Thermodynamic Constraints on Smectite and Iron Oxide Formation at Gale Crater, Mars: Insights into Potential Free Energy from Aerobic Fe Oxidation in Lake Water–Groundwater Mixing Zone

Sakiko Kikuchi,Takazo Shibuya

doi:10.3390/min11040341

Abstract

The presence of saponite and iron oxides in Sheepbed mudstone of Yellowknife Bay at Gale crater on Mars is considered as evidence of a habitable fluvio-lacustrine environment for chemolithoautotrophy. However, the energetic availability for metabolic reactions is poorly constrained. Herein, we propose the possible mixing of surface water and groundwater that (i) explains the formation of magnetite and hematite detected in Sheepbed mudstone and (ii) may work as a potential habitable zone for aerobic Fe2+-oxidizing microbes. Our thermodynamic modeling of water–rock reactions revealed that the formation of abundant saponite in Sheepbed mudstone may occur under various conditions of water-to-rock mass ratios, temperatures (5–200 °C), and initial fluid compositions. In contrast, the formation of iron oxides in the mudstone can be explained only by the mixing of Fe2+-rich groundwater and more oxidized surface waters, where the Fe2+-rich groundwater can be generated by the low-temperature water–rock reactions with a CO2-bearing initial fluid. The calculated bioavailable energy of aerobic Fe2+ oxidation in the fluid-mixing zone on Mars is similar to that estimated for a fluid-mixing zone on Earth actually inhabited by aerobic Fe2+-oxidizing microbes. The findings will contribute to a better understanding of potential habitability on Mars.

Highlights

The search for microbial life on Mars—both past and present—is of significant interest in astrobiological research
We propose the presence of a mixing zone between surface water and groundwater, which can explain the mineral assemblage of sediments observed in Gale crater and the potential habitability of the mixing zone
The thermodynamic database required for the calculations was assembled using SUPCRT92 [38], which comprises thermodynamic data for minerals, aqueous species, and complexes taken from previous works [39,40,41,42,43], and calculated using the estimation techniques of Wilson et al (2006) [44]

Summary

Introduction

The search for microbial life on Mars—both past and present—is of significant interest in astrobiological research. In this regard, the identification of former presence of liquid water, which is a fundamental requirement for any known life, advanced our understanding about the potential habitability on Mars. The representative evidence of liquid water is the fluvial systems and the widespread presence of clay minerals in Noachian terrains. This evidence was obtained from geological, geochemical, and geomorphological analyses based on orbiting spacecraft [1,2]. The evidence suggests that early Mars had environments favorable for microbial lives [3,4]

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