Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars

The 174 km diameter Terby impact crater (28.0°S-74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100 m to 1 km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6-10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000 km³ (a large amount considering classical martian fan deltas such as Eberswalde (6 km³)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential “reference geologic cross-section” for Mars stratigraphy.     

Characterization of hydrated silicate-bearing outcrops in Tyrrhena Terra,Mars: Implications to the alteration history of Mars

The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1 km to 42 km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites–smectites and chlorite–vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20 km in diameter). Most detected minerals formed over relatively high temperatures (100–300 °C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. This is in contrast with some other phyllosilicate-bearing regions like Mawrth Vallis where smectites, kaolinites and hydrated silica were mainly identified, pointing to a predominance of surface/shallow sub-surface alteration; and where excavation by impacts played only a minor role. Smooth plains containing hydrated silicates are observed at the boundary between the Noachian altered crust, dissected by fluvial valleys, and the Hesperian unaltered volcanic plains. These plains may correspond to alluvial deposition of eroded material. The highlands of Tyrrhena Terra are therefore particularly well suited for investigating the diversity of hydrated minerals in ancient martian terrains.     

Nonuniform cratering of the terrestrial planets

We estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399-433]. After determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination, the impact positions are calculated analytically, assuming the projectiles follow hyperbolic paths during the encounter phase. As the source of projectiles is not isotropic, latitudinal variations of the impact flux are predicted: the calculated ratio between the pole and equator is 1.05 for Mercury, 1.00 for Venus, 0.96 for the Earth, 0.90 for the Moon, and 1.14 for Mars over its long-term obliquity variation history. By taking into account the latitudinal dependence of the impact velocity and impact angle, and by using a crater scaling law that depends on the vertical component of the impact velocity, the latitudinal variations of the cratering rate (the number of craters with a given size formed per unit time and unit area) is in general enhanced. With respect to the equator, the polar cratering rate is about 30% larger on Mars and 10% on Mercury, whereas it is 10% less on the Earth and 20% less on the Moon. The cratering rate is found to be uniform on Venus. The relative global impact fluxes on Mercury, Venus, the Earth and Mars are calculated with respect to the Moon, and we find values of 1.9, 1.8, 1.6, and 2.8, respectively. Our results show that the relative shape of the crater size-frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study. Nevertheless, by neglecting the expected latitudinal variations of the cratering rate, systematic errors of 20-30% in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method.     

Evidence for Hesperian impact-induced hydrothermalism on Mars

Several hydrated silicate deposits on Mars are observed within craters and are interpreted as excavated Noachian material. Toro crater (71.8°E, 17.0°N), located on the northern edge of the Syrtis Major Volcanic Plains, shows spectral and morphologic evidence of impact-induced hydrothermal activity. Spectroscopic observations were used to identify extensive hydrated silicate deposits, including prehnite, chlorites, smectites, and opaline material, a suite of phases that frequently results from hydrothermal alteration in terrestrial craters and also expected on Mars from geochemical modeling of hydrothermal environments. When combined with altimetry and high-resolution imaging data, these deposits appear associated predominantly with the central uplift and with portions of the northern part of the crater floor. Detailed geologic mapping of these deposits reveals geomorphic features that are consistent with hydrothermal activity that followed the impact event, including vent-like and conical mound structures, and a complex network of tectonic structures caused by fluid interactions such as fractures and joints. The crater age has been calculated from the cumulative crater size-frequency distributions and is found to be Early Hesperian. The evidence presented here provides support for impact-induced hydrothermal activity in Toro crater, that extends phyllosilicate formation processes beyond the Noachian era.     

Impact crater formation in icy layered terrains on Mars

Abstract— We present numerical simulations of crater formation under Martian conditions with a single near‐surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects on observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H₂O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well‐preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near‐surface icy layers are widespread on Mars.     

Nature of the Martian uplands: Effect on Martian meteorite age distribution and secondary cratering

Abstract— Martian meteorites (MMs) have been launched from an estimated 5–9 sites on Mars within the last 20 Myr. Some 80–89% of these launch sites sampled igneous rock formations from only the last 29% of Martian time. We hypothesize that this imbalance arises not merely from poor statistics, but because the launch processes are dominated by two main phenomena: first, much of the older Martian surface is inefficient in launching rocks during impacts, and second, the volumetrically enormous reservoir of original cumulate crust enhances launch probability for 4.5 Gyr old rocks. There are four lines of evidence for the first point, not all of equal strength. First, impact theory implies that MM launch is favored by surface exposures of near‐surface coherent rock (≤10² m deep), whereas Noachian surfaces generally should have ≥10² m of loose or weakly cemented regolith with high ice content, reducing efficiency of rock launch. Second, similarly, both Mars Exploration Rovers found sedimentary strata, 1–2 orders of magnitude weaker than Martian igneous rocks, favoring low launch efficiency among some fluvial‐derived Hesperian and Noachian rocks. Even if launched, such rocks may be unrecognized as meteorites on Earth. Third, statistics of MM formation age versus cosmic‐ray exposure (CRE) age weakly suggest that older surfaces may need larger, deeper craters to launch rocks. Fourth, in direct confirmation, one of us (N. G. B.) has found that older surfaces need larger craters to produce secondary impact crater fields (cf. Barlow and Block 2004). In a survey of 200 craters, the smallest Noachian, Hesperian, and Amazonian craters with prominent fields of secondaries have diameters of ?45 km, ?19 km, and ?10 km, respectively. Because 40% of Mars is Noachian, and 74% is either Noachian or Hesperian, the subsurface geologic characteristics of the older areas probably affect statistics of recognized MMs and production rates of secondary crater populations, and the MM and secondary crater statistics may give us clues to those properties.     

Martian cratering VI: Crater count isochrons and evidence for recent volcanism from Mars Global Surveyor

Abstract— This paper develops a methodology to establish absolute Martian ages by deriving isochrons on a plot of Martian impact crater density vs. crater diameter, calibrated by lunar crater/age data. The isochrons illustrated here are based on a Mars/Moon cratering ratio of 1.6 at constant size, but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages. Martian crater diameter distributions are determined in several areas down to diameter D = 16–45 m; the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by Neukum. The youngest areas studied here display the lunar-like production function down to D ～30 m, where saturation equilibrium sets in. Model crater retention ages of several volcanic units are found to be a few hundred million years or less, with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher. The results are consistent with Martian meteorite ages. Volcanism on Mars has probably persisted into the last 10 to 15% of the planet's history and is likely ongoing. Because surfaces as young as a few hundred million years have reached crater saturation equilibrium at D < ～60 to 100 m, Mars is likely to have widespread impact-produced regoliths at least a few meters deep, and this may contribute to the widespread mobile dust and boulder fields of Mars.     

Constraints on the Martian cratering rate based on the SNC meteorites and implications for Mars climatic history

Two constraints placed upon the cratering flux at Mars by the SNC meteorites are examined: crystallization ages as a constraint on surface ages and cosmic ray exposure ages and number of impacts as a constraint on absolute rates. The crystallization ages of the SNC meteorites appear to constrain the Martian cratering rate to be 4xLunar or more if the parent lavas are in the north of Mars and the number of SNC ejecting impacts are small. If the SNCs result from a single impact that formed the Lyot basin then the cratering rate must be at least 7xLunar or higher to produce a basin age less than the SNC crystallization age because the basin ages are themselves determined by crater counting. Assuming multiple uncorrelated impacts for SNC ejection from Mars over 10 million years a cratering rate of approximately 4xLunar is also found for ejecting impacts that form craters over 12km in diameter. Therefore, both crystallization ages and ejection ages and number of impacts appear consistent with a 4xLunar cratering rate at Mars. The effect on Martian chronologies of such a high cratering rate is to place the SNC crystallization ages partly within the epoch of channel formation on Mars and to extend this liquid water epoch over much of Mars history.     

Gully formation on Mars: Two recent phases of formation suggested by links between morphology, slope orientation and insolation history

The unusual 80 km diameter Noachian-aged Asimov crater in Noachis Terra (46°S, 5°E) is characterized by extensive Noachian-Hesperian crater fill and a younger superposed annulus of valleys encircling the margins of the crater floor. These valleys provide an opportunity to study the relationships of gully geomorphology as a function of changing slope orientation relative to solar insolation. We found that the level of development of gullies was highly correlated with slope orientation and solar insolation. The largest and most complex gully systems, with the most well-developed fluvial landforms, are restricted to pole-facing slopes. In contrast, gullies on equator-facing slopes are smaller, more poorly developed and integrated, more highly degraded, and contain more impact craters. We used a 1D version of the Laboratoire de Météorologie Dynamique GCM, and slope geometries (orientation and angle), driven by predicted spin-axis/orbital parameter history, to assess the distribution and history of surface temperatures in these valleys during recent geological history. Surface temperatures on pole-facing slopes preferential for water ice accumulation and subsequent melting are predicted to occur as recently as 0.5-2.1 Ma, which is consistent with age estimates of gully activity elsewhere on Mars. In contrast, the 1D model predicts that water ice cannot accumulate on equator-facing slopes until obliquities exceed 45°, suggesting they are unlikely to have been active over the last 5 Ma. The correlation of the temperature predictions and the geological evidence for age differences suggests that there were two phases of gully formation in the last few million years: an older phase in which top-down melting occurred on equator-facing slopes and a younger more robust phase on pole-facing slopes. The similarities of small-scale fluvial erosion features seen in the gullies on Mars and those observed in gullies cut by seasonal and perennial snowmelt in the Antarctic Dry Valleys supports a top-down melting origin for these gullies on Mars.     

The timing of martian valley network activity: Constraints from buffered crater counting

Valley networks, concentrations of dendritic channels that often suggest widespread pluvial and fluvial activity, have been cited as indicators that the climate of Mars differed significantly in the past from the present hyperarid cold desert conditions. Some researchers suggest that the change in climate was abrupt, while others favor a much more gradual transition. Thus, the precise timing of valley network formation is critical to understanding the climate history on Mars. We examine thirty valley network-incised regions on Mars, including both cratered upland valley networks and those outside the uplands, and apply a buffered crater counting technique to directly constrain when valley network formation occurred. The crater populations that we derive using this approach allow assessment of the timing of the last activity in a valley network independent of the mapping of specific geological units. From these measurements we find that valley networks cluster into two subdivisions in terms characteristics and age: (1) valley network activity in the cratered highlands has an average cessation age at the Noachian-Hesperian boundary and all valleys that we crater counted are Early Hesperian or older. No evidence is found for valley networks in the cratered uplands of Late Hesperian or Amazonian age. The timing of the cessation of cratered upland valley network activity at the Noachian-Hesperian boundary also corresponds to a decline in the intensity of large crater formation and degradation and to the apparent end of phyllosilicate-type weathering. (2) A few valley network-incised regions formed outside of the cratered uplands on volcanic edifices, in association with younger impact craters, and on the rim of Valles Marineris. We applied our buffered crater counting technique to four such valleys, on the volcanoes Ceraunius Tholus, Hecates Tholus, and Alba Patera and on the rim of Echus Chasma, and find that each has distinctive and different Late Hesperian or Early Amazonian ages, indicating that valley networks formed from time to time in the post-Noachian period. Unlike the cratered upland valley networks, these isolated occurrences are very local and have been interpreted to represent local conditions (e.g., snowpack melted during periods of intrusive volcanic activity). In contrast to a gradual cessation in the formation of valley networks proposed by some workers, our new buffered crater counting results indicate a relatively abrupt cessation in the formation of the widespread cratered upland valley networks at approximately the end of the Noachian, followed only by episodic and very localized valley network formation in later Mars history, very likely due to specific conditions (e.g., local magmatic heating). These valley network ages and correlations are thus consistent with a major change in the near-surface aqueous environment on Mars at approximately the Noachian-Hesperian boundary. The Noachian environment supported surface running water and fluvial erosion across Mars in the cratered uplands, enhanced crater degradation, and a weathering environment favoring the formation of phyllosilicates. The Hesperian-Amazonian environment was more similar to the hyperarid cold desert of today, with valley networks forming only extremely rarely and confined to localized special conditions. Sources of water for these latter occurrences are likely to be related to periodic mobilization and equatorward migration of polar volatiles due to variations in spin-axis orbital parameters, and to periodic catastrophic emergence of groundwater.     

Sub-kilometer fans in Mojave Crater, Mars

In the Xanthe Terra region of Mars, two forms of flow fields are observed on the walls of Mojave Crater, a fresh impact site with a maximum age of Late Hesperian. Flow fields with steep, lobate margins are consistent with emplacement of a highly viscous medium. The focus of this report is on fan-shaped landforms that share many morphologic attributes in common with terrestrial alluvial fans, including a semi-conical form, branching tributary networks, distributary channels and incised channels. Collectively, these sub-kilometer-scale landforms have attributes consistent with overland flow of fluids and formation of fans by water and gravity-driven alluvial sedimentation. Superposition and cross-cutting relationships indicate that fan formation occurred in multiple phases that may have been a single event or multiple, temporally distinct episodes. Many aspects of the fan formation are ill-constrained, including the amount and source of fluid as well as the duration of fan formation and modification. Fans are concentrated on the crater walls and the ejecta blanket shows minimal evidence of fluvial erosion. Similar fan-shaped landforms to those in Mojave Crater are extremely rare on Mars. The localization of fans to Mojave Crater implies that the impact event played a role in the formation of these sub-kilometer fans. This is the first geologic evidence on Mars that tentatively supports a link between impact crater events and the liberation of water for surface runoff.     

Martian cratering 8: Isochron refinement and the chronology of Mars

This paper reviews and refines the technique of dating martian surfaces by using impact-crater isochrons (defined as size distributions of impact craters on undisturbed martian surfaces of specified ages). In the 1970s, this system identified not only abundant ancient martian volcanic surfaces, but also extensive lava plains with ages of a few 10⁸ y-old; this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites. The present update utilizes updated estimates of the Mars/Moon cratering ratio (the most important calibration factor), improves treatment of gravity and impact velocity scaling effects, combines aspects of the crater size distribution data from earlier work by both Neukum and Hartmann, and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from Popova et al. (2003, Meteorit. Planet. Sci. 38, 905-925). The updated isochrons are not radically different from the previous “2002 iteration” but fit observed data better and give somewhat older model ages for features dated from small craters (diameter D<100 m). Crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in D, confirming the general isochron shape and giving crater retention ages in the range of some 10⁶ to some 10⁸ y, interpreted as lava flow ages. More complex, older units are also discussed. Uncertainties are greatest if only small craters (D?100 m) are used. Suggestions by other workers of gross uncertainties, due to local secondary craters and deposition/exhumation, are discussed; they do not refute our conclusions of significant volcanic, fluvial, and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation. Indeed, crater count data suggest certain very recent episodes of deposition, exhumation, and ice flow, possibly associated with obliquity cycles of ∼¹⁰⁷ y timescale. Evidence from ancient surfaces suggests higher rates of volcanism, fluvial activity, glaciation, and other processes in Noachian/Hesperian time than in Amazonian time.     

Geology of the Thaumasia region,Mars: plateau development,valley origins,and magmatic evolution

We have constructed the complex geologic history of the Thaumasia region of Mars on the basis of detailed geologic mapping and relative-age dating of rock units and structure. The Thaumasia plateau dominates the region and consists of high lava plains partly surrounded by rugged highlands, mostly of Noachian and Hesperian age. Long-lived faulting centered near Syria Planum and at lesser sites produced radiating narrow grabens during the Noachian through Early Amazonian and concentric wrinkle ridges during the Late Noachian and Early Hesperian. Fault activity peaked during the Noachian and waned substantially during Late Hesperian and Amazonian time. Volcanism on the Thaumasia plateau was particularly active in comparison with other martian cratered highlands, resulting in fourteen volcanoes and numerous outcrops of smooth, ridged, and lobate plains materials. A particularly extensive set of overlapping lava-flow units was emplaced sequentially from Thaumasia Planum to Syria Planum, spanning from the Late Noachian to the Late Hesperian; lobate flows succeeded smooth flow at the beginning of the Late Hesperian. Deep crustal intrusion and a thickened, buoyant crust may have caused the uplift of the plateau during the Noachian and Early Hesperian, resulting in outward-verging fold-and-thrust plateau margins. This structural style appears similar to that of the young ranges of the Rocky Mountains in the western U.S. Within the plateau, several sites of volcanotectonic activity and valley erosion may be underlain by large and perhaps long-lived magmatic intrusions. One such site occurs at the headland of Warrego Valles. Here, at least two episodes of valley dissection from the Noachian to Early Hesperian occurred during the formation of two nearby rift systems. The site also is a locus of intersection for regional narrow grabens during the Late Noachian and Early Hesperian. However, at the site, such faults diverge or terminate, which suggests that a resistant body of rock occurs there. The overall volcanotectonic history at Thaumasia fits into a model for Tharsis as a whole in which long-lived Syria Planum-centered activity is ringed by a few significant, shorter-lived centers of activity like the Thaumasia plateau. Valley formation, like tectonism in the region, peaked during the Noachian and declined substantially during the Hesperian and Amazonian. Temporal and spatial associations of single erosional valleys and valley networks with volcanoes, rift systems, and large impact craters suggest that the majority of valleys formed by hydrothermal, deformational, and seismic-induced processes. The origin of scattered, mainly Noachian valleys is more conjectural; possible explanations include local precipitation, seismic disturbance of aquifers, or unrecognized intrusions.     

Influences of atmospheric variations on Mars's record of small craters

Planetary atmospheres influence cratering rates at small diameters (∼2-250 m) by filtering impactor populations via ablation, aerobraking and breakup of entering objects. The atmosphere of Mars undergoes rapid and drastic obliquity-driven variations in density, corresponding to pressure variations between zero and several tens of millibars. Here a simulation is used to assess the fate of a large population of impactors interacting with the present and predicted past and future martian atmospheres. We find that even Mars's present atmosphere significantly reduces crater production rates at small diameters (<30 m) and past denser atmospheres would have affected cratering even more strongly, and to considerably larger diameters. These effects are increased if the inner Solar System's small impactor population contains significant numbers of icy, cometary bodies. Evidence of recent atmospheric density variations may be detectable in the martian small cratering record with future planned imaging capabilities. Because of martian atmospheric effects and variations, surface ages derived from counts of craters of less than about 250 m on Mars may be underestimated.     

Cratering on Mars I. Cratering and obliteration history

Computerized cratering-obliteration models are developed for use in interpreting planetary surface histories in terms of the diameter-frequency relations for craters classified by morphology. An application is made to a portion of the lunar uplands, revealing several episodes of blanketing, presumably due to the formation of some of the major basins.Application to Martian craters leads to the following picture of Martian cratering and obliteration history. During a probable period of intense early bombardment, craters were degraded by two processes: a depositional-type process connected with the declining cratering rate, and a process tending to flatten the largest craters (e.g., isostatic adjustment). During late stages of the early bombardment, or subsequent to it, there occurred a major relative episode of obliteration (probably atmosphere related), but it ceased concurrently with the massive (presumably volcanic) resurfacing of the cratered plains. Subsequent resurfacing episodes have occurred in the smooth plain terrains, but obliteration processes have been virtually absent in the low-latitude cratered terrains.Recent global Martian cratering interpretations of Hartmann and Soderblom are compared. Absolute cratering chronologies are only so good as knowledge of the absolute cratering flux on Mars. The crater data of Arvidson, Mutch, and Jones do not confirm the basis, whereby Soderblom requires the dominant Martian crater obliteration process to be coincident in time with the early bombardment. If the asteroidal-cometary impact flux on Mars has averaged five times the lunar flux during post-lunar-mare epochs, then the obliterative episode lasted about half a billion years and occurred about 1.5 × 10⁹ yr ago.     

Characterization of fluvial activity in Parana Valles using different age-dating techniques

Martian valley networks provide the best evidence that the climate on Mars was different in the past. Although these features are located primarily in heavily cratered terrain of Noachian age (>3.7 Ga), the ages of the features and the time when they were active is not well understood. From superposed craters several recent global studies determined that most valley networks formed during the Late Noachian to Early Hesperian; however, there were some disparities between the techniques. In this study, our principal objective was to test the reliability of the different age-dating techniques to better understand their accuracy and limitations. We applied these techniques to Parana Valles using a variety of high-resolution images taken from different instruments that allow us to identify smaller craters (D > 125 m) while providing sufficient coverage to support a statistically reliable sampling of crater populations, which is necessary to reduce the uncertainties in age determination. Our results indicate that Parana Valles formed during the Early Hesperian Period but that the crater density (D > 353 m) is heterogeneous inside the Parana Valles basin. The crater population decreases from the headwaters downstream recording a resurfacing event that is most likely related to the erosion of downstream sub-basins. The terrain near the source area is Late Noachian to Early Hesperian in age while terrains closer to the outlet are Early to Late Hesperian in age. Crater densities (D > 125 m) inside the valley are also heterogeneous and record several resurfacing events on the valley floor. Where the width of the valley network narrows to <2 km we found evidence of an Amazonian age eolian deposit that is a relatively thin layer of only few meters that was probably deposited as a result of topographic influences. Our results validate the reliability of several proposed age-dating techniques, but we also determined the accuracy and applicability of these techniques. Our results also demonstrate that crater populations can be used to not only determine the relative ages of valley networks, but also to map the distribution of sedimentary materials and the extent of resurfacing events that occurred after valley network formation.     

Breccia dikes and crater‐related faults in impact craters on Mars: Erosion and exposure on the floor of a crater 75 km in diameter at the dichotomy boundary

Abstract— Environmental conditions on Mars are conducive to the modification and erosion of impact craters, potentially revealing the nature of their substructure. On Earth, postimpact erosion of complex craters in a wide range of target rocks has revealed the nature and distribution of craterrelated fault structures and a complex array of breccia and pseudotachylyte dikes, which range up to tens of meters in width and tens of kilometers in length. We review the characteristics of fault structures, breccia dikes, and pseudotachylyte dikes on Earth, showing that they occur in complex network‐like patterns and are often offset along late‐stage crater‐related faults. Individual faults and dikes can undulate in width and can branch and bifurcate along strike. Detailed geological analyses of terrestrial craters show that faults and breccia dikes form during each of the major stages of the impact‐cratering process (compression, excavation, and modification). We report here on the discovery of prominent, lattice‐like ridge networks occurring on the floor of a highly modified impact crater 75 km in diameter near the dichotomy boundary of the northern lowland and southern upland. Interior fill and crater‐floor units have been exhumed by fluvial and eolian processes to reveal a unit below the crater floor containing a distinctive set of linear ridges of broadly similar width and forming a lattice‐like pattern. Ridge exposures range from ?1–4 km in length and ?65–120 m in width, are broadly parallel, straight to slightly curving, and are cross‐cut by near‐orthogonal ridges, forming a box or lattice‐like pattern. Ridges are exposed on the exhumed crater floor, extending from the base of the wall toward the center. On the basis of the strong similarities of these features to terrestrial crater‐related fault structures and breccia dikes, we interpret these ridges to be faults and breccia dikes formed below the floor of the crater during the excavation and modification stages of the impact event, and subsequently exhumed by erosion. The recognition of such features on Mars will help in documenting the nature of impact‐cratering processes and aid in assessment of crustal structure. Faults and breccia dikes can also be used as data for the assessment of post‐cratering depths and degrees of landform exhumation.     

The structural inventory of a small complex impact crater: Jebel Waqf as Suwwan,Jordan

The investigation of terrestrial impact structures is crucial to gain an in‐depth understanding of impact cratering processes in the solar system. Here, we use the impact structure Jebel Waqf as Suwwan, Jordan, as a representative for crater formation into a layered sedimentary target with contrasting rheology. The complex crater is moderately eroded (300–420 m) with an apparent diameter of 6.1 km and an original rim fault diameter of 7 km. Based on extensive field work, IKONOS imagery, and geophysical surveying we present a novel geological map of the entire crater structure that provides the basis for structural analysis. Parametric scaling indicates that the structural uplift (250–350 m) and the depth of the ring syncline (<200 m) are anomalously low. The very shallow relief of the crater along with a NE vergence of the asymmetric central uplift and the enhanced deformations in the up‐range and down‐range sectors of the annular moat and crater rim suggest that the impact was most likely a very oblique one (~20°). One of the major consequences of the presence of the rheologically anisotropic target was that extensive strata buckling occurred during impact cratering both on the decameter as well as on the hundred‐meter scale. The crater rim is defined by a circumferential normal fault dipping mostly toward the crater. Footwall strata beneath the rim fault are bent‐up in the down‐range sector but appear unaffected in the up‐range sector. The hanging wall displays various synthetic and antithetic rotations in the down‐range sector but always shows antithetic block rotation in the up‐range sector. At greater depth reverse faulting or folding is indicated at the rim indicating that the rim fault was already formed during the excavation stage.     

Early Mars climate near the Noachian–Hesperian boundary: Independent evidence for cold conditions from basal melting of the south polar ice sheet (Dorsa Argentea Formation) and implications for valley network formation

Currently, and throughout much of the Amazonian, the mean annual surface temperatures of Mars are so cold that basal melting does not occur in ice sheets and glaciers and they are cold-based. The documented evidence for extensive and well-developed eskers (sediment-filled former sub-glacial meltwater channels) in the south circumpolar Dorsa Argentea Formation is an indication that basal melting and wet-based glaciation occurred at the South Pole near the Noachian–Hesperian boundary. We employ glacial accumulation and ice-flow models to distinguish between basal melting from bottom-up heat sources (elevated geothermal fluxes) and top-down induced basal melting (elevated atmospheric temperatures warming the ice). We show that under mean annual south polar atmospheric temperatures (?100 °C) simulated in typical Amazonian climate experiments and typical Noachian–Hesperian geothermal heat fluxes (45–65 mW/m²), south polar ice accumulations remain cold-based. In order to produce significant basal melting with these typical geothermal heat fluxes, the mean annual south polar atmospheric temperatures must be raised from today’s temperature at the surface (?100 °C) to the range of ?50 to ?75 °C. This mean annual polar surface atmospheric temperature range implies lower latitude mean annual temperatures that are likely to be below the melting point of water, and thus does not favor a “warm and wet” early Mars. Seasonal temperatures at lower latitudes, however, could range above the melting point of water, perhaps explaining the concurrent development of valley networks and open basin lakes in these areas. This treatment provides an independent estimate of the polar (and non-polar) surface temperatures near the Noachian–Hesperian boundary of Mars history and implies a cold and relatively dry Mars climate, similar to the Antarctic Dry Valleys, where seasonal melting forms transient streams and permanent ice-covered lakes in an otherwise hyperarid, hypothermal climate.