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.

Hundreds of ancient palaeolake basins have been identified and catalogued on Mars, indicating the distribution and availability of liquid water as well as sites of astrobiological potential. Palaeolakes are widely distributed across the Noachian aged terrains of the southern highlands, but Arabia Terra hosts few documented palaeolakes and even fewer examples of open-basin palaeolakes. Here we present a detailed topographic and geomorphological study of a previously unknown set of seven open-basin palaeolakes adjacent to the planetary dichotomy in western Arabia Terra. High resolution topographic data were used to aid identification and characterisation of palaeolakes within subtle and irregular basins, revealing two palaeolake systems terminating at the dichotomy including a ~160 km chain of six palaeolakes connected by short valley segments. Analysis and correlation of multiple, temporally distinct palaeolake fill levels within each palaeolake basin indicate a complex and prolonged hydrological history during the Noachian. Drainage catchments and collapse features place this system in the context of regional hydrology and the history of the planetary dichotomy, showing evidence for the both groundwater sources and surface accumulation. Furthermore, the arrangement of large palaeolakes fed by far smaller palaeolakes, indicates a consistent flow of water through the system, buffered by reservoirs, rather than a catastrophic overflow of lakes cascading down through the system.

There is conflicting evidence for an ancient ocean which occupied the northern hemispheric basin on Mars. Along different regions of the dichotomy boundary, sediment fans have been interpreted as either forming into a large water body or a series of smaller paleolake basins. Here, we investigate fluvial systems in the Memnonia Sucli region of Mars, set along the dichotomy, which comprise erosional valley networks, paleolake basins, inverted channel systems, and sediment fans. We focus our analysis on the evolution of the upslope catchment and characterizing the ancient environment of a large, downslope basin, bound by the topographic dichotomy and the Medusae Fossae Formation. The catchment fluvial systems comprise highly degraded valley networks and show a complex history of incision and filling, influenced by paleolake basin overflow, impact crater damming, aggradation, and possibly a downstream water body. The morphology of the sediment fans is consistent with either fluvial fans or deltas and they form at discrete elevations, rather than a common elevation plane. Our analysis is consistent with the sediment fans forming into a series of paleolake basins set along the dichotomy, rather than into a large inner sea or ocean-sized water body. The fluvial systems were likely active between the mid Noachian and early Hesperian periods. Our results demonstrate the complex, multi-phase evolution of fluvial systems on ancient Mars and highlight the importance of regional and local studies when characterising ancient regions of the dichotomy.