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It has been proposed that groundwater was released onto the early martian surface in several locations, including Meridiani Planum, NE Hellas, Terra Sirenum, the northern lowlands, and Gale crater. We assume that groundwater was released in a manner similar to a groundwater-fed lake on Earth and then use a combination of a global climate model and mathematical relationships to place lower limit estimates on the early climate and subsurface water inventory that are required for groundwater release to occur at all of these locations. We find that the minimum required conditions are global mean annual temperature >284 K and a subsurface water inventory >496 m global equivalent layer. Additionally, thermal models predict that if such warm conditions occurred from punctuated heating in an otherwise "cold and icy" climate, the warm conditions must have persisted for thousands of years in order to remove the cryosphere and permit groundwater release.

Plain Language Summary Geologic evidence has been interpreted to mean that groundwater was released from the subsurface to the early martian surface at multiple locations. Groundwater release requires both a warm climate and sufficient water in the subsurface. In this work, we estimate the climate and subsurface water inventory that would be required for groundwater to be released at these locations. The plausibility of the predicted climate and subsurface water inventory can act as a test for these groundwater-related interpretations. We find that early Mars would be required to have had average global temperatures similar to present-day Earth and a very large subsurface water inventory.