Investigating gully flow emplacement mechanisms using apex slopes

The origin of the martian gullies has been much debated since their discovery by the Mars Orbiter Camera (MOC, Malin, M.C., Edgett, K.S. [2000]. Science 288, 2330-2335). Several previous studies have looked at slope gradients in and around gullies, but none have used Digital Elevation Models (DEMs) from the High Resolution Imaging Science Experiment (HiRISE, McEwen, A.S., and 14 colleagues [2007]. J. Geophys. Res. 112 (E05), E0505S02), which has a pixel scale down to 25 cm/pixel. We use five 1 m/post HiRISE DEMs to measure gully apex slopes, the local channel gradient at the upslope extent of the gully debris apron, which marks a shift from erosion to deposition. The apex slope provides information about whether a flow was likely a typical dry granular flow (begins depositing on slopes ∼21°) or fluidized by some extra mechanism (depositing on shallower slopes). We find that 72% of the 75 gully fans studied were likely emplaced by fluidized flows. Relatively old gullies appear more likely to have hosted fluidized flows than relatively fresh gullies. This suggests a time and location dependent fluidizing agent, possibly liquid water produced in a different climate as previously proposed. Our results do not provide evidence for water-rich flows in gullies today.     

Martian hillside gullies and icelandic analogs

We report observations of Icelandic hillside gully systems that are near duplicates of gullies observed on high-latitude martian hillsides. The best Icelandic analogs involve basaltic talus slopes at the angle of repose, with gully formation by debris flows initiated by ground water saturation, and/or by drainage of water from upslope cliffs. We report not only the existence of Mars analog gullies, but also an erosional sequence of morphologic forms, found both on Mars and in Iceland. The observations support hypotheses calling for creation of martian gullies by aqueous processes. Issues remain whether the water in each case comes only from surficial sources, such as melting of ground ice or snow, or from underground sources such as aquifers that gain surface access in hillsides. Iceland has many examples of the former, but the latter mechanism is not ruled out. Our observations are consistent with the martian debris flow mechanism of F. Costard et al. (2001c, Science295, 110-113), except that classic debris flows begin at midslope more frequently than on Mars. From morphologic observations, we suggest that some martian hillside gully systems not only involve significant evolution by extended erosive activity, but gully formation may occur in episodes, and the time interval since the last episode is considerably less than the time interval needed to erase the gully through normal martian obliteration processes.     

Observations of martian gullies and constraints on potential formation mechanisms

The discovery of presumably geologically recent gully features on Mars (Malin and Edgett, 2000, Science 288, 2330-2335) has spawned a wide variety of proposed theories of their origin including hypotheses of the type of erosive material. To test the validity of gully formation mechanisms, data from the Mars Global Surveyor spacecraft has been analyzed to uncover trends in the dimensional and physical properties of the gullies and their surrounding terrain. We located 106 Mars Orbiter Camera (MOC) images that contain clear evidence of gully landforms, distributed in the southern mid and high latitudes, and analyzed these images in combination with Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) data to provide quantitative measurements of numerous gully characteristics. Parameters we measured include apparent source depth and distribution, vertical and horizontal dimensions, slopes, orientations, and present-day characteristics that affect local ground temperatures. We find that the number of gully systems normalized to the number of MOC images steadily declines as one moves poleward of 30° S, reaches a minimum value between 60°-63° S, and then again rises poleward of 63° S. All gully alcove heads occur within the upper one-third of the slope encompassing the gully and the alcove bases occur within the upper two-thirds of the slope. Also, the gully alcove heads occur typically within the first 200 meters of the overlying ridge with the exception of gullies equatorward of 40° S where some alcove heads reach a maximum depth of 1000 meters. While gullies exhibit complex slope orientation trends, gullies are found on all slope orientations at all the latitudes studied. Assuming thermal conductivities derived from TES measurements as well as modeled surface temperatures, we find that 79% of the gully alcove bases lie at depths where subsurface temperatures are greater than 273 K and 21% of the alcove bases lie within the solid water regime. Most of the gully alcoves lie outside the temperature-pressure phase stability of liquid CO₂. Based on a comparison of measured gully features with predictions from the various models of gully formation, we find that models involving carbon dioxide, melting ground ice in the upper few meters of the soil, dry landslide, and surface snowmelt are the least likely to describe the formation of the martian gullies. Although some discrepancies still exist between prediction and observation, the shallow and deep aquifer models remain as the most plausible theories. Interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies.     

Observations of martian gullies and constraints on potential formation mechanisms: II. The northern hemisphere

The formation process(es) responsible for creating the observed geologically recent gully features on Mars has remained the subject of intense debate since their discovery. We present new data and analysis of northern hemisphere gullies from Mars Global Surveyor data which is used to test the various proposed mechanisms of gully formation. We located 137 Mars Orbiter Camera (MOC) images in the northern hemisphere that contain clear evidence of gully landforms and analyzed these images in combination with Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) data to provide quantitative measurements of numerous gully characteristics. Parameters we measured include apparent source depth and distribution, vertical and horizontal dimensions, slopes, orientations, and present-day characteristics that affect local ground temperatures. Northern hemisphere gullies are clustered in Arcadia Planitia, Tempe Terra, Acidalia Planitia, and Utopia Planitia. These gullies form in craters (84%), knobby terrain (4%), valleys (3%), other/unknown terrains (9%) and are found on all slope orientations although the majority of gullies are equator-facing. Most gullies (63%) are associated with competent rock strata, 26% are not associated with strata, and 11% are ambiguous. Assuming thermal conductivities derived from TES measurements as well as modeled surface temperatures, we find that 95% of the gully alcove bases with adequate data coverage lie at depths where subsurface temperatures are greater than 273 K and 5% of the alcove bases lie within the solid water regime. The average alcove length is 470 m and the average channel length is 690 m. Based on a comparison of measured gully features with predictions from the various models of gully formation, we find that models involving carbon dioxide, melting ground ice in the upper few meters of the soil, dry landslide, and surface snowmelt are the least likely to describe the formation of the martian gullies. Although some discrepancies still exist between prediction and observation, the shallow and deep aquifer models remain as the most plausible theories. Interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies. These findings gleaned from the northern hemisphere data are in general agreement with analyses of gullies in the southern hemisphere [Heldmann, J.L., Mellon, M.T., 2004. Icarus 168, 285-304].     

Preservation of Late Amazonian Mars ice and water-related deposits in a unique crater environment in Noachis Terra: Age relationships between lobate debris tongues and gullies

The Amazonian period of Mars has been described as static, cold, and dry. Recent analysis of high-resolution imagery of equatorial and mid-latitude regions has revealed an array of young landforms produced in association with ice and liquid water; because near-surface ice in these regions is currently unstable, these ice-and-water-related landforms suggest one or more episodes of martian climate change during the Amazonian. Here we report on the origin and evolution of valley systems within a degraded crater in Noachis Terra, Asimov Crater. The valleys have produced a unique environment in which to study the geomorphic signals of Amazonian climate change. New high-resolution images reveal Hesperian-aged layered basalt with distinctive columnar jointing capping interior crater fill and providing debris, via mass wasting, for the surrounding annular valleys. The occurrence of steep slopes (>20°), relatively narrow (sheltered) valleys, and a source of debris have provided favorable conditions for the preservation of shallow-ice deposits. Detailed mapping reveals morphological evidence for viscous ice flow, in the form of several lobate debris tongues (LDT). Superimposed on LDT are a series of fresh-appearing gullies, with typical alcove, channel, and fan morphologies. The shift from ice-rich viscous-flow formation to gully erosion is best explained as a shift in martian climate, from one compatible with excess snowfall and flow of ice-rich deposits, to one consistent with minor snow and gully formation. Available dating suggests that the climate transition occurred >8 Ma, prior to the formation of other small-scale ice-rich flow features identified elsewhere on Mars that have been interpreted to have formed during the most recent phases of high obliquity. Taken together, these older deposits suggest that multiple climatic shifts have occurred over the last tens of millions of years of martian history.     

The formation and evolution of youthful gullies on Mars: Gullies as the late-stage phase of Mars’ most recent ice age

Gullies are extremely young erosional/depositional systems on Mars that have been carved by an agent that was likely to have been comprised in part by liquid water [Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330-2335; McEwen, A.S. et al., 2007. A closer look at water-related geologic activity on Mars. Science 317, 1706-1709]. The strong latitude and orientation dependencies that have been documented for gullies require (1) a volatile near the surface, and (2) that insolation is an important factor for forming gullies. These constraints have led to two categories of interpretations for the source of the volatiles: (1) liquid water at depth beneath the melting isotherm that erupts suddenly (“groundwater”), and (2) ice at the surface or within the uppermost layer of soil that melts during optimal insolation conditions (“surface/near-surface melting”). In this contribution we synthesize global, hemispheric, regional and local studies of gullies across Mars and outline the criteria that must be met by any successful explanation for the formation of gullies. We further document trends in both hemispheres that emphasize the importance of top-down melting of recent ice-rich deposits and the cold-trapping of atmospherically-derived H₂O frost/snow as important components in the formation of gullies. This provides context for the incorporation of high-resolution multi-spectral and hyper-spectral data from the Mars Reconnaissance Orbiter that show that (1) cold-trapping of seasonal H₂O frost occurs at the alcove/channel-level on contemporary Mars; (2) gullies are episodically active systems; (3) gullies preferentially form in the presence of deposits plausibly interpreted as remnants of the Late Amazonian emplacement of ice-rich material; and (4) gully channels frequently emanate from the crest of alcoves instead of the base, showing that alcove generation is not necessarily a product of undermining and collapse at these locations, a prediction of the groundwater model. We interpret these various lines of evidence to mean that the majority of gullies on Mars are explained by the episodic melting of atmospherically emplaced snow/ice under spin-axis/orbital conditions characteristic of the last several Myr.     

Martian gullies in the southern mid-latitudes of Mars: Evidence for climate-controlled formation of young fluvial features based upon local and global topography

A new survey of Mars Orbiter Camera (MOC) narrow-angle images of gullies in the 30°-45° S latitude band includes their distribution, morphology, local topographic setting, orientation, elevation, and slopes. These new data show that gully formation is favored over a specific range of conditions: elevation (−5000 to +3000 m), slope (>10°), and orientation (83.8% on pole-facing slopes). These data, and the frequent occurrence of gullies on isolated topographic highs, lead us to support the conclusion that climatic-related processes of volatile accumulation and melting driven by orbital variations are the most likely candidate for processes responsible for the geologically recent formation of martian gullies.     

Evidence for debris flow gully formation initiated by shallow subsurface water on Mars

The morphologies of some martian gullies appear similar to terrestrial features associated with debris flow initiation, erosion, and deposition. On Earth, debris flows are often triggered by shallow subsurface throughflow of liquid water in slope-mantling colluvium. This flow causes increased levels of pore pressure and thus decreased shear strength, which can lead to slide failure of slope materials and subsequent debris flow. The threshold for pore pressure-induced failure creates a distinct relationship between the contributing area supplying the subsurface flow and the slope gradient. To provide initial tests of a similar debris flow initiation hypothesis for martian gullies, measurements of the contributing areas and slope gradients were made at the channel heads of martian gullies seen in three HiRISE stereo pairs. These gullies exhibit morphologies suggestive of debris flows such as leveed channels and lobate debris fans, and have well-defined channel heads and limited evidence for multiple flows. Our results show an area-slope relationship for these martian gullies that is consistent with that observed for terrestrial gullies formed by debris flow, supporting the hypothesis that these gullies formed as the result of saturation of near-surface regolith by a liquid. This model favors a source of liquid that is broadly distributed within the source area and shallow; we suggest that such liquid could be generated by melting of broadly distributed icy materials such as snow or permafrost. This interpretation is strengthened by observations of polygonal and mantled terrain in the study areas, which are both suggestive of near-surface ice.     

The role of the wind-transported dust in slope streaks activity: Evidence from the HRSC data

Slope streaks are gravity-driven albedo features observed on martian slopes since the Viking missions. The debated mechanism of formation could involve alternatively dry granular flow or wet mass wasting. A systematic mapping of slope streaks from the High Resolution Stereo Camera is presented in this paper. Two regions known for their slope streaks activity have been studied, the first one is located close to Cerberus lava flow, and the second one is inside the Olympus Mons Aureole. The statistics of slope streaks shapes measured from orthorectified images confirm previous results from Mars Orbiter Camera surveys. Preferential orientations of slope streaks are reported. Slope streaks occur preferentially on west facing slopes at latitudes lower than 30° N for Olympus and on south-west facing slopes for Cerberus. Wind directions derived from a General Circulation Model during the dusty season correlate with these orientations. Furthermore, west facing slopes at Olympus have a thicker dust cover. These observations indicate that slope streaks are dust avalanches controlled by the preferential accumulation of dust in the downstream side of the wind flow. The paucity of slope streaks at high latitudes and their preferential orientation on south-facing slopes have been presented as an evidence for a potential role of H₂O phase transition in triggering or flow. The potential role of H₂O cannot be ruled out from our observations but the dust avalanche model together with the atmospheric circulation could potentially explain all observations. The role of H₂O might be limited to a stabilizing effect of dust deposits on northward facing slopes at intermediate latitudes (30° N-33° N) and on all slopes further north.     

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

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

Evidence of gully formation by regional groundwater flow in the Gorgonum-Newton region (Mars)

The discovery of gullies on Mars suggests liquid water activity near the surface of the planet in recent times. Since liquid water is unstable under the present-day P-T martian conditions, the formation mechanisms of the gullies, and the source of the putative water, have been a matter of debate for the last years. To provide new insights into these matters, we have approached the problem studying the gullies in relation to their regional setting. A major point in our study relates to the geographic orientation of gullies, an aspect that has been previously regarded as a crucial matter in different models, and has profound implications regarding their origin. We present a comprehensive and detailed survey of the Gorgonum-Newton region, and a study of the Dao and Nirgal Vallis regions. The survey was carried out with the aid of 965 high-resolution MOC images (752 for Gorgonum-Newton, 102 for Nirgal Vallis and 111 for Dao Vallis regions), and MOLA-derived DEMs. We found that gullies display a clear regional pattern, geographically and topographically consistent with a decreasing regional slope. We interpret the results in terms of the existence of several groundwater flow systems operating at different scales, which ultimately may have led to gully formation by seepage at the slopes of craters and canyons. We suggest that aquifers discharging at gully systems may have recharged from the surface, in response to the melting of young partially eroded ice-rich deposits.     

Evaporation effects on the formation of martian gullies

In order to investigate the formation of martian gullies and the stability of fluids on Mars, we examined about 120 gully images. Twelve HiRISE images contained a sufficient number of Transverse Aeolian Ridges (TARs) associated with the gullies to make the following measurements: overall gully length, length of the alcove, channel and apron, and we also measured the frequency of nearby TARs. Six of the 12 images examined showed a statistically significant negative correlation between overall gully length (alcove, channel and apron length) and TAR frequency. Previous experimental work from our group has shown that at temperatures below ∼200 K, evaporation rate increases by about an order of magnitude as wind speed increases from 0 to ∼15 m/s. Thus the negative correlations we observe between gully length and dune frequency can be explained by formation at temperatures below ∼200 K where wind speed/evaporation is a factor governing gully length. In these cases evaporation of the fluid carving the gully was a constraint on their dimensions. Cases where there is no correlation between gully length and TAR frequency, can be explained by formation at temperatures >200 K. The temperatures are consistent with Global Circulation Model and Thermal Emission Spectrometer (TES) data for these latitudes. The temperatures suggested by these trends are consistent with the fluid responsible for gully formation being a strong brine, such as Fe₂(SO₄)₃ which has a eutectic temperature of ∼200 K. We also find that formation timescales for gullies are 10⁵-10⁶ years.     

Martian flow features, moraine-like ridges, and gullies: Terrestrial analogs and interrelationships

Ridges that resemble terrestrial moraines are commonly visible at the foot of many mid-latitude crater walls in Mars Global Surveyor Mars Orbiter Camera images. These moraine-like ridges are often associated with hillside gullies, mantling material, and glacier-like flows, and are usually in contact with crater fill, suggesting possible interrelationships. We consider terrestrial glacier systems that may be analogs of martian moraine-like ridges and glacier-like flows and suggest that the formation of some gullies and crater fill is intimately tied to ice deposition, ice flow, and rock-glacier processes. Upper limits on age suggest the possibility that many of these features formed during the last, or last few, high obliquity cycles.     

Stability of mid-latitude snowpacks on Mars

Christensen [2003. Nature 422, 45-48] suggested that runoff from melting snowpacks on martian slopes might be responsible for carving gullies. He also suggested that snowpacks currently exist on Mars, for example on the walls of Dao Valles (approximately 33° S). Such snowpacks were presumably formed during the last obliquity cycle, which occurred about 70,000 years ago. In this paper we investigate a specific scenario under conditions we believe are favorable for snowpack melting. We model the rate at which a snowpack located at 33° S on a poleward-facing slope sublimates and melts on Mars, as well as the temperature profile within the snowpack. Our model includes the energy and mass balance of a snowpack experiencing diurnal variations in insolation. Our results indicate that a dirty snowpack would quickly sublimate and melt under current martian climate conditions. For example a 1 m thick dusty snowpack of moderate density (550 kg/m³) and albedo (0.39) would sublimate in less than two seasons, producing a small amount of meltwater runoff. Similarly, a cleaner snowpack (albedo 0.53) would disappear in less than 9 seasons. These results suggest that the putative snowpack almost certainly could not have survived for 70,000 years. For most of the parameter settings snowpack interior temperatures at this latitude and slope do reach the melting point. Under most conditions melting occurs when the snowpack is less than 10 cm thick. The modeled snowpack will not melt if it is covered by a 1 cm dust lag. In general, these findings raise interesting possibilities regarding gully formation, but perhaps mostly during a past climate regime when snowfall was expected to have occurred. If there currently are exposed snowpacks on martian mid-latitude slopes, then these ice sheets cannot last long. Hence they might be time variable features on Mars and should be searched for.     

Groundwater discharge and gully formation on martian slopes

Young gullies and gully deposits on walls of martian craters have been cited as evidence that liquid water flowed on the surface of Mars relatively recently. Effects of variable environmental conditions at the surface of Mars are modeled and applied to the case of groundwater emergence from shallow aquifers to investigate whether groundwater is a viable source to enable the erosion of these gullies. The model includes detailed treatment of ice growth in the aquifer. Model results indicate that groundwater discharge can be maintained under the current environmental conditions if the aquifer permeability is like that of terrestrial gravel or higher, if the aquifer is 350 K or warmer, or if the aquifer is a brine with a freezing point depressed to 250 K or below. Groundwater discharge cannot be maintained for the conservative case of a cold, pure water, semi-pervious aquifer. Cold (275 K) pure water pervious (gravel) aquifers, warm (350 K) pure water semi-pervious aquifers, and cold (275 K) CaCl₂ brine semi-pervious aquifers all exhibit a dependence of discharge on season, latitude and slope orientation in our modeling. Seasonal, latitudinal and azimuthal discharge variations are strongest for cold CaCl₂ brine semi-pervious aquifers, with discharges from this aquifer type favoring equator-facing slopes at mid and high southern latitudes. At all latitudes and slope azimuths under our nominal conditions, the cold pure water pervious aquifer, the cold pure water semi-pervious aquifer and the cold CaCl₂ brine semi-pervious aquifer all freeze completely shortly after the simulations are started. Discharge restarts in the summer for the cold pure water pervious aquifer and the cold brine aquifer, but discharge does not restart for the cold pure water semi-pervious aquifer. The warm pure water semi-pervious aquifer maintains daily seeps throughout the year at all but high latitudes. In the case of the cold pure water pervious aquifer, approximately 500,000 m³ of water could be discharged from a mid-latitude, 150-m thick aquifer with a 20-m wide seepage face orientated towards the equator or the pole after a single undermining-induced event before ice growth seals the seepage face. For a brine semi-pervious aquifer with the same dimensions, 200-300 m³ of water could be released from a mid-latitude 20-m wide equator-facing seepage face before the fresh exposure is sealed for the fall and winter seasons. Our results do not rule out groundwater emergence as a means of creating some recent gullies, but they indicate that rather special and perhaps unusual conditions would be required.     

Seasonal melting of surface water ice condensing in martian gullies

In this work we consider when and how much liquid water during present climate is possible within the gullies observed on the surface of Mars. These features are usually found on poleward directed slopes. We analyse the conditions for melting of H₂O ice, which seasonally condenses within the gullies. We follow full annual cycle of condensation and sublimation of atmospheric CO₂ and H₂O, accounting for the heat and mass transport in the soil. During the summer, once the facets of the gullies are exposed to the Sun the water ice can melt and evaporate. Two mid latitude locations in both hemispheres are considered. The model includes both the rough geometry of the gullies as well as the slope of the surface where the gullies appear. It is an extension of the model developed to calculate condensation of CO₂ ice in troughs of different sizes, including polygonal features on Mars (Kossacki and Markiewicz, 2002, Icarus 160, 73; Kossacki et al., 2003, Planet. Space Sci. 51, 569). We have found, that water ice accumulated during winter can undergo transition to the liquid phase after complete sublimation of CO₂ ice. The amount of liquid water depends on water content in the atmosphere and on the local wind speed. It is probably not enough to destabilise the slope and cause flow of the surface material. However, even the small amounts of liquid water predicted, can play an important role in surface chemistry, in increasing the cohesive strength of the soil's surface layer and possibly may have some exobiological implications.     

A 1 Gyr climate model for Mars: new orbital statistics and the importance of seasonally resolved polar processes

New results from a 1 Gyr integration of the martian orbit are presented along with a seasonally resolved energy balance climate model employed to illuminate the gross characteristics of the long-term atmospheric pressure evolution. We present a new analysis of the statistical variation of the martian obliquity and precession prior to and subsequent to the formation of the Tharsis uplift, and explore the long term effects on the martian climate. We find that seasonal polar cycles have a critical influence on the ability for the regolith to release CO₂ at high obliquities, and find that the atmospheric CO₂ actually decreases at high obliquities due to the cooling effect of polar deposits at latitudes where seasonal caps form. At low obliquity, the formation of massive, permanent polar caps depends critically on the values of the frost albedo, A_frost, and frost emissivity, ?_frost. Using our model with values of A_frost=0.67 and ?_frost=0.55, matched to the NASA Ames General Circulation Model (GCM) results (Haberle et al., 1993, J. Geophys. Res. 98, 3093-3123, and Haberle et al., 2003, Icarus 161, 66-89), we find that permanent caps only form at low obliquities (<13°), suggesting that any permanent deposits on the surface of Mars today may be residuals left over from a period of very low obliquity, or are the result of mechanisms not represented by this model. Thus, contrary to expectations, the martian atmospheric pressure is remarkable static over time, and decreases both at high and low obliquity. Also, from our one billion year orbital model, we present new results on the fraction of time Mars is expected to experience periods of low obliquity and high obliquity.     

Experimental simulation of martian gully forms

Gullies are widespread on slopes on the surface of Mars and have been investigated by numerous authors, yet their formation processes remain elusive. In an attempt to understand the possibility of a water-based origin for these forms, we undertook a series of flume experiments at Earth surface temperatures and pressures. Our objectives were to produce forms that resemble those most commonly observed on Mars, documenting their morphometric characteristics and identifying any statistically significant relationships between form and controlling factors of slope and flow rate. Experiments were conducted in a 1×1.5 m² flume filled with medium grain size sand. The experiments were run over a slope angle range of 10–30°, corresponding to the range for gullies on Mars. Water from a constant-head tank fed through 5 mm silicone hose to a rotameter and then released just below the surface at the top of the slope. Gullies were produced at slope angle values of 10°, 20°, and 30° and flow rate values of 445, 705, 965, and 1260 mL min^?1 at each angle. Eighteen parameters were identified and subsequently measured on each gully produced in the flume. Gully forms were successfully reproduced and displayed development of the fundamental morphological components observed on Mars: alcove, channel, and apron. Slope–gully form relationships for each component revealed the following results: higher slope angles formed shorter gullies with thicker apron deposits. Moreover, longer gullies were seen at higher flow rates. We concluded that forms visually similar to those observed on Mars can be created by water in the laboratory flume under terrestrial conditions. Morphometric parameters can be measured and permit identification of controlling factors. Experimental simulation of gullies appears possible with proper scaling of experimental parameters. Although not directly scalable to Mars, flume gully parameters may be used to develop numerical models in the future.     

Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water

Our study investigates possible formation mechanisms of the very recent bright gully deposits (BGDs) observed on Mars in order to assess if liquid water was required. We use two models in our assessment: a one-dimensional (1D) kinematic model to model dry granular flows and a two-dimensional (2D) fluid-dynamic model, FLO-2D (O’Brien et al., 1993, FLO Engineering), to model water-rich and wet sediment-rich flows. Our modeling utilizes a high-resolution topographic model generated from a pair of images acquired by the High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter. For the 1D kinematic modeling of dry granular flows, we examine a range of particle sizes, flow thicknesses, initial velocities, flow densities, and upslope initiation points to examine how these parameters affect the flow run-out distances of the center of mass of a flow. Our 1D modeling results show that multiple combinations of realistic parameters could produce dry granular flows that travel to within the observed deposits’ boundaries. We run the 2D fluid-dynamic model, FLO-2D, to model both water-rich and wet sediment-rich flows. We vary the inflow volume, inflow location, discharge rate, water-loss rate (water-rich models only), and simulation time and examine the resulting maximum flow depths and velocities. Our 2D modeling results suggest that both wet sediment-rich and water-rich flows could produce the observed bright deposits. Our modeling shows that the BGDs are not definitive evidence of recent liquid water on the surface of Mars.     

Variability of the south polar cap of Mars in Mars years 28 and 29

We have used Mars Reconnaissance Orbiter data from 2007 and 2009 to compare summer behaviors of the seasonal and residual south polar caps of Mars in those two years. We find that the planet-encircling dust storm that occurred in the first of the two Mars years enhanced the loss of seasonal CO₂ deposits relative to the second year but did not have a large effect on the continuing erosion of the pits and mesas within the residual cap materials. This suggests that the increase of bright frost in some regions of the residual cap observed between Mariner 9 and Viking can be accommodated within observed martian weather variability and does not require unknown processes or climate change.