occurrence of post-depositional alterations via acidic-oxidizing fluid intrusions into alkaline-reducing sediments, possibly in the late Hesperian or later. The alterations would have been triggered by instant melting of surface ice that brought strong oxidants to the subsurface. Gale's water chemistry records dynamic climates on early Mars, which provided redox disequilibrium and chemical energy to aqueous environments upon episodic warming.