Most previous studies on Mars relevant groundwater have proposed models, but few have looked at the geological evidence of groundwater upwelling in deep closed basins in the northern hemisphere equatorial region. Geological evidence of groundwater upwelling in these deep basins is a key point that will help to validate present‐day models and to better constraint them in the future. Observations in the northern hemisphere show evidence of a planet‐wide groundwater system on Mars. The elevations of these water‐related morphologies in all studied basins lie within the same narrow range of depths below Mars datum and notably coincide with the elevation of some ocean shorelines proposed by previous authors.

The scale of groundwater upwelling on Mars, as well as its relation to sedimentary systems, remains an ongoing debate. Several deep craters (basins) in the northern equatorial regions show compelling signs that large amounts of water once existed on Mars at a planet‐wide scale. The presence of water‐formed features, including fluvial Gilbert and sapping deltas fed by sapping valleys, constitute strong evidence of groundwater upwelling resulting in long term standing bodies of water inside the basins. Terrestrial field evidence shows that sapping valleys can occur in basalt bedrock and not only in unconsolidated sediments. A hypothesis that considers the elevation differences between the observed morphologies and the assumed basal groundwater level is presented and described as the “dike‐confined water” model, already present on Earth and introduced for the first time in the Martian geological literature. Only the deepest basins considered in this study, those with bases deeper than −4000 m in elevation below the Mars datum, intercepted the water‐saturated zone and exhibit evidence of groundwater fluctuations. The discovery of these groundwater discharge sites on a planet‐wide scale strongly suggests a link between the putative Martian ocean and various configurations of sedimentary deposits that were formed as a result of groundwater fluctuations during the Hesperian period. This newly recognized evidence of water‐formed features significantly increases the chance that biosignatures could be buried in the sediment. These deep basins (groundwater‐fed lakes) will be of interest to future exploration missions as they might provide evidence of geological conditions suitable for life.

1 Introduction Early in its history Mars was characterized by an active hydrosphere (Baker, 2001; Carr & Head, 2003, 2010; Fairen et al., 2003; Head et al., 1999; Kreslavsky & Head, 2002; Malin & Edgett, 1999; Parker et al., 1993; J. A. P. Rodriguez, Zarroca et al., 2015; J. A. Rodriguez et al., 2016; Tanaka et al., 2005), followed by the retreat of surface water into subsurface aquifers underneath a thick ice‐rich permafrost zone (Clifford & Parker, 2001) where it became part of groundwater stores (Andrews‐Hanna et al., 2007). Groundwater could have been a major factor in the origin and evolution of water‐related sedimentary systems on Mars (Andrews‐Hanna et al., 2007, 2010; Goldspiel & Squyres, 2000; Howard, 1986; Malin & Carr, 1999; Siebach et al., 2014; Stack et al., 2014). Treiman (2008) found evidence of groundwater flow during the Hesperian in Valles Marineris, and Andrews‐Hanna et al. (2010) presented a model showing that the playa environment in Meridiani Planum was controlled by a zone of groundwater upwelling on a regional scale, occurring first in the deepest basins. According to their model, regional‐ to global‐scale groundwater flow might explain the formation and location of the light‐toned layered deposits (LLDs) in Arabia Terra in a playa depositional setting. The existence of extensive groundwater aquifers in Arabia Terra (Michalski et al., 2013; Pondrelli et al., 2015; J. A. P. Rodriguez, Zarroca et al., 2015) and Meridiani Planum (Grotzinger et al., 2005; Squyres et al., 2009) is also supported by the presence of spherical hematite inclusions informally called “blueberries,” discovered by the Mars Exploration Rover Opportunity in Meridiani Planum (Squyres et al., 2004), as well as the sulfate mineralogy of the cements and the presence of current ripples in the upper Burns formation (Chan et al., 2004; Lamb et al., 2012). In recent years, several authors (Cabrol & Grin, 1999; De Hon, 1992; Goudge et al., 2015; Scott et al., 1995, 1991) presented catalogues of closed basins across the surface of Mars, but they did not include candidate groundwater‐fed closed‐basin lakes (Goudge et al., 2015), despite that they represent an important component of the paleolake record on Mars (Michalski et al., 2013; Wray et al., 2011). Furthermore, previous surveys of ancient deep craters have shown no evidence for widespread groundwater upwelling. Based on their analysis, Michalski et al. (2013, 2012) raised several arguments against a global‐scale groundwater upwelling scenario because of the lack of extensive groundwater‐driven sedimentation (e.g., LLD) inside these deep basins and the deposits' age (Michalski et al., 2013). In contrast with the interpretation of Andrews‐Hanna et al. (2010) regarding regional to global groundwater activity, Michalski et al. (2013) analyzed 95 large basins in the northern hemisphere and inferred that groundwater upwelling on Mars may have occurred sporadically and locally in basins deeper than −4,000 m, such as McLaughlin crater. According to Michalski et al. (2013), this activity would have predated the groundwater upwelling related deposition proposed by Andrews‐Hanna et al. (2010) because 80% of the basins that were analyzed in the northern hemisphere impacted Noachian terrain. However, the ages of these 95 craters were not investigated in detail and the study only used the global superordinate stratigraphic units (Noachian, Hesperian, and Amazonian), considering the craters found within these units to be the same age as the units. This is arguably a misconception however, as every geomorphological unit includes craters formed at a later time. This means that the Noachian unit includes numerous craters of Hesperian and Amazonian age as well. Furthermore, the authors did not find all of this geomorphological and sedimentological evidence of groundwater upwelling and standing bodies of water within the studied basins, with the exception being McLaughlin crater.This study aims to present new evidence that explains the nature and origin of the deposits and morphologies inside these deep craters, as well as their spatial distribution, to infer the controls on deposition, and establish their global significance. In order to understand if groundwater influences are expressed on a local, regional, or global scale, we focus our detailed geological analyses on the structure and stratigraphy of enclosed craters located in the northern equatorial regions, near the dichotomy boundary with floors below −4,000‐m elevation. We have only considered and analyzed enclosed basins that were not fed externally, so as to ensure that all processes within the craters are related to groundwater alone. The studied craters show little or no evidence of aqueous activity in the terrains surrounding their outer rims, so they are not fed by surface drainage. In fact, they lack breached rims, with the exception of short sapping valleys that are a result of retrograding erosion. All the selected basins are located between 0°N and 37°N to limit the effects of current day ice on the observed morphological forms and deposits as the north cap reaches about 37°N at its maximum extent (Michaux & Newburn, 1972; Slipher, 1962). We focused on the northern hemisphere because this is where a large fraction of groundwater upwelling is predicted to have occurred (Andrews‐Hanna et al., 2007, 2010; Michalski et al., 2013). Our work reveals evidence of some previously unrecognized water‐formed features that have never been considered holistically, from the scale of the individual basin to the planet‐scale context. These features are likely to have resulted from a process of groundwater upwelling followed by water table fluctuations and eventual groundwater recession. The presence of a large number of water‐formed features in all these deep basins (Figure 1) is a compelling sign that Mars once had large amounts of water stored as groundwater that debouched into the intercepting craters to form lacustrine systems (Palucis et al., 2016). The purpose of this work is to identify these water‐related features at a basin level, define their depositional environments and then analyze them holistically at a planetary scale to identify whether they are the result of a single planet‐wide control (Figure 1). Figure 1 Open in figure viewer PowerPoint Distribution of the studied basins on Mars based on Mars Orbiter Laser Altimeter topography (blue indicates high elevations). Their distribution follows the dicotomy boundary, and they are clustered in the Arabia and Amazonia quadrangles along two parallel lines aligned NNE‐SSW in the first case and NNW‐SSE, respectively, with high concentration in Arabia Terra and Amazonia. The geological evidence presented in this work corroborates the Andrews‐Hanna et al. (2010) theory of global‐scale groundwater upwelling and supports the model presented by Michalski et al. (2013) but contests the assertion that McLaughlin crater is the only basin presenting geological evidence of groundwater upwelling. Although they focused on McLaughlin crater due to the strong spectroscopic evidence for upwelling, the authors also suggest there could be other craters presenting geomorphological evidence of groundwater upwelling that were not investigated in their work Michalski et al. (2013). If life once existed on Mars, it could have been preferentially confined to some protective water related niches (e.g., Carrozzo et al., 2017; Hamilton et al., 2018). Evidence of the past existence of long‐standing bodies of water on Mars, such as lakes or deltas, has significant implications both for climate and life: groundwater‐fed lakes could warm Mars' climate (Tosca et al., 2018) and increase the chance that life forms might have existed and presently remain buried in the sediment. These deep basins (due to a lower gravity that implies less compaction of the pore space and lower heat flow that reduces the temperature constraints, see Michalski et al., 2013) arguably offer the best chance of finding evidence of past prebiotic chemistry or even past microbial life on the Red Planet.

2 Methodology Basins are repositories of sediments that store information and aid interpretations of the geological history because they provide us with a window into the past. We focused on the identification and analysis of water‐related features in selected deep closed basins and then extended the analysis to a regional level to arrive at a holistic interpretation of the groundwater processes that could have taken place on Mars. We investigated the geology of deep impact craters in the northern hemisphere of Mars, between 0°N and 37°N, using imaging data from High Resolution Imaging Science Experiment (HiRISE; McEwen et al., 2007), Context Camera (CTX; Malin et al., 2007), High Resolution Stereo Camera (HRSC; Jaumann et al., 2007; Neukum & Jaumann, 2004), and the digital terrain model from HRSC and Mars Orbiter Laser Altimeter (MOLA; D. E. Smith et al., 1999) data. The sparse distribution of hyperspectral observations, such as those made by the Compact Reconnaissance Imaging Spectrometer for Mars (Murchie et al., 2007), does not allow the systematic use of mineralogy as a tool to interpret the depositional processes/environments. Still, where published data were available, they have been considered to strengthen the genetic inferences. Deep craters (typically diameters of 28 to 114 km; depths of 1.4 to 3.1 km and floor elevations between −4,215 and −5,770 m below the Mars datum) were identified using a database of all Martian craters published by Robbins and Hynek (2012) and integrated with further observations. We focused on the northern hemisphere because this is where a large fraction of groundwater upwelling is predicted to have occurred (Andrews‐Hanna & Lewis, 2011; Andrews‐Hanna et al., 2007, 2010; Michalski et al., 2013). We selected craters with floor depths less than −4,000 m below the Mars datum; in particular, we selected craters below 37°N to limit ice noise or deposits associated with it, as the north cap reaches about 37°N at its maximum extent (Michaux & Newburn, 1972; Slipher, 1962). Furthermore, this was done to avoid confusion with landforms on Mars that are morphologically similar to terrestrial glacial and periglacial landforms occurring in the midlatitudes, between about 35°N and 60°N (Hauber et al., 2011, and references therein). Moreover, based on current surface conditions, the location of a subpermafrost groundwater table is presumed to occur below the subsurface melting isotherm estimated by Clifford and Parker (2001), which is <5 km at 30° latitude. We looked for evidence of intracrater fluvial activity and lacustrine deposits. In particular, we searched for locations where evidence of aqueous activity was present in the crater but absent in the terrain outside the crater (i.e., the crater was not fed by surface drainage, excluding short sapping valleys). As we wanted to investigate craters that have most likely formed during episodes of groundwater upwelling, prior to the Amazonian and Late Hesperian, we restricted our study to regions determined to be of Noachian to Early Hesperian age in the global stratigraphic chart recently updated by Platz et al. (2013) and Tanaka et al. (2014). Although crater‐based absolute model age estimates for each of the 24 investigated craters could constrain the temporal occurrence of groundwater upwelling activity on Mars, we decided not to perform crater counts due to little prospects of deriving reliable age data. The reasons for our decision are explained in detail in the supplementary material. The identified 24 craters that fulfill our requirements are listed in Table 1. Table 1. List of the Studied Craters ID Crater name Latitude longitude Diameter (km) Floor elevation (m) Crater depth (km) Landslide (slump/debris flow/hummocky) Sapping Valley Delta (front elevation) Channel (base elevation) Shoreline (elevation) Fan (front elevation) Inverted morphology Flat floor Mineralogy/sedimentology CRISM/HRSC DEM Depositional inner crater slope Floor slope Delta surface (km2)/volume (km3) Slump (e.g., El Hierro) Debris flow Hummocky terrain 1 Nameless 1 27°54′19.36″N 69 −4,690 3.0 ✓ ✓ ✓ ✓(−4,000; stpd) ✓ Possible Phyllosilicates on fan (to check in detail) Yes/No 20% 0.32% 2.96/0.59‐2.65/0.53‐9.44/2.36‐6.88/1.38 11°29′42.67″E 2 Pettit 12°15′8.64″N 92.49 −5,770 3.1 ✓ ✓ ✓ ✓ Yes/Yes 20% 0.26% 173°51′58.60″O 3 Sagan 10°43′22.75″N 90.26 −4,927 2.9 ✓ ✓ ✓ ✓(−4,200) ✓(−4,600) ✓(−4,300) ✓ (Starting from −4,500) ✓ Yes/Yes 12% 0.09% 30°36′11.55″O 4 Nicholson 0°12′39.97″N 102.5 −5,124 3.1 ✓ ✓ ✓ ✓ ✓(−4,200) ✓(−4,200) ✓(−4,000) ✓ Yes/Yes 25% 0.27% 1/0.1 164°25′49.81″O 5 McLaughlin 21°54′28.66″N 90.92 −5,050 2.2 ✓ ✓ ✓ (−4,200 start −4,700 end) ✓(−4,400) ✓(partially) Clays + Carbonates Yes/Yes 24% 0.30% 22°21′16.17″O 6 Du Martheray 5°27′15.55″N 102 −4,753 2.3 ✓ ✓(−4,500) ✓(−4,000) ✓(−4,000 and −4,400/−4,500) ✓ Possible Phyllosilicates Yes/No 11,5% 0.16% 93°34′51.24″E 7 Nameless (Du Martheray) 6° 6′10.31″N 36.15 −4,575 2.1 ✓ ✓(−4,000/−4,400; stpd) ✓ Sedimentary deposits (floor) No/No 12,5% 0.28% 31/2.48 94° 0′38.44″E 8 Nameless 2 1°12′23.79″N 44 −4,215 2.7 ✓(−4,000) ✓ Possible Phyllosilicates Light toned layered deposits on the floor Yes/No 14% 0.21% 109/3.27 133°56′32.31″E 9 Nameless 3 (in Trouvelot) ex candidate Mars rover landing site 15°53′2.15″N 28 −4,415 1.4 ✓ ✓ ✓ ✓(−4,000; stpd) ✓(−4,200) ✓ Light toned layered deposits (floor) No/Yes 33% 0.24% 3.86/1 12°44′59.28″O 10 Nameless 4 15° 8′51.65″N 60 −4,275 2.4 ✓ ✓(−4,000) ✓(−4,000) ✓ Layered deposits (floor) No/Yes 18% 0.19% 26°43′13.10″O 11 Nameless 5 18°20′20.33″N 44 −4,912 2.2 ✓ ✓ ✓ ✓ ✓(−4,600) ✓(−4,000–4,700) ✓(−4,400) ✓ No/No 25% 0.12% 178°46′41.77″O 12 Nameless 6 (in Becquerel) 22° 5′3.78″N 53.8 −4,447 1.6 ✓(−4,100) ✓(−4,250) ✓ Layered deposits (floor) No/Yes 21% 0.05% 9.35/1.87‐1.91/0.28 8°15′17.00″O 13 Tombaugh 3°33′28.93″N 60.3 −4,542 2.2 ✓ ✓ ✓ ✓ ✓(−4,300) ✓(−4,400) ✓(−4,400) ✓ No/No 11% 0.32% 5.31/1.06 161°55′25.33″E 14 Nameless 7 12°19′50.23″N 68.5 −4,380 2.1 ✓(−4,200) ✓ ✓ Layered deposits (floor) No/Yes 15% 0.01% 34°31′1.21″O 15 Nameless 8 17°55′35.10″N 57 −5,103 2.0 ✓ ✓ ✓ ✓(−4,800, stpd) ✓(−4,400) ✓(−4,400) ✓ Layered deposits (floor) No/No 11% 0.18% 8.29/2.49‐4.9/0.68 169°42′49.89″O 16 Nameless 9 24° 2′59.53″N 46.3 −5,215 2.1 ✓ ✓ ✓ ✓(4,700; stpd) ✓(−4,700) ✓(−4,700) ✓(−4,700) ✓(Starting from −4,700) ✓ Layered deposits & erosional windows (floor) Yes/No 14% 0.12% 2.6/0.1 177°20′54.00″O 17 Mojave 7°29′2.78″N 58 −5,190 2.6 ✓(−4,600) ✓(−4,600) ✓ Yes/Yes 20% 0.11% 32°59′10.92″O 18 Eden Patera 33°46′9.22″N 52 −4,990 2.8 ✓ ✓ ✓(−4,300) ✓ ✓ ✓(partially) No/Yes 30% 0.26% 0.33/0.13 11° 3′26.75″O 19 Nameless 10 31°26′37.37″N 40.1 −5,167 2.2 ✓ ✓ ✓ ✓(−4,500; stpd) ✓ ✓(partially) No/No 28% 0.07% 6.4/2.51 12°55′55.55″O 20 Curie 28°46′59.38″N 114.1 −4,547 2.3 ✓(−4,000) ✓ ✓ ✓(−4,300) ✓(−4,400) ✓(−4,400) ✓ Layered deposits No/Yes 14% 0.14% 2.25/0.16 4°45′2.88″O 21 Nameless 11 10°13′3.21″N 45.4 −5,200 2.5 ✓(−4,000) ✓ ✓ ✓(−4,200) ✓(−4,700) ✓ Yes/No 20% 0.35% 0.37/0.008 94°18′16.00″E 22 Oyama 23°33′58.32″N 100 −4,358 2.1 ✓(−4,100) ✓(−4,000) ✓(−4,000) ✓(partially) Clay‐rich layers and layered deposits on the floor Yes/Yes 14% 0.07% 1.04/0.11 20° 6′43.20″O 23 Wahoo 23°14′0.36″N 62 −5,091 2.0 ✓(−4,200) ✓ ✓ ✓(−4800) ✓(−4,600) ✓(−4,300 and −4,600) ✓(−4,500) ✓ Possible Phyllosilicates (floor) Layered deposits Yes/Yes 17% 0.2% 0.35/0.07 33°40′53.76″O 24 Nameless 12 15°23′22.17″N 64.2 −4,520 2.1 ✓ ✓ ✓ ✓(−4,300) ✓ Remnant of megaslump on the floor No/No 28% 0.12% 178°26′21.82″O

4 Discussion The simultaneous presence of a variety of water‐formed features such as sapping valleys, deltas, channels, alluvial fans, landslides, flat floors, and exhumed channels are all found in close association in the studied craters. Taken together, these landforms suggest a formation in fluviolacustrine depositional settings. We further note a consistent range of elevation for all of these morphologies, as shown in Table 1 and particularly in Figure 11, which allows us to infer an associated (although fluctuating) water level. Figure 11 Open in figure viewer PowerPoint X axis shows the crater ID as in Table Y axis shows the elevation in meters. Most of the morphologies are between −4,000 and −4,400 m and in several cases they are superposed in the diagrams, indicating that they occurred at same elevation in the same basin. The solid line represents the elevation of the crater floors. This diagram synthetizes the purpose of this work: Our aim is to consider all of these water related morphologies holistically and in a planetary‐wide scale. Except for the delta top, which gives us a confident estimation of the water level within the basin, in some cases the morphologies present different elevations within the same basin; this is due to the fact that they register the last stage of life of these basins. Morphologies developed at different stages, registering the water level fluctuations and/or drop. Elevations of most of the morphologies described in the main text and listed in detail in Table 1 . Theaxis shows the crater ID as in Table 1 , and theaxis shows the elevation in meters. Most of the morphologies are between −4,000 and −4,400 m and in several cases they are superposed in the diagrams, indicating that they occurred at same elevation in the same basin. The solid line represents the elevation of the crater floors. This diagram synthetizes the purpose of this work: Our aim is to consider all of these water related morphologies holistically and in a planetary‐wide scale. Except for the delta top, which gives us a confident estimation of the water level within the basin, in some cases the morphologies present different elevations within the same basin; this is due to the fact that they register the last stage of life of these basins. Morphologies developed at different stages, registering the water level fluctuations and/or drop. What emerges from Figure 11 is that most morphologies (including delta) oscillate between −4,000 and −4,500 m, which is a notably limited range given the distribution of various sites throughout the northern equatorial hemisphere. For some craters the symbols in Figure 11 overlap each other to indicate that those morphologies in that site are at the same elevation. The study, and in particular the holistic consideration of all these laterally related morpholiges in a landscape containing different depositional environments, supports the idea that in the past these craters contained water that progressively receded leaving behind landforms in a specific chronological order. Nevertheless, our research still leaves some questions open, which we will discuss below, including the following: Is seepage or is water flow erosion the more likely mechanism for valley formation? Is there any inconsistency between the lack of sediment in the deep basins and groundwater upwelling? What was the main control on deposition, groundwater and/or precipitation? 4.1 Sapping Valleys Genesis The distinction between seepage and water flow erosion in valley formation is subtle. The valleys analyzed in this work have roughly uniform widths and show steep amphitheater heads. According to Sharp and Malin (1975), amphitheater‐headed valleys are a common indication of groundwater‐seepage erosion and, more recently, Harrison and Grimm (2005) interpreted these valleys as markers of an increased relative contribution of groundwater to erosion from the late Noachian into the Hesperian period. Nonetheless, other formation mechanisms of these valleys cannot be excluded on the basis of their morphology alone. Several mechanisms such as groundwater seepage (Goldspiel & Squyres, 2000; Howard, 1986; Kochel & Piper, 1986; Malin & Carr, 1999; Schumm et al., 1995), overland flow (Lamb et al., 2008; Lapotre & Lamb, 2018), and combinations of the two (Amidon & Clark, 2015; Laity & Malin, 1985; Pelletier & Baker, 2011) have been suggested to explicate the genesis of amphitheater‐shaped valleys both on Earth and Mars. Pelletier and Baker (2011) applied numerical models to show that amphitheater headwalls are diagnostic of seepage erosion. Nevertheless, more recently Lapotre and Lamb (2018) proposed that valley formation by seepage erosion can only occur in sand because of its relatively unique mobility and permeability properties, which is consistent with experimental observations (Howard & Mclane, 1988; Marra et al., 2014). In competent rock, the authors find that a more plausible scenario for large valleys is erosion by overland water flows. Lapotre and Lamb (2018) while reporting in their paper evidence of groundwater‐seepage valleys engraved in the volcanic bedrock, they ignore terrestrial field evidence of groundwater upwelling and sapping valleys carved in volcanic consolidated deposits such as those reported by several authors (Izuka et al., 2018; Kochel & Piper, 1986; Macdonald et al., 1983; Stearns, 1966). Porosity and permeability drive the groundwater occurrence and movement within rocks. Basalts generally have moderate porosities (10–30%) but include some of the most permeable formations on Earth (Macdonald et al., 1983). Basalt's permeability is mostly due to its internal structures such as clinker layers in “aa” lava, lava tubes in pahoehoe, irregular openings between flow surfaces, gas vesicles, and vertical contractions joints formed by cooling lavas (Peterson, 1972). On Earth, such as in Hawaii, for example, sapping valleys form even when the basal groundwater does not reach the valleys' head. This is due to two different processes, the so‐called “high level groundwater” and the more important “dike‐confined water” (Izuka et al., 2018; Macdonald et al., 1983). The high‐level groundwater forms when the descending water reaches an impermeable bed (perched groundwater) such as dense lava or volcanic ash. Where erosional valleys cut across the perching members, the perched water may run out. In the presence of numerous dikes, which constitute nearly vertical and less permeable walls, the water accumulates in compartments of more permeable rocks between the dikes as it rises in level. In some dike complexes on Earth, water is held between the dikes to a height of more than 600 m. This is the case of dike‐confined water, more relevant for the Martian case (see Figure 2 for graphical explanation; please compare to terrestrial field example by Macdonald et al., 1983, Figure 11d). The valleys that tap these aquifers incise at accelerated rates compared to valleys fed only by runoff. Sapping valleys evolve by gradual headward growth, maintaining a steep theater head. Sapping valleys combined with efficient fluvial transport in the channels can accumulate sediment at their mouth (Kochel & Piper, 1986). It is very likely that the hidden Martian stratigraphy is mainly composed by basaltic lava flows, favoring the presence of dike‐confined water as well as “perched water.” Evidence of Martian Hesperian age basaltic lava flows associated to subvertical (cooling) joints carved by valleys were already shown by Mangold et al. (2008), and the deep Martian stratigraphy is still unknown. Since only a few valleys on Earth (e.g., Schumm et al., 1995) are thought to be affected exclusively by seepage erosion (e.g., Lamb et al., 2006) and the component of water flow erosion cannot be excluded, we believe that a composite discharge model is a plausible formation mechanism for the amphitheater‐headed valleys studied in this work. There is no evidence of large surficial floods in the intercrater plains around the craters or of flows cutting the craters from outside, as also suggested by Amidon and Clark (2015). Furthermore, a composite discharge model (overland flow and groundwater) is also consistent with the model proposed by Andrews‐Hanna et al. (2007, 2010) and terrestrial field observation (Kochel & Piper, 1986). The composite discharge model for amphitheater‐headed valleys also has important implications for the Martian hydrologic cycle. It implies that valleys could be formed by groundwater discharge on a more rapid time scale than required by a seepage weathering mechanism (Lamb et al., 2006). It also suggests that the lower latitude valleys studied in this work could potentially be attributed to groundwater recharge or from nearby craters (Amidon & Clark, 2015). 4.2 Lack of Sediment Inside Deep Basins The lack of sediment does not imply lack of geological events: stratigraphic record is “msore gap than record,” as famously stated by Ager (1981) and agreed by (Dott, 1983). Especially within a hydrologically closed basin the volume of sediments available may not be high. Processes such as the ones recorded in closed lacustrine systems controlled by groundwater upwelling could occupy most of the geological time but may contribute volumetrically far less sediment to the sedimentary record, compared to more catastrophic events (e.g., sediment gravity flow and flash floods; Ager, 1986; Miall, 2016; Sadler, 1981). More recently, Kemp and Sadler (2014) demonstrated that the number and duration of intervals of nondeposition or erosion increases over time. In stratigraphy it is also now recognized that the duration of the gaps, the distribution of layer thicknesses and the sedimentation rate have fractal properties ( Bailey & Smith, 2005; Plotnick, 1986; Sadler, 1999; Schlager, 2004; D. G. Smith et al., 2015). Furthermore, a recent Martian model proposed by Horvath and Andrews‐Hanna (2017) predicts the presence of numerous small lakes around Gale crater even if there are no sediments preserved within them or other geological evidence that supports past groundwater upwelling events. Some of the craters studied in this work record a limited amount of deposits (i.e., not comparable with the layered mounds in shallower craters inside Arabia Terra and Meridiani Planum). The evidence supporting groundwater upwelling discussed by Andrews‐Hanna et al. (2007, 2010) and Andrews‐Hanna and Lewis (2011) are largely related to the layered sediments in the plains and within some craters in Arabia Terra. Different depositional rates depend on the different depositional processes involved even in the presence of a common control (in this case groundwater fluctuations). The craters presented here show interior valleys that could potentially be attributed to sapping and also extremely flat floors that could potentially indicate the former presence of a groundwater‐fed lake. Regardless, the lack of extensive sedimentary succession in deep craters, which would have experienced the first upwelling events compared to the shallower craters and intercrater plains ( Andrews‐Hanna & Lewis, 2011; Andrews‐Hanna et al., 2007, 2010; Zabrusky et al., 2012), might appear to question the occurrence of groundwater upwelling. Zabrusky et al. (2012) hypothesized that erosional processes could have played a significant role in the lack of sedimentary deposits inside these deep basins, even if it would require erosional rates higher than expected (Golombek et al., 2006; Salese et al., 2016). Again, the key question resides in understanding the nature of the depositional environments. The deepest basins do not necessarily share the same geological evolution (and consequently the same sedimentation rate) with the shallower basins in Arabia Terra and Meridiani Planum or the same erosional and depositional conditions. The basins and the intercrater plains of Arabia Terra and Meridiani Planum appear to host mostly evaporitic deposition in playas (Grotzinger et al., 2005; Squyres et al., 2009) and possibly fluid upwelling in the same regions, such as spring mounds, as is suggested by the overall sulfate‐bearing composition (Allen & Oehler, 2008; Pondrelli et al., 2015, 2011). Here the model used the groundwater upwelling scenario (Andrews‐Hanna et al., 2010). In the deepest basins, groundwater upwelling would have resulted in the formation of clastic‐dominated fluviolacustrine depositional environments (with related subenvironments), while in the shallower basins/plains evaporitic deposition would have predominated. Clastic‐dominated fluviolacustrine depositional environments entail the accumulation of most of the sediments in the proximal part of the basin (delta and shorelines), whereas the distal parts (i.e., central part of the lacustrine system/crater) can show very limited sediment accumulations. Moreover, as a consequence of the low sedimentation rate, diagenesis might have not occurred. The observed landforms and their relation to groundwater upwelling processes are also supported with mineralogical observations that suggest spectral evidence for Mg‐rich clay minerals—possibly smectites and smectite/talc interlayered clays—as well as Mg carbonates (e.g., McLaughlin crater; Michalski et al., 2013). These minerals represent the typical manifestation of ancient groundwater processes on Mars, and they corroborate the hypothesis of lacustrine activity in these basins. This would imply a more complex geological relation between phyllosilicate and sulfate‐bearing deposits than the mere sharp vertical transition that has been previously hypothesized (Bibring et al., 2006). The lateral transitions and interfingering found are also consistent with the detailed stratigraphies observed by the in situ analyses performed by rovers (Cino et al., 2017). 4.3 Groundwater or Precipitation Control? In our work we found a sustained water level variation of at least 800 m between the investigated craters. We do not take into account the apex of sapping valleys because they could be affected by a certain degree of uncertainty due to recessive erosion of the valleys, especially if we take into account a composite discharge model in the development of the valleys. The range of sustained water level variation that we estimated from several geological observations is of the same order of magnitude estimated by the model of Horvath and Andrews‐Hanna (2017) for Gale crater. There is a notable consistency between the stationary water level at approximately −4,000 m in the basins analyzed and previous works that suggests the presence of a northern Late Hesperian ocean shoreline between 3,940 and −4,100 m (Costard et al., 2017; Ivanov et al., 2017J. A. Rodriguez et al., 2016; Webb, 2004). Ivanov et al. (2017) also date the Deuteronilus shoreline to 3.6 Ga, which is close to the model ages of several of the craters studied in this work (see Table S1 for crater age estimation). This supports putative post Noachian sedimentary deposits within the studied basins and the notion that there was a link between the ocean and a planet‐wide groundwater system as well. The outflow channels also form part of this emerging picture of a planet‐wide groundwater table linked to the ocean. The outflow channels might have been fed by this groundwater system as suggested by previous authors for specific areas (Marra et al., 2015; J. A. P. Rodriguez, Kargel, et al., 2015). As proposed in previous work (Baker, 2001; Baker et al., 1991; Clifford & Parker, 2001; Fairen et al., 2003; Parker et al., 1993, 1989; Tanaka et al., 2005), groundwater outbursts at the end of the Hesperian may have generated catastrophic floods flowing through the outflow channels into the ocean in the northern lowlands. Different climatic conditions can strongly influence the variation of the groundwater level as shown by Horvath and Andrews‐Hanna (2017). The parameters that most affect the groundwater level are the aridity index and the annual precipitation. For a given aridity index, the lack or reduction of precipitation leads to a lower recharge of the aquifers and therefore a lowering of the groundwater level. A recent precipitation‐based theory work by Seybold et al. (2018) analyzed branching geometry of valley networks both on Earth and Mars concluding that Martian valleys formed primarily by overland flow erosion and that groundwater seepage played a minor role. Nevertheless, valleys that fed the basins studied in our work are not branched and their morphology is consistent with a sapping‐like and/or mixed origin. Permeability also controls the aquifer: as it increases, subsurface flow becomes a prominent source to the lake, allowing a given lake to persist even in arid climates. Furthermore, different degrees of aqueous‐related activity in the craters could be due to lateral and vertical variations in permeability because of different primary and/or postdepositional lithological characters. Horvath and Andrews‐Hanna (2017) estimated a possible variation of the water surface within Gale crater of about 600 m depending on different climatic conditions. Nevertheless, the authors did not find any geological evidence of lakes inside the craters surrounding Gale, which could corroborate and confirm their results. Our studied basins can be an excellent case study to test in future works the Horvath and Andrews‐Hanna (2017) model in the presence of integrated geological evidence.

5 Conclusions The studied basins show evidence of water‐related landforms below −4,000‐m Mars datum. This suggests that the water‐saturated zone and any long‐term stationary water level remained below an elevation of −4,000 m. Nevertheless, the existence of such a deep aquifer does not exclude the presence of other aquifers at lower elevation, such as the one inferred for the Endeavour crater explored by MER Opportunity. Other aquifers could have existed in Martian history depending on either lateral or vertical stratigraphy. In the deepest basins, groundwater upwelling would have resulted in the formation of clastic‐dominated fluvio‐lacustrine depositional environments (with related subenvironments), as also demonstrated by the presence of Gilbert delta, while in the shallower basins/plains evaporitic deposition would have predominated. These observations suggest that only the deepest basins, those with floors deeper than −4,000 m below the Mars datum, intercepted this deep water‐saturated zone. Furthermore, they point toward the existence of a planet‐wide groundwater table between −4,000 and −5,000 m during the past Martian history. This evidence fully or partially supports the models proposed by Andrews‐Hanna and Lewis (2011); Michalski et al. (2013), which predicted regional to global groundwater upwelling and the existence of a water saturated zone around −4,000 m below Mars datum respectively. Basins with bases deeper than −4000 m below Mars datum intercepted the water‐saturated zone were predicted by the model of Michalski et al. (2013) and exhibit evidence of groundwater fluctuations. The analysis of these morphologies shows that the topographic elevation of the water surface fluctuated at least between −4,000 and −4,800 m (Table 1), but at a certain point in time it became sufficiently stable to allow the formation of deltas. The range of water level variations that we estimated from several geological observations is of the same order of magnitude as the one estimated by the model of Horvath and Andrews‐Hanna (2017) for Gale crater. Fifteen of the 24 deep basins analyzed in our study show clear evidence of the existence of deltas or stepped deltas (Figure 1). Other morphological features also reinforce the idea of a stable water table, for example, the lowest part of the channels and the highest part of the terraces are notably at the same elevation as the deltas. Other morphologies that resulted from groundwater processes are sapping valleys which, given the lack of surface drainage recharge and the absence of breached rims, represent the most compelling evidence that these basins were groundwater fed. The formation of sapping valleys at higher elevations than the groundwater basal level could have been possible due to the presence of the dike‐confined water that can make the groundwater level shallower than the basal one. This concept is introduced for the first time in the Martian geological literature by the proposed conceptual model (Figure 2). In some craters we found stepped retrograding deltas indicating that over time the water level stabilized at different elevations and when the water level fell, the deltas were partially carved by channels and destroyed by erosion. It is notable that the position of the water table varied within a range between −4,800 and −4,000 m in all 24 basins that exhibit water related landforms. This may suggest that the aquifer in all basins was interconnected; nevertheless, the evidence obtained from this work is not enough to affirm with certainty the interbasins connection. Our observations show that the extent of this aquifer is very significant and it leads us to support the thesis that it could have been planet wide. Supporting information: It is linked to the online version of the paper.

Acknowledgments Francesco Salese was supported by Marie Curie Individual Postdoctoral Fellowship (WET_MARS, Grant Agreement 795192). We would like to thank Monica Bufill for her time spent on reviewing our manuscript and her comments helping us improving the article and two anonymous reviewers. The data (HRSC, CTX, HiRISE, and MOLA) that support the findings of this study were obtained freely from the Planetary Data System (PDS) and are publicly available online at https://pds.nasa.gov/index.shtml. The authors wish to thank MRO and HRSC team for these data. Satellite imagery and the Extended Data were generated with ISIS 3 (Integrated Software for Imagers and Spectrometers) available online at https://isis.astrogeology.usgs.gov. All these data were integrated into ArcGIS 10.6 project. Correspondence and requests for materials should be addressed to F. S (f.salese@uu.nl).

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