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Mars' controversial hypothesized ocean shorelines have been found to deviate significantly from an expected equipotential surface. While multiple deformation models have been proposed to explain the wide range of elevations, here we show that the historical locations used in the literature and in these models vary widely. We find that the most commonly used version of the Arabia Level does not follow the originally described contact and can deviate laterally by ∼500 km in Deuteronilus Mensae. A meta‐analysis of different published maps shows that, globally, the minimum lateral offsets between the locations of the putative Arabia and Deuteronilus shorelines vary by an average of 141 ± 142 km and 180 ± 177 km, respectively. This leads to mean elevations of the Arabia Level that vary by up to 2.2 km between different mappings, and topographic ranges within each global mapping ranging from 2.7 to 7.7 km. The younger Deuteronilus Level has less topographic variation as it largely follows a formal contact (the Vastitas Borealis Formation) within the relatively flat northern plains. Given the high variance in position (spatial and topographic) of the maps, the use of such data and conclusions based on them are potentially problematic. Plain Language Summary: Whether oceans ever existed on Mars is controversial, with support largely coming from hypothesized ancient shorelines. As with modern shorelines on Earth, possible ancient martian shorelines are expected to be approximately level, but past studies found that the two main global shoreline mappings have elevation ranges from about one to several kilometers, respectively. Here, we remap segments of the proposed shorelines based on their original geomorphic definitions and find that modern maps vary laterally by hundreds of kilometers from our segments mapped using higher‐resolution data. Additionally, we compare maps of potential shorelines over time. We find that maps are both inconsistent and inaccurate with their placement of hypothesized shorelines. Lateral offsets between different maps locally exceed a thousand kilometers. This disagreement with the poorly understood location of the potential shorelines can explain, in part, the observed elevation differences. Our results question the usefulness of putative shorelines as evidence for ancient martian oceans and implies the need for more detailed, revised mappings and scrutiny. Key Points: Remapping segments of the putative Mars shorelines finds modern interpolated maps diverge up to 500 km from original geomorphic mapsPublished maps of the Arabia and Deuteronilus Levels have similar mean lateral offsets of 140 and 180 km with 1,000 km max offsetsA large portion of the topographic disparity of the Arabia Level may be explained through these inconsistent mappings over time [ABSTRACT FROM AUTHOR]

Whether extant life exists in the martian subsurface is an open question. High concentrations of photochemically produced CO and H2 in the otherwise oxidizing martian atmosphere represent untapped sources of biologically useful free energy. These out-of-equilibrium species diffuse into the regolith, so subsurface microbes could use them as a source of energy and carbon. Indeed, CO oxidation and methanogenesis are relatively simple and evolutionarily ancient metabolisms on Earth. Consequently, assuming CO- or H2-consuming metabolisms would evolve on Mars, the persistence of CO and H2 in the martian atmosphere sets limits on subsurface metabolic activity. In this study, we constrain such maximum subsurface metabolic activity on Mars using a one-dimensional photochemical model with a hypothetical global biological sink on atmospheric CO and H2. We increase the biological sink until the modeled atmospheric composition diverges from observed abundances. We find maximum biological downward subsurface sinks of 1.5 × 108 molecules/(cm2·s) for CO and 1.9 × 108 molecules/(cm2·s1) for H2. These convert to a maximum metabolizing biomass of ≲1027 cells or ≤2 × 1011 kg, equivalent to ≤10−4–10−5 of Earth's biomass, depending on the terrestrial estimate. Diffusion calculations suggest that this upper biomass limit applies to the top few kilometers of the martian crust in communication with the atmosphere at low to mid-latitudes. This biomass limit is more robust than previous estimates because we test multiple possible chemoautotrophic ecosystems over a broad parameter space of tunable model variables using an updated photochemical model with precise atmospheric concentrations and uncertainties from Curiosity. Our results of sparse or absent life in the martian subsurface also demonstrate how the atmospheric redox pairs of CO-O2 and H2-O2 may constitute antibiosignatures, which may be relevant to excluding life on exoplanets. [ABSTRACT FROM AUTHOR]

Mars today has no active volcanism and its atmosphere is oxidizing, dominated by the photochemistry of CO 2 and H 2 O. Mars experienced widespread volcanism in the past and volcanic emissions should have included reducing gases, such as H 2 and CO, as well as sulfur-bearing gases. Using a one-dimensional photochemical model, we consider whether plausible volcanic gas fluxes could have switched the redox-state of the past martian atmosphere to reducing conditions. In our model, the total quantity and proportions of volcanic gases depend on the water content, outgassing pressure, and oxygen fugacity of the source melt. We find that, with reasonable melt parameters, the past martian atmosphere (∼3.5 Gyr to present) could have easily reached reducing and anoxic conditions with modest levels of volcanism, >0.14 km 3 yr −1 , which are well within the range of estimates from thermal evolution models or photogeological studies. Counter-intuitively we also find that more reducing melts with lower oxygen fugacity require greater amounts of volcanism to switch a paleo-atmosphere from oxidizing to reducing. The reason is that sulfur is more stable in such melts and lower absolute fluxes of sulfur-bearing gases more than compensate for increases in the proportions of H 2 and CO. These results imply that ancient Mars should have experienced periods with anoxic and reducing atmospheres even through the mid-Amazonian whenever volcanic outgassing was sustained at sufficient levels. Reducing anoxic conditions are potentially conducive to the synthesis of prebiotic organic compounds, such as amino acids, and are therefore relevant to the possibility of life on Mars. Also, anoxic reducing conditions should have influenced the type of minerals that were formed on the surface or deposited from the atmosphere. We suggest looking for elemental polysulfur (S 8 ) as a signature of past reducing atmospheres. Finally, our models allow us to estimate the amount of volcanically sourced atmospheric sulfate deposited over Mars’ history, approximately ∼10 6 –10 9 Tmol, with a spread depending on assumed outgassing rate history and magmatic source conditions. [ABSTRACT FROM AUTHOR]