The surface of Mars is currently cold and very dry, conditions that are inhospitable to life as we know it. This may not have always been the case, however. A key question in martian history is whether Mars has always been cold and dry or if it once was warm and wet. Based on semi-global exposures of possible paleo shorelines, low roughness of the surface, purported tsunami deposits, and fractal analysis of topography, it has been suggested that Mars hosted an ocean in the northern lowlands in its deep past. However, concurrent with evidence of this northern ocean, there appear to be widespread glacial deposits throughout the southern highlands. Consequently, serious questions remain about the longevity of the putative ocean, what sorts of global climatic conditions could have supported an ocean, and how Mars could exhibit both glacial and fluvial deposits contemporaneously.

To address these questions, Frédéric Schmidt (Université Paris-Saclay), Michael Way (NASA Goddard Institute for Space Studies), and colleagues use three-dimensional models for global circulation of the coupled ocean and atmosphere to simulate conditions of early Mars. These models include both hydrological cycles and simplified glacial advance/retreat schemes, allowing for land-based glaciation and sea ice development. Results indicate that a global martian ocean can be stable despite mean surface temperatures falling below 0°C (the freezing point of water at sea level on Earth) if the atmosphere is at 1 bar total pressure (equivalent to the pressure of Earth’s atmosphere), is dominated by carbon dioxide, and contains 10% hydrogen_, a potent, though short-lived, greenhouse gas. While there may be moderate rainfall near the ocean, the southern highlands remain glaciated. This dichotomy of warm/wet northern lowlands and cold/dry southern highlands matches geologic observations of both fluvial and glacial landforms, including tsunami deposits, consistent with observed martian geology. READ MORE