Groundwater Sapping Research Articles

The stratigraphy of the Amenthes region, Mars: Time limits for the formation of fluvial, volcanic and tectonic landforms

We present results of our morphologic and stratigraphic investigations in the Amenthes region for which our observations suggest a complex spatial and temporal interrelation between volcanic and possibly water-related processes. We have produced a series of self-consistent geological maps and a stratigraphic correlation chart that show the spatial and temporal distribution of volcanic, fluvial and tectonic processes. The Amenthes region consists of a broad trough-like topographic depression that has served as a path for the supply of materials from Hesperia Planum to Isidis Planitia. It is most likely that Hesperia Planum and, in particular the area north of Hesperia Planum, including Tinto Vallis, Palos crater and the surrounding dissected highlands have acted as a source region for materials that were transported into the Amenthes trough and farther into the Isidis basin. The Amenthes trough, as well as the graben of Amenthes Fossae were formed after the Isidis impact in the Noachian and represent likely the oldest features in the Amenthes region. Dendritic valley networks, that bear evidence for surface runoff, have dissected the highlands adjacent to Amenthes Planum and within the Tinto Vallis and Palos crater region before ∼3.7 Ga. The ridged volcanic plains located near the Palos crater and Tinto Vallis region, within Amenthes Planum as well as within the Isidis transitional plains were formed between ∼3.5 and 3.2 Ga and represent the volcanic activity which resulted in the flooding of the Amenthes trough. The sinuous channel of Tinto Vallis was formed in the Hesperian (≲3.5 Ga) and shows characteristics, which are consistent with both ground water sapping and igneous processes. The Palos crater outflow channel was formed nearly at the same time as Tinto Vallis, between ∼3.5 Ga and ∼3.2 Ga and postdates the volcanic flooding of the Amenthes trough in the Hesperian. Small valleys (∼3.4–2.8 Ga) incised into the ridged plains of Amenthes Planum appear also within the transitional plains located between the Amenthes plains and the Isidis interior plains. Our model ages show that Tinto Vallis, the Palos crater outflow channel as well as the small valleys are unlikely formed at the same time and by the same processes as the dendritic valley networks and represent an episode that clearly postdates the volcanic activity.

Titan's fluvial valleys: Morphology, distribution, and spectral properties

Titan's fluvial channels have been investigated based on data obtained by the Synthetic Aperture Radar (SAR) instrument and the Visible and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft. In this paper, a database of fluvial features is created based on radar-SAR data aiming to unveil the distribution and the morphologic and spectral characteristics of valleys on Titan on a global scale. It will also study the spatial relations between fluvial valleys and Titan's geologic units and spectral surface units which have become accessible thanks to Cassini-VIMS data. Several distinct morphologic types of fluvial valleys can be discerned by SAR-images. Dendritic valley networks appear to have much in common with terrestrial dendritic systems owing to a hierarchical and tree-shaped arrangement of the tributaries which is indicative of an origin from precipitation. Dry valleys constitute another class of valleys resembling terrestrial wadis, an indication of episodic and strong flow events. Other valley types, such as putative canyons, cannot be correlated with rainfall based on their morphology alone, since it cannot be ruled out that they may have originated from volcanic/tectonic action or groundwater sapping. Highly developed and complex fluvial networks with channel lengths of up to 1200km and widths of up to 10km are concentrated only at a few locations whereas single valleys are scattered over all latitudes. Fluvial valleys are frequently found in mountainous areas. Some terrains, such as equatorial dune fields and undifferentiated plains at mid-latitudes, are almost entirely free of valleys. Spectrally, fluvial terrains are often characterized by a high reflectance in each of Titan's atmospheric windows, as most of them are located on Titan's bright ‘continents’. Nevertheless, valleys are spatially associated with a surface unit appearing blue due to its higher reflection at 1.3μm in a VIMS false color RGB composite with R: 1.59/1.27μm, G: 2.03/1.27μm, and B: 1.27/1.08μm; the channels either dissect pure bluish surface units or they are carved into terrain with a mixed spectral signature between bright and bluish surface materials. The global picture of fluvial flows clearly indicates a high diversity of parameters controlling fluvial erosion, such as climatic processes, as well as surface and bedrock types. Recent fluvial activity is very likely in the north polar region in contrast to more arid conditions at lower latitudes and at the south pole of Titan. This divergence is probably an indication of seasonal climatic asymmetries between the hemispheres. However, traces of previous fluvial activity are scattered over all latitudes of Titan, which is indicative of previous climatic conditions with at least episodic rainfall.

Mapping and analysis of a chain of channeled basins in the eastern rim region of Hellas basin, Mars

The study is a detailed look on one of the several fluvial systems located on the eastern rim region of the Hellas basin on Mars. We analyzed the morphologic and morphometric characteristics of an extensive channel system, which extends for over 650 km from 35.8°S, 106.4°E in Hesperia Planum to Reull Vallis at 39.5°S, 98.1°E, and has a drainage area of 35,000–40,000 km 2. During its traverse the channel changes its characteristics many times, indicating variations in the surface properties. Based on cross-cutting relations, the fluvial system post-dates the emplacement of the early Hesperian lava plains in Hesperia Planum but predates the Amazonian deposits. We describe the geomorphology and evolution of the system and provide evidence of both surface flow and groundwater sapping processes. A chain of channeled paleolake basins in the central parts of the system (38°S, 102°E) provides a rough estimate for the water volume (250–300 km 3) which was required to form the system. The minimum volume of surface materials eroded by the channel system is ∼74 km 3. Although this study presents the detailed analysis of only one fluvial system, the presence of many similar channel systems along the margin of Hellas suggests that late-stage surface runoff has played a significant role in the degradation of the rim of the basin and also in the transportation of materials towards Hellas floor.

How to find a habitable planet

Instead of diving into the Intergovernmental Panel on Climate Change Reports, astrobiologists interested in the physics behind global warming could start with Jim Kasting’s new book, How to Find a Habitable Planet. Kasting not only describes how to find another habitable planet but also clearly explains the long-term evolution of terrestrial planet atmospheres. This is the long-term context needed to understand short-term global warming. Kasting explains the behavior of terrestrial planet atmospheres on billion-year timescales and shows us how, over the long-term on Earth, greenhouse gases come and go and the increasing luminosity of the Sun eventually humidifies the stratosphere, where H2O is photolyzed and lost to space. Thus, the evolution of the Sun produces a leak in Earth’s hydrological cycle that eventually sinks the boat of life. In blurbs for this book, NASA Ames astrobiologist Chris McKay wrote ‘‘I learned several new things from this book,’’ while MIT’s Sara Seager wrote ‘‘everything you need to know about habitable worlds.’’ As in any dialectical marketing scheme, the truth is in the middle. If you read this book you will learn more than ‘‘several new things’’ but certainly not ‘‘everything you need to know about habitable worlds.’’ Among the new things you might learn are that the Sun’s luminosity increases about 1% every 100 million years; that methane, not ammonia, is the greenhouse gas of choice that most plausibly solves the faint early Sun problem; that tropospheric cold traps keep stratospheres dry. You might learn how to distinguish flashflood outflow channels on Mars from runoff channels due to ground water sapping; that Rayleigh scattering is proportional to l 4 and why this explains how high CO2 abundance acts as an anti-greenhouse gas by increasing Earth’s albedo at short wavelengths; or how the Snowball Earth hypothesis is based on equatorial dropstones, diamictites, and cap carbonates from 710 and 630 million years ago. You might also learn the strengths and weaknesses of the various techniques for finding exoplanets and the important difference between a false positive and a false negative. Kasting writes with the pace and authority of having taught this material many times to undergraduates at Pennsylvania State University. During a Jet Propulsion Laboratory sabbatical he compiled his lecture notes into this book and included the most important insights garnered from NASA Terrestrial Planet Finder meetings in 2004–2005. The extensive set of bibliographic notes are just what the overburdened academic needs to direct an enthusiastic but misinformed classroom of astrobiology students. Kasting has found and organized the essential references that will guide the beginner. For example, for a treatment of the Milankovich cycles, he leads the reader to Imbrie and Imbrie (1979) Ice Ages: Solving the Mystery. So much information is clearly dispensed that it is a pleasure to read. The digressions in the *100 footnotes provide some relief when the text seems too simple. The book includes 8 pages of color figures and an impressive 18 pages of bibliographic notes. In addition to providing an excellent teaching resource, the book has much to offer to the seasoned astrobiologist. In chapter 10, Kasting describes, with authority and familiarity, the history of circumstellar habitable zone research. He traces the increasingly sophisticated research on the circumstellar habitable zone as it moves from Harlow Shapley’s (1953) ‘‘Liquid Water Belt’’ to Hubertus Strughold’s (1953–1955) ‘‘Ecosphere of the Sun’’ to the work of Huang in 1959–1960 to Dole’s (1964) Habitable Planets for Man. Kasting describes how Hart (1978–1979) inappropriately used global glaciations to set the outer boundary of his habitable zone (we now know that we have probably lived through several such Snowball Earth episodes). Kasting reviews and extends the oft-cited habitable zone research of Kasting et al. (1993). He compares the atmospheric evolution of Venus, Earth, and Mars—a comparison that helps provide constraints on the inner and outer boundaries of the habitable zone. He describes how the increasing luminosity of host stars leads to the concept of a continuous habitable zone, and he gives us the pros and cons of habitable zones around M stars. The work of Franck et al. (2000a,b) on the continental-crust-dependent habitable zone is also discussed. In several chapters Kasting describes climate models and their dependence on a zoo of feedback mechanisms: stabilizing negative feedback, runaway positive feedback, feedback at different timescales. Some feedback loops involving the same components act in different directions on different timescales. CO2 seems to be linked to all kinds of complicated feedback. Trying to make a prediction based on a short-term positive feedback superimposed on a long-term negative feedback sounds as confusing and nonlinear as parenting a teenager. Kasting articulately walks us through this minefield and points out some instructive exceptions to this unpredictability. The buildup of CO2 from ongoing

A tale of two planets: geomorphology applied to Mars' surface, fluvio‐deltaic processes and landforms

AbstractDeltas on planet Mars record past climate, but so far a wide range of hypotheses for their formation have been proposed. The objective of this paper is to understand martian fan deltas, their formative conditions, evolution and formative duration, and implications for the past climate.As an introduction to Mars, physiographic provinces are described and unambiguous proof is listed for the presence of flowing water in the past, such as certain minerals, groundwater, catastrophic outflow channels, alluvial fans and fan deltas, distributary networks and glaciers. The climate history of Mars differs from that of Earth by having had much drier conditions than on Earth, extreme intermittency and extreme events, most of them billions of years ago.Tens of fan deltas, unchannelized fan deltas and stepped fans have been found in impact crater and other lakes. The stepped fans were likely formed by backstepping under fast rising lake levels and have no known terrestrial equivalent. The fan deltas were formed once the lake overflowed. Alluvial fans are much more numerous and formed with less water.The delta studies illustrate how major challenges of martian morphology and reconstruction of past conditions can be taken up most effectively by combinations of the available high‐resolution images and digital elevation models, terrestrial analogues, laboratory experiments and physics‐based models gleaned from geomorphology.To resolve formative mechanisms and time scale of martian fans and deltas, morphological distinctions between dense debris flows and dilute fluvial flows must be identified for both source and sink areas. Furthermore, the properties of the martian surface material are very poorly constrained but can be explored by modelling various mass wasting, fluvial and glacial phenomena and hydrology, and by experimentation with slightly cohesive sediment. Finally, the highly debated role of groundwater sapping in valley and delta formation on Mars should be explored experimentally. Copyright © 2010 John Wiley & Sons, Ltd.

Sedimentary deposits in Xanthe Terra: Implications for the ancient climate on Mars

A variety of sedimentary deposits is observed in Xanthe Terra, Mars, including Gilbert-type deltas, fan deltas dominated by resedimentation processes, and alluvial fans. Sediments were provided through deeply incised valleys, which were probably incised by both runoff and groundwater sapping. Mass balances based on High-Resolution Stereo Camera (HRSC) digital terrain models show that up to ∼30% of the material that was eroded in the valleys is present as deltas or alluvial fan deposits. Stratigraphic relationships and crater counts indicate an age of ∼4.0 to ∼3.8 Ga for the fluvial activity. Hydrologic modeling indicates that the deposits were probably formed in geologically very short time scales. Our results point to episodes of a warmer and wetter climate on early Mars, followed by a long period of significantly reduced erosion rates.

Formation of Box Canyon, Idaho, by Megaflood: Implications for Seepage Erosion on Earth and Mars

Amphitheater-headed canyons have been used as diagnostic indicators of erosion by groundwater seepage, which has important implications for landscape evolution on Earth and astrobiology on Mars. Of perhaps any canyon studied, Box Canyon, Idaho, most strongly meets the proposed morphologic criteria for groundwater sapping because it is incised into a basaltic plain with no drainage network upstream, and approximately 10 cubic meters per second of seepage emanates from its vertical headwall. However, sediment transport constraints, 4He and 14C dates, plunge pools, and scoured rock indicate that a megaflood (greater than 220 cubic meters per second) carved the canyon about 45,000 years ago. These results add to a growing recognition of Quaternary catastrophic flooding in the American northwest, and may imply that similar features on Mars also formed by floods rather than seepage erosion.

Wind Scour of Navajo Sandstone at the Wave (Central Colorado Plateau, U.S.A.)

At the Wave, a photogenic landform on the Utah–Arizona border, modern, southwesterly, sand-carrying winds abrade the Navajo Sandstone. Abundant trains of centimeter-scale, transverse, upwind-facing treads and risers cut sedimentary structures at a high angle. Central to the formation of these erosional steps are crusts produced by microbes lying just beneath exposed sandstone surfaces. Treads and risers are present on the walls of smoothly curved troughs at the Wave, on the walls of nearby circular scour pits, and on bedrock domes found at the center of scour pits. Because of their locations and orientations, the large-scale troughs and scour pits could not have been formed by flowing water or groundwater sapping; treads and risers indicate sculpting by the wind.

Formation of amphitheater-headed valleys by waterfall erosion after large-scale slumping on Hawai'i

Amphitheater-headed valleys are common on the surfaces of Earth and Mars. The abrupt terminations of these valleys at their headwalls have been used extensively to argue for valley erosion from springs (i.e., seepage erosion or groundwater sapping) rather than surface runoff. This interpretation has significant implications for Martian hydrology and the associated prospects for life. A connection between channel form and the erosion processes induced by groundwater, however, has not been demonstrated in resistant rock. Perhaps the most widely cited terrestrial analogs for Martian amphitheater-headed valleys in basalt are the spectacular canyons of Kohala, Hawai‘i. Here we present new field observations and topographic analyses of the amphitheater-headed Kohala valleys. We found no evidence for intensively weathered rocks or alcoves around springs at valley headwalls. Instead, valley-head erosion appears to be dominated by waterfall plunge pools. Stream flow from peak annual precipitation events exceeds spring discharge by more than an order of magnitude, and such flow is responsible for evacuation of the coarse sediment that lines the streams. Bathymetric surveys along the Kohala coast have revealed a large submarine landslide, the Polol Slump, directly offshore of the Kohala valleys. We propose that the headscarp of this massive landslide is expressed as the present-day 400 m Kohala sea cliffs. As dominant streams poured over this headscarp as waterfalls, vertical plunge pool erosion and undercutting caused upstream propagation of knickpoints, eventually producing amphitheater-headed valleys. Island subsidence rates and the ages of volcanic eruptions and submarine terraces indicate that the average rate of valley headwall advance is as high as 60 mm/yr. We propose a simple expression for upslope headwall propagation by vertical waterfall erosion based on abrasion by impacting sediment particles in plunge pools. This model indicates that head-wall propagation depends nonlinearly on the sediment flux passing over the waterfall and linearly on the ratio of kinetic versus potential energy of sediment impacts. After the Polol Slump, many streams did not form upslope-propagating waterfalls because they had smaller discharges due to a radial drainage pattern and fault-bounded drainage divides, which prevented runoff from the wetter summit of the volcano. A threshold for headwall propagation due to sediment supply or sediment-transport capacity is consistent with the model. Island subsidence following valley formation has resulted in alluviation of the valley floors, which has created the observed U-shaped valley cross sections. Our interpretation implies that surface runoff can carve amphitheater-headed valleys and that seepage erosion cannot be inferred based solely on valley form on Earth, Mars, or other planets.

Can springs cut canyons into rock?

Amphitheater‐headed valleys on Earth and Mars are often assumed to result from erosion by emerging spring water (i.e., seepage erosion or groundwater sapping) rather than by surface runoff. The origin of such valleys has implications for landscape evolution on Earth and the hydrologic cycle and associated potential for life on other planets. In this paper we explore the evidence for seepage erosion in bedrock to address whether valley morphology can be used as a diagnostic indicator of seepage erosion. Seepage erosion is an important process in loose sediment where hydraulic forces cause grain detachment, often resulting in amphitheater‐headed valleys. However, the extension of these processes to resistant rock is uncertain. In sedimentary rocks, groundwater might control the shape and rate of valley formation. It is possible, however, that seepage plays only a secondary role to runoff processes. This seems likely in basaltic valleys on Earth, where little evidence exists for seepage erosion. Since the ability of seepage to erode bedrock valleys remains unclear and because many amphitheater‐headed valleys were probably carved by other processes, seepage erosion should not be inferred based solely on valley form.

Groundwater drainage and recharge by networks of irregular channels

[1] A linear complementarity theory is proposed for the coupled treatment of groundwater seepage and surface runoff along a sloping plane ground perturbed by irregular channels. Steady downslope routing is applied to the two-dimensional overland flow, while Green functions are used to relate the three-dimensional groundwater motion to the surface drainage and recharge distribution. The coupling between the surface and subsurface components is formulated as a linear complementarity problem. The algorithms needed to compute useful solutions are presented and validated. Two variants of the theory are addressed, one applicable to large-scale problems in which capillary effects can be neglected and the other to small-scale processes for which the range of variation of the topography does not exceed the capillary fringe. Linearized theories for these two cases are found to be very close in their structure and in their predictions, providing a rigorous basis for the representation of field-scale processes by laboratory-scale analogues. Predictions from the theory are compared with detailed experiments in which groundwater sapping was monitored using a video camera and a laser sheet scanning apparatus.

Morphometric analysis of Martian valley network basins using a circularity function

This paper employs a circularity function to quantify the internal morphology of Martian watershed basins in Margaritifer Sinus region and to infer the primary erosional processes that led to their current geomorphologic characteristics and possible climatic conditions under which these processes operated. The circularity function describes the elongation of a watershed basin at different elevations. We have used the circularity functions of terrestrial basins that were interpreted as having been modified by (1) erosion related to primarily groundwater sapping and (2) erosion related to primarily rainfall and surface run‐off, as well as the circularity functions of cratering basins on the Moon, in order to formulate discriminant functions that are able to separate the three types of landforms. The spatial pattern of the classification of Martian basins based on discriminant functions shows that basins that look morphologically similar to terrestrial fluvial basins are mostly clustered near the mainstream at low elevation, while those that look morphologically similar to terrestrial basins interpreted as groundwater sapping origin are located near the tributaries and at higher elevation. There are more of the latter than the former. This spatial distribution is inconsistent with a continuous Earth‐like warm and wet climate for early Mars. Instead, it is more aligned with an overall early dry climate punctuated with episodic wet periods. Alternatively, the concentrated erosion in the mainstream could also be caused by a change of water source from rainfall to snowfall or erosion cut through a duricrust layer.

Morphology of drainage basins as an indicator of climate on early Mars

We show that drainage basin morphology correlates with climate. Computational analysis of the 46 basins extracted from the western slopes of the Andes reveals the existence of four different basin morphologic classes. This purely geomorphic partition correlates with division of the same basins on the grounds of climate types. Basins are compared using circularity functions as their formal representations. Self‐organizing maps and dendrograms are employed to provide basin classification. One class of basin morphologies corresponds to sites in the arid Atacama Desert, and the other class corresponds to sites in the Atacama exhibiting groundwater sapping landforms. Using the same technique, we study a larger sample of 94 basins that, in addition to the Andean basins, includes other terrestrial basins and 26 basins from Martian sites that show prominent valley networks. The classification of this larger set shows that morphologies of Martian and terrestrial basins bifurcate at the root of the dendrogram, forming two separate domains of basin morphologies. The similarity map reveals that, of all the terrestrial basins, the Atacama Desert basins are morphologically closest to the Martian basins. Extrapolating the terrestrial morphology‐climate linkage to Mars points to formation of valley networks in a hyperarid climatic environment. We submit that the Atacama Desert provides the best possible terrestrial morphologic analog to valley network sites on Mars. We discuss climatic and hydrologic particularities of the Atacama Desert and hypothesize that a similar environment existed on early Mars.

Groundwater‐controlled valley networks and the decline of surface runoff on early Mars

Fluvial erosion on early Mars was dominated by valley networks created through a combination of groundwater processes and surface runoff. A reduced greenhouse effect due to CO2 loss, together with a declining geothermal heat flux, promoted the growth of a cryosphere and a Hesperian hydrologic regime dominated by outflow channel formation. We test the hypothesis that the transition from valley network to outflow channel formation was preceded by a more subtle evolution characterized by a weakening of surface runoff, leaving groundwater processes as the dominant, final source of valley network erosion. Our hypothesis, supported by a terrestrial analog in the Atacama desert of Chile, is related to the groundwater sapping reactivation hypothesis for densely dissecting highland valley networks on Mars suggested by Baker and Partridge in 1986 and focuses on the age analysis of large, sparsely dissecting valley networks such as Nanedi Valles, Nirgal Vallis, valleys in fretted terrain, and tributaries of outflow channels and Valles Marineris chasmata. We find that these features are consistently late Noachian to Hesperian in age, younger than Noachian densely dissecting dendritic valley networks in the southern highlands. In the Tharsis region the observation of dense and sparse valley network morphologies on Hesperian terrain suggests that while surface runoff gave way to groundwater processes consistent with our hypothesis, the transition may have occurred later than elsewhere on the planet. The volcanic nature of Tharsis suggests that geothermal heat and volatile production led to episodically higher volumes of surface runoff in this region during the Hesperian.

Dynamic river channels suggest a long‐lived Noachian crater lake on Mars

Recent images of the Noachian‐age Holden Northeast Crater show spectacular meandering channels that form a complex 150 meter thick lacustrine river delta deposit confined to the northwest margin of the crater. We identify 11 major avulsion events in the topmost layer and suggest an avulsion frequency of a few hundred years. The meandering nature of the channels and numerous avulsions is incompatible with deposition on an alluvial fan and clearly reflects a complex history of multiple sustained flows. Assuming an average sedimentation rate of 1mm/year, we suggest that Holden NE Crater contained a lake that persisted for at least 150,000 years. Our analysis is incompatible with this feature being the result of a major bolide impact that would have released a catastrophic flood by groundwater sapping and supports the hypothesis that early Mars was both warmer and wetter during the Noachian.

Implications of bank failures and fluvial erosion for gully development: Field observations and modeling

Gully erosion is most commonly triggered by fluvial erosion following natural and anthropogenic disturbances or as a response to changes in climate and tectonic forcing and base level drop. Field observations attribute the headward growth and widening of many gully systems to gravitational mass‐wasting processes of oversteepened sidewalls. Soil saturation, groundwater sapping, and tension crack development contribute to the instability. Recent landscape evolution models treat such mass failures as slope‐dependent continuous sediment transport processes, sometimes conditioned on a slope threshold or with nonlinear dependence on slope gradient. In this study we first present an explicit physically based theory for the stability analysis of gully heads and walls. The theory is based on the force balance equation of an assumed planar failure geometry of a steep gully wall, with a potential failure plane dipping into the incised gully bed and tension cracks developing behind the scarp face. Then, we test the theory against field data collected in our field site in Colorado and against other published data. Second, the theory is implemented in a one‐dimensional hillslope profile development model and the three‐dimensional channel‐hillslope integrated landscape development (CHILD) to study the effects of soil cohesion, erosion thresholds, and stochastic climate on the tempo of gully development and morphology. Preliminary results indicate that wider and shallower gullies develop and integrate, forming wide valleys, when soil cohesion is small. As soil cohesion increases, erosion slows down, gullies become deeper with vertical walls, and episodic mass failures occur. Differences in storm intensity‐duration characteristics and erosion thresholds are predicted to have a significant impact on gully development. Vertical gully walls develop rapidly, and gullies enlarge by slab failures in a climate characterized by high‐intensity, short‐duration storm pulses. However, under low‐intensity, long‐duration storms, gullies quickly stabilize, and vertical walls are eliminated and rounded, forming diffusion‐dominated hilltops. Erosion thresholds have a similar impact on the tempo of gully erosion but in the opposite direction. Lowering the erosion threshold enhances gully widening by slab failures. Gully walls stabilize when the erosion threshold is high due to a reduction in the erosion of the failure material on the toe of gully walls.

Hypsometric analysis of Margaritifer Sinus and origin of valley networks

The formation of Martian valley networks has been debated since their discovery. The relative roles of groundwater sapping and fluvial runoff processes have different implications on Martian paleoclimate. This paper uses a hypsometric analysis technique that has been tested on terrestrial landforms to analyze the hypsometric attributes of watershed basins in the Margaritifer Sinus region on Mars using Mars Orbiter Laser Altimeter (MOLA) gridded topographic data. On the basis of quantitative characteristics of their hypsometric attributes, the majority of the basins (covering ∼3/5 of the study area) look more like terrestrial basins of sapping origin, and a significant number of basins (covering ∼1/5 of the study area) are more fluvial like. The classification of the rest of the basin is less certain at the presently available resolution of the digital elevation model (DEM). Overlay with Viking photomosaic indicates that the classification is generally consistent with the morphology shown in Viking images. Although this analysis alone cannot make a sure determination of valley network origin, combined with other morphometric analysis, the results from this study are consistent with a valley network formation by precipitation‐recharged groundwater sapping, suggesting a warm and wet climate for early Mars.

Drainage evolution in the Margaritifer Sinus region, Mars

Margaritifer Sinus, Mars, records a complex history of water transport, storage, and release extending from the late Noachian into at least the mid‐Hesperian. Collection, transport, and discharge of water were accomplished by systems of differing character flanking opposite sides of the Chryse Trough. Drainage on the western side of the trough was via the segmented Uzboi‐Ladon‐Margaritifer mesoscale outflow system that heads in Argyre Basin and incises and fills as it crosses ancient multiringed impact basins. By contrast, Samara and Parana‐Loire Valles, two of the largest and best integrated valley systems on Mars, dominated drainage on the eastern trough flank. Valley morphometry suggests formation was due to precipitation‐recharged groundwater sapping. All systems discharged into Margaritifer Basin, located along the Chryse Trough axis, and caused ponding that persisted into the early Hesperian. The Uzboi‐Ladon‐Margaritifer system dominated discharge that was coincident with widespread geomorphic activity on Mars. As channel and valley formation ended, some water in Margaritifer Basin infiltrated the subsurface. Collapse and release of water began shortly thereafter and persisted into mid‐Hesperian times, thereby forming Margaritifer and Iani Chaos and incising Ares Vallis.

New England tidal inlets with special reference to riverine-associated inlet systems

Abstract Tidal inlets along the glaciated coast of New England exhibit a diverse morphology due to widely different physical and geological settings and sediment abundance. Except for the glacial sediment coasts of Cape Cod, Nantucket Island, and Martha's Vineyard, most regions of New England are rocky, and barrier and tidal inlet development is related to isolated glacial and riverine sediment supplies. Inlet-connected bays and marshes encompass many different origins, including drowned river valleys, glacial lake discharge channels, indented bedrock and glacial sediment coasts, kettles, and groundwater sapping channels. Generally, inlet size is correlated to tidal range; the largest inlets (width >300 m) occur along mesotidal coasts, whereas most inlets along microtidal coasts are small (width In spite of their diversity, New England tidal inlets can be grouped into three broad categories based on their morphology, hydrographic regime, and sediment transport characteristics. These classes include the wave-dominated and mixed-energy tidal inlets, and riverine-associated tidal inlets. Wave-dominated inlets tend to occur in microtidal settings and are found primarily in southern New England. They contain flood-tidal deltas of variable morphology and poorly developed or absent ebb-tidal deltas. They have histories of migration, closure, and reopening during storms. Mixed-energy tidal inlets occur mostly along the mesotidal shorelines of central and northern New England. These inlets exhibit well-developed ebb- and flood-tidal deltas. They are backed by extensive marshes and tidal creeks and, in some cases, by a broad system of tidal flats. Riverine-associated tidal inlets coincide with major rivers that have significant freshwater discharge, especially during late winter and early spring. The throat cross-sections are large in comparison to other inlets in New England and are a function of their tidal prism. They contain well-developed subtidal ebb-tidal deltas and variable developed flood-tidal deltas. Ebb-oriented bedforms, grain size, and mineralogical analyses of channel sediments, and hydraulic data have been used to demonstrate that the estuaries are exporting sand to the nearshore, a process that has likely been ongoing since deglaciation. The mechanism for downstream movement of sand in the estuaries is moderate to large spring freshets and major floods when freshwater discharge supplants the saltwater tidal prism. During these periods, there are unidirectional seaward currents in the estuaries that may last for several days or weeks.

Drainage basins and channel incision on Mars

Measurements acquired by the Mars Orbiter Laser Altimeter on board the Mars Global Surveyor indicate that large drainage systems on Mars have geomorphic characteristics inconsistent with prolonged erosion by surface runoff. We find the topography has not evolved to an expected equilibrium terrain form, even in areas where runoff incision has been previously interpreted. By analogy with terrestrial examples, groundwater sapping may have played an important role in the incision. Longitudinally flat floor segments may provide a direct indication of lithologic layers in the bedrock, altering subsurface hydrology. However, it is unlikely that floor levels are entirely due to inherited structures due to their planar cross-cutting relations. These conclusions are based on previously unavailable observations, including extensive piece-wise linear longitudinal profiles, frequent knickpoints, hanging valleys, and small basin concavity exponents.