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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. Key Points: Several locations on Mars have been proposed to have undergone groundwater release to the surface in the Noachian/HesperianWe estimate the climate, subsurface water inventory, and duration of warm conditions required for such groundwater releaseRequirements for release are global mean annual temperature >284 K for approximately thousands of years and >496 m global equivalent layer of groundwater [ABSTRACT FROM AUTHOR]

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 CO 2 -H 2 O atmosphere; these models instead predict "cold and icy" conditions, characterized by MAT ~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 (~30 K above the nominal ambient "cold and icy" climate) and the subsurface contained >63 m global equivalent layer (GEL) of groundwater, then groundwater could be released onto the surface and pond on the floor of the Hellas basin; global MAT >258 K and a subsurface water inventory of >170 m GEL are required for release of groundwater into the topographically lowest regions of Utopia Planitia. If a subsurface water reservoir of ≥770 m GEL existed and local MAT was ≥273 K at Utopia, groundwater release could occur at Utopia and lead to flooding of the northern lowlands and formation of an ocean up to the level of Contact 1. Further, in a "cold and icy" MAT 225 K climate, short-term punctuated heating that raised global MAT to >255 K for at least a few thousand years could lead to groundwater release and flooding of the lowest points on Mars. Under the influence of these very cold climates (global MAT ~255–260 K), these bodies of water 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. • A groundwater-fed ocean may have existed in the Noachian. • We explore the required climatic conditions for formation of the Noachian ocean. • The ocean could have formed in a globally sub-freezing Noachian climate. [ABSTRACT FROM AUTHOR]

Pit-floored craters (PFCs) have been observed in the highlands surrounding the Hellas impact basin on Mars. The pits consist of large irregularly shaped depressions (∼10s of km wide and ∼100s of m deep), often with steep layered walls and terraced interiors. We analyze a previously recognized population of circum-Hellas PFCs and nearby layered terrains using high-resolution visible imagery and stereo topography data and compare them to current models of Noachian–Hesperian geologic and climatic evolution. We find little evidence for regionally integrated fluvial runoff or crater rim breaching; crater interiors instead display fine-scale layering potentially related to pyroclastic, aeolian, or glacial deposition. Steep pit scarps, terraced pit floors, and a lack of traditional sediment transport pathways implicate sublimation of ice and removal into the atmosphere as the major pit-forming process. Radiating ridges interpreted as inverted fluvial channels emanating from raised pit boundaries further suggest water ice as a dominant crater floor fill material. Latitudinal distinctions in PFC morphology and evidence for both recent and past crater mantling and degradation events suggest a climate-controlled origin and evolution of circum-Hellas layered terrains dominated by the emplacement and removal of snow and ice. • Pit-floored craters (PFCs) and layered terrains are observed around Hellas basin. • Layered PFC walls are potentially related to ancient ice and dust deposition. • PFCs were likely formed through sublimation of ice to the atmosphere. • Origin, evolution of circum-Hellas layered terrains was modulated by early climate. [ABSTRACT FROM AUTHOR]