Repeated Aqueous Flooding from the Cerberus Fossae: Evidence for Very Recently Extant, Deep Groundwater on Mars

The geomorphology and topography of the Cerberus Plains region of Mars show three spatially and temporally distinct, young, aqueous flood channel systems. Flood geomorphology in each of these channels, as seen in Mars Orbiter Camera images, consists of streamlined forms, longitudinal lineations, and a single occurrence of transverse dunes, features similar to those in the flood-carved terrain of the Channeled Scabland in the northwestern United States. As additional geomorphic evidence of flooding, small cones (interpreted as phreatic) are found preferentially in the channels or at their distal ends. Glaciers, lava flows, and CO₂-charged density flows are each inconsistent with these geomorphic features. Mars Orbiter Laser Altimeter data show two of the three channel systems (Athabasca Valles and an unnamed northern channel system) emanating from the Cerberus Fossae; we suggest that the third channel system (Marte Vallis) also originated at the fissures. The discharges for two of the three systems (Athabasca Valles and Marte Vallis) have been estimated from surface topography to have been on the order of 10⁶ m³/s. Crater counts indicate that the channels are not only young (extreme Late Amazonian), but also were carved asynchronously. Geomorphic evidence suggests that two of the channels (Athabasca and Marte Valles) experienced more than one flood. Emanation from volcanotectonic fissures instead of chaotic terrain distinguishes these Cerberus Plains channels from the larger, older circum-Chryse channels. Groundwater must have collected in a liquid state prior to flood onset to flow at the estimated discharge rates. Lack of large-scale subsidence near the channels' origination points along the Cerberus Fossae indicates that this groundwater was at least several kilometers deep.     

Comparisons of the hydraulics of water flows in Martian outflow channels with flows of similar scale on earth

Comparisons are undertaken between the hydraulics of channelized water flows on Mars, large terrestrial rivers, deep-sea turbidity currents, and the catastrophic flow of Lake Missoula floods. Expected bottom shear stresses, velocities and discharges, flow powers, and other parameters are computed for each. Sand transport rates and the times required for channel erosion are estimated for Mangala Channel. These calculations indicate that the turbidity currents and Lake Missoula floods were similar to channelized water flow on Mars in their flow characteristics and in their abilities to erode and transport sediments. Like the Lake Missoula floods, deep-sea turbidity currents are catastrophic in character, being formed by the slumping of large masses of sediment trapped in submarine canyons or deposited on the continental slope. The repeated flows originating from submarine canyons have formed deep-sea channels similar in scale and overall morphology to the Martian outflow channels. The submarine canyon can be viewed as the counterpart of the chaotic terrain or crater which serves as sources for many Martian channels. Like most Martian outflow channels, the deep-sea channels generally lack tributaries or have only minor tributaries, instead consisting of a single pronounced channel extending for several hundred kilometers from its origin at the submarine canyon to deep abyssal depths. The channels vary considerably in dimensions, but most commonly have widths in the range 2 to 15 km with reliefs of 50 to 450 meters, again similar in scale to the Martian channels. Other similarities include sections of anastomosing channels, a general lack of pronounced meandering, and a lack of an apparent “delta” where the transported sediments are deposited. The similarities of channel morphology and flow hydraulics indicate the deep-sea channels and turbidity currents can be useful in furthering our understanding of the Martian outflow channels. Physical processes in the deep-sea occur under a reduced effective gravity because of the overlying water with its buoyancy. The deep-sea channels provide another set of Earth-based channels which can be studied to determine the effects of gravity on such factors as channel meandering and anastomosing characteristics.     

Geological evolution of Ares Vallis on Mars: Formation by multiple events of catastrophic flooding, glacial and periglacial processes

Ares Vallis is one of the greatest outflow channels of Mars. Using high-resolution images of recent missions to Mars (MGS, 2001 Odyssey, and Mars Express), we investigated Ares Vallis and its valley arms, taking advantage of 3-dimensional analysis performed using the high-resolution stereo capability of the Mars Express High Resolution Stereo Camera (HRSC). In our view, Ares Vallis is characterized by catastrophic flood landscapes partially superimposed by ice-related morphologies. Catastrophic flood landforms include erosional terraces, grooved terrains, streamlined uplands, giant bars, pendant bars, and cataract-like features. Ice-related morphologies include probable kame features, thermokarstic depressions, and patterned grounds. Our investigations outline that throughout the Hesperian age, Ares Vallis and its valley arms had been sculpted by several, time-scattered, catastrophic floods, originating from Iani, Hydaspis and Aram Chaos. Geomorphological evidence suggests that catastrophic floods were ice-covered, and that climatic conditions of Mars at this time were similar to those of the present day. At the end of each catastrophic flood, ice masses grounded, forming a thick stagnant dead-ice body. Each catastrophic flood was followed by a relatively brief period of warmer-wetter climatic conditions, originated as a consequence of catastrophic flooding. During such periods thermokarstic depressions originated, liquid water formed meandering channels, and ice-contact deposits were emplaced by ice-walled streams. Finally, the climate turned into cold-dry conditions similar to the present-day ones, and ice masses sublimated.     

Martian channels and valleys: Their characteristics,distribution, and age

All Martian channels and valleys visible at a resolution of 125 to 300 meters between 65°N and 65°S were mapped at a scale of 1:5,000,000 and the maps then digitized. Correlations of valley presence with other surface features show that almost all valleys are in the old cratered terrain. preferentially in areas of low albedo, low violet/red ratios, and high elevation. The networks are open, the individual drainage basins are small relative to Earth, and large distances separate the basins, features which all suggest an immature drainage system. The simplest explanation of the correlations and the restriction of valley networks to old terrain is that the channels themselves are old, and that the climatic conditions necessary for their formation did not prevail for long after the decline in the cratering rate around 3.9 billion years ago. Two types of outflow channel are distinguished: unconfined, in which broad swaths of terrain are scoured, and confined, in which flow is restricted to discrete channels. The outflow channels have a wide range of ages and may form under present climatic conditions. Fretted channels are largely restrited to two latitude belts centered on 40°N and 45°S, where relatively rapid erosion along escarpments results from mass wasting. They probably form by enlargement of preexisting channels by escarpment retreat.     

Secondary chaotic terrain formation in the higher outflow channels of southern circum-Chryse, Mars

Higher outflow channel dissection in the martian region of southern circum-Chryse appears to have extended from the Late Hesperian to the Middle Amazonian Epoch. These outflow channels were excavated within the upper 1 km of the cryolithosphere, where no liquid water is expected to have existed during these geologic epochs. In accordance with previous work, our examination of outflow channel floor morphologies suggests the upper crust excavated by the studied outflow channels consisted of a thin (a few tens of meters) layer of dry geologic materials overlying an indurated zone that extends to the bases of the investigated outflow channels (1 km in depth). We find that the floors of these outflow channels contain widespread secondary chaotic terrains (i.e., chaotic terrains produced by the destruction of channel-floor materials). These chaotic terrains occur within the full range of outflow channel dissection and tend to form clusters. Our examination of the geology of these chaotic terrains suggests that their formation did not result in the generation of floods. Nevertheless, despite their much smaller dimensions, these chaotic terrains are comprised of the same basic morphologic elements (e.g., mesas, knobs, and smooth deposits within scarp-bound depressions) as those located in the initiation zones of the outflow channels, which suggests that their formation must have involved the release of ground volatiles. We propose that these chaotic terrains developed not catastrophically but gradually and during multiple episodes of nested surface collapse. In order to explain the formation of secondary chaotic terrains within zones of outflow channel dissection, we propose that the regional Martian cryolithosphere contained widespread lenses of volatiles in liquid form. In this model, channel floor collapse and secondary chaotic terrain formation would have taken place as a consequence of instabilities arising during their exhumation by outflow channel dissection. Within relatively warm upper crustal materials in volcanic settings, or within highly saline crustal materials where cryopegs developed, lenses of volatiles in liquid form within the cryolithosphere could have formed, and/or remained stable.In addition, our numerical simulations suggest that low thermal conductivity, dry fine-grained porous geologic materials just a few tens of meters in thickness (e.g., dunes, sand sheets, some types of regolith materials), could have produced high thermal anomalies resulting in subsurface melting. The existence of a global layer of dry geologic materials overlying the cryolithosphere would suggest that widespread lenses of fluids existed (and may still exist) at shallow depths wherever these materials are fine-grained and porous. The surface ages of the investigated outflow channels and chaotic terrains span a full 500 to 700 Myr. Chaotic terrains similar in dimensions and morphology to secondary chaotic terrains are not observed conspicuously throughout the surface of Mars, suggesting that intra-cryolithospheric fluid lenses may form relatively stable systems. The existence of widespread groundwater lenses at shallow depths of burial has tremendous implications for exobiological studies and future human exploration. We find that the clear geomorphologic anomaly that the chaotic terrains and outflow channels of southern Chryse form within the Martian landscape could have been a consequence of large-scale resurfacing resulting from anomalously extensive subsurface melt in this region of the planet produced by high concentrations of salts within the regional upper crust. Crater count statistics reveal that secondary chaotic terrains and the outflow channels within which they occur have overlapping ages, suggesting that the instabilities leading to their formation rapidly dissipated, perhaps as the thickness of the cryolithosphere was reset following the disruption of the upper crustal thermal structure produced during outflow channel excavation.     

Fluvial channels on Titan: Initial Cassini RADAR observations

Cassini radar images show a variety of fluvial channels on Titan's surface, often several hundreds of kilometers in length. Some (predominantly at low- and mid-latitude) are radar-bright and braided, resembling desert washes where fines have been removed by energetic surface liquid flow, presumably from methane rainstorms. Others (predominantly at high latitudes) are radar-dark and meandering and drain into or connect polar lakes, suggesting slower-moving flow depositing fine-grained sediments. A third type, seen predominantly at mid- and high latitudes, have radar brightness patterns indicating topographic incision, with valley widths of up to 3 km across and depth of several hundred meters. These observations show that fluvial activity occurs at least occasionally at all latitudes, not only at the Huygens landing site, and can produce channels much larger in scale than those observed there. The areas in which channels are prominent so far amount to about 1% of Titan's surface, of which only a fraction is actually occupied by channels. The corresponding global sediment volume inferred is not enough to account for the extensive sand seas. Channels observed so far have a consistent large-scale flow pattern, tending to flow polewards and eastwards.     

Mass and energy flow near sunspots

Sunspots block the flow of energy to the solar surface. The blocked energy heats the volume beneath the spot, producing a pressure excess which drives an outflow of mass. Linear numerical models of the mass and energy flow around spots were constructed to estimate the predictions of this physical picture against the observed properties of sunspot bright rings and moat flows. The width of the bright ring and moat are predicted to be proportional to the depth of the spot penumbra, in conflict with the observed proportionally of the moat width to the spot diameter. Postulating that spot depths are proportional to spot diameters would bury the moat flow too deeply to be observed, because the radial velocity at the surface is found to be inversely proportional to the depth of the spot penumbra. The radial velocity at the surface is of order a few hundred meters per second after 1 day, in agreement with the observed excess of moat velocities over supergranule velocities.     

Auroral heating and the composition of the neutral atmosphere

Heating of the neutral atmosphere by auroral particle fluxes and by orthogonal electric fields is responsible for large changes in the thermospheric composition that have been observed by satellite mass spectrometers. Vertical winds of a few meters per second are produced in the region subject to auroral heating; this vertical upwelling drives circulation cells that extend the effects of heating in the auroral region on a global scale. Our analysis focuses on the initial phase of a magnetic storm within the auroral region.     

Glacial,periglacial and glacio-volcanic structures on the Echus Plateau,upper Kasei Valles

An extensive region of low, sinuous ridges occupies the Hesperian plateau above Echus Chasma in the upper Kasei Valles, Mars. The ridges have lengths of up to 270 km, heights of 100 m and widths of 10 km. The total volume of the ridge material is 6×10¹¹ m³. In this paper, volcanic flows, depositional and erosional features are discussed using Mars Observer Laser Altimeter (MOLA), THEMIS and Mars Orbiter Camera (MOC) imagery and a chronology that places the ridge formation in the Late Hesperian is developed.The plateau is bounded to the north and west by more recent Late Hesperian and Amazonian lava flows. The plateau floor suddenly changes from being relatively smooth, to elevated, rough, hummocky terrain that extends eastwards to Echus Chasma. This rough terrain is penetrated by 2 km broad, shallow entrant channels that join with the canyons of Echus Chasma. The sinuous ridges appear to control the surface drainage associated with the entrant channels.The sinuous ridges’ size and morphology are similar to those associated with volcanic ridge eruptions. Their degraded structure is reminiscent of Moberg ridges. The rough, hummocky terrain is interpreted as glacial outwash, subsequently eroded by short-lived floods associated with ridge eruptions. The presence of both volcanic and glacial structures on the Echus Plateau raises the possibility that the ridge system arose from subglacial, volcanic events. The resulting jokulhlaups eroded the broad, entrant channels. As surface flow declined, groundwater flows dominated and canyon heads eroded back along the entrant channels, by sapping.     

Episodic bright and dark spots on Uranus

The northern mid-latitudes of Uranus produce greater episodes of bright cloud formation than any other region on the planet. Near 30°N, very bright cloud features were observed in 1999, 2004, and 2005, with lifetimes of the order of months. In October 2011, Gemini and HST observations revealed another unusually bright cloud feature near 23°N, which was subsequently identified in July 2011 observations and found to be increasing in brightness. Observations obtained at Keck in November 2011 revealed a second bright spot only 2°N of the first, but with a substantially different drift rate (?9.2°E/day vs ?1.4°E/day), which we later determined would lead to a close approach on 25 December 2011. A Hubble Target of Opportunity proposal was activated to image the results of the interaction. We found that the original bright spot had faded dramatically before the HST observations had begun and the second bright spot was found to be a companion of a new dark spot on Uranus, only the second ever observed. Both spots exhibited variable drift rates during the nearly 5 months of tracking, and both varied in brightness, with BS1 reaching its observed peak on 26 October 2011, and BS2 on 11 November 2011. Altitude measurements based on near-IR imaging in H and Hcont filters showed that the deeper BS2 clouds were located near the methane condensation level (≈1.2 bars), while BS1 was generally ～500 mb above that level (at lower pressures). Large morphological changes in the bright cloud features suggest that they are companion clouds of possibly orographic nature associated with vortex circulations, perhaps similar to companion clouds associated with the Great Dark Spot on Neptune, but in this case at a much smaller size scale, spanning only a few degrees of longitude at their greatest extents.     

Emplacement of the youngest flood lava on Mars: A short, turbulent story

Recently acquired data from the High Resolution Imaging Science Experiment (HiRISE), Context (CTX) imager, and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) spacecraft were used to investigate the emplacement of the youngest flood-lava flow on Mars. Careful mapping finds that the Athabasca Valles flood lava is the product of a single eruption, and it covers 250,000 km² of western Elysium Planitia with an estimated 5000-7500 km³ of mafic or ultramafic lava. Calculations utilizing topographic data enhanced with MRO observations to refine the dimensions of the channel system show that this flood lava was emplaced turbulently over a period of only a few to several weeks. This is the first well-documented example of a turbulently emplaced flood lava anywhere in the Solar System. However, MRO data suggest that this same process may have operated in a number of martian channel systems. The magnitude and dynamics of these lava floods are similar to the aqueous floods that are generally believed to have eroded the channels, raising the intriguing possibility that mechanical erosion by lava could have played a role in their incision.     

CO2 permafrost and Martian topography

The role of CO₂ permafrost as an erosive agent on Mars is considered. In the CO₂H₂O system, with a CO₂ triple point at 217°K and 5.1-bar pressure, carbon dioxide solid, liquid, or gas, CO₂ clathrate, and ice are possible stable phases in the range of temperatures and pressures likely to be encountered in the Martian regolith. It is argued that conditions may exist in which CO₂ permafrost is extensive on Mars, provided that adequate CO₂ is available: the maximum ratio of H₂O:CO₂ required in the subsurface pore space system is 17:3. Erosional processes likely to result from such permafrost are block slumping, leading to canyon development; pit chains along faults; chaotic terrain where massive permafrost destruction has occured; large-scale flows of slurry; and perhaps even the flash floods which create channels.     

Orbits supporting bars within bars

High-resolution observations of the inner regions of barred disc galaxies have revealed many asymmetrical, small-scale central features, some of which are best described as secondary bars. Because orbital time-scales in the galaxy centre are short, secondary bars are likely to be dynamically decoupled from the main kiloparsec-scale bars. Here we show that regular orbits exist in such doubly barred potentials, and that they can support the bars in their motion. We find orbits in which particles remain on loops : closed curves which return to their original positions after two bars have come back to the same relative orientation. Stars trapped around stable loops could form the building blocks for a long-lived, doubly barred galaxy. Using the loop representation, we can find which orbits support the bars in their motion, and the constraints on the sizes and shapes of self-consistent double bars. In particular, it appears that a long-lived secondary bar may exist only when an inner Lindblad resonance is present in the primary bar, and that it would not extend beyond this resonance.     

Topography and geomorphology of the Huygens landing site on Titan

The Descent Imager/Spectral Radiometer (DISR) aboard the Huygens Probe took several hundred visible-light images with its three cameras on approach to the surface of Titan. Several sets of stereo image pairs were collected during the descent. The digital terrain models constructed from those images show rugged topography, in places approaching the angle of repose, adjacent to flatter darker plains. Brighter regions north of the landing site display two styles of drainage patterns: (1) bright highlands with rough topography and deeply incised branching dendritic drainage networks (up to fourth order) with dark-floored valleys that are suggestive of erosion by methane rainfall and (2) short, stubby low-order drainages that follow linear fault patterns forming canyon-like features suggestive of methane spring-sapping. The topographic data show that the bright highland terrains are extremely rugged; slopes of order of 30° appear common. These systems drain into adjacent relatively flat, dark lowland terrains. A stereo model for part of the dark plains region to the east of the landing site suggests surface scour across this plain flowing from west to east leaving ∼100-m-high bright ridges. Tectonic patterns are evident in (1) controlling the rectilinear, low-order, stubby drainages and (2) the “coastline” at the highland–lowland boundary with numerous straight and angular margins. In addition to flow from the highlands drainages, the lowland area shows evidence for more prolific flow parallel to the highland–lowland boundary leaving bright outliers resembling terrestrial sandbars. This implies major west to east floods across the plains where the probe landed with flow parallel to the highland–lowland boundary; the primary source of these flows is evidently not the dendritic channels in the bright highlands to the north.     

The morphology of galactic rings exterior to evolving bars: test-particle simulations

The morphology of the outer rings of early-type spiral galaxies is compared to integrations of massless collisionless particles initially in nearly circular orbits. Particles are perturbed by a quadrupolar gravitational potential corresponding to a growing and secularly evolving bar. We find that outer rings with R1R2 morphology and pseudo-rings are exhibited by the simulations even though they lack gaseous dissipation. Simulations with stronger bars form pseudo-rings earlier and more quickly than those with weaker bars. We find that the R1 ring, perpendicular to the bar, is fragile and dissolves after a few bar rotation periods if the bar pattern speed increases by more than ∼8 per cent, bar strength increases (by ≳140 per cent) after bar growth or the bar is too strong  ( Q_T > 0.3)  . If the bar slows down after formation, pseudo-ring morphology persists and the R2 ring perpendicular to the bar is populated due to resonance capture. The R2 ring remains misaligned with the bar and increases in ellipticity as the bar slows down. The R2 ring becomes scalloped and does not resemble any ringed galaxies if the bar slows down more than 3.5 per cent suggesting that bars decrease in strength before they slow down this much. We compare the morphology of our simulations to B -band images of nine ringed galaxies from the Ohio State University Bright Spiral Galaxy Survey, and we find a reasonable match in morphologies to R1R2' pseudo-rings seen within a few bar rotation periods of bar formation. Some of the features previously interpreted in terms of dissipative models may be due to transient structure associated with recent bar growth and evolution.     

Evidence for explosive volcanic density currents on certain Martian volcanoes

A number of Martian volcanoes, especially Ceraunius Tholus, Uranius Tholus, Uranius Patera, and Hecates Tholus, show morphological features strikingly different from those of shield volcanoes but analogous to those of terrestrial cones and composite volcanoes such as Barcena Volcano, Mexico. The most distinguishing overall features are steep slope angles, and Krakatoa-type caldera morphologies. Erosional features comprise numerous radial channels which extend from below the rim toward the base of the dome, and in some cases, patterns of anastamosing gullies which contribute to the main radial channels. Constructional features include blanketed flanks interpreted as dune or fan-like deposits of ash, and perhaps lava deltas. A possible explanation for the morphological features associated with these volcanoes is that they were formed by explosive volcanic density currents. Such eruptions would be expected on Mars where a rising magma came in contact with a thick layer of permafrost generating a base surge or after a Vulcanian explosion of a separate gas phase producing a nuée ardente. Crater age data from the surface of Martian domes and shields indicate that such explosive activity occurred more frequently early in Martian geologic history. This is more consistent with the view that the volcanic density flows were base surges rather than nuées ardentes, the melting of permafrost supplying the water required in base surge generation. If atmospheric conditions were more clement at the time, allowing the recycling of water back to the ground water, then the length of duration of phreatic activity would have been longer, not being limited by depletion time of the local permafrost reservoir.     

Floods on Mars released from groundwater by impact

On Earth, large earthquakes commonly cause saturated soils to liquefy and streamflow to increase. We suggest that meteoritic impacts on Mars may have repeatedly caused similar liquefaction to enable violent eruption of groundwater. The amount of erupted water may be comparable to that required to produce catastrophic floods and to form outflow channels.     

Digital multichannel spectrometer for solar microwave observations

Observations of solar microwave bursts have shown fine structures (e.g., the millisecond spikes), not resolvable in time and frequency by existing instruments. In order to investigate these features in greater detail we have developed a spectrometer with high temporal and spectral resolution. The frequency range from 3000 to 4000 MHz is covered by 32 channels with different bandwidths (0.1, 5, and 20 MHz). The instrument is fully controlled by a multiprocessor computer system and allows the recording of about 200 000 measurements per second. Thus it is possible to observe the intensity and the circular polarization of all the 32 channels with a time resolution of about 350 s. A very flexible frequency selection system allows the use of many different observation modes.     

Formation and disruption of aquifers in southwestern Chryse Planitia, Mars

We present geologic evidence suggesting that after the development of Mars' cryolithosphere, the formation of aquifers in southwestern Chryse Planitia and their subsequent disruption led to extensive regional resurfacing during the Late Hesperian, and perhaps even during the Amazonian. In our model, these aquifers formed preferentially along thrust faults associated with wrinkle ridges, as well as along fault systems peripheral to impact craters. The characteristics of degraded wrinkle ridges and impact craters in southwestern Chryse Planitia indicate a profound role of subsurface volatiles and especially liquid water in the upper crust (the upper one hundred to a few thousands of meters). Like lunar wrinkle ridges, the martian ones are presumed to mark the surface extensions of thrust faults, but in our study area the wrinkle ridges are heavily modified. Wrinkle ridges and nearby plains have locally undergone collapse, and in other areas they are associated with domical intrusions we interpret as mud volcanoes and mud diapirs. In at least one instance, a sinuous valley emanates from a modified wrinkle ridge, further indicating hydrological influences on these thrust-fault-controlled features. A key must be the formation of volatile-rich crust. Primary crustal formation and differentiation incorporated juvenile volatiles into the global crust, but the crustal record here was then strongly modified by the giant Chryse impact. The decipherable rock record here begins with the Chryse impact and continues with the resulting basin's erosion and infilling, which includes outflow channel activity. We propose that in Simud Vallis surface flow dissection into the base of the cryolithosphere-produced zones where water infiltrated and migrated along SW-dipping strata deformed by the Chryse impact, thereby forming an extensive aquifer in southwestern Chryse Planitia. In this region, compressive stresses produced by the rise of Tharsis led to the formation of wrinkle ridges. Zones of high fracture density within the highly strained planes of the thrust faults underlying the wrinkle ridges formed regions of high permeability; thus, groundwater likely flowed and gathered along these tectonic structures to form zones of elevated permeability. Volatile depletion and migration within the upper crustal materials, predominantly along fault systems, led to structurally controlled episodic resurfacing in southwestern Chryse Planitia. The erosional modification of impact craters in this region is linked to these processes. This erosion is scale independent over a range of crater diameters from a few hundred meters to tens of kilometers. According to our model, pressurized water and sediment intruded and locally extruded and caused crustal subsidence and other degradational activity across this region. The modification of craters across this wide range of sizes, according to our model, implies that there was intensive mobilization of liquid water in the upper crust ranging from about one hundred to several thousand meters deep.     

Generation of Martian chaos and channels by debris flows

A debris flow mechanism is proposed to account for the formation of chaos and the large channels debouching into Chryse Planitia from the adjacent southern uplands of Mars. The debris is thought to have originated through a mechanism of collapse in the chaotic terrains which exist at the head of these channels as well as locally along the channels. This proposition is based on the detailed morphologic similarities between Martian channel source areas and the heads of both subaerial and subaqueous terrestrial debris flows. The downslope movement of the debris produced the channels through (a) modification of earlier collapse areas, (b) active bed erosion, and (c) loading-induced collapse. The large-scale channel geometry and the assemblage of related morphologic features on Mars correspond tto that observed in subaqueous debris flow chutes on the Mississippi delta front. Through various mechanisms of strain-dependent viscosity decrease the debris flow gained mobility downstream, turned into a debris avalanche, and moved onto Chryse Planitia at very high velocities. This high-velocity avalanche eroded a series of streamlined remnants near the channel mouths and deposited its load as a thin blanket over a large area of the basin creating virtually no depositional relief.