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In deserts, the interplay between occasional fluvial events and persistent aeolian erosion can form composite modern and relict surfaces, especially on the distal portion of alluvial fans. There, relief inversion of alluvial deposits by differential erosion can form longitudinal ridges. We identified two distinct ridge types formed by relief inversion on converging alluvial fans in the hyperarid Chilean Atacama Desert. Although they are co-located and similar in scale, the ridge types have different ages and formation histories that apparently correspond to minor paleoclimate variations. Gravel-armored ridges are remnants of deflated alluvial deposits with a bimodal sediment distribution (gravel and sand) dated to a minor pluvial phase at the end of the Late Pleistocene (~12 kyr). In contrast, younger (~9 kyr) sulfate-capped ridges formed during a minor arid phase with evaporite deposition in a pre-existing channel that armored the underlying deposits. Collectively, inverted channels at Salar de Llamara resulted from multiple episodes of surface overland flow and standing water spanning several thousand years. Based on ridge relief and age, the minimum long-term deflation rate is 0.1-0.2 m/kyr, driven primarily by wind erosion. This case study is an example of the equifinality concept whereby different processes lead to similar landforms. The complex history of the two ridge types can only be generally constrained in remotely sensed data. In situ observations are required to discern the specifics of the aqueous history, including the flow type, magnitude, sequence, and paleoenvironment. These findings have relevance for interpreting similar landforms on Mars.

Understanding planetary landforms, including the theater-headed valleys (box canyons) of Mars, usually depends on interpreting geological processes from remote-sensing data without ground-based corroboration. Here we investigate the origin and development of two Mars-analog theater-headed valleys in the hyperarid Atacama Desert of northern Chile. Previous workers attributed these valleys to groundwater sapping based on remote imaging, topography, and publications on the local geology. We evaluate groundwater sapping and alternative hypotheses using field observations of characteristic features, strength measurements of strata exposed in headscarps, and estimates of ephemeral flood discharges within the valleys. The headscarps lack evidence of recent or active seepage weathering, such as spring discharge, salt weathering, alcoves, or vegetation. Their welded tuff caprocks have compressive strengths multiple times those of the underlying epiclastic strata. Flood discharge estimates of cubic meters to tens of cubic meters per second, derived using the Manning equation, are consistent with the size of transported clasts and show that the ephemeral streams are geomorphically effective, even in the modern hyperarid climate. We interpret that headscarp retreat in the Quebrada de Quisma is due to ephemeral flood erosion of weak Miocene epiclastic strata beneath a strong welded tuff, with erosion of the tuff facilitated by vertical jointing. The Quebrada de Humayani headscarp is interpreted as the scar of a giant landslide, maintained against substantial later degradation by similar strong-over-weak stratigraphy. This work suggests that theater-headed valleys on Earth and Mars should not be attributed by default to groundwater sapping, as other processes with lithologic and structural influences can form theater headscarps.

Felsic rocks have not been identified on Mars, a planet that lacks plate tectonics to drive the magmatic processes that lead to evolved silica-rich melts. Spectral observations by the Mars Reconnaissance Orbiter indicate that felsic lithologies occur at multiple localities on Mars and suggest prolonged magmatic activity on ancient Mars.

[1] Most of the geomorphic changes on Mars occurred during the Noachian Period, when the rates of impact crater degradation and valley network incision were highest. Fluvial erosion around the Noachian/Hesperian transition is better constrained than the longer-term landscape evolution throughout the Noachian Period, when the highland intercrater geomorphic surfaces developed. We interpret highland resurfacing events and processes using a new global geologic map of Mars (at 1:20,000,000 scale), a crater data set that is complete down to 1 km in diameter, and Mars Orbiter Laser Altimeter topography. The Early Noachian highland (eNh) unit is nearly saturated with craters of 32–128 km diameter, the Middle Noachian highland (mNh) unit has a resurfacing age of ~4 Ga, and the Late Noachian highland unit (lNh) includes younger composite surfaces of basin fill and partially buried cratered terrain. These units have statistically distinct ages, and their distribution varies with elevation. The eNh unit is concentrated in the high-standing Hellas basin annulus and in highland terrain that was thinly mantled by basin ejecta near 180° longitude. The mNh unit includes most of Arabia Terra, the Argyre vicinity, highland plateau areas between eNh outcrops, and the Thaumasia range. The lNh unit mostly occurs within highland basins. Crater depth/diameter ratios do not vary strongly between the eNh and mNh units, although crater losses to Noachian resurfacing appear greater in lower lying areas. Noachian resurfacing was spatially non-uniform, long-lived, and gravity-driven, more consistent with arid-zone fluvial and aeolian erosion and volcanism than with air fall mantling or mass wasting.

Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft flybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent of the Huygens probe to the surface. Interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan’s fluvial networks imply a variety of mechanical controls, including structural influence, on channelized flows. At the reach scale, the various morphologies of individual fluvial features, implying a broad range of fluvial processes, suggest that (paleo-)flows did not occupy the entire observed width of the features. DISR images provide a spatially limited view of uplands dissected by valley networks, also likely formed by overland flows, which are not visible in lower-resolution SAR data. This high-resolution snapshot suggests that some fluvial features observed in SAR data may be river valleys rather than channels, and that uplands elsewhere on Titan may also have fine-scale fluvial dissection that is not resolved in SAR data. Radar-bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine-scale fluvial dissection. Fluvial deposition is inferred to occur in braided channels, in (paleo)lake basins, and on SAR-dark plains, and DISR images at the surface indicate the presence of fluvial sediment. Flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fluvial processes. With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to flow on Titan for fully turbulent flow and rough boundaries. For low-Reynolds-number flow over smooth boundaries, however, knowledge of fluid kinematic viscosity is necessary. Sediment movement and bed form development should occur at lower bed shear stress on Titan than on Earth. Scaling bedrock erosion, however, is hampered by uncertainties regarding Titan material properties. Overall, observations of Titan point to a world pervasively influenced by fluvial processes, for which appropriate terrestrial analogs and formulations may provide insight.

Uzboi Vallis (centered at ∼28°S, 323°E) is ∼400 km long and comprises the southernmost segment of the northward-draining Uzboi–Ladon–Morava (ULM) meso-scale outflow system that emerges from Argyre basin. Bond and Holden craters blocked the valley to the south and north, respectively, forming a Late Noachian-to-Hesperian paleolake basin that exceeded 4000 km 3 . Limited CRISM data suggest lake deposits in Uzboi and underlying basin floor incorporate relatively more Mg-clays and more Fe-clays, respectively. The short-lived lake overflowed and breached Holden crater’s rim at an elevation of −350 m and rapidly drained into the crater. Fan deltas in Holden extend 25 km from the breach and incorporate meter-sized blocks, and longitudinal grooves along the Uzboi basin floor are hundreds of meters long and average 60 m wide, suggesting high-discharge drainage of the lake. Precipitation-derived runoff rather than regional groundwater or overflow from Argyre dominated contributions to the Uzboi lake, although the failure of most tributaries to respond to a lowering of base level indicates their incision largely ended when the lake drained. The Uzboi lake may have coincided with alluvial and/or lacustrine activity in Holden, Eberswalde, and other craters in southern Margaritifer Terra, where fluvial/lacustrine activity may have required widespread, synoptic precipitation (rain or snow), perhaps associated with an ephemeral, global hydrologic system during the Late Noachian into the Hesperian on Mars.

The rate of movement for 3- and 10-cm-high granule ripples was documented in September of 2006 at Great Sand Dunes National Park and Preserve during a particularly strong wind event. Impact creep induced by saltating sand caused ∼24 granules min −1 to cross each cm of crest length during wind that averaged ∼9 m s −1 (at a height well above 1 m), which is substantially larger than the threshold for saltation of sand. Extension of this documented granule movement rate to Mars suggests that a 25-cm-high granule ripple should require from hundreds to thousands of Earth-years to move 1 cm under present atmospheric conditions.

Fluvial systems can be preserved in inverted relief on both Earth and Mars. Few studies have evaluated the applicability of various paleohydrological models to inverted fluvial systems. The first phase of this investigation focused on an extensive (spanning ∼ 12 km) inverted paleochannel system that consists of four sandstone-capped, carbonate-cemented, sinuous ridges within the Early Cretaceous Cedar Mountain Formation located southwest of Green River, Utah. Morphologic and sedimentologic observations of the exhumed paleochannels were used to evaluate multiple numerical models for reconstructing paleofluvial hydrological parameters. Another objective of the study was to determine whether aerial or orbital observations yield model results that are consistent with those constrained by field data. The models yield an envelope of plausible dominant discharge values (100–500 m 3 /s), reflecting the limitations of the approach, and no single model can be used to reliably estimate paleodischarge. On Mars, landforms with attributes consistent with inverted channels have been identified. In spite of differences in the formation history between these martian landforms and the terrestrial analog described here, including potential differences in cement composition and the erosional agent that was responsible for relief inversion, these numerical models can be applied (with modification) to the martian landforms and yield an envelope of plausible values for dominant discharge.

The hemispheric dichotomy is a fundamental feature of Mars, expressed by a physiographic and geologic divide between the heavily cratered southern highlands and the relatively smooth plains of the northern lowlands. The origin of the dichotomy, which may have set the course for most of the subsequent geologic evolution of Mars, remains unclear. Internally driven models for the dichotomy form the lowlands by mantle convection, plate tectonics, or early mantle overturn. Externally driven models invoke one giant impact or multiple impacts. Areal densities of buried basins, expressed by quasi-circular depressions and subsurface echoes in radar sounding data, suggest that the dichotomy formed early in the geologic evolution of Mars. Tectonic features along the dichotomy boundary suggest late-stage modification by flexure or relaxation of the highlands after volcanic resurfacing of the northern lowlands. Subsequent deposition and erosion by fluvial, aeolian, and glacial processes shaped the present-day dichotomy boundary.

We have studied a terraced fan deposit with unique characteristics located within a trough of Coprates Catena. The fan has an average length of 6.8 km, and is approximately 44 km2 in area and 18 km3 in volume. The fan's broad contributing valley is approximately 35 km long and it noticeably increases in depth about 12.8 km before it intersects the trough, where a rounded knickpoint marks the transition between flat-floored upstream and V-shaped downstream cross-sections. A 14-km-long channel with no apparent source enters the contributing valley from the south. A much smaller sinuous channel has incised along a smaller V-shaped valley in the uppermost eastern portion of the fan deposit. We explored several possible origins for the terraced fan, including mass wasting, volcanic flow, alluvial fan, and delta. We propose that water sourced from volcanic melting of ice eroded and transported material along the contributing valley. This material was then deposited as a delta in a lake within the trough. The concentric terraces are most likely the result of shoreline or ice cover erosion during drops in lake level. A light-toned layered deposit to the east of the fan deposit along the floor of the trough may represent a sedimentary unit formed during the terminal stages of the lake. Although other terraced fans have been identified on Mars, the Coprates Catena fan is unique because it has many more terraces and its surface was incised by a channel and associated valley. The identification of several other valleys to the east suggests that volcanic melting of volatiles during the Hesperian Period created favorable conditions for water flow along the plains in this region.

Points of origin and downgradation of tropical cyclones in the eastern North Pacific (ENP) east of 160°W are related to the Southern Oscillation Index (SOI) during the hurricane season. All ENP tropical storms and hurricanes from 1966 through 1997 were grouped into three SOI-based categories: those occurring during strong El Nino events (mean SOI 0.6), or near zero periods (−0.6 < mean SOI < 0.6). During El Nino storm seasons, ENP tropical cyclones originated approximately 5.7° (617 km) west and downgraded 7.5° (780 km) west of the long-term mean longitudes for the positive SOI group. Near zero group storms also followed more northerly tracks than the negative SOI group storms. However, no significant differences in storm track are evident between the positive SOI and near zero groups, and the track length is not significantly different for any storm group.

[1] A number of studies have attempted to characterize Martian valley and channel networks. To date, however, little attention has been paid to the role of lithology, which could influence the rate of incision, morphology, and hydrology as well as the characteristics of transported materials. Here, we present an analysis of the physical and hydrologic characteristics of drainage networks (gullies and channels) that have incised the Keanakāko‘i tephra, a basaltic pyroclastic deposit that occurs mainly in the summit area of Kīlauea Volcano and in the adjoining Ka‘ū Desert, Hawai‘i. The Keanakāko‘i tephra is up to ∼10 m meters thick and largely devoid of vegetation, making it a good analog for the Martian surface. Although the scales are different, the Keanakāko‘i drainage networks suggest that several typical morphologic characteristics of Martian valley networks may be controlled by lithology in combination with ephemeral flood characteristics. Many gully headwalls and knickpoints within the drainage networks are amphitheater shaped, which results from strong-over-weak stratigraphy. Beds of fine ash, commonly bearing accretionary lapilli (pisolites), are more resistant to erosion than the interbedded, coarser weakly consolidated and friable tephra layers. Because the banks of the gullies and channels are easily eroded widths vary downslope, similar to Martian valley networks that have been characterized as “degraded.” The floors of the gullies and channels tend to be low-relief with few prominent bed forms, reflecting the large proportion of sediment transported as bed load in high-energy but short-lived flood events. We calculate that the average flow velocities within the drainage networks are typically 1 m/24 h. Given some recent modeling of the early Martian climate, our observations imply that rainfall on early Mars could also be associated with large intense events and that Martian valley network formation may be related to similar cyclonic storms.

[1] Many basins with relict contributing valley networks and outlet valleys in the Martian highlands indicate past flowing and ponded water on the surface. These likely paleolakes motivate an investigation of pluvial shore landforms in the Great Basin region of the western United States, as confident identification of strandlines on Mars would facilitate analyses of its past hydrology and climate. The purpose of this study is to characterize the scale of Late Pleistocene erosional and depositional shore landforms in an endorheic setting, determine the preservation potential of similar forms from multiple epochs in early Martian history, and identify the data products that would be necessary to detect them. We use Differential Global Positioning System field surveys to measure the dimensions and elevations of shore landforms; compare shore platform widths to theoretical maxima; and note the minimum scale of landforms that have survived since the Late Noachian and Early Hesperian Epochs on Mars. We find that due to impact gardening and aeolian erosion, Martian highland paleolakes like those in the pluvial Great Basin would likely not have well-preserved shore landforms, unless they were unusually large and formed in the Late Hesperian or later. Individual strandlines are often not equally well expressed around an entire basin, so correlating shore landforms in plan view imaging and using their consistency in elevation as a hypothesis test for paleolakes can be challenging. Detection of younger shore landforms like those examined here would require meter-resolution imaging and topography such as stereo digital elevation models.

[1] Some morphometric differences between terrestrial and Martian valley networks may reflect the precursor topography on Mars, particularly impact basins or tectonic slopes. To evaluate these possible influences, we mapped highland watersheds in nine study areas that sample a range of geographic and topographic settings. We collected data including latitude, longitude, watershed length, divide and terminal elevations, watershed relief and slope, slope orientation, and qualitative descriptors including whether a drainage basin was open or closed. The longest valley networks and most overflowed basins occur on preexisting intercrater slopes of 0.1–1°, particularly on north facing slopes associated with the crustal dichotomy. The control of watershed length by earlier Noachian topographic features, which the relict networks did not significantly modify, suggests that the Early to Middle Noachian geomorphic environment was nominally much drier than the later Noachian to Hesperian transition. The distribution of fluvial valleys and likely orographic effects created by the crustal dichotomy suggest that evaporation from the northern lowlands was an important source of atmospheric humidity over short time scales. Much of the highland plateau consists of smaller enclosed watersheds, which (along with cooler temperatures) detained surface water at high elevations, lengthening or impeding the global water cycle. Ponding and evaporation may have partly offset a continentality effect of the highland landmass. Prolonged modification of the intercrater geomorphic surface prior to incision of valley networks included substantial weathering, reduction of relief, and gravity-driven sediment transport, indicating a long-term role for surface water in a transport-limited, arid to hyperarid Noachian paleoclimate.

[1] The contrast in crustal thickness, surface age, elevation, and morphology between the southern cratered highlands and northern lowland plains of Mars is termed the crustal dichotomy. The oldest exposed sections of the crustal dichotomy boundary are ancient cratered slopes, which influenced post-Noachian fresh crater morphometry, Late Noachian valley network planform, and the degradation patterns of Middle to Late Noachian (∼3.92–3.7 Ga) impact craters. Noachian visible and topographically defined impact craters at the top of the cratered slope show no evidence of flexure-induced normal faulting. These observations and published geophysical data collectively require an Early to Pre-Noachian age for the crustal dichotomy, prior to the largest recognized impact basins. Late Noachian plateau deposits and more prolonged Tharsis volcanism appear to have buried parts of the old cratered slope, and fretted terrain developed in this transition zone during the Early Hesperian Epoch (∼3.7–3.6 Ga). Fretted/knobby terrains, lowland plains, and most visible structures (wrinkle ridges, fractures, and normal faults) postdate Noachian crater modification and are several hundred million years younger than the cratered slope of the crustal dichotomy, so they provide no valid basis or constraint for models of its formation. Long-wavelength topography in cratered terrain dates to Early to Pre-Noachian time and provides a useful model constraint. Geological and geophysical observations are thus reconciled around an early age and relatively rapid development of the Martian crustal dichotomy.

Holden crater in southwestern Margaritifer Terra likely contained two distinct lakes of differing character and sources during the Late Noachian on Mars. The first lake may have included distal alluvial deposits, was longer-lived, and formed when drainage from within the crater ponded on the crater floor. The second lake was shorter-lived and created when water impounded in Uzboi Vallis breached Holden's rim and rapidly drained into the crater. Holden crater is partially filled by an impressive sequence of more than 150 m of finely bedded, sedimentary deposits that are best exposed in the southwestern portion of the crater but are also observed in the central and eastern portions of the crater. The sedimentary beds comprising the lower and upper units exposed in Holden record two intervals of ponding within the crater. Various processes have been proposed to account for the origin of the lower unit deposits in Holden crater. These processes include one or multiple lacustrine episodes on the crater floor, air-fall or glacial origin. The thin, laterally continuous bedding that is confined below –1960m within the crater is most consistent with a water-lain origin for all three members of the lower unit. While non-aqueous depositional processes for the lower unit appear improbable, distinguishing between a distal alluvial versus lacustrine depositional environment is challenging.

[1] We present evidence that a final epoch of widespread fluvial erosion and deposition in the cratered highlands during the latest Noachian or early to mid-Hesperian was characterized by integration of flow within drainage networks as long as 4000 km and trunk valley incision of 50 to 350 m into earlier Noachian depositional basins. Locally deltaic sediments were deposited where incised valley systems debouched into basins. Large alluvial fans of sediment deposited from erosion of alcoves in steep crater walls probably formed contemporaneously. The depth of incision below Noachian surfaces correlates strongly with the gradient and the total valley length, suggesting consistent regional hydrology. Estimated discharges from channel dimensions indicate flow rates equivalent to mean annual floods in terrestrial drainage basins of equivalent size. Such high flow rates imply either runoff directly from precipitation or rapid melting of accumulated snow. Development of duricrusts on the Noachian landscape may have contributed to focusing of late-stage erosion within major trunk drainages. This late-stage epoch of intense fluvial activity appears to be fundamentally different than the fluvial environment prevailing during most of the Noachian Period, which was characterized by widespread fluvial erosion of highlands and crater rims, deeply infilling crater floors, and intercrater basins through ephemeral fluvial activity, and development of local rather than regionally integrated drainage networks.

To explain the much higher denudation rates and valley network development on early Mars (more than approximately 3.6 Gyr ago), most investigators have invoked either steady state warm/wet (Earthlike) or cold/dry (modern Mars) end-member paleoclimates. Here we discuss evidence that highland gradation was prolonged, but generally slow and possibly ephemeral during the Noachian Period, and that the immature valley networks entrenched during a brief terminal epoch of more erosive fluvial activity in the late Noachian to early Hesperian. Observational support for this interpretation includes (1) late-stage breaching of some enclosed basins that had previously been extensively modified, but only by internal erosion and deposition; (2) deposition of pristine deltas and fans during a late stage of contributing valley entrenchment; (3) a brief, erosive response to base level decline (which was imparted as fretted terrain developed by a suite of processes unrelated to surface runoff) in fluvial valleys that crosscut the highland-lowland boundary scarp; and (4) width/contributing area relationships of interior channels within valley networks, which record significant late-stage runoff production with no evidence of recovery to lower-flow conditions. This erosion appears to have ended abruptly, as depositional landforms generally were not entrenched with declining base level in crater lakes. A possible planetwide synchronicity and common cause to the late-stage fluvial activity are possible but remain uncertain. This increased activity of valley networks is offered as a possible explanation for diverse features of highland drainage basins, which were previously cited to support competing warm, wet and cold, dry paleoclimate scenarios.

Ma'adim Vallis, one of the largest valleys in the Martian highlands, appears to have originated by catastrophic overflow of a large paleola ke located south of the valley heads. Ma'adim Vallis debouched to Gus ev crater, 900 km to the north, the landing site for the Spirit Mars Exploration Rover. Support for the paleolake overflow hypothesis come s from the following characteristics: (I) With a channel width of 3 km at its head, Ma'adim Vallis originates at two (eastern and western) gaps incised into the divide of the approximately 1.1 M km(exp 2) enc losed Eridania head basin, which suggests a lake as the water source. (2) The sinuous course of Ma'adim Vallis is consistent with overland flow controlled by preexisting surface topography, and structural con trol is not evident or required to explain the valley course. (3) The nearly constant approximately 5 km width of the inner channel through crater rim breaches, the anastomosing course of the wide western tri butary, the migration of the inner channel to the outer margins of be nds in the valley's lower reach, a medial sedimentary bar approximate ly 200 m in height, and a step-pool" sequence are consistent with modeled flows of 1-5 x l0 (exp 6) m(exp 3)/s. Peak discharges were likely higher but are poorly constrained by the relict channel geometry. (4 ) Small direct tributary valleys to Ma'adim Vallis have convex-up lon gitudinal profiles, suggesting a hanging relationship to a valley that was incised quickly relative to the timescales of tributary developm ent. (5) The Eridania basin had adequate volume between the initial d ivide and the incised gap elevations to carve Ma'adim Vallis during a single flood. (6) The Eridania basin is composed of many overlapping , highly degraded and deeply buried impact craters. The floor materials of the six largest craters have an unusually high internal relief ( approximately 1 km) and slope (approximately 0.5-1.5 degrees) among d egraded Martian craters, which are usually flat-floored. Long-term, fluvial sediment transport appears to have been inhibited within these craters, and the topography is inconsistent with basaltic infilling. (7) Fluvial valleys do not dissect the slopes of these deeper crater floor depressions, unlike similar slopes that are dissected at higher levels in the watershed. These characteristics (6, 7) suggest that wa ter mantled at least the lower parts of the Eridania basin floor thro ughout the period of relatively intense erosion early in Martian hist ory. The lake level increased and an overflow occurred near the close of the Noachian (age determined using >5 km crater counts). Initially , the Eridania basin debouched northward at two locations into the in termediate basin, a highly degraded impact crater approximately 500 k m in diameter. As this intermediate basin was temporarily filled with water, erosion took place first along the lower (northern) reach of Ma'adim Vallis, debouching to Gusev crater. The western overflow point was later abandoned, and erosion of the intermediate basin interior was concentrated along the eastern pathway. Subsequent air fall depos ition, impact gardening, tectonism, and limited fluvial erosion modified the Eridania basin region, so evidence for a paleolake is restrict ed to larger landforms that could survive post-Noachian degradation p rocesses.

[1] The evolution of the Martian crustal dichotomy boundary, which separates the southern cratered highlands from the northern lowland plains by 1–3 km of elevation, remains among the fundamental outstanding issues in Mars research. For a study area at Aeolis Mensae we show that fretted terrain formed exclusively in a >2 km thick, late Noachian (∼3.7 Ga) sedimentary deposit that overlies the base of an older, cratered dichotomy boundary slope. In this equatorial study area, fretted terrain does not exhibit the debris aprons or lineated valley fills that are attributed to ground ice in otherwise similar, midlatitude fretted terrain in Arabia Terra. The massive deposit of fine sand or loess was probably transported from the north by wind and trapped against the precursor dichotomy slope, producing a similar initial form to the younger Medusae Fossae layered materials that occur east of Aeolis Mensae. Contemporary with the latest Noachian to early Hesperian decline in fluvial erosion, the fretting process likely initiated as the massive layer's indurated surface was compromised by fracture, cratering, or collapse into possible voids. In these depressions, grain impact or contact with water disaggregated the fine sedimentary materials, which were then largely deflated by wind. The fretting process largely ended when liquid water was no longer widely available for weathering during the early Hesperian period, although some degradation of the region by aeolian and slope processes has continued to the present.

At 8 to 15 kilometers wide, Ma'adim Vallis is one of the largest valleys in the martian highlands. Although a groundwater source was previously suggested, the channel originates at a spillway in the divide of a approximately 3,000,000-square-kilometer closed drainage basin. The interior morphology of this source basin, including likely shoreline features following topographic contours, suggests that Ma'adim Vallis was created through catastrophic overflow of a approximately 1,100,000-square-kilometer highland lake. The size, constant levels, and interior morphology of three regional paleolake basins require a warmer paleoclimate and a long-term, recharged, stable highland water table more than approximately 3.5 billion years ago.

[1] Geomorphic mapping of a ∼1 million square kilometer section of Terra Cimmeria on Mars (1:1M scale) indicates that prolonged, intense fluvial erosion occurred during the period of heavy bombardment. Crater counts date the termination of ubiquitous, intense erosion to the late Noachian, although some valleys may have continued downcutting into the early Hesperian. Stratigraphic and topographic relationships indicate that early erosional processes created large, integrated drainage basins, affected primarily by large impact basin structures and regional slopes. This terrain is not consistent with an origin purely by volcanic or impact processes. Cratering competed with drainage basin development, minimizing valley length, catchment area, and valley network integration. Drainage basin disruption resulted from impacts on valley thalwegs, or when the ejecta of large (usually >75 km) craters created low divides. Near-level upland intercrater plains are lightly dissected, in part because smaller craters could interrupt flow paths on gently sloping or flat terrain. Some closed drainage basins became integrated by continued erosion of drainage divides and stream capture, overflow of the divides, or headward growth of valleys fed by groundwater collected in closed basins. Drainage divide breaching was most effective on steeper (>0.5°) regional slopes, where observed drainage density is also highest. This is due to greater runoff volumes and velocity encouraging valley incision, as well as steep slopes allowing valleys to bypass or breach superimposed craters. Valley systems commonly extend nearly to the crests of sharp drainage divides, and spatially ubiquitous valley source points throughout the higher elevations suggest that runoff derived largely from precipitation.

High Resolution Imaging Science Experiment (HiRISE) images of Holden crater, Mars, resolve impact megabreccia unconformably overlain by sediments deposited during two Noachian-age phases of aqueous activity. A lighter-toned lower unit exhibiting phyllosilicates was deposited in a long-lived, quiescent distal alluvial or lacustrine setting. An overlying darker-toned and often blocky upper unit drapes the sequence and was emplaced during later high-magnitude flooding as an impounded Uzboi Vallis lake overtopped the crater rim. The stratigraphy provides the first geologic context for phyllosilicate deposition during persistent wet and perhaps habitable conditions on early Mars.

The highland valley networks are perhaps the most compelling evidence for widespread fluvial activity on Mars .3.5 Ga. However, determining the hydrology of these features has been difficult owing to poor image resolution and the lack of available topographic data. New orbital imaging reveals 21 late-stage channels within valley networks, which we use to estimate formative discharges and to evaluate water supply mechanisms. We find that channel width and associated formative discharge are comparable to terrestrial valley networks of similar area and relief. For 15 narrow channels in basin-filling networks, likely episodic runoff production rates up to centimeters per day and first-order formative discharges of ;300‐3000 m 3 /s are similar to terrestrial floods supplied by precipitation. Geothermal melting of ground ice would produce discharges ;100 times smaller per unit area and would require pulsed outbursts to form the channels. In four large valleys with few tributaries, wider channels may represent large subsurface outflows or paleolake overflows, as these four channels originate at breached basin divides and/or near source regions for the catastrophic outflow channels.