Semiarid climate and hyposaline lake on early Mars inferred from reconstructed water chemistry at Gale

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.

Highly Oxidizing Aqueous Environments on Early Mars Inferred From Scavenging Pattern of Trace Metals on Manganese Oxides

“…Since all of the oxidants capable of generating high Eh > 0.5 V would have been generated via the atmospheric chemistry, our results further suggest the previous idea that the oxidants were transported from the surface possibly through fluid activity (Fukushi et al, ; Rampe et al, ). If photochemically produced perchloric acids and nitrates accumulated on the surface (Lasne et al, ; Smith et al, ), ice melting upon transient warming (e.g., Bishop et al, ; Kite et al, ; Wordsworth et al, ) could have provided a quantity of these oxidants to the subsurface (Fukushi et al, ). Given the Cl contents of the Mn enrichments (0.4–3.3 wt.%), nevertheless, the measured MnO 2 (3.7–6.0 wt.%) may not be generated solely by reactions of Mn 2+ with perchloric acids (Lanza et al, ).…”

“…Based on the above discussion in sections 4.1 and 4.2, we suggest that, at least in the Gale crater, MnO 2 precipitated from Mn 2+ -bearing reducing groundwater (e.g., Eh~0 V) by an interaction with high-Eh oxidants (e.g., Eh > 0. Journal of Geophysical Research: Planets acids and nitrates accumulated on the surface (Lasne et al, 2016;Smith et al, 2014), ice melting upon transient warming (e.g., Bishop et al, 2018;Kite et al, 2017;Wordsworth et al, 2017) could have provided a quantity of these oxidants to the subsurface (Fukushi et al, 2018). Given the Cl contents of the Mn enrichments (0.4-3.3 wt.%), nevertheless, the measured MnO 2 (3.7-6.0 wt.%) may not be generated solely by reactions of Mn 2+ with perchloric acids (Lanza et al, 2016).…”

“…The recent experimental study shows that formation of akaganeite requires oxidizing, low‐to‐moderate pH (2–8) aqueous environments with moderate Cl concentrations ([Cl] > 0.05 mol/L; Peretyazhko et al, ). Thermochemical equilibrium calculations suggest that akaganeite and magnetite can coexist at high Eh in possible aqueous environments at Gale (e.g., 0.5–0.6 V; Fukushi et al, ). These results on the formation conditions of akaganeite are consistent with the high‐Eh near‐surface aqueous environments required for the deposition of MnO 2 .…”

Constraining the preservation of organic compounds in Mars analog nontronites after exposure to acid and alkaline fluids

“…In the same way, this divalent saturated smectite could have also better stabilized chemical interactions with organic molecules, in agreement with the divalent cation-bridging theory (DCBT) 63,64 . Results from a recent study indicate that the Yellowknife Bay sediments would have interacted with different fluids in an early post-depositional stage 23 . In this sense, we have shown here that the interlayer space of nontronite, which is very sensitive to external fluids, is a key variable to understand the preservation of organic compounds in the long-term under current Mars surface conditions.…”

Hydrogeochemical Study on Closed-Basin Lakes in Cold and Semi-Arid Climates of the Valley of the Gobi Lakes, Mongolia: Implications for Hydrology and Water Chemistry of Paleolakes on Mars

“…Early Mars is thought to have been warmer and wetter than today, evidenced by the widespread presence of valley networks, deltas, lake deposits, clay minerals, and evaporates on the surface across Noachian to early Hesperian crust (3.8-3.5 billion years ago (Ga)) (e.g., see a review by [1]). Formation models of valley networks, closed-basin lakes, and lake deposits on early Mars suggest (cold and) semi-arid climates at places when the surface temperatures were above the freezing point of liquid water, e.g., [2][3][4][5][6][7][8][9][10][11]. However, hydrogeochemical cycles that occurred in the semi-arid climates on early Mars remain poorly constrained.…”