摘要 :
No sand transport pathways are visible in a study performed in Noachis Terra, a 60° × 35° region in the southern highlands of Mars known for its many intracrater dune fields. Detailed studies were performed of five areas in Noa...
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No sand transport pathways are visible in a study performed in Noachis Terra, a 60° × 35° region in the southern highlands of Mars known for its many intracrater dune fields. Detailed studies were performed of five areas in Noachis Terra, using Mars Orbiter Camera (MOC) wide-angle mosaics, Thermal Emission Imaging System (THEMIS) daytime and nighttime infrared mosaics, MOLA digital elevation and shaded relief maps, and MOC narrow-angle images. The lack of observable sand transport pathways suggests that such pathways are very short, ruling out a distant source of sand. Consistent dune morphology and dune slipface orientations across Noachis Terra suggest formative winds are regional rather than local (e.g., crater slope winds). A sequence of sedimentary units was found in a pit eroded into the floor of Rabe Crater, some of which appear to be shedding dark sand that feeds into the Rabe Crater dune field. The visible and thermal characteristics of these units are similar to other units found across Noachis Terra, leading to the hypothesis that a series of region-wide depositional events occurred at some point in the Martian past and that these deposits are currently exposed by erosion in pits on crater floors and possibly on the intercrater plains. Thus the dune sand sources may be both regional and local: sand may be eroding from a widespread source that only outcrops locally. Sand-bearing layers that extend across part or all of the intercrater plains of Noachis Terra are not likely to be dominated by loess or lacustrine deposits; glacial and/or volcanic origins are considered more plausible.
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Aeolian sediment transport, deposition, and erosion have been ongoing throughout Mars's history. This record of widespread aeolian processes is preserved in landforms and geologic units that retain important clues about past envir...
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Aeolian sediment transport, deposition, and erosion have been ongoing throughout Mars's history. This record of widespread aeolian processes is preserved in landforms and geologic units that retain important clues about past environmental conditions including wind patterns. In this study we describe landforms within Melas Chasma, Valles Marineris, that occur in distinct groups with linear to crescentic shapes, arranged with a characteristic wavelength; some possess slope profiles analogous to modern sand dunes yet show evidence for lithification. Based on the features' dimensions, asymmetry, and spatial patterns relative to modern equivalents, we interpret these landforms to be two classes of aeolian bedforms: decameter-scale megaripples and sand dunes. The presence of superposed erosional features and depositional units indicates that these landforms were cemented and likely ancient. Melas paleodunes are found atop Hesperian-aged layered deposits, but we estimate them to be younger, likely lithified in the Amazonian period. Although a range of degradation was observed, some paleodunes are >10 m tall and maintain steep lee sides (>25°), an uncommon scenario for terrestrial examples as other geologic processes lead to dune obliteration. The preserved paleobedform geometries are largely consistent with those of modern aeolian indicators, suggesting no major shifts in wind regime or contributing boundary conditions. Finally, we propose that their appearance and context require sequential periods of dune migration, stabilization following catastrophic burial, cementation, differential erosion, exposure, and burial. The presence of wholly preserved duneforms appears to be more common on Mars compared to the Earth and may signal something important about Martian landscape evolution.
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In a study area spanning the martian surface poleward of 50° S., 1190 dune fields have been identified, mapped, and categorized based on dune field morphology. Dune fields in the study area span ～116 400 km~2, leading to a globa...
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In a study area spanning the martian surface poleward of 50° S., 1190 dune fields have been identified, mapped, and categorized based on dune field morphology. Dune fields in the study area span ～116 400 km~2, leading to a global dune field coverage estimate of ～904 000 km～2, far less than that found on Earth. Based on distinct morphological features, the dune fields were grouped into six different classes that vary in interpreted aeolian activity level from potentially active to relatively inactive and eroding. The six dune field classes occur in specific latitude zones, with a sequence of reduced activity and degradation progressing poleward. In particular, the first signs of stabilization appear at ～60° S., which broadly corresponds to the edge of high concentrations of water-equivalent hydrogen content (observed by the Neutron Spectrometer) that have been interpreted as ground ice. This near-surface ground ice likely acts to reduce sand availability in the present climate state on Mars, stabilizing high latitude dunes and allowing erosional processes to change their morphology. As a result, climatic changes in the content of near-surface ground ice are likely to influence the level of dune activity. Spatial variation of dune field classes with longitude is significant, suggesting that local conditions play a major role in determining dune field activity level. Dune fields on the south polar layered terrain, for example, appear either potentially active or inactive, indicating that at least two generations of dune building have occurred on this surface. Many dune fields show signs of degradation mixed with crisp-brinked dunes, also suggesting that more than one generation of dune building has occurred since they originally formed. Dune fields superposed on early and late Amazonian surfaces provide potential upper age limits of ～100 My on the south polar layered deposits and ～3 Ga elsewhere at high latitudes. No craters are present on any identifiable dune fields, which can provide a lower age limit through crater counting: assuming all relatively stabilized dune fields represent a single noncontiguous surface of uniform age, their estimated crater retention age is <～10000 years. An average-sized uncratered dune field (94 km~2) has a crater retention age <～8My. This apparent youth suggests that present-day climate conditions are responsible for the observed degradation and reduced level of aeolian activity. A lack of observed transport pathways and the absence of large dune fields in the largest basins (Hellas and Argyre Planitiae) are consistent with the previously proposed idea that dune sands are not typically transported far from their source regions on Mars.
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Both atmospheric modeling and spacecraft imagery of Mars are now of sufficient quality that the two can be used in conjunction to acquire an understanding of regional- and local-scale aeolian processes on Mars. We apply a mesoscal...
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Both atmospheric modeling and spacecraft imagery of Mars are now of sufficient quality that the two can be used in conjunction to acquire an understanding of regional- and local-scale aeolian processes on Mars. We apply a mesoscale atmospheric model adapted for use on Mars (the Mars MM5) to Proctor Crater, a 150 km diameter crater in the southern highlands. Proctor Crater contains numerous aeolian features that indicate wind direction, including a large dark dune field with reversing transverse and star dunes containing three different slipface orientations, small and older bright bedforms that are most likely transverse granule ripples, and seasonally erased dust devil tracks. Results from model runs spanning a Martian year, with a horizontal grid spacing of 10 km, predict winds aligned with two of the three dune slipfaces as well as spring and summer winds matching the dust devil track orientations. The primary (most prevalent) dune slipface orientation corresponds to a fall and winter westerly wind created by geostrophic forces. The tertiary dune slipface orientation is caused by spring and summer evening katabatic flows down the eastern rim of the crater, influencing only the eastern portion of the crater floor. The dunes are trapped in the crater because the tertiary winds, enhanced by topography, counter transport from the oppositely oriented primary winds, which may have originally carried sand into the crater. The dust devil tracks are caused by light spring and summer westerly winds during the early afternoon caused by planetary rotation. The secondary dune slipface orientation is not predicted by model results from either the Mars MM5 or the Geophysical Fluid Dynamics Laboratory Mars general circulation model. The reason for this is not clear, and the wind circulation pattern that creates this dune slipface is not well constrained. The Mars MM5 model runs do not predict stresses above the saltation threshold for dune sand of the appropriate size and composition. As with previous work, the calculated wind velocities are too low, which may be caused by too large of a grid spacing.
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Inverse maximum gross bedform-normal transport (IMGBNT) analysis has been applied to Context Camera (CTX) images of the largest dune field in Ganges Chasma on Mars. The dune field was selected for its position in a likely complex,...
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Inverse maximum gross bedform-normal transport (IMGBNT) analysis has been applied to Context Camera (CTX) images of the largest dune field in Ganges Chasma on Mars. The dune field was selected for its position in a likely complex, multi-directional wind regime. Results indicate that four main winds are responsible for simultaneous construction of the dune field, including along-chasma winds from the ESE, winds blowing down a nearby re-entrant from the NE, winds blowing down the adjacent chasma wall from the NW, and chasma floor winds from the SW. Each wind represents a transport vector that dominates dune morphology at its respective edge of the dune field, such that the central axis of the dune field reflects the convergence of the three most prominent winds (ESE, NW, and SW). The Mars Regional Atmospheric Modeling System (MRAMS) was run at twelve times throughout the martian year to provide context for the local wind patterns. Potential sand fluxes calculated from MRAMS output show that three major air flows from the ENE-E, NNE-NE, and NNW-N converge near the location of the dune field. These flows likely correspond to the ESE, NE, and NW winds identified from IMGBNT analysis, respectively. MRAMS output shows that the flows with major northerly components are produced by larger-scale Hadley return flow constructively combining with nighttime downslope winds; the flow with a major easterly component is likely produced by equatorial easterly "trade" winds constructively combining with the diurnal tide and/or local topography. Although the model correctly predicts the major elements of the local wind pattern, some aspects are either over- or underrepresented, demonstrating the value of using aeolian morphological analysis to conclusively constrain the major sand-moving winds on Mars. Overlapping High Resolution Imaging Experiment (HiRISE) images of barchanoid dunes at the northernmost edge of the dune field indicate that these dunes are currently migrating southward at ~5 m/Mars year (~2.6 m/Earth year); the direction of migration is consistent with both MRAMS predictions in this location and the NW/NE winds found from IMGBNT analysis. Dune morphology suggests that sand in the northwestern part of the dune field is likely to be derived from the adjacent chasma wall to the north and northwest, and sand in the southeastern part of the dune field was probably transported from farther east along the main chasma floor.
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A new Mars Global Digital Dune Database (MGD3) constructed using Thermal Emission Imaging System (THEMIS) infrared (IR) images provides a comprehensive and quantitative view of the geographic distribution of moderate- to large-siz...
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A new Mars Global Digital Dune Database (MGD3) constructed using Thermal Emission Imaging System (THEMIS) infrared (IR) images provides a comprehensive and quantitative view of the geographic distribution of moderate- to large-size dune fields (area >1 km2) that will help researchers to understand global climatic and sedimentary processes that have shaped the surface of Mars. MGD3 extends from 65°N to 65°S latitude and includes ～550 dune fields, covering ～70,000 km2, with an estimated total volume of ～3,600 km3. This area, when combined with polar dune estimates, suggests moderate- to large-size dune field coverage on Mars may total ～800,000 km2, ～6 times less than the total areal estimate of ～5,000,000 km2 for terrestrial dunes. Where availability and quality of THEMIS visible (VIS) or Mars Orbiter Camera narrow-angle (MOC NA) images allow, we classify dunes and include dune slipface measurements, which are derived from gross dune morphology and represent the prevailing wind direction at the last time of significant dune modification. For dunes located within craters, the azimuth from crater centroid to dune field centroid (referred to as dune centroid azimuth) is calculated and can provide an accurate method for tracking dune migration within smooth-floored craters. These indicators of wind direction are compared to output from a general circulation model (GCM). Dune centroid azimuth values generally correlate to regional wind patterns. Slipface orientations are less well correlated, suggesting that local topographic effects may play a larger role in dune orientation than regional winds.
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The plains ripples of Meridiani Planum are the first paleo-aeolian bedforms on Mars to have had their last migration episode constrained in time (to ~50-200 ka). Here we test how variations in orbital configuration, air pressure, ...
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The plains ripples of Meridiani Planum are the first paleo-aeolian bedforms on Mars to have had their last migration episode constrained in time (to ~50-200 ka). Here we test how variations in orbital configuration, air pressure, and atmospheric dust loading over the past 400 kyr affect bedform mobility and crest alignment. Using the National Aeronautics and Space Administration Ames Mars Global Climate Model, we ran a series of sensitivity tests under a number of different conditions, seeking changes in wind patterns relative to those modeled for present-day conditions. Results indicate that enhanced sand drift potential in Meridiani Planum correlates with (1) high axial obliquity, (2) a longitude of perihelion (L_p) near southern summer solstice, and (3) a greater air pressure. The last pulse of westward plains ripple migration likely occurred during the most recent obliquity (relative) maximum, from 111 to 86 ka. At L_p coinciding with southern summer solstice, the Mars Global Climate Model produced a westward resultant drift direction, consistent with the observed north-south plains ripple crest alignment. However, smaller superposed ripples, aligned NNE-SSW, are consistent with a strengthened northern summer Hadley return flow, occurring when L_p coincided with northern summer solstice. The superposed NNE-SSW ripples likely formed as the axial obliquity decreased during the last relative maximum and L_p swung toward northern summer, from 86 to 72 ka. The timeline of bedform activity supports the proposed sequence of CO_2 sequestration in the south polar residual cap over the past 400 kyr.
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Change detection analyses of aeolian bedforms (dunes and ripples), using multitemporal images acquired by the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE), can reveal migration of bedforms on Mar...
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Change detection analyses of aeolian bedforms (dunes and ripples), using multitemporal images acquired by the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE), can reveal migration of bedforms on Mars. Here we investigated bedform mobility (evidence of wind-driven migration or activity), from analysis of HiRISE temporal image pairs, and dune field modification (i.e., apparent presence/lack of changes or degradation due to nonaeolian processes) through use of a dune stability index or SI (1-6; higher numbers indicating increasing evidence of stability/modification). Combining mobility data and SI for 70 dune fields south of 40°S latitude, we observed a clear trend of decreasing bedform mobility with increasing SI and latitude. Both dunes and ripples were more commonly active at lower latitudes, although some high-latitude ripples are migrating. Most dune fields with lower SIs (≤3) were found to be active while those with higher SIs were primarily found to be inactive. A shift in prevalence of active to apparently inactive bedforms and to dune fields with SI ≥ 2 occurs at ~60°S latitude, coincident with the edge of high concentrations of H_2O-equivalent hydrogen observed by the Mars Odyssey Neutron Spectrometer. This result is consistent with previous studies suggesting that stabilizing agents, such as ground ice, likely stabilize bedforms and limit sediment availability. Observations of active dune fields with morphologies indicative of stability (i.e., migrating ripples in SI = 3 dune fields) may have implications for episodic phases of reworking or dune building, and possibly geologically recent activation or stabilization corresponding to shifts in climate.
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Martian aeolian dunes preserve a record of atmosphere/surface interaction on a variety of scales, serving as ground truth for both Global Climate Models (GCMs) and mesoscale climate models, such as the Mars Regional Atmospheric Mo...
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Martian aeolian dunes preserve a record of atmosphere/surface interaction on a variety of scales, serving as ground truth for both Global Climate Models (GCMs) and mesoscale climate models, such as the Mars Regional Atmospheric Modeling System (MRAMS). We hypothesize that the location of dune fields, expressed globally by geographic distribution and locally by dune centroid azimuth (DCA), may record the long-term integration of atmospheric activity across a broad area, preserving GCM-scale atmospheric trends. In contrast, individual dune morphology, as expressed in slipface orientation (SF), may be more sensitive to localized variations in circulation, preserving topographically controlled mesoscale trends. We test this hypothesis by comparing the geographic distribution, DCA, and SF of dunes with output from the Ames Mars GCM and, at a local study site, with output from MRAMS. When compared to the GCM: 1) dunes generally lie adjacent to areas with strongest winds, 2) DCA agrees fairly well with GCM modeled wind directions in smooth-floored craters, and 3) SF does not agree well with GCM modeled wind directions. When compared to MRAMS modeled winds at our study site: 1) DCA generally coincides with the part of the crater where modeled mean winds are weak, and 2) SFs are consistent with some weak, topographically influenced modeled winds. We conclude that: 1) geographic distribution may be valuable as ground truth for GCMs, 2) DCA may be useful as ground truth for both GCM and mesoscale models, and 3) SF may be useful as ground truth for mesoscale models.
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It has been a goal of aeolian science to use bedforms as indicators of local and regional sediment transport and atmospheric circulation, but even with the application of the rule of maximum gross-bedform normal transport (MGBNT),...
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It has been a goal of aeolian science to use bedforms as indicators of local and regional sediment transport and atmospheric circulation, but even with the application of the rule of maximum gross-bedform normal transport (MGBNT), the underdetermined nature of the problem has precluded its application in all but the most simple cases. We present a method to apply the rule of MGBNT and its inverse (IMGBNT) from analysis of aeolian dune crestlines derived from aerial imagery. Although the solutions to IMGBNT analysis are non-unique, the possible transport vectors influencing bedform morphology can often be constrained by making inferences regarding bedform type (e.g., transverse, oblique, or longitudinal), resultant drift direction, and the ratio of transport vector magnitudes. The technique is demonstrated on the Great Sand Dunes, located in Colorado, USA. This dune field has a wide array of dune morphologies; eight crestline sets were identified and mapped. IMGBNT analysis and the subsequent constraint of possible solutions suggests that transport vectors from the southeast and southwest, with a SE:SW transport ratio of ~1:2, produce oblique north-south oriented dunes that dominate the main dune field. These results compare favorably with MGBNT analysis of meteorologic measurements from three stations located adjacent to the Great Sand Dunes, which predict dune types and orientations similar to those observed in their vicinity.
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