Hence at 3 Ga the flux at Mars was set to 452.8 W We assume the solar luminosity to be ∼79% ( 25) of its current value. R3D allows us to estimate the interaction between atmosphere and ocean circulation but also encompasses a surface hydrological scheme. We present fully coupled ocean/atmosphere 3D-GCM simulations based on ROCKE-3D (Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics, hereafter R3D) (23), which is based upon a parent Earth climate model known as ModelE2 ( 24). The Lomonosov crater morphology is coherent with an impact in shallow water, at the very same age as the tsunami deposits ( 13), and it is thus the most probable source. Along this shoreline, candidate tsunami deposits have been identified at an age of 3 Ga ( 11, 12) with at least two impact events. Detailed studies in Kasei Valles imply that such an ocean rose in elevation (∼1,000 m) between ca. Crater count dating of the Vastitas Borealis Formation near Deuteronilus Mensae is 3.5 Ga ( 9) but the ocean may have been more recent. A northern ocean is also supported by specific radar properties ( 6), smooth surface roughness ( 7), and a fractal analysis of the topography ( 8). The Deuteronilus shoreline seems to have formed during the last stage of the true polar wander induced by Tharsis ( 5). There is evidence of Martian paleoshorelines ( 3) in Deuteronilus Mensae (sometimes noted contact 2) in a geometry closely corresponding to the current equipotential height ( 4). A recent review ( 2) discusses this controversy. The possibility of a late Martian ocean is a topic of debate with strong implications on the habitability of the Red Planet ( 1). Under this scenario of 3 Ga, the geologic evidence of a shoreline and tsunami deposits along the ocean/land dichotomy are compatible with ice sheets and glacial valleys in the southern highlands. This climate is achieved with a 1-bar CO 2-dominated atmosphere with 10% H 2. The southern plateau is mostly covered by ice with a mean temperature below 0 ☌ and a glacier return flow back to the ocean. Rainfall is moderate near the shorelines and in the ocean. Using an advanced general circulation model (GCM), we demonstrate that an ocean can be stable, even if the Martian mean surface temperature is lower than 0 ☌. Here, we provide insights from numerical climate simulations in agreement with surface geological features to demonstrate that the Martian climate could have been both cold and wet. But this would prevent tsunami formation, for which there is some evidence. A moderate cold climate would have transferred the water from the ocean to the land in the form of snow and ice. A too cold climate would have kept any northern ocean frozen most of the time. A warm and wet climate would have produced extensive fluvial erosion but few valley networks have been observed at the age of the Late Hesperian. What was the nature of the Late Hesperian climate, warm and wet or cold and dry? Formulated this way the question leads to an apparent paradox since both options seem implausible.
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