The great oxidation event on early Mars : evidence from remote sensing and Mars rover data

Abstract

The redox evolution of Earth’s atmosphere from reducing to oxidizing, known as the Great Oxidation Event, significantly changed many aspects of our planet’s systematic evolution. Mars is cold and dry now, but ~ 3.5 billion years ago, it was warmer and wetter with the formation of river channels, lakes, and hydrous minerals that need the existence of liquid water. The chemically reducing atmosphere can warm early Mars and Earth efficiently and maybe the answer for the faint young Sun paradox, while there is no direct geological evidence that a reduced atmosphere actually existed on Mars. My Ph.D. study demonstrates that the atmosphere of early Mars was reducing and that the planet Mars also experienced a Great Oxidation Event of its own, although for different reasons than Earth. My work uses infrared spectroscopy and remote sensing to explore the ancient rocks on early Mars. Through the study of a thick weathering basaltic sequence at Hainan island, South China, I established spectral, mineralogical, and geochemical indexes for weathering intensity of basalts, which can be used to identify weathering sequences on Mars. Remote sensing spectral indexes for abundance and valence state of iron (Fe) are also established. By linking remote sensing and laboratory analyses, I demonstrate that most surface Martian clay minerals were formed by chemical weathering. These ancient soils on Mars experienced substantial Fe loss, indicating that ancient Mars possessed a reducing atmosphere like ancient Earth. Because the redox state of modern Mars’s atmosphere is oxidizing, it implies that Mars has also undergone a Great Oxidation Event. In addition, my study presents a challenge to the paleolake model for Gale crater, which was mainly proposed based on the existence of layered structures and hydrous minerals. I show that the geochemical, mineralogical, and textural observations in Gale crater can be best explained by chemical weathering. The substantial depletion of Fe and manganese (Mn) in the upper sequence of Gale weathering sequences confirms the remote sensing interpretation that Fe was leached from paleosols on Mars. Most of the stratigraphic section in Gale explored to date can be explained as eolian and/or volcaniclastic sediments subaerially chemically weathered by acidic precipitation in a reduced atmosphere. The widespread surface Fe3+-rich hydrous minerals at the Noachian (~4.1-~3.7 Ga) surface on Mars seems contradictory with a reducing atmosphere. The ferric minerals include surface nontronite and Fe3+ oxides/sulfates, e.g., Gale crater. My study suggests the iron was directly oxidized during the pedogenesis of eolian sediments under a reducing atmosphere. Oxyhalogens, e.g., chlorate, can be the oxidants, which are formed and coated on particles by photochemical processes and/or cyclical sand/dust storm events. The oxidants have been accumulated on Mars’ surface from dry deposition. During chemical weathering, oxyhalogens are dissolved and oxidized redox-sensitive elements directly. These observations suggest that the surface of Mars has been oxidized earlier than the atmosphere. We cannot simply use inorganic redox proxies in sedimentary rocks to investigate the redox state of Mars and the terrestrial atmosphere.