Valles Marineris Research Articles

The relationship between Martian gravity and topography

The relationship between gravity and topography of various regions of Mars is used to estimate their effective elastic thicknesses T e using direct measurements of line of sight velocity, rather than spherical harmonic coefficients. Estimates of T e vary from 70 km for Tharsis, 29 km for Elysium, to 14.5 km for the southern hemisphere, and show that the thickness of the Martian lithosphere increases with age as the radioactive isotopes of K, Th, and U decay. A simple parameterised model of the convective thermal history is used to estimate the temperature structure of the lithosphere, and shows that the base of the elastic layer has a temperature of 300±50°C, or similar to the value for terrestrial continents. In both cases the rheology is probably affected by the presence of water. The short wavelength behaviour of the gravity field allows the density of the rocks that form the topography to be estimated, and gives values of about 3.0 Mg/m 3 for Tharsis and Elysium. This value is substantially greater than that of 2.7 Mg/m 3 obtained for Earth, and is in agreement with estimates from SNC (Shergottites–Nahklites–Chassigny) meteorites of 3.3 Mg/m 3. The density of the topography of Valles Marineris is only 2.35 Mg/m 3, and suggests that ice may be present below the surface. In the heavily bombarded southern hemisphere, isostatic compensation occurs at wavelengths as short as 700 km, which requires the effective compensation depth to be no more than 10 km. The gravity field with wavelengths greater than 1500 km may be supported dynamically, by a plume rising beneath the Tharsis region. The difference in temperature between the solidus and the present areotherm is less than 250°C, so melt generation can occur in rising plumes.

Global mapping of Martian hematite mineral deposits: Remnants of water‐driven processes on early Mars

Near‐global (60°S to 60°N) thermal infrared mapping by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor has revealed unique deposits of crystalline gray hematite (α‐Fe2O3) exposed at the Martian surface in Sinus Meridiani, Aram Chaos, and in numerous scattered locations throughout Valles Marineris. The Sinus Meridiani material is an in‐place, rock stratigraphic sedimentary unit characterized by smooth, friable layers composed primarily of basaltic sediments with ∼10–15% crystalline gray hematite. This unit has outliers to the north that appear to have formed by stripping and removal. The hematite within Aram Chaos occurs in a sedimentary layer within a closed basin that was likely formed during the basin infilling and predates the formation of nearby chaos and outflow terrains. This unit appears to be exposed by erosion and may be more extensive beneath the surface. The Valles Marineris occurrences are closely associated with the interior layered deposits and may be in place within the layers or eroded sediments. Overall, crystalline gray hematite is extremely uncommon at the surface, yet in all observed locations it is closely associated with layered, sedimentary units. Here we argue that these hematite deposits have formed by a process involving chemical precipitation from aqueous fluids, under either ambient or hydrothermal conditions. Thus the TES mineralogic data provide evidence that liquid water has been stable at or near the surface, probably for millions of years by analogy with terrestrial iron formations, in specific locations on early Mars.

An improved solution of the gravity field of Mars (GMM‐2B) from Mars Global Surveyor

A spherical harmonic solution of the Mars gravity field to degree and order 80, Goddard Mars Model 2B (GMM‐2B), has been developed using X band tracking data of Mars Global Surveyor (MGS) from October 1997 to February 2000 and altimeter crossovers formed from the Mars Orbiter Laser Altimeter (MOLA) data between March and December 1999. During the mapping mission, MGS was located in a near‐polar (92.9° inclination) and near‐circular orbit at a mean altitude of 400 km. The tracking data from this orbit provide a detailed, global, and high resolution view of the gravity field of Mars. Mars gravity solutions are stable to 60×60 even without application of a Kaula power law constraint. The Valles Marineris is resolved distinctly with lows reaching −450 mGals. Olympus Mons and its aureole are both separately resolved, and the volcano has a peak anomaly of 2950 mGals. The global correlation of the GMM‐2B gravity coefficients with MOLA‐derived topography is 0.78 through degree 60, and the correlation remains above 0.6 through degree 62. The global gravity anomaly error predicted from the GMM‐2B error covariance through 60×60 is 11 mGal. The global geoid error from GMM‐2B through 60×60 is 1.8 m. MGS orbit quality using GMM‐2B, as measured by overlapping orbital arcs, is 1 m in the radial direction and 10 m in total position.

Gravity field of Mars: A 75th Degree and Order Model

Radiometric tracking data collected by NASA's Deep Space Network (DSN) for Mariner 9, Viking 1 and 2 orbiters, and Mars Global Surveyor (MGS) have been analyzed to determine the gravity field of Mars. The gravity solution (MGS75D) is a complete spherical harmonics model to degree and order 75. Also parts of the solution are the gravitational constants of Mars, Phobos, and Deimos. The significant improvement in the MGS75D Mars gravity model is mainly a consequence of globally distributed data set collected from the MGS mapping orbits, the use of optimal weighting and constrained least squares solution techniques, and improved force and measurement models in the orbit determination process. The gravity anomaly map reveals significantly more detail and larger amplitudes than any previous Mars gravity map over the major impact and volcanic areas on Mars. Olympus Mons remains the largest known gravity anomaly in our solar system and Isidis Planitia, the highest‐amplitude mascon. There are mascons in the Hellas, Argyre, and Utopia basins. Valles Marineris has distinctive negative anomalies, and all volcanic features are positive. Gravity anomalies over several craters have also been resolved. The overall correlation of the MGS75D gravity anomaly map with topography derived from the Mars Orbiter Laser Altimeter (MOLA) is very high, while the polar regions display relatively poor correlation.

Primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of Mars

Five main stages of radial and concentric structures formed around Tharsis from the Noachian through the Amazonian as determined by geologic mapping of 24,452 structures within the stratigraphic framework of Mars and by testing their radial and concentric orientations. Tectonic activity peaked in the Noachian (stage 1) around the largest center, Claritas, an elongate center extending more than 20° in latitude and defined by about half of the total grabens which are concentrated in the Syria Planum, Thaumasia, and Tempe Terra regions. During the Late Noachian and Early Hesperian (stage 2), extensional structures formed along the length of present‐day Valles Marineris and in Thaumasia (with a secondary concentration near Warrego Vallis) radial to a region just to the south of the central margin of Valles Marineris. Early Hesperian (stage 3) radial grabens in Pavonis, Syria, Ulysses, and Tempe Terra and somewhat concentric wrinkle ridges in Lunae and Solis Plana and in Thaumasia, Sirenum, Memnonia, and Amazonis are centered northwest of Syria with secondary centers at Thaumasia, Tempe Terra, Ulysses Fossae, and western Valles Marineris. Late Hesperian/Early Amazonian (stage 4) structures around Alba Patera, the northeast trending alignment of Tharsis Montes, and Olympus Mons appears centered on Alba Patera. Stage 5 structures (Middle‐Late Amazonian) represent the last pulse of Tharsis‐related activity and are found around the large shield volcanoes and are centered near Pavonis Mons. Tectonic activity around Tharsis began in the Noachian and generally decreased through geologic time to the Amazonian. Statistically significant radial distributions of structures formed during each stage, centered at different locations within the higher elevations of Tharsis. Secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout Tharsis.

Three‐dimensional normal faulting models of the Valles Marineris, Mars, and geodynamic implications

Inversion of Martian topography, using a three‐dimensional boundary element model, permits revised estimates of fault dip angle, depth of faulting, paleogeothermal gradient, and extensional strain in the Valles Marineris region. The major normal faults dip at 40°–55° to depths of ∼60–75 km with comparatively small footwall uplifts (<500 m for throws to 10 km), implying that paleogeothermal gradients during faulting were ∼10 K km−1 or less, assuming relatively rapid strain rates appropriate to the extending Martian crust. Accumulation of dip‐slip offsets along the main normal faults likely was associated with anticlinal flexure and deformation of preexisting strata within the troughs. The magnitudes of the predicted vertical and horizontal displacements outside the troughs, as well as extensional strains, are spatially variable, depending on both cross‐ and along‐strike position relative to the troughs. The predicted displacements and strains attain maximum values at the troughs and decay to background values at cross‐strike distances of ∼250 km, corresponding to 3–4 times the depth of faulting, with an average province‐wide strain of 4–15%. By implication, the aggregate strain field surrounding Tharsis is a highly variable spatial and temporal composite of the inhomogeneous strain fields, each associated and scaling in size with an individual graben, from the smallest structures through the largest Valles Marineris troughs.

Interior trough deposits on Mars: Subice volcanoes?

Widespread, several‐kilometer‐thick successions of layered deposits occur as mounds that partly fill the troughs or chasmata that compose the Valles Marineris on Mars. Like terrestrial subice volcanoes, the layered deposits occur in a volcano‐tectonic setting within basins that may have held ponded water or ice. On the basis of their dimensions, morphologies, and associated catastrophic floods and other geologic events as shown in Viking and new Mars Global Surveyor (MGS) data sets, we suggest that the interior deposits are volcanic in origin and possibly generated by subice eruptions. A tuya origin for the mounds can explain the lack of external sediment, mound heights that can rival the plateau, local flat‐topped mesas, morphologically distinct mounds of different ages, horizontal to steep dips, fine‐grained materials, indications of rare volcanic vents and lava flows, and spectral composition. The extremely diverse layering of west Candor Chasma and possible volcanic cones in Melas may have formed by related subaerial eruptions. Consistent with the suggestion that interior deposits are eroding out of the wall rock, some deposits could have been erupted from sites along the walls.

Tharsis dome, Mars: New evidence for Noachian‐Hesperian thick‐skin and Amazonian thin‐skin tectonics

A photogeological reconnaissance of Viking mosaics and images of the Tharsis dome has been carried out. Fifteen new areas of transcurrent faulting have been located which, together with other structures previously detected, support a model in which the Thaumasia Plateau, the southeastern part of the Tharsis dome, is proposed to be an independent lithospheric block that experienced buckling and thrust faulting in Late Noachian or Early Hesperian times as a result of an E‐W directed compression. Evidence is presented that this stress field, rather than the Tharsis uplift, was decisive in the inception of Valles Marineris, which we consider a transtensive, dextral accident. The buckling spacing permits us, moreover, to tentatively reconstruct a Martian Hesperian lithosphere similar in elastic thickness to the mean present terrestrial oceanic lithosphere, thus supporting the possibility of a restricted lithospheric mobility in that period. Tharsis lithosphere was again subjected to shear stresses in Amazonian times, a period in which important accidents, such as strike‐slip faults, wrinkle ridges, and straight and sigmoidal graben, were formed under a thin‐skin tectonic regime, while the lithosphere as a mechanical unit had become too thick and strong to buckle. The possible causes of those stresses, and especially their relationships to a putative period of plate tectonics, are discussed.

Morphology, evolution and tectonics of Valles Marineris wallslopes (Mars)

Hillslopes up to 11 km in height can be found along the walls of the Valles Marineris troughs. The widest and deepest troughs are grabens, in which tectonics probably exerted the primary control on the wall morphology. Geographical variations in the wall morphology and profiles show that they result from complex, persistent tectonic influences, and that significant changes in erosional processes occurred during this evolution, from late Hesperian to late Amazonian. Preliminary calculations suggest that about 85–95% of the fault-controlled wall relief probably formed in an “ancient” stage prior to this transitional period. A study of the volatile content of the wall rocks, based upon the morphology and distribution of impact craters on the surrounding plateaus, shows that extreme erosional widening of the Central Valles Marineris troughs occurred during the “ancient” stage of high ground ice content. During the subsequent “recent” stage of tectonic and morphological evolution, the wall materials were partly desiccated.

Strength of Martian lithosphere beneath large volcanoes

This paper provides an estimate of the thermal state of Martian lithosphere established since the formation of the shield volcanoes. Comparison of the spherical harmonic models of Martian topography derived using harmonics truncated at degrees 10, 11, 12, and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degree 11. Therefore intermediate‐scale surface topography and gravity anomalies of Mars, specified by harmonics of degree 11–50, are considered in this study. These harmonics are dominated by shield volcanoes, Alba Patera, and Isidis basin. The central part of Valles Marineris is also well defined by these harmonics. The strong correlation of the topography and gravity anomalies over these harmonics and the positive Bouguer anomalies of the volcanoes suggest significant excess masses beneath these surface features in addition to their topographic masses. This indicates that the Martian lithosphere has been strong enough to support both the topography and the internal excess masses. We determine the rigidity of the lithosphere beneath a given surface feature. The lithosphere is modeled by a thin elastic spherical shell that overlies an inviscid interior of higher density and is subjected to both surface and internal loads. The thickness of the shell and the magnitude of the internal load are calculated such that the deformed structure resulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly. It is shown that the lithosphere beneath Elysium Mons, Alba Patera, and the central part of Valles Marineris can be modeled by elastic shells subjected to topographic loads alone, whereas the lithosphere beneath Olympus and Tharsis Montes cannot. Internal excess masses of at least 40%, and more likely 50%, of the topographic masses are required, and the lithosphere beneath must have an elastic core of ∼100–80 km to support these volcanoes. The mean thermal gradient beneath this region of Tharsis is estimated to be ∼7–10 K/km on the basis of the strength envelopes calculated using diabase rheology for the crust of 50 km thickness and olivine rheology for the underlying mantle.

Near-Infrared Spectral Variations of Martian Surface Materials from ISM Imaging Spectrometer Data

Imaging spectrometer data from the ISM instrument on Phobos 2 were used to characterize spatial variations in near-infrared spectral properties of the martian surface, to determine the correspondence between near-infrared and visible-wavelength spectral variations, and to assess lithologic variations in the surface materials. All data were radiometrically calibrated and corrected for effects of atmospheric gases using previously described methods. The data were also corrected photometrically to a standard geometry, and the estimated contribution of light backscattered by atmospheric aerosols was removed to isolate the reflectance properties of surface materials. At shorter near-infrared wavelengths, the surface varies between three major spectral types which correspond to known visible color units. Dark gray materials have 1- and 2-μm absorptions consistent with a pyroxene-containing lithology, and bright red dust has a shallow 0.9-μm absorption consistent with a poorly crystalline ferric mineralogy. Dark red soils are spectrally similar to dust although lower in albedo. In some cases their 0.9-μm ferric iron absorption is deeper and offset toward longer wavelengths than in dust. These attributes agree well with those determined in situ for comparable materials at the Mars Pathfinder landing site. At longer wavelengths, significant regional heterogeneities are observed in the slope of the spectral continuum and the depth of the 3-μm H 2O absorption. The 3-μm band is stronger in bright red soils than in most dark gray soils, but the strongest absorptions are found in intermediate-albedo dark red soils. Observed spectral variations suggest the presence of at least four surface components, dust, pyroxene-containing rock and sand, one or more crystalline ferric minerals, and a water-bearing phase. These are broadly consistent with four surface components that have been inferred from ground-based, orbital, and landed spectral studies and from in situ compositional measurements. We also conclude from our analysis that most albedo and spectral variations result from the coating of dark mafic rock materials by bright ferric dust. Dark red regions, however, are inferred to have dust-like compositions but lower albedos, due in part to intermixture of a dark, crystalline ferric mineral. Both of these major conclusions are strongly supported by landed investigations by Mars Pathfinder. The layered materials in Valles Marineris are the only geologic formation with distinctive spectral properties, including an enhanced 3-μm H 2O band and pyroxene absorptions which imply a mineralogy distinct from materials in the surrounding highlands. These properties provide important evidence for the layered materials' origins and are most consistent with mechanisms that involve volcanism restricted to the interiors of the chasmata.

An altitude‐normalized magnetic map of Mars and its interpretation

Techniques developed for the reduction and analysis of terrestrial satellite magnetic field data are used to better understand the magnetic field observations made by Mars Global Surveyor. A global distribution of radial (Br) magnetic field observations and associated uncertainties is inverted for an equivalent source magnetization distribution and then used to generate an altitude‐normalized map of Br at 200 km. The observations are well‐represented by a potential function of crustal origin, consistent with a remanent origin for the Martian magnetic features. The correlation between the 40546 Br observations and Br calculated from the magnetization solution at observation locations is 0.978. For a magnetization distribution confined to a 50 km layer, calculated magnetizations range from ‐22 to +17 A/m. We see correlations with tectonics that were only hinted at in earlier maps. Magnetic features appear to be truncated against Valles Marineris and Ganges Chasma, suggestive of a major change in crustal properties associated with faulting.

Fault-population statistics at the Valles Marineris Extensional Province, Mars: implications for segment linkage, crustal strains, and its geodynamical development

The length distribution of normal faults and grabens in the Valles Marineris region of Mars reveals the importance of fault linkage on fault-length data sets. Fault-length data for the normal-fault segments produce a continuously curving distribution on a cumulative-length vs. frequency plot, assuming that the faults all slip independently, with an approximate slope of −2 for lengths 30< L<200 km. When the Martian fault lengths are modified to account for their interaction and linkage, a more shallowly sloping (approximately −1) distribution is produced for fault lengths 30< L<300 km, reflecting the greater importance of large linked faults in accommodating the regional strains. Incremental-frequency and R-plots of fault lengths demonstrate variations in fault density with linkage and the importance of considering grabens as single structures. Unlike some terrestrial earthquake data, the Martian fault population does not reveal persuasive evidence for a small-to-large fault transition associated with lithospheric thickness or rheology, or with long-term strain. However, the decoupling of maximum fault displacement and length over the full range of the data is consistent with the dependence of displacement-size scaling on fault segment aspect ratio, rather than on the total fault length. The important inverse dependence of population power-law exponent and natural inhomogeneous fault strain, obtained previously from terrestrial fault sets and fault-growth simulations, is supported by using this Martian fault array.

The Near-Infrared Continuum Slope of Martian Dark Region Reflectance Spectra

The planet Mars has a bimodal albedo distribution with bright and dark regions. Earth-boundreflectance spectra ofbright Martian regions are generally characterized by a flat to positive continuum slope in the near-infrared (NIR), while those of dark regions tend to have a negative slope. Spectra obtained with the instrument ISM (Imaging Spectrometer for Mars) onboard the Phobos 2 spacecraft usually show the same relationship between brightness and NIR continuum slope. Nevertheless, some ISM spectra show deviations from this relationship, the most conspicuous ones being observed in the Syrtis Major and Valles Marineris regions, where some dark areas are characterized by a flat continuum slope. So far, this was interpreted to indicate a basaltic material covered with a weathering coating or a thin layer of dust. The results of a new spectroscopic investigation of analog materials for the Martian crust revealed that reflectance spectra of bulk samples generally show a preference for a negative NIR continuum slope, in contrast to spectra of powder samples. This suggests a different interpretation of the ISM spectra, namely that the unusual dark regions, which are characterized by a flat NIR continuum slope, are covered by sand-sized basaltic particles. In contrast, the common dark regions, which are characterized by a blue continuum slope, are dominated by bedrock or blocky basalt rocks. This interpretation is in agreement with the findings of other methods of remote sensing.

The gravity field of Mars: results from Mars Global Surveyor.

Observations of the gravity field of Mars reveal a planet that has responded differently in its northern and southern hemispheres to major impacts and volcanic processes. The rough, elevated southern hemisphere has a relatively featureless gravitational signature indicating a state of near-isostatic compensation, whereas the smooth, low northern plains display a wider range of gravitational anomalies that indicates a thinner but stronger surface layer than in the south. The northern hemisphere shows evidence for buried impact basins, although none large enough to explain the hemispheric elevation difference. The gravitational potential signature of Tharsis is approximately axisymmetric and contains the Tharsis Montes but not the Olympus Mons or Alba Patera volcanoes. The gravity signature of Valles Marineris extends into Chryse and provides an estimate of material removed by early fluvial activity.

Hubble Space Telescope observations of the Martian aphelion cloud belt prior to the Pathfinder mission: Seasonal and interannual variations

The presence of a globally extant equatorial belt of water ice clouds on Mars is quantitatively investigated using data from three seasons of our Hubble Space Telescope synoptic monitoring program (1993–1997). A subset of the 1996–1997 images covers the landing site of the Mars Pathfinder including a set of images taken after touchdown. Using multicolor imaging from the Wide Field Planetary Camera and the Wide Field Planetary Camera 2, we characterize both water ice cloud and dust optical depths as a function of latitude at several local times for each observing epoch. The analysis technique models calibrated data using a multiple scattering radiative transfer code. Our results support the initial results of Clancy et al. [1996a] regarding changes between the aphelion and perihelion climate of Mars and provide a more detailed look at the development and decay of the cloud belt. Comparing our dust optical depths to those of the Viking landers for the same seasons, we note a trend toward lower dust loading in late northern winter and in spring. Our observations of the Pathfinder site in July 1997 reveal a dust opacity in good agreement with that reported by Pathfinder [Smith et al., 1997b]. In addition, the serendipitous occurrence of a dust storm in Valles Marineris in late June 1997 allows us to derive a set of dust single scattering albedos for use in more accurately modeling the dust's radiative properties and effects.

Geology of Xanthe Terra outflow channels and the Mars Pathfinder landing site

The Mars Pathfinder landing site (MPLS) was selected for its location at the mouths of Ares and Tiu Valles outflow channels on Mars, where sediments from a variety of geologic units could have accumulated in a lacustrine environment. Recent geologic mapping at 1:2 M scale of the Xanthe Terra channels and Chryse Planitia near MPLS reveals a progression of fluvial erosion and deposition throughout the Hesperian period. From the geology, the watersheds of the channels were inferred, and a sequence of paleogeologic maps was produced. Three outflow episodes are recognized: (1) broad sheetwash during the early Hesperian, emanating from eastern Valles Marineris and equatorial chaos regions, with initial erosion into highland plateau basement, (2) early channel formation, and (3) subsequent flooding which deepened the channels to their current morphologies. The most recent flooding issued from Simud and Tiu Valles during the late Hesperian, indicated by stratigraphic superposition, striations along the floor of the channel, and the orientation of streamlined islands in the flood direction. This suggests that surface sediments deposited at MPLS are most likely to have come from these channels, and to a lesser extent from Ares Vallis. A ranking scheme was developed to assess the potential origins for the sediments found at MPLS. The most abundant materials are inferred to have come from the Noachian and Hesperian highland plains and reworked sediments from previous floods. Underlying materials from earlier floods could also be found at MPLS as ejecta from nearby impact craters.

Voluminous volcanism on early Mars revealed in Valles Marineris

The relative rates and importance of impact cratering, volcanism, erosion, and the deposition of sediments to the early geological history of Mars are poorly known. That history is recorded in the upper crust of the planet, which is best exposed along the 4,000-km-long canyon system called Valles Marineris. Previous studies of the stratigraphy of this region have assumed that it consists of megabreccia and fractured bedrock resulting from impacts, overlain by or interbedded with relatively thin layers of lava, and with the layering restricted to the uppermost level of the crust1,2,3,4,5,6. Here we report new high-resolution images that reveal ubiquitous horizontal layering to depths of at least 8 km in the canyons. Megabreccia should be only coarsely layered and fractured bedrock should be unlayered, so these observations indicate that volcanic or sedimentary processes were much more important in early martian history than previously believed. Morphological and compositional data suggest that the layers were formed mainly by volcanic flood lavas. Mars was therefore probably very volcanically active during at least the first billion years and after the period when the heaviest impact bombardment had ended.

The generation of martian floods by the melting of ground ice above dykes.

The surface of Mars is cut by long linear faults with displacements of metres to kilometres, most of which are thought to have been formed by extension. The surface has also been modified by enormous floods, probably of water, which often flowed out of valleys formed by the largest of these faults. By analogy with structures on Earth, we propose here that the faults are in fact the surface expression of dykes, and not of large-scale tectonic movements. We use a numerical model to show that the intrusion of large dykes can generate structures like Valles Marineris. Such dykes can provide a heat source to melt ground ice, and so provide a source of water for the floods that have been inferred to originate in some of the large valleys.

Mars: Northern hemisphere slopes and slope distributions

We investigate slope distributions in the northern hemisphere of Mars from topographic profiles collected by the Mars Orbiter Laser Altimeter. Analysis of the region from about 12°S to 82°N, over diverse geologic units, indicates that the range of regional‐scale slopes is small, generally &lt;3°. Surface smoothness is most distinctive in the vast northern hemisphere plains, where slopes are typically &lt;1°. Amazonis Planitia is particularly remarkable in its smoothness, exhibiting an rms variation in topography of &lt;2 m over a 100‐km baseline. This relative smoothness is still present when compared with other sampled areas of the Martian northern hemisphere and with volcanically resurfaced terrains elsewhere in the solar system. Planetary surfaces of large areal extent that are most comparable to Amazonis in terms of rms elevation variation over long baselines are depositional in origin and include terrestrial oceanic abyssal plains and certain sedimentary basins. Slopes across the Valles Marineris canyon system show that the upper portion of the walls are significantly and consistently steeper than the lower walls, characteristic of extensive mass wasting. The observed long‐runout is consistent with a high‐energy collapsed flow. In the neighboring Noctis Labyrinthus canyons the duality between the upper and lower walls is reduced, and indicates a lower energy modificational history and/or greater cohesion of wall rock.