Ancient hydrothermal seafloor deposits in Eridania basin on Mars

The Eridania region in the southern highlands of Mars once contained a vast inland sea with a volume of water greater than that of all other Martian lakes combined. Here we show that the most ancient materials within Eridania are thick (>400 m), massive (not bedded), mottled deposits containing saponite, talc-saponite, Fe-rich mica (for example, glauconite-nontronite), Fe- and Mg-serpentine, Mg-Fe-Ca-carbonate and probable Fe-sulphide that likely formed in a deep water (500–1,500 m) hydrothermal setting. The Eridania basin occurs within some of the most ancient terrain on Mars where striking evidence for remnant magnetism might suggest an early phase of crustal spreading. The relatively well-preserved seafloor hydrothermal deposits in Eridania are contemporaneous with the earliest evidence for life on Earth in potentially similar environments 3.8 billion years ago, and might provide an invaluable window into the environmental conditions of early Earth.

T he oldest supracrustal rocks on Earth are early Eoarchean seafloor deposits (Z3.7 Ga) 1 . The presence of isotopically light carbon 2 within biogenic morphologies in these rocks indicates that life may have flourished on the early Earth in hydrothermal seafloor environments 3 . Yet progress in further understanding the actual origin of life or prebiotic chemistry from these rocks, or those of similar age, is severely challenged by the fact that they have experienced multiple generations of metamorphism, metasomatism and deformation 4 . The search for life's origins through empirical geologic evidence might require exploration beyond Earth, where younger geological activity has not overwritten critically important chemical and textural records. This journey could lead to Mars where ancient sedimentary, volcanic and hydrothermal deposits contemporaneous with the origin of life on Earth have escaped deep burial and metamorphism.
The Eridania region, located at the boundary of Terra Cimmeria and Terra Sirenum (Fig. 1), includes exposures of some of the most ancient terrain on Mars 5 . This area exhibits the strongest evidence for remnant magnetism on Mars and could be a site of ancient crustal spreading 6 (Fig. 1a). Geophysical models suggest that the area had a high thermal gradient in the Noachian 7 , consistent with regional magmatism. The presence of a high-potassium anomaly 8 could be an indication of a deep mantle source for ancient volcanism in the area or widespread alteration of the crust 9,10 . Regardless of whether the remnant magnetism is truly indicative of early plate tectonics, the fact that the magnetic signature is observed is an indication that the near surface materials formed when the magnetic field of Mars was strong, and have not been buried as has occurred in other areas 5 . Therefore, the geology observed here provides insights into geological processes that operated in the earliest observable epoch of Martian history 11 .
In addition to containing come of the most ancient crust on Mars, the Eridania region is important because it contains a large basin that was once filled with water. In this study, we examined the geology and mineralogy of the most ancient deposits within this basin. Using infrared spectroscopy and high-resolution imaging, we show that the Eridania basin contains a complex suite of alteration minerals that likely formed in a hydrothermal seafloor volcanic-sedimentary setting.

Results
Geomorphic evidence for an ancient sea in Eridania basin. Eridania basin is composed of a series of connected, smaller, quasi-circular basins (Fig. 1), which potentially originated as very ancient impacts that were resurfaced by volcanism and erosion early in Mars' history 12,13 . The extent of the Eridania basin was previously defined as the 1,100 m elevation contour around these sub-basins 13 (Fig. 1). Irwin et al. 13 deduced that the Eridania basin was once filled to this level because it is at this elevation that the 3-km-wide Ma'adim Vallis outflow channel originates (Fig. 1c) 13 . This morphology, with a complete lack of upstream tributaries, suggests that the channel formed at full width, although a spillway at the edge of the Eridania basin at B1,100 m elevation, a strong indication that the basin was filled with water at the Noachian/Hesperian boundary 13 .
Irwin et al. 13 recognized the unusual hypsometry of the Eridania basins, noting that they have unusual concave topographic profiles. We similarly compare the topographic data of Eridania basins to data of basins elsewhere on Mars ( Supplementary Fig. 1). Most similar sized basins elsewhere on Mars exhibit clear 'U-shaped' topographic profiles which arise from colluvial, fluvial and volcanic resurfacing of the basins in a subaerial setting. The concave structure of the Eridania basins is an indication that, during the only intense period of erosive activity in mars history, these basins were protected beneath water or ice-covered water.
Previous researchers noted that Noachian valley networks also terminate at an elevation of B700-1,100 m (refs 13,14), suggesting the existence of an ancient base level. If a water level existed between 700 and 1,100 m elevation, the basin topography implies that the parts of the lake would have been 1-1.5 km deep. The approximate size of such a body of water would have been B1.1 Â 10 6 km 2 , B3 Â larger than the largest landlocked lake or sea on Earth (Caspian Sea) (Fig. 2). In fact, even a conservative estimate of the volume of the Eridania sea exceeds the total volume of all other lakes on Mars combined (Fig. 2) 15 . Here we synthesize previous work and provide new analyses of the mineralogy, geology, and context of the most ancient deposits in Eridania basin (Fig. 1c), which we argue formed in a deep-water hydrothermal setting.
Multiple types of colles and chaos units in Eridania basin. Unique deposits found only at the centre (deepest part) of each basin (Fig. 1c)    B0.1-10 km diameter (Fig. 3). While these deep basin units are in some cases formally named 'chaos' and in other cases, 'colles 16 ,' there are some clear and important geological differences among the deposits that are not reflected in the naming convention and often confused in previous work (Fig. 3). Most importantly, the fractured blocks in the western and central parts of Eridania, as we argue in this paper, represent ancient, deep basin subaqueous units and those in the eastern parts of the basin are younger, eroded volcanics deposited subaerially. Ariadnes Colles and Atlantis Chaos contain the best examples of deep basin deposits (Fig. 3a-c) that formed in deep water 16 . There, massive blocks of bedrock (lacking observable bedding) Cross-section d-d'  reach up to 400 m elevation above the surrounding dark-toned plains (Fig. 4). The deep basin unit was eroded and dismembered into buttes and mesas, subsequently embayed and blanketed by volcanic materials in the Hesperian, resulting in a kı %puka-like landscape (Fig. 4). By contrast, block-forming units observed to the east in Gorgonum are completely different in terms of texture, bedding 17 , colour and mineralogy ( Fig. 3d-f ). These units are characterized by the presence of smaller blocks composed of a mixture of boulders and friable materials. Texturally, they are smooth and hummocky, and they have been widely eroded to form gullies in many cases 18 , which is rare in the deep basin deposits to the west except where mantling volcanic deposits occur. The younger chaos units never show mottled colour patterns and do not contain evidence for fractures and veins. These are eroded blocks within the younger, superposed volcanic material (likely both ash and lava).
The block-forming basin unit in Gorgonum is substantially younger than the deep basin units in Ariadnes or Atlantis. Crater counting was performed within the deep basin deposits to estimate minimum ages for those deposits. We counted craters with diameters Z500 m throughout the deep basin deposits using Mars Context Imager (CTX) data as the base. The key result is that that deposits in Ariadnes and Atlantis basins are much older than basin deposits in eastern basins, especially Gorgonum. Assuming a crater producton function from Ivanov (2001) and absolute chronology based on Hartmann and Neukum 19,20 , we estimate that minimum exposure age for the Ariadnes deposits at 3.77 Ga and the Gorgonum deposits at 3.47 Ga ( Supplementary  Fig. 2). These ages are consistent with previous results, which suggest that the Eridania basin-forming impacts occurred 44 Ga, the sea existed in the Late Noachian and was resurfaced by subaerial volcanism in the Late Hesperian 5,12,13,16 . A key new conclusion is that, while all of the Eridania sub-basins likely contained deep water environments, the deposits representing those environments are only well exposed in the western basins. They have been too intensely resurfaced in the east basins.
Mineralogy of the Eridania basin. In this work, we analysed the infrared spectra of all Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and all High Resolution Imaging Science Experiment data within the Eridania basin in order to evaluate the detailed mineralogy and geological context of deep basin deposits within the Eridania region (Fig. 5).
CRISM spectra acquired of the kı %pukas throughout the western and central Eridania basin contain absorptions at (l) B1. 4 (Fig. 6). Fe-Mg-rich clays are common on Mars 23,24 , but in detail, the deep basin bedrock units show spectral characteristics unusual for the planet. CRISM spectra of the kı %puka blocks typically show absorptions at 2.31-2.315 mm characteristic of Mg-rich, trioctahedral clay minerals. Specifically, this absorption is indicative of Mg 3 OH and Mg 2 FeOH combination bands in the octahedral sheets of saponite, talc, serpentine and various mixed-layer clays 25 . Pure talc (that is, with little Fe 2 þ or Fe 3 þ substitution for Mg 2 þ ) exhibits pronounced doublet absorptions at 2.29 and 2.31 mm, as do well-ordered examples of saponite and sepiolite.
The deep basin bedrock units show, in some cases, evidence for the doublet at 2.31-2.315 mm attributable to talc or other well-ordered, Mg-rich tetrahedral-octahedral-tetrahedral (TOT) clays such as sepiolite and some saponite, and in others, simply show a sharp absorption that could be attributable to saponite or Fe-rich talc. However, saponite and talc are commonly interstratified at the lattice scale in some seafloor settings 26,27 (mixed-layering), and some of the best matches to these Martian spectra correspond to spectra of mixed-layer seafloor clays on Earth 28 . In addition, the presence of a 1.9 mm H 2 O absorption in many of the detections (Fig. 6) suggests the presence of TOT clays with expandable layers (smectite or smectitic mixed-layer clays). However its absence in other materials suggests non-expandable TOT clays, such as talc, or that expandable clays have been locally dehydrated while others remain hydrated. Absorptions at 1.39, 2.315, 2.43 and 2.51 mm in some deposits suggest the presence of serpentine 29 , which could also include serpentine-smectite mixed-layered clays (Fig. 6).
Fe-rich phyllosilicates are also observed. These deposits show absorptions at 2.295-2.305 mm, which are characteristic of Fe-rich dioctahedral mica or smectite with some Mg-substitution (that is, VI Fe 3 þ /Mg 2 þ molar ratio r4) 28 . Such materials are spectrally similar to Fe-rich seafloor deposits sampled on Earth, and easily distinguishable from Al-bearing nontronite formed in a subaerial/ continental setting 30 . In some deposits, an unusual doublet absorption at 2.32 and 2.38 is observed, and the same spectra display a weak or absent 1.9 mm feature and a very strong spectral slope from 1 to 2 mm, indicative of Fe-rich serpentine-or chloritegroup minerals 31 . In fact, the 1-2 mm slope, which is stronger than is typically observed in Martian clays 23 is likely a reflection of the abundant Fe 2 þ present in many of the clay detections 32 , and is a key indicator that the formation conditions likely involved a very Fe-rich fluid.
While the signature of phyllosilicates dominates the spectral character of the deep basin colles and chaos units, there are also several detections of jarosite occurring along with clay minerals within the chaos blocks (Fig. 6) 33 . The most common formation mechanism for jarosite is through oxidative chemical weathering of sulphide minerals 35 . Sulphides might be present in the deep basin units, but they are very difficult to detect directly because, in the near infrared, they exhibit few or no distinguishing features. In fact, jarosite formed through oxidative weathering 36 is commonly considered a proxy for sulphide-bearing ore deposits 35 .
Spectra extracted from the central peaks and interior walls of impact craters occurring within the basin centres provide information about the mineralogy of the units stratigraphically below the Mg-clay units (for example, Fig. 4d), and other deposits at depth that are poorly exposed. A 17 km diameter impact crater in Caralis Chaos contains structurally and texturally complex blocks of exhumed bedrock in its central uplift that display spectral absorptions at (l) B2.31 and 2.51 mm indicative of Mg-rich or Fe-Mn-Ca-Mg carbonate 37 (Fig. 7). These materials also contain dense networks of crosscutting veins at the same scale (5-20 m-spacing) as is observed in the Mg-clay-bearing colles units (Fig. 7). Similarly, the central peak of an unnamed 15 km diameter crater in Ariadnes colles contains spectral evidence for Ca/Fe-carbonate in its central peak, as does a crater in the western, unnamed basin and the walls of a crater near Simois Colles where carbonates might have been mobilized within younger gullies 38 .
The carbonates might have formed from impact-generated hydrothermal activity, but they appear to occur within coherent bedrock exposed in the uplifted peak and rim (Fig. 7). Depth of exhumation can be assessed assuming that the central peak uplift represents a depth 10% of the final crater diameter 39 (D ¼ 15-17 km in this case). The carbonates were likely uplifted from 1 to 2 km below the floor of the basin where the impact occurred, which has a Mars Orbiter Laser Altimeter (MOLA) elevation of À 50 m. This observation suggests that alteration is present to substantial depth within the basin, the deep basin units might be kilometres thick, and that the phyllosilicate-rich deep basin units might overly or be interbedded with carbonate-rich rocks.
One B10 km diameter impact crater in Ariadnes Colles has exhumed material in its ejecta and rim that contain relatively strong absorptions at 1.9 and 2.24 mm indicating the presence of hydrated silica. But this crater is nested within a larger impact structure and it is possible that it has exhumed hydrothermal crater floor deposits formed from the earlier impact (Fig. 5).
Previous researchers identified chlorides in the region 40 using data from the Thermal Emission Imaging System (THEMIS) (Fig. 5). In contrast to the Mg-and Fe-rich clays, which are concentrated below 300 m MOLA elevation, the chlorides occur at higher elevations (350-1,050 m) (Fig. 8). The average elevation of chlorides is 660 m, which is similar to the low-water level of the Eridania sea (700 m) 13 , suggesting they may have formed through evaporation in shallow seawater near the basin margin. Impact craters as small as 360 m-diametre have chloride-poor ejecta at distances up to 2-3 crater radii, suggesting that the craters have penetrated through a relatively thin (likely o30 m thick) chloride deposit (Fig. 8c).
Some of the eastern and northern parts of the basin contain Fe/Al-rich clays that are interpreted as pedogenic weathering sequences 21 , as has been observed elsewhere on Mars 41,42 . These deposits typically contain 2.28-2.29 mm FeFeOH and FeAlOH absorptions corresponding to aluminous nontronite, and in two cases contain absorptions at 2.2 mm corresponding to kaolinitegroup clays and a broad absorption from 2.3 to 2.5 mm likely interpreted as polyhydrated sulphates 43 . In Eridania, these deposits occur in the superposed volcanic resurfacing unit and are therefore younger than and of different context to the deep basin deposits.
In previous works, many of the pedogenic-type deposits have not been delineated from the true deep basin deposits. Here we point out that the deep basin deposits are clearly distinguishable based on texture, colour, stratigraphic relations and mineralogy. The older deep basin deposits in the western basins contain clear and strong evidence for complex Mg-and Fe-rich clay mineralogy. The younger volcanic resurfacing units (Fig. 3d-f) are generally spectrally unremarkable. But it is in association with these units that pedogenic-type sequences are found.

Origin of deep basin deposits. The deep basin units in Ariadnes,
Atlantis and Caralis basins formed in association with significant amounts of water, as evidenced by the presence of 100 s of metre thick deposits of phyllosilicates containing dense vein networks. It is possible that clay in Eridania could have formed in an alkaline-saline evaporative lake setting 16,43 , but we present challenges to the evaporite hypothesis, the most significant of which is the fact that the deep basin deposits are dominated by silicates rather than salt deposits.
The chemistry of the Eridania sea is unknown, though the volume to watershed ratio argues strongly that the sea was fed by groundwater 13,15 . Such a fluid would have been a Fe 2 þ , Mg 2 þ , Ca 2 þ , Cl À , HCO 3 À and sulphur-rich-brine after interaction with the regional mafic-ultramafic crust 44 . Evaporation of such a fluid could initially produce Fe-carbonates. In fact, the occurrence of carbonates, exhumed from depth within the deep basin deposits is consistent with precipitation from an early phase of evaporation or freezing. But continuation along an evaporative pathway would quickly exhaust Fe 2 þ in solution leading to Mg-sulphate and ultimately to chloride precipitation 45 , which is not observed in the deep basin units. The deep basin deposits (that is, in the basin centres) contain 4400 m thick clay deposits and no detectable hydrated sulphates or chlorides (Figs 5a and 8a), though anhydrite and small amounts of hydrous salts are possible.
Chlorides are present at higher elevations along the interior basin margins at concentrations 10-25% by volume 46 and likely trace evaporitic, shallow water (o100 m) settings 40 . However, these deposits do not include any hydrated sulphates that should have precipitated before the chlorides during the evaporation sequence 44,45 . In most terrestrial playas, chloride deposits are situated in the middle of the basin rather than on the edges.
The lack of Mg, Fe sulphates in these deep basin deposits makes an evaporite-playa origin untenable. However, the chloride deposits on the basin margins may be related to evaporation in coastal, shallow water environments 47 . If the majority of the Eridania sea did not evaporate or freeze (both produce similar evaporite-type deposits), then the fluid was likely lost back into the subsurface due to some fundamental change in the regional groundwater table, perhaps including the formation of a new, deep basin that affected groundwater flow.
A detrital origin of clay-rich, deep basin deposits in Eridania is also unlikely. The concave topography of the deep basins below 700 m elevation is unusual for Martian basins 13 (see Supplementary Fig. 2), most of which have flat floors that formed through subaerial resurfacing. The shape of Eridania basin floors argues strongly that the surfaces were protected below water 13 during the period of intense sedimentary deposition on Mars (Late Noachian) 11,48 . The deep basin deposits are unlikely to have formed by air fall as has been previously concluded 12,16 . The younger Electris deposits, which are layered, are of consistent thickness throughout the region (150-200 m), and occur at a wide range of elevations are consistent with an air fall origin 49 . By contrast, the deep basin deposits in Ariadnes and Atlantis are not layered, are thick, and concentrated at low elevations (Figs 5 and 8a). Ariadnes Colles and Atlantis Chaos contain at least 1-5 Â 10 4 km 3 of altered material (assuming a minimum thickness of 400 m and a likely thickness of 41 km). Deposition of such a thickness and volume of material is possible proximal to explosive vents 50 . It is possible that unrecognized volcanic vents are present 51 , but any air fall origin fails to account for why such thick deposits are found within the basins, but no trace of similar deposits of similar age are found outside the basins. Even if air fall deposition cannot be ruled out as a geological process, this model seemingly requires major volcanic source regions near, but outside the basins while ignoring the fact that volcanism would most likely be localized in the basins themselves, as is observed elsewhere on Mars and on other planets 52 .
The most plausible way to produce such large volumes of deep basin, deep water deposits is through seafloor volcanic-sedimentary processes focused in the basin floors where fractured, thinner crust and higher heat flow would be expected. Large volumes of Hesperian lava present throughout Eridania are proof that significant volcanism occurred within the basins. We argue that this volcanism did not suddenly begin after the sea had ceased to exist in the Early Hesperian, but most likely began in the Noachian, shortly after the basins formed. A sea of the size of Eridania is unlikely to have been ephemeral and therefore, it is nearly inescapable that subaqueous volcanism would have occurred during the period in which the sea existed.
Previous authors have demonstrated that most ancient, large impact basins on Mars were resurfaced by ultramafic to mafic volcanic materials-olivine rich lavas that have erupted through the relatively thin crust of basin environments 53 . The Eridania basins would have likely had the same type of activity. The important difference in Eridania is that a deep sea was present while volcanism occurred. If the Eridania sea level was at the 700 m elevation level (a conservative estimate), it implies that the deep basin deposits formed beneath 500-1,200 m water depth (Fig. 9). The lower gravity of Mars results in lower water pressure in a Martian sea compared to one on Earth, for a given depth (Fig. 9). Seafloor volcanism in Eridania would have occurred at water pressures of 20-50 bars. At these pressures, Martian seafloor volcanism could have included both effusive and explosive elements, in addition to chemical sedimentation from hydrothermal fluids (Fig. 10). The transition from altered deep basin deposits to flood lavas in the Hesperian does not represent the onset of volcanism in the basins, but the transition from subaqueous to subaerial volcanic activity as the Eridania sea came to an end.
Implications of the hydrothermal seafloor model. We conclude that thick, massive, clay-, carbonate-and likely sulphide-bearing deposits in Eridania basin formed in a deep-water hydrothermal environment on ancient Mars (43.8 billion years ago) (Fig. 10). Saponite, talc, talc-saponite, Mg-bearing nontronite, glauconite, serpentine and berthierine are all common in terrestrial seafloor deposits 26,27,54 . The clay assemblages and spectral trends observed in seafloor deposits on Earth provide a good analogue for the deep basin deposits detected remotely in Eridania 28 . Salts only observed at higher elevations likely represent coastal evaporative settings (Fig. 10). Several lines of evidence strongly suggest that Eridania was a sustained inland sea in the late Noachian.
The deep-water environment was likely reducing based on direct evidence for Fe 2 þ -rich clay minerals and indirect evidence for Fe-sulphides. This could be an indication of stratification of an ancient sea beneath an oxidized atmosphere, chemical isolation in an ice-covered sea, or quasi-equilibrium with a reduced atmosphere. The ancient Eridania sea deposits might represent a setting analogous to Fe-rich sea environments present on the early Earth.
Ancient, deep-water hydrothermal deposits in Eridania basin represent a new category of astrobiological target on Mars. To date, the search for habitable environments on Mars has been focused on exploration of ephemeral playa and shallow lacustrine settings. The Eridania deposits represent an ancient environment rich in chemical nutrients and energy sources. Such a deep-water