Oceans on Mars: The possibility of a Noachian groundwater-fed ocean in a sub-freezing martian climate

摘要

The Noachian climate of Mars is thought by many to have been "warm and wet", characterized by a global mean annual temperature (MAT) > 273 K and abundant rainfall infiltrating into the groundwater system. In this "warm and wet" scenario, when the groundwater system was in contact with the surface in the northern lowlands, a groundwater-fed ocean could form. In contrast, 1-dimensional climate models and 3-dimensional general circulation models (GCMs) are unable to reproduce long-lived, continuous "warm and wet" conditions due to the influence of the faint young Sun when considering a pure CO2-H2O atmosphere; these models instead predict "cold and icy" conditions, characterized by MAT similar to 225 K and water trapped as snow and ice in the southern highlands. Furthermore, a kilometers-thick globally continuous cryosphere is predicted in this "cold and icy" scenario, precluding direct connection of the groundwater system and the surface, and disfavoring a Noachian northern lowlands ocean. However, global MAT values lack information about regional and seasonal temperature variations, which may vary significantly from global MAT. In this work, we explore whether regional or seasonal temperature variations could permit the formation of a Noachian ocean in a climate with global MAT < 273 K. We use a combination of a 3D climate model and a 1D thermal model to determine the climatic conditions in which the required criteria for groundwater release could be met: (1) temperatures above 273 K, required for there to be no cryosphere preventing the groundwater from being released onto the surface, and (2) a sufficiently large groundwater reservoir, required for the groundwater table to intersect the surface. Our results suggest that the formation of a groundwater-fed ocean does not require a continuous and long-lived "warm and wet" climate with global MAT > 273 K. If the long-lived Noachian global MAT was > 255 K (similar to 30 K above the nominal ambient "cold and icy" climate