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

Abundant evidence for flowing water on early Mars has led many researchers to conclude that the Noachian climate may have been "warm and wet", characterized by global mean annual temperature (MAT) above the melting point of water and abundant rainfall infiltrating into the groundwater system. In this "warm and wet" scenario, the groundwater system could be in contact with the surface, leading to the formation of a groundwater-fed ocean in the northern lowlands (NL). However, global climate models (GCMs) are unable to reproduce "warm and wet" conditions due to the faint young Sun; instead, they predict that the climate was "cold and icy", characterized by MAT 225 K and an adiabatic cooling effect which resulted in water being distributed as ice in the highlands. A kilometers-thick globally continuous cryosphere is predicted in this scenario, precluding direct connection of the groundwater system to the surface and disfavoring a Noachian NL ocean.

However, global MAT does not describe variations in time and space, such as temperature variations with season, latitude and elevation. Simulations of the early climate have shown that the lowest elevation regions, such as in the Hellas basin and Utopia Planitia, can have local MAT much higher than global MAT. Thus, we re-examine whether globally-averaged long-lived, sub-freezing conditions (global MAT <273 K) preclude an ocean. In this contribution, we use the 3D LMD GCM and a basic thermal model to ask: What is the coldest possible global MAT at which groundwater release could occur? Where would it occur? What subsurface water volumes and timescales would be required? What might be the lifetime and fate of such bodies of water?

We find that groundwater release is possible in a globally sub-freezing climate. If global MAT was > 255 K and the subsurface contained >63 m GEL water, groundwater could be released onto the surface and pond within the Hellas basin. If global MAT was > 258 K and the subsurface contained >170 m GEL water, an additional groundwater-fed body of water would form in the NL at Utopia. An ocean sourced at Utopia would have been preceded by and concomitant with a sea in Hellas. The ocean would remain ice-free for a geologically short period of time, possibly only a few hundred days, and would completely freeze and sublime in only a few thousand years.