Inlet Valleys Research Articles

Paleolake Inlet Valley Formation: Factors Controlling Which Craters Breached on Early Mars

AbstractThe ancient surface of Mars is dominated by degraded impact craters with reduced or eliminated rim relief. Some degraded craters have an inlet valley, while many remain fluvially isolated. Despite controlling Martian fluvial connectivity, few constraints exist on why some—but not all—degraded craters possess inlets. We compared a suite of properties around degraded Martian craters with and without inlets to ascertain what topographic and hydrologic factors influenced inlet formation. Slope and surface roughness are similar, but topographic inset within the catchment, drainage density, and potential contributing areas diverge for breached and non‐breached craters. We suggest that the importance of basin hydrology‐related factors over topographic factors is the result of the former less frequently surpassing inlet incision thresholds than the latter. We conclude that greater topographic inset (i.e., craters deeper within regional depressions) promoted higher discharge, and that inlet valley formation was ultimately controlled by Mars' crater‐dominated topography.

Constraining the formation of paleolake inlet valleys across crater rims

Impact crater lakes with an inlet valley were common on Mars during past epochs. However, it has not been established exactly how impact craters with initially high-standing rims were breached by an inlet. We investigated four potential mechanisms for inlet valley breach formation: (1) rim erosion, (2) depositional rim burial, (3) drainage head erosion, and (4) overflow. We used remote sensing measurements to assess the relative likelihood of these mechanisms for 39 crater paleolake inlet systems, keeping in mind that either a single mechanism or multiple mechanisms may have occurred. Our findings indicate that crater degradation associated with (1) rim erosion and/or (2) depositional rim burial played an important role in establishing connections between exterior fluvial systems and crater interiors, especially during the late Noachian - early Hesperian valley network era. Isolated fluvial valleys, interpreted to post-date the main valley network era, breached craters in a wider range of degradation states compared to valley network-fed craters. Interestingly, in many cases, these younger systems accomplished a comparable magnitude of fluvial erosion via drainage head erosion and across crater rims as the valley network inlets, despite differences in broader landscape dissection. This finding suggests fluvial activity was able to accomplish comparable geomorphic work during later periods of Mars' history, at least locally. Our catalog of analyzed paleolakes includes Jezero crater, and so our work provides broader context for in situ rover analyses at this site.

Overspilling small craters on a dry Mars: Insights from breach erosion modeling

Understanding when, where, and how frequently liquid water was stable on Mars since the Late Noachian/Early Hesperian (3.2-3.9 Ga) is important for understanding the evolution of Mars' climate and hydrology. Some relatively young features on Mars require multiple wetting events to form, whereas others are consistent with single wetting events. Small and rare exit breach craters or “pollywogs” are craters between 0.5 and 15 km in diameter with valleys leading away from the lowest point on their rims but no visible inlet valleys. These craters must have been filled with water to the point of overspill to form the observed valleys. The two possible water sources are precipitation and groundwater. In this paper we use measurements from Digital Elevation Models (DEMs) of 18 pollywog craters (21 outlet valleys) and a fixed channel width 0-D breach erosion model to determine whether pollywog exit breach valleys are consistent with a single crater overspill event, or if their formation requires multiple overspill events. Our model, which we compare to a selection of dam breaching events on Earth, predicts runaway erosion for two pollywog exit breaches. No runaway erosion is observed. We discuss potential explanations for this mismatch between the data and our model. We show that the majority of pollywog craters on Mars are consistent with formation during a single crater overspill event, incorporating a work around for the long-standing problem of unknown grainsize into our approach. Three pollywog craters require either multiple events or sustained water supply to drive erosion. We discuss potential source mechanisms for crater-filling water and conclude that pollywogs either formed in a single erosion event, driven by groundwater discharge, or through many small erosion events, driven by draining of small meltwater lakes formed on crater-filling bodies of ice.

Fluvial Regimes, Morphometry, and Age of Jezero Crater Paleolake Inlet Valleys and Their Exobiological Significance for the 2020 Rover Mission Landing Site.

Jezero crater has been selected as the landing site for the Mars 2020 Perseverance rover, because it contains a paleolake with two fan-deltas, inlet and outlet valleys. Using the data from the High Resolution Stereo Camera (HRSC) and the High Resolution Imaging Science Experiment (HiRISE), we conducted a quantitative geomorphological study of the inlet valleys of the Jezero paleolake. Results show that the strongest erosion is related to a network of deep valleys that cut into the highland bedrock well upstream of the Jezero crater and likely formed before the formation of the regional olivine-rich unit. In contrast, the lower sections of valleys display poor bedrock erosion and a lack of tributaries but are characterized by the presence of pristine landforms interpreted as fluvial bars from preserved channels, the discharge rates of which have been estimated at 103-104 m3s-1. The valleys' lower sections postdate the olivine-rich unit, are linked directly to the fan-deltas, and are thus formed in an energetic, late stage of activity. Although a Late Noachian age for the fan-deltas' formation is not excluded based on crosscutting relationships and crater counts, this indicates evidence of a Hesperian age with significant implications for exobiology.

The mineral diversity of Jezero crater: Evidence for possible lacustrine carbonates on Mars

Noachian-aged Jezero crater is the only known location on Mars where clear orbital detections of carbonates are found in close proximity to clear fluvio-lacustrine features indicating the past presence of a paleolake; however, it is unclear whether or not the carbonates in Jezero are related to the lacustrine activity. This distinction is critical for evaluating the astrobiological potential of the site, as lacustrine carbonates on Earth are capable of preserving biosignatures at scales that may be detectable by a landed mission like the Mars 2020 rover, which is planned to land in Jezero in February 2021. In this study, we conduct a detailed investigation of the mineralogical and morphological properties of geological units within Jezero crater in order to better constrain the origin of carbonates in the basin and their timing relative to fluvio-lacustrine activity. Using orbital visible/near-infrared hyperspectral images from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) along with high resolution imagery and digital elevation models, we identify a distinct carbonate-bearing unit, the “Marginal Carbonates,” located along the inner margin of the crater, near the largest inlet valley and the western delta. Based on their strong carbonate signatures, topographic properties, and location in the crater, we propose that this unit may preserve authigenic lacustrine carbonates, precipitated in the near-shore environment of the Jezero paleolake. Comparison to carbonate deposits from terrestrial closed basin lakes suggests that if the Marginal Carbonates are lacustrine in origin, they could preserve macro- and microscopic biosignatures in microbialite rocks like stromatolites, some of which would likely be detectable by Mars 2020. The Marginal Carbonates may represent just one phase of a complex fluvio-lacustrine history in Jezero crater, as we find that the spectral diversity of the fluvio-lacustrine deposits in the crater is consistent with a long-lived lake system cataloging the deposition and erosion of regional geologic units. Thus, Jezero crater may contain a unique record of the evolution of surface environments, climates, and habitability on early Mars.

Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters

Cross crater is a 65 km impact crater, located in the Noachian highlands of the Terra Sirenum region of Mars (30°S, 158°W), which hosts aluminum phyllosilicate deposits first detected by the Observatoire pour la Mineralogie, L’Eau, les Glaces et l’Activitie (OMEGA) imaging spectrometer on Mars Express. Using high-resolution data from the Mars Reconnaissance Orbiter, we examine Cross crater’s basin-filling sedimentary deposits. Visible/shortwave infrared (VSWIR) spectra from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) show absorptions diagnostic of alunite. Combining spectral data with high-resolution images, we map a large (10 km × 5 km) alunite-bearing deposit in southwest Cross crater, widespread kaolin-bearing sediments with variable amounts of alunite that are layered in <10 m scale beds, and silica- and/or montmorillonite-bearing deposits that occupy topographically lower, heavily fractured units. The secondary minerals are found at elevations ranging from 700 to 1550 m, forming a discontinuous ring along the crater wall beneath darker capping materials. The mineralogy inside Cross crater is different from that of the surrounding terrains and other martian basins, where Fe/Mg-phyllosilicates and Ca/Mg-sulfates are commonly found. Alunite in Cross crater indicates acidic, sulfurous waters at the time of its formation. Waters in Cross crater were likely supplied by regionally upwelling groundwaters as well as through an inlet valley from a small adjacent depression to the east, perhaps occasionally forming a lake or series of shallow playa lakes in the closed basin. Like nearby Columbus crater, Cross crater exhibits evidence for acid sulfate alteration, but the alteration in Cross is more extensive/complete. The large but localized occurrence of alunite suggests a localized, high-volume source of acidic waters or vapors, possibly supplied by sulfurous (H_2S- and/or SO_2-bearing) waters in contact with a magmatic source, upwelling steam or fluids through fracture zones. The unique, highly aluminous nature of the Cross crater deposits relative to other martian acid sulfate deposits indicates acid waters, high water throughput during alteration, atypically glassy and/or felsic materials, or a combination of these conditions.

Insights into surface runoff on early Mars from paleolake basin morphology and stratigraphy

We present observations on the morphology and stratigraphy of more than 400 paleolake basins on Mars. We show that there are two distinct classes of Martian paleolake basins: (1) paleolakes fed by regionally integrated valley networks (N = 251), and (2) paleolakes fed by isolated inlet valleys not integrated into broader regional drainage systems (N = 174). We conclude that valley network–fed paleolakes primarily formed prior to approximately the Noachian-Hesperian boundary, ca. 3.7 Ga, while isolated inlet valley paleolakes primarily formed later in Martian history. All 174 isolated inlet valley paleolakes are closed-basin lakes; however, there are surprisingly few (31) valley network–fed closed-basin lakes compared to a large number (220) of valley network–fed open-basin lakes. This observation is consistent with declining levels of fluvial activity over time on the Martian surface. Our results imply that during the era of valley network formation, ∼90% of topographic basins breached by an inlet valley had sufficiently high ratios of water influx to losses to fill, overtop, and form an outlet valley. This conclusion provides an important constraint on the balance between surface runoff production and water losses on early Mars that must be satisfied by any model of the early Martian climate and hydrologic cycle.

Classification and analysis of candidate impact crater-hosted closed-basin lakes on Mars

We present a new catalog of 205 candidate closed-basin lakes contained within impact craters across the surface of Mars. These basins have an inlet valley that incises the crater rim and flows into the basin but no visible outlet valley, and are considered candidate closed-basin lakes; the presence of a valley flowing into a basin does not necessitate the formation of a standing body of water. The major geomorphic distinction within our catalog of candidate paleolakes is the length of the inlet valley(s), with two major classes – basins with long (>20km) inlet valleys (30 basins), and basins with short (<20km) inlet valleys (175 basins). We identify 55 basins that contain sedimentary fan deposits at the mouths of their inlet valleys, of which nine are fed by long inlet valleys and 46 are fed by short inlet valleys. Analysis of the mineralogy of these fan deposits suggests that they are primarily composed of detrital material. Additionally, we find no evidence for widespread evaporite deposit formation within our catalog of candidate closed-basin lakes, which we conclude is indicative of a general transience for any lakes that did form within these basins. Morphometric characteristics for our catalog indicate that as an upper limit, these basins represent a volume of water equivalent to a ∼1.2m global equivalent layer (GEL) of water spread evenly across the martian surface; this is a small fraction of the modern water ice reservoir on Mars. Our catalog offers a broader context within which results from the Mars Science Laboratory Curiosity rover can be interpreted, as Gale crater is a candidate closed-basin lake contained within our catalog. Gale is also one of 12 closed-basin lakes fed by both long and short inlet valleys, and so in␣situ analyses by Curiosity can shed light on the relative importance of these two types of inlets for any lacustrine activity within the basin.

Constraints on the history of open‐basin lakes on Mars from the composition and timing of volcanic resurfacing

Abundant evidence exists for valley network‐related fluvial activity near the Noachian‐Hesperian transition on Mars, and areally significant quantities of volcanic ridged plains were emplaced during this period as well. Thus, it is worthwhile to explore the hypothesis that lava‐water interaction occurred on the surface of Mars at this time. We analyzed the morphology, physical properties, composition, and surface ages of thirty open‐basin lakes (topographic lows with both an inlet valley and an outlet valley) that were also resurfaced by volcanic flows. Hyperspectral imaging data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) instruments indicate that of the 30 basins, 12 exhibit the spectral properties of basaltic lithologies with diagnostic absorptions of olivine, high‐calcium pyroxene and low‐calcium pyroxene. An olivine/high‐calcium pyroxene mixture is the most commonly identified mineral assemblage, consistent with other Hesperian‐aged volcanic units. Therefore, our mineralogical results for over a third of the open‐basin lake floors analyzed support prior interpretations that the basins were resurfaced by volcanic flows. Supporting evidence for resurfacing by volcanic flows is also given by the observed morphology and physical properties (e.g., surface roughness, thermal inertia) of the resurfaced open‐basin lake floors. In these 30 examples, however, no evidence was found for lava‐water interaction. The ages of emplacement, derived through counts of superposed craters, of all 30 of the open‐basin lake volcanic resurfacing units show that resurfacing began in the Late Noachian, near the Noachian‐Hesperian boundary, was concentrated in the Hesperian, and continued into the Early Amazonian. The lack of geologic features indicative of lava‐water interaction suggests that the basins were likely to have been mostly devoid of water at the time of the latest phase of volcanic resurfacing. We conclude that there is no geologic evidence that suggests the fluvial activity associated with the studied paleolakes was coeval with the emplacement of the observed volcanic resurfacing units, several of which date to the Late Noachian‐Early Hesperian.

An analysis of open-basin lake deposits on Mars: Evidence for the nature of associated lacustrine deposits and post-lacustrine modification processes

A large number of candidate open-basin lakes (low-lying regions with both inlet valleys and an outlet valley) have been identified and mapped on Mars and are fed by valley network systems that were active near the Noachian–Hesperian boundary. The nature of processes that modified the open-basin lake interiors subsequent to lacustrine activity, and how frequently sedimentary deposits related to lacustrine activity remain exposed, has not been extensively examined. An analysis of 226 open-basin lakes was undertaken to identify evidence for: (1) exposed deposits of possible lacustrine origin and (2) post-lacustrine-activity processes that may have modified or resurfaced open-basin lakes. Spectroscopic data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument were analyzed over identified exposed open-basin lake deposits to assess the mineralogy of these deposits. Particular attention was paid to the possible detection of any component of aqueous alteration minerals (e.g. phyllosilicates, hydrated silica, zeolites) or evaporites (e.g. carbonates, sulfates, chlorides) associated with these exposed deposits. The aim of this paper is to act as a broad survey and cataloguing of the types of lacustrine and post-lacustrine deposits that are present within these 226 paleolake basins. Results of the morphologic classification indicate that 79 open-basin lakes (∼35% of the population) contain exposed deposits of possible lacustrine origin, identified on the basis of fan/delta deposits, layered deposits and/or exposed floor material of apparent lacustrine origin. Additionally, all 226 open-basin lakes examined appear to have been at least partially resurfaced subsequent to their formation by several processes, including volcanism, glacial and periglacial activity, impact cratering and aeolian activity. Results from the analysis of CRISM data show that only 10 (∼29% of the 34 deposits with CRISM coverage) of the exposed open-basin lake deposits contain positively identified aqueous alteration minerals, with one deposit also containing evaporites. The identified hydrated and evaporite minerals include Fe/Mg-smectite, kaolinite, hydrated silica and carbonate, with Fe/Mg-smectite the most commonly identified mineral. These results indicate that hydrated and evaporite minerals are not as commonly associated with lacustrine deposits on Mars as they are on Earth. This suggests in situ alteration and mineral precipitation, a common source of such minerals in terrestrial lakes, was not a major process occurring in these paleo-lacustrine systems, and that the observed minerals are likely to be present as transported material within the lacustrine deposits. The lack of widespread in situ alteration also suggests that either the water chemistry in these paleolake systems was not conducive to aqueous alteration and mineral precipitation, or that the open-basin lake systems were relatively short-lived.

Evidence for paleolakes in Erythraea Fossa, Mars: Implications for an ancient hydrological cycle

There is now widespread agreement that the surface of Mars underwent some degree of fluvial modification, but there is not yet full understanding of its surface hydrological cycle and the nature of standing bodies of water, rivers, and precipitation that affected its surface. In this paper we explore Erythraea Fossa (31.5 W, 27.3 S), a graben adjacent to Holden crater, which exhibits strong evidence that it once housed a chain of three lakes, had overland water flow, and was subject to precipitation. The inlet valley, outlet valley, and fan morphologies in the paleolakes are used to qualitatively discern the hydrologic history of the paleolakes; based on topography constraints, the three basins combined once held 56 km 3 of water. Depositional features within the basins that change with drainage area and nearby valleys that start near drainage divides indicate that the paleolakes may have been fed by precipitation driven runoff. This suggests the presence of an atmosphere, at least locally, that was capable of supporting a hydrological cycle.