The oxygenation of Earth’s atmosphere 2.43 billion years ago (Ga) in the ‘Great Oxidation Event’ (GOE), set Earth on the trajectory to being able to support complex life. Planetary surface oxidation set the stage for atmospheric oxygenation, so understanding when, where, and how Earth’s surface was oxidized is fundamental to understanding habitability. In this work we explore aspects of early Earth and Mars surface redox evolution, emphasizing novel uses of iron geochemistry. We first address controversial evidence for atmospheric oxygenation at around 2.95 Ga. In a study of an ancient weathering horizon, we show that continental weathering took place under an anoxic atmosphere, despite widespread evidence for the coincident oxygen production in the oceans. Next, we show that oxic and sulfidic iron sinks in the oceans both grew in the runup to the GOE and resolve a resolve a longstanding controversy about the iron isotope record of Archean pyrites. An increase in pyrite burial after 2.65 Ga, likely driven by increased volcanic sulfur supply to sulfur-starved early oceans, might have driven the iron cycle in high-productivity ocean environments to become a net source of environmental oxygen that overwhelmed nearby iron oxidation buffers. We also investigated columnar stromatolites in the shallowest facies of an iron formation deposited prior to the GOE. Combined geochemical and textural evidence suggest that these stromatolites were formed where iron-rich waters inundated a shallow shelf occupied by cyanobacterial communities, providing a unique snapshot of a ground zero site for early oxygen production. Lastly in our investigation of ancient Earth, we consider an alternative model for the deposition iron formations, long assumed to be an oxide-rich sedimentary sink in the early oceans, that instead elevates the role of ferrous iron silicates. We determine the equilibrium iron isotope behavior of the ferrous silicate greenalite and find surprising similarity between the iron isotope systematics of greenalite and ferrihydrite, a representative hydrous ferric oxide. For Mars, we first consider the impacts of long-term processes that affected the surface volatile budget. We indirectly quantify the post 3.5 Ga loss of CO2 from Mars by independently constraining the loss of hydrogen and oxygen to space, and to surface oxidation, and inferring carbon loss from the oxygen loss unexplained by depletion of the Mars hydrosphere. We find that the late surface oxidation sink on Mars is small, and oxygen loss can by predominantly attributed to water, necessitating a missing Martian (sub)surface CO2 sink. Finally, we conducted new experiments to constrain the kinetics of iron photooxidation and tested whether this process can quantitatively oxidize iron from solution. By modeling iron photooxidation in Gale Crater Lake, we find that it supports iron deposition rates consistent with sedimentary textures, relaxing the need for other chemical oxidants. Taking lessons from chemical trends seen on pre-GOE Earth, we show that extensive iron oxidation can explain the iron and manganese systematics of Gale Lake sediments and produce manganese rich residual fluids that explain localized elemental enrichments observed by Curiosity.