Northern Hemisphere Of Mars Research Articles

Three‐dimensional simulations of the southern polar giant impact hypothesis for the origin of the Martian dichotomy

AbstractWe demonstrate via numerical simulations that the impact of a ~lunar‐sized body with Mars is capable of creating a hemispherical magma ocean that upon cooling and solidification resulted in the formation of the southern highlands and thus the Martian dichotomy. The giant impact may have contributed a significant amount of iron to the Martian core and generated a deep thermal anomaly that led to the onset and development of the volcanism in the southern highlands. Our model predicts several mantle plumes converging to the South Pole from the equatorial regions as well as new plumes forming in the equatorial region and also an absence of significant large‐scale volcanism in the northern lowlands. The core heat flux evolution obtained from our numerical models is consistent with the decline of the magnetic field. We argue that such a scenario is more consistent with a range of observations than a northern giant impact (excavating the Borealis basin) for the formation of the Martian dichotomy.

Channel slope reversal near the Martian dichotomy boundary: Testing tectonic hypotheses

Faults along the Martian dichotomy boundary are evidence of tectonic activity, which by analogy with terrestrial tectonism may cause changes in or even reversal of fluvial longitudinal profiles. In the eastern hemisphere between 30° E and 150° E, this tectonic activity has been hypothesized to result from lower crustal flow or from lithospheric flexure, for which loading (e.g., by material deposition) of the northern lowlands is a possible cause (Watters, 2003a; Nimmo, 2005). The topographic (slope) changes resulting from these two different mechanisms are distinct and can provide a means for distinguishing between them, although other processes may complicate interpretations of reversed longitudinal profiles. Two fan-shaped networks of inverted channels are located near 150° E in the Aeolis Dorsa region just north of the dichotomy boundary. Their original flow directions, inferred from planform morphology, suggest flow to the northeast in contrast to their current longitudinal profiles sloping down to the southwest. This contrast indicates slope reversal. We investigate the lower crustal flow and flexure mechanisms for slope reversal by testing three different hypotheses: 1) lower crustal flow, 2) flexure caused by material erosion from the highlands and deposition in the lowlands, and 3) flexure caused by highland erosion and deposition in the lowlands plus deposition of the Medusae Fossae Formation in the lowlands. We test these three hypotheses by comparing the inferred magnitudes of the slope reversals with predicted slope changes from geophysical models for these processes. Taking the possibility of non-tectonic (i.e., collapse) processes into account, our results suggest that, among these three models, the slope reversal is most consistent with the predicted tectonic response to erosion and deposition of highland material in conjunction with deposition of the Medusae Fossae Formation. Contrary to previous findings, our results do not support the mechanism of lateral crustal flow, although they do not rule it out because lower crustal flow and erosion may both have been operating, but on different time scales. The result of this hypothesis-testing provides insight into the evolution of the dichotomy boundary and Martian crust in this location.

Petrological constraints on the density of the Martian crust

New insights into the chemistry of the Martian crust have been made available since the derivation of crustal thickness maps from Mars Global Surveyor gravity and topography data that used a conservative range of density values (2700–3100 kg/m3). A new range of crustal density values is calculated from the major element chemistry of Martian meteorites (3100–3700 kg/m3), igneous rocks at Gusev crater (3100–3600 kg/m3) and from the surface concentration of Fe, Al, Ca, Si, and K measured by the Gamma-Ray Spectrometer on board Mars Odyssey (3250–3450 kg/m3). In addition, the density of mineral assemblages resulting from low-pressure crystallization of primary melts of the primitive mantle are estimated for plausible conditions of partial melting corresponding to the Noachian to Amazonian periods (3100–3300 kg/m3). Despite the differences between these approaches, the results are all consistent with an average density above 3100 kg/m3 for those materials that are close to the surface. The density may be compatible with the measured mass of Mars and the moment of inertia factor, but only if the average crustal thickness is thicker than previously thought (approaching 100 km). A thicker crust implies that crustal delamination and recycling could be possible and may even control its thickness, globally or locally. Alternatively, and considering that geoid-to-topography ratios argue against such a thick crust for the highlands, our results suggest the existence of a buried felsic or anorthositic component in the southern hemisphere of Mars.

Searching for springtime zonal liquid interfacial water on Mars

We analyzed the spatial and temporal characteristics of the surface temperature at the northern water ice annulus on Mars that is left behind the receding seasonal carbon dioxide cap in springtime. Using OMEGA hyperspectral images we show that water ice without carbon dioxide ice coverage lasts for 10–30days between 55° and 70°N. The longest water ice coverage without CO2 ice is observed between 40–55°N and 300–330°E and lasts 80–110days in ideal case. Using TES temperature data, we show that thin interfacial liquid water may be present at the water ice annulus. Higher spatial resolution THEMIS temperature data shows that the above mentioned finding is relevant to a spatial scale of 100m. Although the exact near surface water vapor concentration is not known, beside the average 10pr-μm we used two elevated values and corresponding threshold temperatures for interfacial liquid water formation: 190 and 199K beside the average 180K. While the area of interfacial liquid water is substantially smaller in the case of higher threshold temperature values, even for 199K terrains exist at THEMIS and OMEGA scale of resolution where such thin interfacial liquid water could be present on the surface. Summarizing: good chance exists for the presence of liquid interfacial water in the warmest part of the day on at the northern hemisphere of Mars at extended areas – although firm evidence requires better targeted future observations.

Possible ice-wedge polygons and recent landscape modification by “wet” periglacial processes in and around the Argyre impact basin, Mars

The Argyre basin and associated rim-materials in the southern hemisphere of Mars are ancient, having been formed by the impact of a large body ∼4Gya. This notwithstanding, the regional landscape continues to be altered by a multiplicity of geological and geomorphological processes.Three landforms, whose close spatial-association is identified in a new geological map of the Argyre region (∼290–360°E; ∼30–72°S), feature prominently in the list of very Late Amazonian Epoch alterations:(a)Small-sized (⩽∼20m in diameter) and unsorted polygons that exhibit metre to sub-metre elevated margins or shoulders, giving them a low-centred appearance; in “wet” permafrost environments on Earth low-centred polygons (LCPs) often are underlain by ice-wedges.(b)Gullies, seemingly formed by “wet” flow(s) and incised by the LCPs.(c)A putative (possibly ice-rich) latitude-dependent mantle (LDM) that underlies the LCPs and spatially-convergent “wet” gullies in all of our observations.These landforms occur from the middle to the high (near-polar) latitudes of the region and overlie geological units of all ages.Hitherto, the presence of the LCPs has been reported only fleetingly in the literature and only in as much as they have been observed on the walls of a few scalloped depressions in mid-Utopia Planitia. By contrast, we report the ubiquitous occurrence of the LCPs in and around the Argyre impact-basin on gully-margins and adjacent slopes as well as on relatively-flat inter-crater terrain.On the basis of three separate but related lines of reasoning we hypothesise that ice wedges could be present beneath LCP margins in our study region. If we are correct then these LCP sites are geomorphological expressions of boundary conditions that were relatively warm and wet in the recent past.First, substantial ice-wedging in permafrost environments on Earth requires the availability of meltwater. If, as some researchers propose, the formation of some “wet” gullies at the martian mid-latitudes is induced by the localised (slope-side and crater-wall) thaw of the underlying LDM at the gully sites themselves, then meltwater also should be available for ice-wedging and the formation of LCPs at these sites. Interestingly, LCPs are observed on gully-channel walls as expected if meltwater is associated with gully formation; however, in some instances the LCPs are also observed on the slope-side terrain that extends for hundreds of metres beyond the channel walls and even above the gully alcoves. This suggests that the distribution of icy terrain affected by thaw could be much more substantial than has been suggested hitherto.Second, LCPs that are identical in shape and scale to the slope-side LCPs are observed on relatively flat inter-crater terrain (also underlain by the LDM) that is distal from the “wet” gullies. By contrast, here, their distribution extends for kilometres. This too could be indicative of meltwater being more extensive in the regional landscape than most workers have thought possible.Third, on Earth the “dryness” or the “wetness” of a permafrost environment determines whether LCP margins are underlain by sand or by ice. When the observed LCPs and major deposits of sand in the study region are plotted on our new map of the Argyre impact-basin, we see that these deposits are rarely proximal. On the other hand, the LDM underlies the LCPs in all instances and, if modified by thaw, seems to be a likelier source of margin fill for the LCPs than sand.

Evidence for Hesperian glaciation along the Martian dichotomy boundary

Here we provide geologic and geomorphologic evidence of Hesperian glacial activity along the Martian topographic dichotomy in Aeolis Mensae. Our geologic investigation focuses on a fretted plateau unit with networks of deep, flat-bottomed valleys, some of which extend from cirque-like scarps. Based on cross-sectional elevation profiles of the valleys and the resemblance to terrestrial analogue features, we propose that these fretted terrains were dissected by outlet glaciers emanating from the dichotomy boundary. The fretted terrains are spatially and temporally linked to deposits with concentrically ridged lobate fronts, providing evidence for ductile flow along a canyon floor, similar to debris-covered glaciers found on Earth and in other glaciated regions of Mars. We estimate a minimum thickness of 1500 m for the ice cover in that region during the Hesperian, equivalent to recent mid-latitude glaciations in other parts of the dichotomy boundary. Collectively, our observations suggest that glacial activity could have been an important mechanism of modification of the topographic dichotomy boundary since the Hesperian.

Mapping of the water ice content within the Martian surficial soil on the periphery of the retreating seasonal northern polar cap based on the TES and the OMEGA data

Analysis of seasonal data from the Mars Global Surveyor Thermal Emission Spectrometer (TES) shows a significant increase in thermal inertia during autumn, winter and spring in the middle and high latitudes of Mars. At each stage of the northern seasonal polar cap's recession in spring a distinct high thermal inertia (HTI) annulus arises around the cap's edge. Within this annulus, we estimated and mapped the springtime water ice content in the daily thermal skin depth layer using spring and summer values of the thermal inertia in TES surface footprints. The results show that the average water content in the surface soil within the HTI annulus varies from ∼5 vol % at the early stages of the seasonal polar cap retreating (Ls = 340°–360°) to ∼1 vol % at later stages (Ls = 60°–70°). Maximum values of water ice content within the HTI annulus occur at Ls = 0°–20° (2–6 vol %) and Ls = 20°–40° (4–10 vol %). We analyzed the temporal and spatial relationship between the HTI annuli and the water ice (WI) annuli at the edge of the northern seasonal polar cap. The water ice within the WI annuli was mapped using a water ice spectral index (the absorption band depth at the 1.5 μm wavelength) derived from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) imaging spectrometer aboard the Mars Express spacecraft. Recent OMEGA observations show that the WI annuli formation arises only around the retreating northern seasonal polar cap, never around the retreating southern seasonal cap. For this reason our study is confined only to the northern hemisphere. The observed relationship between the HTI and WI annuli in the northern hemisphere of Mars indicates a close physical interdependence between these two phenomena. Our results confirm that the seasonal permafrost exposed by the retreating northern polar cap (within the HTI annuli) is actively involved today in the condensation and sublimation processes in the modern water cycle on Mars. The water abundance in this annual condensation/sublimation cycle within the active layer may be an order of magnitude higher than all the annual water vapor abundance in the Martian atmosphere. Our results are consistent with existence of the positive mass balance of the northern permanent polar cap in the modern climatic period.

Martian crustal dichotomy and Tharsis formation by partial melting coupled to early plume migration

A recently proposed model links the formation and early evolution of the Tharsis volcanic province on Mars to the preexisting hemispheric dichotomy (Zhong, 2009). A key aspect of this model is the assumption of a deep lithospheric root below the thicker crust of the southern highlands. We implemented a parameterization of partial melting into the 3‐D spherical shell mantle convection code CitcomS in order to investigate whether the required lithospheric thickness variation can be generated self‐consistently by partial melting when stiffening of the melt residue due to devolatilization is considered. The rate of melt production strongly depends on the mantle temperature, and additional strong coupling between the flow and partial melting is introduced through the stiffening effect on the melt residue. We find that it is possible to generate a lithospheric keel by partial melting above a single upwelling that excites a relative rotation between the one‐plate lithosphere and the mantle below while producing the amount of melt distributed in a broad region constrained to one hemisphere that is necessary to form the crustal dichotomy. This scenario thus offers an internal mechanism for the Martian dichotomy formation and validates the hypothesis of Zhong (2009).

Origin of the martian dichotomy and Tharsis from a giant impact causing massive magmatism

The origin of the ancient martian crustal dichotomy and the massive magmatic province of Tharsis remains an open problem. Here, we explore numerically a hypothesis for the origin of these two features involving both exogenic and endogenic processes. We propose a giant impact event during the late stage of planetary formation as the source of the southern highland crust. In a second stage, the extraction of excess heat by vigorous mantle convection on the impacted hemisphere leads to massive magmatism, forming a distinct Tharsis-like volcanic region. By coupling short-term and long-term numerical simulations, we are able to investigate both the early formation as well as the 4.5 Gyr evolution of the martian crust. We demonstrate numerically that this exogenic–endogenic hypothesis is in agreement with observational data from Mars.

Dual-spacecraft observation of large-scale magnetic flux ropes in the Martian ionosphere

[1] We here report the first dual-spacecraft detection of planetary flux ropes in the ionosphere of Mars. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board Mars Express, can measure the magnetic field magnitude near the spacecraft. Typically, these measurements track the known crustal magnetic field strength very well; however, occasionally, MARSIS detects transient, intense magnetic fields that deviate significantly from the known crustal fields. Two such magnetic field enhancements occur in near-coincidence with flux rope detections by the Mars Global Surveyor Magnetometer and Electron Reflectometer, which provides vector magnetic field measurements, allowing us to clearly identify the enhancements as flux ropes. The flux ropes detected are quasi-stable for at least a half hour, have peak magnetic field strengths of ∼50 and 90 nT, and are ∼650–700 km in diameter. Both occur downstream of the region of strong crustal fields. In addition, MARSIS has detected 13 other magnetic enhancements over a 5 year period, which we infer to be flux ropes. These structures have peak field strengths up to 130 nT and measured horizontal dimensions of several hundred to over a thousand kilometers. They are clustered around the intense crustal fields in the southern hemisphere of Mars. The large spatial scale of these flux ropes distinguishes them from small-scale flux ropes, with diameters of tens of kilometers, that have been seen in the ionospheres of Venus and Mars. These large-scale flux ropes are believed to be caused by solar wind stretching and shearing of the Martian crustal fields.

Lineated valley fill at the Martian dichotomy boundary: Nature and history of degradation

The fretted terrain of the Martian dichotomy boundary is a key region for investigating landforms related to creep of ice and debris as it exhibits landforms comparable to morphologies of periglacial environments. Although features known as lobate debris aprons, lineated valley fills and concentric crater fills have been studied in great detail, basic questions concerned with the composition and the style of emplacement and degradation still remain unanswered. This study focuses on morphologies which are located in a near‐circular depression located at the dichotomy escarpment in Deuteronilus Mensae. Analysis of high‐resolution image data suggests an early formation of these features as the result of backward thermokarstic degradation of highland terrain. Geologically younger processes caused deposition and degradation of an ice‐rich mantling deposit, which ultimately led to formation of creep morphologies that might have even been active in the geologically recent past. Intermixing of both degradational landform units form complex patterns that cannot be explained by a late stage (glacial) process alone. Morphological comparisons of lineated valley fill units with concentric crater fill landforms in Utopia Planitia strongly suggest comparable emplacement and degradation styles of these features. The sequential development consisting of an initial probably widespread thermokarstic degradation followed by later cyclic deposition of volatile material and continual thermokarstic degradation suggests that the development of parts of the Martian fretted terrain is directly related to climatic variations in the planet's history.

Concentric crater fill in the northern mid-latitudes of Mars: Formation processes and relationships to similar landforms of glacial origin

Hypotheses accounting for the formation of concentric crater fill (CCF) on Mars range from ice-free processes (e.g., aeolian fill), to ice-assisted talus creep, to debris-covered glaciers. Based on analysis of new CTX and HiRISE data, we find that concentric crater fill (CCF) is a significant component of Amazonian-aged glacial landsystems on Mars. We present mapping results documenting the nature and extent of CCF along the martian dichotomy boundary over −30 to 90°E latitude and 20–80°N longitude. On the basis of morphological analysis we classify CCF landforms into “classic” CCF and “low-definition” CCF. Classic CCF is most typical in the middle latitudes of the analysis area (∼30–50°N), while a range of degradation processes results in the presence of low-definition CCF landforms at higher and lower latitudes. We evaluate formation mechanisms for CCF on the basis of morphological and topographic analyses, and interpret the landforms to be relict debris-covered glaciers, rather than ice-mobilized talus or aeolian units. We examine filled crater depth–diameter ratios and conclude that in many locations, hundreds of meters of ice may still be present under desiccated surficial debris. This conclusion is consistent with the abundance of “ring-mold craters” on CCF surfaces that suggest the presence of near-surface ice. Analysis of breached craters and distal glacial deposits suggests that in some locations, CCF-related ice was once several hundred meters higher than its current level, and has sublimated significantly during the most recent Amazonian. Crater counts on ejecta blankets of filled and unfilled craters suggests that CCF formed most recently between ∼60 and 300 Ma, consistent with the formation ages of other martian debris-covered glacial landforms such as lineated valley fill (LVF) and lobate debris aprons (LDA). Morphological analysis of CCF in the vicinity of LVF and LDA suggests that CCF is a part of an integrated LVF/LDA/CCF glacial landsystem. Instances of morphological continuity between CCF, LVF, and LDA are abundant. The presence of formerly more abundant CCF ice, coupled with the integration of CCF into LVF and LDA, suggests the possibility that CCF represents one component of the significant Amazonian mid-latitude glaciation(s) on Mars.

New and recent gully activity on Mars as seen by HiRISE

Fresh‐appearing gully deposits are found at tens of sites in the southern hemisphere of Mars. These deposits have latitudinal and azimuthal dependences similar to the overall preferences of southern‐hemisphere gullies, suggesting that most gullies can undergo such events. Definite changes are seen at ten sites, including two previously reported. These include visible modification of gully channels and aprons. Those formation intervals constrained to better than one Mars year tend to include winter and exclude summer, suggesting seasonal activity. This seasonal activity is consistent with proposed models for gully formation driven by CO2 frost, although at least some of the new deposits are probably due to dry granular flow with no volatile involvement. As these deposits are capable of effecting distinct topographic changes to gully aprons and channels, they represent a significant component of recent gully evolution.

Indications of brine related local seepage phenomena on the northern hemisphere of Mars

Springtime low albedo features, called Dark Dune Spots, on the seasonal frost covered dunes on Mars between 77°N and 84°N latitude have been analyzed. Two groups of these spots have been identified: “small” and “large” ones, where large spots have diameters above 4 m, and complex internal structure. From these “large” spots branching seepage-like features emanate and grow on the steep slopes. They show a characteristic sequence of changes: first only wind-blown features emanate from them, while later a bright circular and elevated ring forms, and dark seepage-features start from the spots. These streaks grow with a speed between 0.3 m/day and 7 m/day respectively, first only from the spots, later from all along the dune crest. During this “seepage period” the temperature is between 150 K and 180 K at a 3–9 km spatial resolution scale, indicating that CO 2 ice-free parts must be present at the observed dark spots. Around the receding northern seasonal CO 2 cap, an annulus of water ice lags behind, which is probably present in the spots too where the CO 2 frost has sublimated. Our model estimates show in the present work and in Kereszturi et al. (Kereszturi, A., Möhlmann, D., Berczi, Sz., Ganti, T., Kuti, A., Sik, A., Horvath, A. [2009b]. Icarus 201, 492–503) that the warming driven by solar insolation may produce not only interfacial water, but also bulk brines around the dune grains. The brine can support the movement of liquids and dune grains, enhances the sublimation of CO 2 frost, and produce the dark features, as well as liquid modifies the optical properties of the surface. Signs of movement of dune material after the total defrosting of the terrain is also visible but it is uncertain because of the limit of resolution. In our previous work (Kereszturi et al., 2009b) we showed that resembling seepage-like streaks at the southern hemisphere might have been formed by ephemeral interfacial water, as well as these northern features. Such wet environments may have astrobiological importance too.

Further observations of regional dust storms and baroclinic eddies in the northern hemisphere of Mars

Abstract We have investigated the near-surface meteorology in the northern hemisphere of Mars through detailed analysis of data obtained with Mars Global Surveyor in January–August 2005. The season in the northern hemisphere ranged from midsummer through winter solstice of Mars Year (MY) 27. We examined composite, wide-angle images from the Mars Orbiter Camera and compiled a catalog of the dust storms that occurred in this interval. As in previous martian years, activity in the northern hemisphere was dominated by regional “flushing” dust storms that sweep southward through the major topographic basins, most frequently in Acidalia Planitia. We also used atmospheric profiles retrieved from radio occultation experiments to characterize eddy activity near the surface at high northern latitudes. There are strong correlations between the two sets of observations, which allowed us to identify three factors that influence the timing and location of the regional dust storms: (1) transitions among baroclinic wave modes, which strongly modulate the intensity of meridional winds near the surface, (2) storms zones, which impose strong zonal variations on the amplitude of some baroclinic eddies, and (3) stationary waves, which further modulate the wind field near the surface. The flushing dust storms ceased abruptly in midautumn, possibly in response to source depletion, CO 2 condensation, a shift in the period of the baroclinic eddies, and changes in the tidal wind field near the surface. Our results extend the meteorological record of the northern hemisphere, substantiate the findings of previous investigations, and further illuminate the climatic impact of baroclinic eddies.

The sedimentology and dynamics of crater-affiliated wind streaks in western Arabia Terra, Mars and Patagonia, Argentina

Wind streaks comprise recent aeolian deposits that have been extensively documented on Venus, Earth and Mars. Martian wind streaks are among the most abundant surface features on the planet and commonly extend from the downwind margins of impact craters. Previous studies of wind streaks emerging from crater interior deposits suggested that the mode of emplacement was primarily related to the deposition of silt-sized particles as these settled from plumes. We have performed geologic investigations of two wind streaks clusters; one situated in western Arabia Terra, a region in the northern hemisphere of Mars, and another in an analogous terrestrial site located in southern Patagonia, Argentina, where occurrences of wind streaks emanate from playas within maar craters. In both these regions we have identified bedforms in sedimentary deposits on crater floors, along wind-facing interior crater margins, and along wind streaks. These observations indicate that these deposits contain sand-sized particles and that sediment migration has occurred via saltation from crater interior deposits to wind streaks. In Arabia Terra and in Patagonia wind streaks initiate from crater floors that contain lithic and evaporitic sedimentary deposits, suggesting that the composition of wind streak source materials has played an important role in development. Spatial and topographic analyses suggest that regional clustering of wind streaks in the studied regions directly correlates to the areal density of craters with interior deposits, the degree of proximity of these deposits, and the craters' rim-to-floor depths. In addition, some (but not all) wind streaks within the studied clusters have propagated at comparable yearly (Earth years) rates. Extensive saltation is inferred to have been involved in its propagation based on the studied terrestrial wind streak that shows ripples and dunes on its surface and the Martian counterpart changes orientation toward the downslope direction where it extends into an impact crater.

Distribution and orientation of northern-hemisphere gullies on Mars from the evaluation of HRSC and MOC-NA data

Martian gullies show morphologic characteristics that indicate the presence of a flowing liquid during their formation process. However, the current climate in the northern hemisphere of Mars does not allow the existence of substantial amounts of liquid water and, therefore, an assessment of the regional distribution and characteristics of gullies is required in order to understand the most recent climatic development of Mars. We examined High Resolution Stereo Camera (HRSC) and Mars Orbiter Camera—Narrow Angle (MOC-NA) data covering the northern hemisphere in order to determine the distribution, geological setting and orientation of gullied slopes. We used these results to test the consistency of formation models proposed in earlier works and to define the climate dependence. Gully orientations were analyzed only at impact crater walls, because craters have an azimuthally uniform distribution of slopes. This is ideal when investigating the influence of insolation on the distribution and development of landforms. We examined 230 HRSC and ~ 21,000 MOC-NA images and identified more than 2200 gullies on HRSC and more than 3100 gullies in MOC-NA data. Both surveys show the highest densities of gullies between 35°N and 55°N and a shift of gully orientation at 40°N from pole-facing slopes to slopes facing the equator. Orientation trends as well as the latitudinal distribution of identified gullies strongly support the view of gully formation caused by insolation/climatic conditions. This result suggests that formation mechanisms based on the assumption of atmospheric water-ice deposition proposed in earlier work are more likely than processes related to groundwater flow. Further support for a theory related to deposition is gained through observation of the high occurrence of gullies at isolated knobs and hills, which cannot be associated with formation models based upon groundwater flow.

Towards self-consistent modeling of the martian dichotomy: The influence of one-ridge convection on crustal thickness distribution

In order to find an explanation for the origin of the martian crustal dichotomy, a number of recent papers have examined the effect of layered viscosity on the evolution of a degree-1 mantle convection, e.g. Roberts and Zhong [Roberts, J.H., Zhong, S., 2006. J. Geophys. Res. 111. E06013] and Yoshida and Kageyama [Yoshida, M., Kageyama, A., 2006. J. Geophys. Res. 111, doi:10.1029/2005JB003905. B03412]. It was found that a mid-mantle viscosity jump, combined with highly temperature- and depth-dependent rheology, are effective in developing a degree-1 convection within a short timescale. Such a layered viscosity profile could be justified by martian mineralogy. However, the effect of a degree-1 convective planform on the crustal thickness distribution has not yet been demonstrated. It is not obvious whether a thinner crust, due to sublithospheric erosion and crustal thinning, or a thicker crust, due to enhanced crustal production, would form above the hemisphere of mantle upwelling. Also, the general shape of the dichotomy, which is not strictly hemispherical, has not yet been fully investigated. Here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection, using the finite-volume multigrid code StagYY [Tackley, P.J., 2008. Phys. Earth Planet. Int. 107, 7–18, doi:10.1016/j.pepi.2008.08.005] A self-consistent treatment of melting, crustal formation and chemical differentiation has been added to models of three-dimensional thermal convection. This allows us to obtain global maps of the crustal thickness distribution as it evolves with time. The obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree-1 mantle convection early in Mars' history. We find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy. In all models, the region of thick crust is located over the region of mantle upwelling, which itself is a ridge-like structure spread over roughly one half of the planet, a planform we describe as “one-ridge convection.”

Analysis of Dark Albedo Features on a Southern Polar Dune Field of Mars

We observed 20-200 m sized low-albedo seepage-like streaks and their annual change on defrosting polar dunes in the southern hemisphere of Mars, based on the Mars Orbiter Camera (MOC), High Resolution Stereo Camera (HRSC), and High Resolution Imaging Science Experiment (HiRISE) images. The structures originate from dark spots and can be described as elongated or flowlike and, at places, branching streaks. They frequently have another spotlike structure at their end. Their overall appearance and the correlation between their morphometric parameters suggest that some material is transported downward from the spots and accumulates at the bottom of the dune's slopes. Here, we present possible scenarios for the origin of such streaks, including dry avalanche, liquid CO(2), liquid H(2)O, and gas-phase CO(2). Based on their morphology and the currently known surface conditions of Mars, no model interprets the streaks satisfactorily. The best interpretation of only the morphology and morphometric characteristics is only given by the model that implies some liquid water. The latest HiRISE images are also promising and suggest liquid flow. We suggest, with better knowledge of sub-ice temperatures that result from extended polar solar insolation and the heat insulator capacity of water vapor and water ice, future models and measurements may show that ephemeral water could appear and flow under the surface ice layer on the dunes today.

High Resolution Imaging Science Experiment (HiRISE) observations of glacial and periglacial morphologies in the circum‐Argyre Planitia highlands, Mars

The landscape of the Argyre Planitia and adjoining Charitum and Nereidum Montes in the southern hemisphere of Mars has been heavily modified since formation of the Argyre impact basin. This study examines morphologies in the Argyre region revealed in images acquired by the High Resolution Imaging Science Experiment (HiRISE) camera and discusses the implications for glacial and periglacial processes. Distinctive features such as large grooves, semicircular embayments in high topography, and streamlined hills are interpreted as glacially eroded grooves, cirques, and whalebacks or roche moutonnée, respectively. Large boulders scattered across the floor of a valley may be ground moraine deposited by ice ablation. Glacial interpretations are supported by the association of these features with other landforms typical of glaciated landscapes such as broad valleys with parabolic cross sections and stepped longitudinal profiles, lobate debris aprons interpreted as remnant debris covered glaciers or rock glaciers, and possible hanging valleys. Aligned boulders observed on slopes may also indicate glacial processes such as fluting. Alternatively, boulders aligned on slopes and organized in clumps and polygonal patterns on flatter surfaces may indicate periglacial processes, perhaps postglaciation, that form patterned ground. At least portions of the Argyre region appear to have been modified by processes of ice accumulation, glacial flow, erosion, sediment deposition, ice stagnation and ablation, and perhaps subsequent periglacial processes. The type of bedrock erosion apparent in images suggests that glaciers were, at times, wet based. The number of superposed craters is consistent with geologically recent glacial activity, but may be due to subsequent modification.