Early Hesperian Research Articles

A probable ancient nearshore zone in southern Utopia on Mars unveiled from observations at the Zhurong landing area

The Chinese Mars rover Zhurong successfully landed in southern Utopia Planitia on Mars in May 2021. Previous research suggested a Hesperian ocean may have existed in the northern lowland on Mars. Recent research observed water-related features at the Zhurong landing site from in situ data. In this study, we conducted a comprehensive geomorphological analysis of the landing area using remote sensing data, supplemented by in situ observations, and unveiled features consistent with the existence of a nearshore zone in southern Utopia. Different types of water-related geomorphological features were separated by specific topographic contours, suggesting different types of marine environments. The area was subdivided into a foreshore highland-lowland transition unit, a shallow marine unit and a deep marine unit. In situ observations including sedimentary deposit rocks, water-related lamination features, and subsurface sedimentary layers, also indicate past water activities. Results suggested an evolution scenario of the nearshore zone in southern Utopia: (1) Flooding of the Utopia Planitia in Late Noachian around 3.65–3.68 Ga reached the foreshore unit; (2) formation of the shallow and deep marine units after the flooding was completed by about 3.5 Ga and 3.42 Ga in Early Hesperian, respectively; (3) gradual loss of subsurface volatiles during the Amazonian epoch.

Multiple Overspill Flood Channels from Young Craters Require Surface Melting and Hundreds of Meters of Midlatitude Ice Late in Mars’s History

Mars’s tadpole craters are small, young craters whose crater rims are incised by one or more exit breaches but lack visible inlets. The tadpole-forming climate records the poorly understood drying of Mars since the Early Hesperian. A third of tadpole craters have multiple breaches; therefore, the climate must have been able to generate crater rim incision in multiple locations. We use HiRISE data for four multiple-breach tadpole craters to measure their crater fill, rims, and exit breaches. We compare these measurements and other data with our calculations of liquid water supply by rain, surface melting, groundwater discharge, and basal ice sheet melting to discriminate between four proposed formation hypotheses for tadpole breaches, favoring scenarios with ice-filled craters and supraglacial melting. We conclude that multiple-breach tadpole craters record hundreds of meters of midlatitude ice and climate conditions enabling intermittent melting in the Late Hesperian and Amazonian, suggesting that liquid water on Mars has only been available in association with water ice for billions of years.

Magmatic Origins of Extensional Structures in Tempe Terra, Mars

AbstractNumerous graben features transect the Tempe Terra plateau in the northeastern Tharsis Rise, Mars, making it one of the most heavily structured regions of Tharsis. The origin of the complex fault geometries, generated over three distinct stages of tectonic activity, is poorly understood. This work distinguishes between Tempe Terra structures of local and regional origin, to isolate regional deformation patterns related to the general development of the Tharsis Rise from the patterns due to effects of local stress mechanisms. Comparison of structural observations to predicted deformation patterns from different drivers of graben formation in the Martian crust demonstrates the important role of magmatic activity at a variety of scales in driving tectonism in Tempe Terra. Noachian (Stage 1) faulting resulted from local magmatic underplating and associated heating and uplift, which formed part of an incipient stage of widespread Tharsis volcanism that predated development of the main Tharsis Rise. Early Hesperian (Stage 2) faults reflect the interaction of regional stresses from growth of Tharsis with magmatic activity highly localized along the Tharsis Montes Axial Trend—a linear volcanotectonic trendline including the alignment of the Tharsis Montes volcanoes. Early–Late Hesperian (Stage 3) faulting resulted from a series of dyke swarms from a Tharsis‐centered plume, which propagated in a regional stress field generated by growth of the Tharsis Rise. As only Stage 2 NNE faults and Stage 3 ENE faults are linked to regional, Tharsis‐related stresses, other observed Tempe Terra fault trends can be excluded when evaluating models of Tharsis's tectonic evolution.

A giant volcanic island in an early Martian Ocean?

Tracking early oceans on Mars has fundamental climatic and exobiological implications but requires unambiguous datable markers. Here we show that the Olympus Mons giant volcano shares morphological similarities with active volcanic islands on Earth where major constructional slope breaks systematically occur at the sea-air transition in response to sharp lava viscosity contrasts. We propose that the upper rim of the 6-km high concentric main escarpment surrounding Olympus Mons most likely formed by lava flowing into liquid water when the edifice was an active volcanic island during the late Noachian - early Hesperian. Similar features on the northern flank of the 1800 km-distant Alba Mons volcano further support an extended early Ocean initially occupying Martian lowlands. Abnormal current shoreline heights imply major surface uplift promoted by internal dynamics during the main phase of development of the Tharsis Bulge at some time during the Hesperian. After ocean retreat, the uplifted shorelines were partly blanketed by late sub-aerial activity during the Amazonian. The newly proposed volcanic shorelines can be dated by radiometric methods, constituting exceptional targets to track early liquid water at key temporal steps and provide valuable insights into the timing and fate of Martian Oceans.

The history of global strain and geodynamics on Mars

The tectonic record of Mars is dominated by compressional wrinkle ridges on volcanic surfaces, and these structures have been widely used as a record of tectonic and geodynamic evolution. This study analyzes the density of compressional tectonic structures and inferred strain as functions of time, using the lengths and heights of compressional ridges together with geologic estimates of surface age. Our analyses confirm an apparent peak in compressional strain in the late Noachian and early Hesperian, and comparatively lower values before and after. The lower tectonic strain in the early and middle Noachian relative to the early Hesperian reflects the incompleteness of the ancient compressional tectonic record of strain, as older surfaces should accumulate more strain than younger surfaces. This strain deficit necessitates other means of accommodating contractional strain in the ancient crust, including distributed strain and small-scale faulting. The abrupt decrease in the accumulated tectonic strain and strain rate after the early Hesperian reflects a rapid episode of global contraction followed by much lower rates, in conflict with models of steady-state mantle evolution. The decreasing strain rate may be explained by a changing mantle rheology due to volcanic outgassing. Alternatively, the strain history may be explained by the dominance of mantle plumes in the late Noachian and early Hesperian associated with the formation of Tharsis and the early Hesperian volcanic provinces, which could have led to a pulse of rapid contraction caused by disequilibrium cooling and volcanic outpourings. The tectonic record of strain on Mars – including the incomplete ancient record and evidence for strong departures from steady-state models – may have implications for the interpretation of the tectonic record and thermal evolution of other bodies as well.

Inverted fluvial channels in Terra Sabaea, Mars: Geomorphic evidence for proglacial paleolakes and widespread highlands glaciation in the Late Noachian–Early Hesperian

Inverted fluvial channels in Terra Sabaea, Mars: Geomorphic evidence for proglacial paleolakes and widespread highlands glaciation in the Late Noachian–Early Hesperian

Structural Architecture and Deformation History of Tempe Terra, Mars

AbstractThe structurally complex region of Tempe Terra, located in the northeast of the Tharsis Rise on Mars, preserves deformation related to the growth of Tharsis and lies along the trendline formed by the Tharsis Montes volcanoes. We characterize the spatiotemporal tectonic evolution of Tempe Terra based on comprehensive structural mapping. From this mapping, we identified 16 cross‐cutting fault sets and placed these in relative time order, based on a hybrid approach using cross‐cutting relationships and buffered crater counting. We are thus able to provide a broad framework for understanding the timing of development for the Tharsis Rise and Tharsis Montes axial trend (TMAT). Our work shows that Tempe Terra has experienced three distinct stages of tectonic activity from the Middle Noachian to the Late Hesperian. Stage 1 involved E–W extension followed by localized NE–SW extension, which produced local zones of N and NW faulting through the center and west of Tempe Terra in the Noachian. Stage 2 produced intense NE‐oriented faulting concentrated along the TMAT in the Early Hesperian as a result of a discrete period of NW–SE extension and local volcanism. Stage 3 involved NW–SE extension coinciding with Tharsis volcanic activity, which generated a regional fabric of ENE‐trending graben distributed across Tempe Terra from the Early to Late Hesperian. We observe an overall peak in tectonic activity in the Early Hesperian and find that Tharsis‐related extensional deformation in the form of NE‐oriented radial faulting did not start in Tempe Terra until this time.

Episodic and declining fluvial processes in Noctis Fossae, Syria Planum Province, Mars

The Noachian surfaces across Mars bear abundant evidence for aqueous processes; however, the transformation of the planet from warm and wet to the present-day cold and dry is unclear. A key question is whether these processes continued by the same or different magnitudes in the Hesperian and Amazonian times. The equatorial region of Mars has extensive sedimentary records of Hesperian time. The area includes Valles Marineris, Echus Plateau, Melas Basin, etc., with a probable water source from the glacial melt in the southern highland caused by heating due to Tharsis volcanism. The Noctis Fossae with extensive fluvial erosional channels situated on the transition zone between Valles Marineris and Tharsis has preserved evidence of flowing water. Our dataset includes mosaics of ∼6 m/pixel resolution Context Camera (CTX) images mapped in Arc-GIS software. The area has recorded NNE-SSW trending channels maintaining U-shaped glacial valleys at higher altitudes and V-shape fluvial valleys at lower altitudes forming parallel to subparallel drainage system, fluvial terraces, surfaces, and deep gorges on the Hesperian volcanic rocks. The progression of the faulting episodes centered at Syria Planum, Pavonis-I, and Pavonis-II has influenced the fluvial activity in the Noctis Fossae. The first phase of faulting centered at Syria Planum formed NNE-SSW trending grabens in the early Hesperian and created weak zones for the water to drain. The second phase of faulting centered at Pavonis-I induced the collapse of Noctis Labyrinthus in the south, concentrating water to the troughs, implying the decline in the fluvial processes in the early stages of Amazonian time. The third phase trending in the NW-SE direction cross-cuts all geomorphic units except younger scoria cones, which mark the most recent geological process in Noctis Fossae.

Possible widespread occurrence of vermiculite on Mars

On Earth, trioctahedral (Mg-rich) vermiculite occurs as a transient alteration product of Mg/Fe2+-mica and chlorite, pyroxene, olivine, amphibole and even plagioclase where there is a nearby Mg source, indicating mild/short-lived weathering or hydrothermal conditions. On Mars, the abundant mafic rocks and low water availability suggest the likelihood of widespread vermiculite formation from the Noachian to the early Hesperian. Although it can be challenging to identify vermiculite by remote sensing or in situ infrared spectroscopy alone, we show that numerous previous remote sensing results suggest that vermiculite is widespread on the martian surface and perhaps a more common mineral on that planet than on Earth. Investigation of rocks from Riotinto (SW Spain) provides an example of how chlorite alters to vermiculite in an acidic environment. Similar incomplete vermiculite replacements of Mg/Fe2+-mica, chlorite and mafic minerals could have resulted on Mars from acidic to neutral weathering at low water regimes. While possible bias exists towards remote sensing detection of vermiculite on Mars in sites (1) with advanced alteration and (2) where vermiculite is not mixed (at pixel level) with other phyllosilicates, we find that vermiculite occurs in Noachian terrains, in sites (wider than pixel level) where other phyllosilicates are detected, and that vermiculite has more Fe3+ than is typical on Earth.

Clay sediments derived from fluvial activity in and around Ladon basin, Mars

The morphology and mineralogy of light-toned layered sedimentary deposits were investigated using multiple orbital datasets across the Ladon basin region, including within northern Ladon Valles, southern Ladon basin, and the southwestern highlands of Ladon basin. Light-toned layered deposits are particularly widespread in Ladon Valles and Ladon basin, ranging laterally for distances over 200 km, with the thickest exposure (54 m) located at the mouth of Ladon Valles. The restriction of layered sediments below a common elevation (−1850 m) in Ladon Valles and Ladon basin and their broad conformable distribution with bedding dips between 1 and 4° favor a lacustrine environment within this region during the Late Noachian to Early Hesperian. The Ladon layered deposits have spectral signatures consistent with Mg-smectites, even when the morphology of the layering varies considerably in color and brightness. These phyllosilicates were most likely eroded from the highlands upstream to the south, but the lacustrine environment may have also been favorable for in situ alteration and formation of clays. The southwestern highlands also display light-toned layered deposits within valleys and small basins. These sediments predominantly have signatures of Mg-smectites, although we also identified Fe/Mg-smectites and additional hydrated phases in some deposits. One of these altered deposits was found within a younger Holden crater secondary chain, possessing a Late Hesperian to Early Amazonian age for valleys and sediments that postdate the deposits within Ladon Valles and Ladon basin. Phyllosilicate signatures were also detected in the ejecta from two fresh craters that exposed highland materials upstream of Arda Valles, revealing that the highlands are clay-bearing and may be the most plausible source of the clay-bearing fluvial-derived sediments found within the valleys and basins downstream. Some of the highland deposits are likely coeval to similar clay-bearing sediments found to the south within Holden and Eberswalde craters, indicating late, widespread fluvial activity and deposition of allochthonous clays within the broader Margaritifer Terra region when Mars was thought to be colder and drier.

In Situ and Orbital Stratigraphic Characterization of the InSight Landing Site—A Type Example of a Regolith‐Covered Lava Plain on Mars

AbstractThe InSight lander rests on a regolith‐covered, Hesperian to Early Amazonian lava plain in Elysium Planitia within a ∼27‐m‐diameter, degraded impact crater called Homestead hollow. The km to cm‐scale stratigraphy beneath the lander is relevant to the mission's geophysical investigations. Geologic mapping and crater statistics indicate that ∼170 m of mostly Hesperian to Early Amazonian basaltic lavas are underlain by Noachian to Early Hesperian (∼3.6 Ga) materials of possible sedimentary origin. Up to ∼140 m of this volcanic resurfacing occurred in the Early Amazonian at 1.7 Ga, accounting for removal of craters ≤700 m in diameter. Seismic data however, suggest a clastic horizon that interrupts the volcanic sequence between depths of ∼30 and ∼75 m. Meter‐scale stratigraphy beneath the lander is constrained by local and regional regolith thickness estimates that indicate up to 10–30 m of coarse‐grained, brecciated regolith that fines upwards to a ∼3 m thick loosely‐consolidated, sand‐dominated unit. The maximum depth of Homestead hollow, at ∼3 m, indicates that the crater is entirely embedded in regolith. The hollow is filled by sand‐size eolian sediments, with contributions from sand to cobble‐size slope debris, and sand to cobble‐size ejecta. Lander‐based observations indicate that the fill at Homestead hollow contains a cohesive layer down to ∼10–20 cm depth that is visible in lander rocket‐excavated pits and the HP3 mole hole. The surface of the landing site is capped by a ∼1 to 2 cm‐thick loosely granular, sand‐sized layer with a microns‐thick surficial dust horizon.

Noachian Proglacial Paleolakes on Mars: Regionally Recurrent Fluvial Activity and Lake Formation within Closed-source Drainage Basin Craters

Abstract A body of geologic evidence suggests that Noachian-aged craters on Mars were modified primarily by runoff from rainfall in a warm and wet early Mars climate. Although melting and runoff of frozen water ice have been suggested as plausible alternatives, supporting geomorphic evidence of Noachian glaciation on Mars has been elusive. We previously identified a Noachian-aged crater in the southern highlands that contained evidence of glacially derived fluvial and lacustrine features but was hydrologically disconnected from its surroundings. This closed-source drainage basin (CSDB) crater and proglacial paleolake provided the first candidate evidence of Noachian cold-based glaciation on Mars. Here, we describe a second nearby CSDB crater that contains similar evidence of glacial melting leading to the formation of proglacial fluvial channels and paleolakes on the crater floor. This new evidence suggests that CSDB formation was occurring episodically at regional scales throughout the Late Noachian and into the Early Hesperian. Recurrent episodes of glacial melting are consistent with climate model predictions of a cold and icy early Mars highlands with limited melting of glacial ice during periods of punctuated warming.

Evidence of Regionally Distributed Tectono‐Volcanism in a Floor Fractured Crater of North‐Central Arabia Terra, Mars

AbstractArabia Terra features many tectonic structures, which have been correlated to episodes of incipient tectonics, massive impacts, and controls on fluvial systems. However, the extent of regionally distributed tectono‐volcanism and its effects on local structures remains unknown. To characterize the prevalence of volcanically driven processes in this region, we investigate the geomorphic features of an unnamed ∼85‐km diameter and 3.6 Ga old floor‐fractured crater (FFC) in North‐Central Arabia Terra. Widespread crater floor rocks resemble layered sedimentary deposits. Nevertheless, the central crater floor hosts mounds and linear ridges, which have higher thermal inertia than the surroundings, indicating a provenance distinct from the sedimentary units. Crosscutting relationships suggest that the mounds and ridges are stratigraphically overlying the layered sedimentary rocks. Morphologically and morphometrically, the mounds and ridges resemble known Martian and terrestrial volcanic cone and dike systems. Unlike other FFCs of Arabia Terra, the dikes and cone azimuths consistently orient in the NW‐SE direction, implying that regional tectonic controls on their formation overrode localized effects of the crater‐forming impact. Our observations collectively support magma intruding and erupting along regionally controlled tectonic structures; the migration of magma was perhaps facilitated by the impact that formed the host crater. Consequently, the studied FFC may represent a category of hitherto unrecognized numerous, small‐scale volcanic centers controlled by regional tectono‐volcanism within Arabia Terra. This is also consistent with regionally distributed magmatic systems in the late Noachian to early Hesperian, associated with a thermally eroded crust.

The Timing of Alluvial Fan Formation on Mars

Abstract The history of rivers on Mars is an important constraint on Martian climate evolution. The timing of relatively young, alluvial fan-forming rivers is especially important, as Mars’s Amazonian atmosphere is thought to have been too thin to consistently support surface liquid water. Previous regional studies suggested that alluvial fans formed primarily between the Early Hesperian and the Early Amazonian. In this study, we describe how a combination of a global impact crater database, a global geologic map, a global alluvial fan database, and statistical models can be used to estimate the timing of alluvial fan formation across Mars. Using our global approach and improved statistical modeling, we find that alluvial fan formation likely persisted into the last ∼2.5 Gyr, well into the Amazonian period. However, the data we analyzed were insufficient to place constraints on the duration of alluvial fan formation. Going forward, more crater data will enable tighter constraints on the parameters estimated in our models and thus further inform our understanding of Mars’s climate evolution.

A global investigation of wrinkle ridge formation events; Implications towards the thermal evolution of Mars

Wrinkle ridges (WR) are well-established paleo-stress indicators providing information on the compressional history and thermal evolution of Mars. Therefore, constraining their timing of formation on a global scale could yield better insight into the tectonic history, evolution of the mantle, and the paleoclimate of the planet. Here, we have updated the existing global map of the Martian WRs (up to a latitude of 70o on either hemisphere) using high-resolution Context (CTX) Camera images from twenty-seven different regions and inferred their ages using the Buffer Crater Counting (BCC) technique. The BCC technique enables us to conduct age estimation even when WRs propagated through more than one geological unit, or even the accountable compressional stress is younger than the underlying geologic unit, giving an additional advantage over the conventional relative age estimation technique. Globally, we have found that the ages of the WRs are almost entirely of the Early Hesperian (3.66–3.4 Ga) phenomenon, with exceptions from the Sinai Planum (~ 3.14 Ga; Late Hesperian) or dichotomy perpendicular ridges from the Amenthes Planum (~2.52 Ga; Amazonian) which possibly produced due to the horizontal forces that emerged due to isostatic adjustments. We have identified at least five distinct phases (~3.66, 3.62, 3.59, 3.48, and 3.14 Ga) of compressional deformations, related to the crustal growth in response to isostatic adjustments and flexural loading from the Noachian/Hesperian boundary period to the Late Hesperian period in and around the Tharsis region. In the eastern hemisphere, we have observed that the WR formation-stresses are global (peak global contraction ~3.59–3.55 Ga; responsible for the formation of WRs within southern highlands, parts of northern lowlands) to regional (around Syrtis Major and Malea Planum) in scale. Additionally, the estimated WR-ages indicate multiple phases of volcanic activity (~3.7, 3.54, and 3.40 Ga) eventuated at several provinces of the planet. Therefore, we conclude from the observation of WR-formation ages that the Martian mantle was vastly active during the Noachian/Hesperian boundary and the intensity of the mantle activity decreased through time, giving rise to multiple pulses of regional volcano-tectonic events, as a part of thermal evolution over the Hesperian period.

Global climate and river transport simulations of early Mars around the Noachian and Hesperian boundary

Over the past few decades, numerous geochemical and geological observations of early Mars have suggested the presence of surface river systems during the late Noachian and early Hesperian. A large volume of liquid water combined with stable and long-term fluvial activity is required to explain these river systems, suggesting that early Mars must have experienced a long period during which the climate was sufficiently warm and wet for stable surface liquid water and precipitation to occur. We performed 3-dimensional climate simulations for the late Noachian and early Hesperian using a paleo-Mars global climate model (PMGCM), assuming a CO2/H2O/H2 atmosphere under the “Faint Young Sun” condition and using the assumed topography before the formation of the Tharsis load. Cases of surface pressures of up to 2 bar and obliquities of 20°, 40°, and 60° were investigated. We also developed a river transport model, CRIS (Catchment-based RIver Simulator), which enabled us to estimate global river discharge and the transport of bedload and suspended load sediments from the PMGCM output for a variety of sediment particle sizes.Our simulation results revealed that CO2 atmospheres with surface pressures of above 2 bar and a few percent of H2 composition produced a hospitable surface environment that could support the stable liquid water on the surface for long periods. Annual precipitation increased with surface pressure, and intense precipitation occurred around the low to mid-latitudes, where the majority of the valley networks are currently observed. Our river model CRIS produced river runoff, which agrees with some parts of the observed valley distribution, and revealed that these valleys could be formed in ~104 years, which agrees with previous geological studies. However, our simulation results also suggested that Martian valleys were not all created as a result of precipitation-fed river activity, and other mechanisms such as seasonal or transient snow melting should be simultaneously considered to explain Martian valley networks.

Chronological Analysis and Remote Sensing of Craters on the Surface of Mars

In the present research, we carried out detailed chronological and compositional analyses along with detailed spectral analysis of three unnamed craters on the surface of Mars. Knowledge on chronology/age analysis and compositional analysis of Mars’ surface is essential for future manned and unmanned missions. The study area is near the landing site of previous landed missions, which could be used for future landing. The area is interesting to be studied because of its high elevation in the northeastern side and low elevation in the southern side, consisting of three major geological boundaries, i.e., Hesperian, Noachian, and Amazonian, which are further subdivided into fourteen units. Chronological investigations were carried out using the active machine learning approach and Craterstats 2.0 software, which revealed the age plot of 3.09 ± 0.04 Ga for Amazonian, 3.63 ± 0.0 Ga for Hesperian, and 3.73 ± 0.0 Ga for Noachian geological units, stating that N(1) craters’ density corresponds to the early Amazonian, early Hesperian, and late Noachian/early Hesperian periods according to the established crater density boundaries, respectively. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)-derived browse products are used for the compositional study of the surface characteristics of Mars. A spectral investigation was performed on an unnamed crater belonging to the Amazonian period, which showed to be majorly composed of oxides as the primary mineral, indicating the spectra of hematite, boehmite, and akaganeite. A Hesperian unit-unnamed crater shows the signature of monohydrated sulfates, melilite, illite, and kaolinite minerals in the region. For the unnamed crater 3, which belongs to the Noachian period, it has diagnostic absorptions of clay minerals in their extracted spectra, indicating the sign of long-term water–rock interactions in the period. Derived chronology results and compositional studies of craters help in better understanding the geological formation units of Mars’ surface.

Record of environmental changes based on a low latitude martian crater

An early fluvial and potential lacustrine event was identified in Xanthe Terra at 11.6 N 308.7E on Mars, during the early Hesperian around the termination of the main fluvial era. After a possible sudden sediment input a sharp and steep edge fan formed by fluvial activity at the termination of this channel, possibly indicating a lacustrine period, although other signatures of the ancient lake could have been covered by later sediments. The water ould come from another basin, a neighbouring crater to the south fed by another valley indicating a moderately complex hydrological system.Almost 3 Ga passed by without observable sedimentary signature, then two depositional and erosional periods followed each other (about 0.3 Ga), when dust sedimentation from the atmosphere produced a some 10 m thick, and later another few m thick layers in the crater. These periods were possibly connected to climatic variations by orbital tilt changes. Around the same periods the slope stability might also decreased, possibly rejuvenating wall debris fields at the steep crater perimeter. The eroded mantles found in this crater are closer to the equator and older than the so-called latitude dependent mantle on Mars. The stratigraphically lower- and older member of the mantles (bright one) is substantially thicker than this latitude dependent mantle on Mars. These sedimentary units straighten the idea that mantles could have formed relatively recently on Mars possibly from dust sedimentation even at a nearly equatorial location. Such mantles might hide the signatures of any possible lacustrine period thus increase the difficulty in observing old lacustrine features too. The climatic context of the formation and erosion of these mantles in terms of general models of climate change on Mars should be investigated in the near future.

Sinuous ridges in Chukhung crater, Tempe Terra, Mars: Implications for fluvial, glacial, and glaciofluvial activity

We present a geomorphic map of Chukhung crater (38.47°N, 72.42°W) in central Tempe Terra, Mars. Chukhung crater formed ~3.6–2.1 Ga, between the early Hesperian and early Amazonian periods of Mars' geologic history. It hosts dendritic networks of crater wall valleys, broad crater floor valleys, mid- to late-Amazonian-aged debris-covered glaciers, moraine-like deposits, and a radial assemblage of sinuous ridge landforms. We explore the origins of landforms in Chukhung crater, focusing in particular upon the sinuous ridges. In northern Chukhung crater, sinuous ridges extend downslope from fluvial valley systems on the northern crater wall. We interpret the northern sinuous ridges as inverted paleochannels: ridges formed by exhumation of resistant and/or indurated fluvial channel fill deposits. The origins of sinuous ridges on the southern floor of Chukhung crater are more ambiguous. They emerge from beneath moraine-like ridges which bound extant debris-covered glaciers extending from the southern wall of the crater. The southern sinuous ridges have numerous morphological and contextual similarities to eskers: ridges of glaciofluvial sediment deposited in meltwater conduits within or beneath wet-based glaciers. The close proximity of the northern and southern sinuous ridges, however, calls into question an interpretation which ascribes a different origin to each set. The similarity in the overarching process between esker and inverted channel formation (i.e., exposure by the removal of a bounding medium, be that ice or sediments/rock) results in convergence of form between eskers and inverted paleochannels. We discuss the esker-like characteristics of the southern sinuous ridges in detail, and argue that one of two ridge populations in southern Chukhung crater is best explained by the esker hypothesis while the other could be explained under either the esker or the inverted paleochannel hypothesis. Regardless of the specific formation mechanism for the southern sinuous ridges, we find that Chukhung crater has undergone significant modification by liquid water since its formation. The northern sinuous ridges and associated crater-wall valleys provide evidence for subaerial drainage of precipitation and/or snowmelt. This suggests that Chukhung crater, and possibly the surrounding region, experienced unusually warm and wet episodes between the early Hesperian and mid Amazonian. If some or all of the southern sinuous ridges are eskers, they could provide evidence for an additional influence of glacial meltwater in Chukhung crater during the mid-to-late Amazonian. If wet-based glaciation did occur in Chukhung crater, the location of the crater between major branches of the Tempe Fossae tectonic rift system would add to the growing body of evidence that elevated geothermal heat flux was an important driver of localized occurrences of recent wet-based glaciation on Mars.

Reconstructing paleolakes in Nepenthes Mensae, Mars, using the distribution of putative deltas, coastal-like features, and terrestrial analogs

Nepenthes Mensae is an equatorial region situated north of the Martian dichotomy, northwest of Gale crater. It is characterized by a NW-SE-oriented belt of interconnected depressions, Late Noachian to Early Hesperian in age, with knobby terrains with residual relief. The highlands south of Nepenthes Mensae, Middle Noachian in age, correspond to the Licus Vallis region, which is dissected by drainage networks with valley mouths located along the dichotomy boundary scarp. This work presents a detailed geomorphological analysis of ten fan-shaped and fifty-four coastal-like benches identified in Nepenthes Mensae. The combination of detailed mapping, morphological and morphometric analyses, spatial-altitudinal distribution relationships, crater counting, spectral analysis, and comparison with terrestrial analogs suggest that (1) the fan-shaped and coastal-like benches are likely putative Gilbert-type deltas and paleoshore platforms, respectively, and (2) these features may be attributable to paleoshorelines developed along the margins of an ancient inner sea or interconnected paleolakes. These findings reveal the important morphogenetic role that liquid water played in the evolution of Nepenthes Mensae and Licus Vallis regions during the Late Noachian-Early Hesperian transition, and contribute to contextualize the continuous findings on the environmental and climatic conditions of the nearby Gale crater during such time period.