P31B-0427
Large Ripples and Small Dunes on the Floor of Gamboa Impact Crater on Mars
A diverse array of aeolian features are revealed in unprecedented detail in the initial release of High Resolution Imaging Science Experiment (HiRISE) data from the Primary Science Phase of the MRO mission. HiRISE image 002721-2210 was targeted on the central peak of Gamboa crater (centered on 40.8 deg. N lat., 315.7 deg. E long.), a 33-km-diameter impact feature in Acidalia Planitia. We mapped the distribution of aeolian bedforms on the floor of Gamboa crater around the central peak, measured the dimensions of the aeolian features, and categorized the features into classes that are likely related to differing processes of formation: dark broad dunes, dark linear ripples, bright linear ripples, and complex regions that are most likely combinations of two or more of the classes. Crest orientations appear to be sensitive to both local and regional relief, including some regions where the crest complexity defies any simple categorization. We interpret the dark dunes to consist of low albedo sand-sized particles much like those that have been observed during the traverses of both Mars Exploration Rovers (MERs). We also interpret the bright linear features to be granule-coated sand-cored features, again analogous to features observed by both MERs, but especially at the Opportunity site in Sinus Meridiani. Dark linear features are tentatively identified as granule ripples that have been coated by a dark (sand) layer sufficiently thick to mask the granule albedo but not thick enough to obscure the relief of the ripples. The complex regions likely also involve dark sand over bright granules, but with a very intricate crest orientation that appears to reflect both wind direction and perhaps underlying surface texture or relief. The diversity of aeolian bedforms observed within this single impact crater indicates that future HiRISE images likely will greatly expand the diversity of aeolian features observed across the planet.
P31B-0428
Reconstruction of Eolian Bedforms from Cross-Bedded Strata at Victoria Crater, Meridiani Planum, Mars
Outcrop exposures imaged by the Opportunity rover at Meridiani Planum have depicted cross-bedded strata with geometries and scales similar to eolian and subaqeous deposits on Earth. On Earth preserved cross-strata are rich in geologic information, providing insight into the depositional environment and sediment transport directions. The high-resolution stratigraphy of these cross-bedded strata can be used to reconstruct sedimentary bedforms on Mars to infer formation process and describe the depositional environment. Meter-scale cross bedding at Victoria Crater is similar to terrestrial eolian deposits and is interpreted as a dry dune field. Sets of cross-strata in the Cape St. Vincent and Cape St. Mary sections of Victoria Crater are comparable to Jurassic-age eolian deposits of the western US. The Opportunity Rover has spent ~300 sols traversing 90 degrees of the rim of Victoria crater, obtaining images of rock outcrops exposed by several promontories along the way. The outcrops at the Cape St. Mary and Cape St. Vincent promontories, which are located at opposite ends of the traverse, have proven to be the best examples of meter scale cross-bedding observed on Mars to date. Super-resolution imaging techniques and long baseline stereo observations were utilized during an extended imaging campaign of both outcrop faces. Cape St. Mary is characterized by meter-scale trough-style cross bedding, suggesting sinuous crested dunes with scour pits migrating perpendicular to the outcrop face. Cape St. Vincent, which is striking 110° away from Cape St. Mary, has layering indicative of a single climbing bedform with dune heights of several meters. The findings at Cape St. Mary and Cape St. Vincent are combined with other bedding faces to produce an eolian deposition model for layering exposed at Victoria Crater. Any depositional model used to explain the bedding must conform to an observed N-S paleo-flow direction. In addition to bedded layering, a bright band is observed which lies on an equipotential surface and is ubiquitous around the crater. The super-resolution images of Cape St. Vincent show that bedforms can be followed through the layer, suggesting that it is diagenetic in origin.
P31B-0429
Degradation of Victoria Crater, Meridiani Planum, Mars
Victoria crater (2.05N, 354.51E) is ~750 m in diameter and the largest crater on Mars observed in situ. The Mars Exploration Rover Opportunity traversed NW to SE across a broad annulus dominated by dark sand that at least partially surrounds the crater before navigating the northern crater rim. Rover observations of the crater and ejecta deposits are complemented by images with 26-52 cm/pixel scales from the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter and enable assessment of degradation state. The present depth/diameter ratio for Victoria is 0.1, less than the 0.2 expected for a pristine primary impact structure. Together with the eroded, serrated rim, this implies an originally smaller crater diameter and/or considerable infilling consistent with occurrence of a large dune field and few exposed rocks on the crater floor. The height and width of the raised rim is generally 4-5 m and 150-225 m, respectively, less than the 30 m and 500-600 m, respectively, expected for a pristine 750 m diameter crater. Ejecta thicknesses around the rim were derived using rover-based and HiRISE images and yield consistent estimates averaging ~3 m. The serrated rim plan creates a series of promontories extending up to 50 m into the crater and generally fronted by 30-60 degree slopes that are locally vertical and are separated by bays whose floors typically slope 15-25 degrees. A crater originally on order of 600-650 m in diameter and subsequently enlarged by mass wasting and aeolian erosion may yield a structure resembling Victoria today. The steep expression of the promontories and local outcroppings of rocks in the ejecta blanket points to some ongoing mass wasting, but the relative paucity of associated flanking talus indicates derived blocks of sulfate sandstone are not resistant to saltating sand and are rapidly broken down by the wind or are completely covered/filled in by aeolian drift. At Cape St. Vincent, the promontory appears undercut by aeolian erosion; thereby leading to mass wasting and accumulation of the large angular blocks observed on the slopes below By contrast, the shallow slope in the bays implies mass wasting is currently limited. Most likely, early mass wasting and aeolian stripping of the walls was gradually superseded by aeolian activity that proceeded most quickly along structural weaknesses and resulted in formation of the bays. Redistributed wall material contributes to crater infilling and/or was transported out of the crater. Occurrence of dark basaltic sands in the crater and in wind streaks along the north rim indicates additional transport into and out of the crater from other sources. The diminished expression of the rim, planed-off appearance of the ejecta blanket, paucity of exposed blocks, and dark wind streaks outside the crater imply efficient aeolian modification of Victoria crater, likely contributing to down-wasting and sediment transport that may be responsible for thin, local outcrops of finely bedded sediments capping the ejecta. The annulus appears to have resulted from combined erosion and planing of the ejecta, leading to evolution of a resistant lag of hematite concretions along with infilling and covering by the basaltic sand. There is no evidence for water-related erosional contributions to the present form of Victoria crater.
P31B-0430
HiRISE Observations and Slope Stability Analysis of Rotational Landslides in Bahram Vallis, Mars
Bahram Vallis is a narrow-winding valley that terminates at the circum-Chyrse basin (20-22 N, 301-304 E) on Mars. The valley has numerous circular to sub-circular alcoves along its slopes, some with large deposits directly below on the valley floor. These features are evident from earlier image datasets of Mars including THEMIS, MOC, and HRSC, and are indicative of mass-wasting processes that have occurred along the valley walls. A portion of Bahram Vallis was recently imaged by the HiRISE camera at 25 cm/pixel resolution (PSP_003460_2015 and PSP_003605_2015) in a location with both alcoves and valley floor deposits. Several features are apparent, a well-defined circular crown and main scarp (i.e., failure area), tensional cracks along the crown margin, and a well-defined accumulation zone with slump deposits. The general appearance of these landslides is similar to terrestrial rock and/or loose earth rotational landslides. The Bahram landslides are also different from other well-documented landslides on Mars, such as those in the Valles Marineris canyon system that have lobate forms and longitudinal and/or transverse ridges on their surfaces. To assess the possible landslide surface and stability of valley walls in Bahram Vallis, we have applied a "method of slices" modeling technique commonly used in geotechnical engineering along two MOLA topographic profiles across the valley and alcove walls. A landslide slip surface (circular arc) is fitted between two points, a point at the top of the slope closest to the landslide crown, and another at the base of the slope near the deposit terminus. The valley wall materials are divided into slices and the sum of their moments (driving moment) is divided by the total resisting moment of the mass to produce a "factor of safety" ratio value (F.S. < 1 = failure). Assuming the wall materials are a layer of regolith above a infinitely deep mass of rock, the initial estimated F.S. for the two profiles is 84-87, indicating relatively stable walls. Although these values suggest current wall stability, a different combination(s) of rock and regolith properties or the addition of saturated materials in the model could produce significantly different (i.e., lower F.S.) results. Future analysis of these landslides may include using a stereo-derived digital elevation model from HiRISE images and software for landslide modeling that can apply horizontal ground accelerations (seismic impulses) as an additional parameter.
P31B-0431
Mapping and Characterization of the Eastern Elysium Planitia Deposits
We present SHARAD investigation results on an extensive region of Mars near the equator that show evidence of shallow radio- transparent deposits. The first radar sounding data over this area was collected by MARSIS showing evidence of radio-transparency but we were not able to resolve the depth since the depth resolution of MARSIS is approximately 80 meters. SHARAD is able to resolve these deposits thanks to its higher bandwidth providing approximately 10 times better depth resolution. We have obtained observations with sufficient density to allow mapping the bed/deposit interface. Also, the reflection properties of the radio waves are analyzed to provide clues about the nature of the material filling the Elysium Planitia. Elysium Planitia is located east of the "Frozen Sea" region where is the focus of debate regarding its nature. The MARSIS radar sounder operates over 4 bands between 1.3 MHz and 5.5 MHz and has a maximum bandwidth of 1 MHz. SHARAD operates between 15 MHz and 25 MHz with a maximum bandwidth of 10 MHz.
P31B-0432
Rigorous Photogrammetric Processing of HiRISE Stereo Images for Topographic and Geomorphologic Analysis at MER landing sites
High-precision topographic information is critical to Mars surface exploration. Such information can be derived from both orbital and ground rover data. The availability of HiRISE stereo images makes a great progress in high resolution imaging and topographic and morphological information derivation. This presentation gives the necessary rigorous photogrammetric model for HiRISE stereo image processing and results achieved at the Mars Exploration Rover mission sites. HiRISE is a push-broom imaging sensor. For short segments of the orbital trajectory, 2nd-order polynomials can be used to model change in the exterior orientation (EO) parameters with respect to time. Since all of the 14 CCD lines (10 red, 2 blue-green and 2 NIR) share the same EO parameters at a specific time, only one set of polynomial parameters is needed to model the EO parameters for all the CCD arrays. The orbit's initial position and pointing angles are provided in the SPICE kernels. For any given ephemeris time, the EO parameters can be retrieved by interpolating the spacecraft's trajectory and pointing vectors. Based on the developed rigorous sensor model, we have developed a method for bundle adjustment (BA) of HiRISE stereo images that removes or reduces measurement inconsistency and improves mapping precision. We have also developed a hierarchical stereo matching process. Based on the original images, an image pyramid with 5 levels is constructed by sub-sampling of each previous level. Interest points are generated by Foerstner operator at every image scale. Matching starts from the images of the lowest resolution; results are transferred to the next higher level, with more interest points being extracted and matched. After matching the highest resolution images, evenly distributed matched interest points are selected as tie points between the stereo images. In the end, grid points (4-pixel spacing) are defined and matched to generate a DTM of the terrain. Using the HiRISE images TRA_000873_1780 and PSP_001414_1780 of Victoria Crater, 136 evenly distributed tie points and 135 check points were automatically selected for BA. Before BA, the mean residual in the along- track direction was 24.6 pixels with a standard deviation of 0.77 pixel, and the mean residual in the cross-track direction was 0.017 pixel with a standard deviation of 0.23 pixel. After BA, the mean residual in the along-track and cross-track directions are 0.30 pixel and 0.0047 pixel, respectively, with standard deviations of 0.37 pixel and 0.024 pixel. This improved precision ensures the high quality of topographic and morphological information to be derived. Currently, we are processing a stereo pair of HiRISE images that cover Husband Hill and the Home Plate area. The presentation should provide more detailed information and results.
P31B-0433
HiRISE observations of potential MSL landing sites
The Mars Reconnaissance Orbiter's (MRO) High Resolution Imaging Science Experiment (HiRISE) began acquiring data in the fall of 2006. Some of the images of the past year of operations include support for the 2009 Mars Science Laboratory (MSL) potential landing sites, which were proposed at the first landing site workshop in October of 2006. There are 28 proposed sites that were prioritized at the first workshop and they are located across elevations and latitudes ranging from -6 km to approximately +1 km, and 29.3°N to 48.5°S respectively. These sites emphasize a range of science themes including mineralogy (i.e. phyllosilicates, sulfates, or hematite), layered materials, or presence of fluvial, erosional or depositional landforms. Each site was originally proposed with a 20 km landing ellipse in an area that appeared smooth and flat in pre-HiRISE images. Many of the sites are "go to" sites where there is a proposed landing ellipse adjacent to a nearby science target of interest. HiRISE has acquired images at ~30 cm/pixel for all the proposed sites, including a few additional images of the northern hemisphere landing ellipses at low sun, used to better constrain the distribution of slopes at small scales. Each HiRISE image is approximately 6 km wide by at least 10 km long and includes a 1.2 km-wide blue-green and NIR swath down the middle of the image. For each HiRISE footprint, there is also a co-located Context Image (CTX) and CRISM hyperspectral image. The HiRISE images show a variety of landforms at high resolution, such as layered deposits, dunes, channels, ridges, rocks, polygonal terrain and craters. Each of these sites will be extensively studied to assess the safest landing site, as well as what is best suited to achieving mission science objectives. The acquired images have been released to the PDS and are available to all interested parties, including MSL project and science advocates for the purpose of characterizing potential hazards to safe landing and operations and to assess to science potential of the sites relative to stated mission objectives and planetary protection requirements. http://marsoweb.nas.nasa.gov/landingsites/ http://webgis.wr.usgs.gov/msl/
P31B-0434
Evidence of multiple flooding events or pulses from erosional features in Martian outflow channels
Cataracts, steep knickpoints in large outflow channels, were once large waterfalls on the martian surface. New images of cataracts in the Kasei, Ares, and Mangala Valles obtained by the High Resolution Imaging Science Experiment (HiRISE) onboard the Mars Reconnaissance Orbiter (MRO) in conjunction with topography from the Mars Orbiter Laser Altimeter (MOLA) are used to further constrain channel outflow rates and durations. Both the morphology and geometry of these erosional features help us to further elucidate the history of these outflow channels. Morphologically, all of the cataract systems have horseshoe-shaped headcuts and longitudinal grooves. These morphological features are strikingly similar to terrestrial flood erosional features from the Missoula Flood, Jokulsa a Fjollum, and English Channel flood. Like terrestrial cataracts, the Martian cataracts have significantly lower width-to-depth ratios compared to the rest of the channel, consistent with erosion into bedrock. Knickpoint recession is also evident from these images. The Ares Valles cataract has migrated more than 35 km, an order of magnitude more than the terrestrial examples of catastrophic floods. Most estimates from Ares Vallis suggest similar discharge rates to the terrestrial floods, though some estimates of Martian flood discharge are significantly higher. High resolution imaging shows that nearly all of the cataract systems have multiple generations of erosion, with smaller subchannels within the cataract system. Based on the length of the recession and the morphological evidence, multiple flooding events are possible in most of these Martian channels, either by discrete events or pulses of flow. All of the terrestrial examples resulted from the breaching of a rock or ice dam, releasing a glacial flood. Most of the Martian outflow channel discharges likely came from the subsurface. DEM's of these cataracts from HiRISE and CTX stereo pairs will be input into a 2.5D hydraulic flood model to provide more precise discharge calculations. HiRISE stereo pairs have a vertical precision of better than 25 cm.
P31B-0435
Complex Processes and Environments as seen in the Scarp and Aureole Deposit of Olympus Mons
Enigmatic features previously identified off the western slope of Olympus Mons and recently imaged with Hi-RISE provide clues to the history of the global dust cycle and changing surface environments. The most effective means of interpreting the surface history and physical properties of unique surface materials is from a multi- spectral point of view, in which geomorphology, composition, and surface properties are combined from different instruments and observations. In combining THEMIS with higher-resolution observations from MOC and Hi-RISE, we are able to compare the texture, brightness and thermophysical characteristics to take a more comprehensive perspective, and develop a detailed relative surface history. A MOC mosaic was used to map the layers, dust mantle, and blocks of uniform materials on a broad scale. New Hi-RISE images (PSP-003450-1975, PSP-003806-1975, and PSP-002461-1975) provide information that make the interpretation of these surfaces more complex, as the increase in resolution reveals additional episodes of modification and smaller-scale features. The juxtaposition of moderate-inertia features with low-inertia, dust-mantled surfaces allows for detailed mapping and interpretation of processes and associated erosional or depositional surfaces. Some of the most interesting features are the string of moderate-inertia dark spots in the aureole deposit and the extremely low- inertia layer exposed above the Olympus Rupes scarp. Evidence of homogeneous resistant blocks surrounded by a field of ripples on eroded dunes and other features indicate that these surfaces are the most recent and active in the low-inertia regions. Information from on and off the dark spots reveal complex morphologies in which cross-cutting relationships and correlations can be used to resolve a minimum of seven surface environments or events since the original emplacement. This history includes erosion of the original surface, deposition of regional layered deposits, and at least three separate episodes of eolian modification. The most recent process is complex, involving the activation of dark sand which stands in stark contrast to the surrounding dust-mantled surfaces.
P31B-0436
Wind a potential mechanism of Mars gully formation
Since Mars gullies were first revealed with the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) in 2000, they rapidly became a hotspot in Mars studying in that some of them are very young features on Mars surface. The previous studies focused on their formation and erosion mechanisms. As a result, several mechanisms have been proposed. But none of them can interpret the formation of all gullies perfectly. High resolution HiRISE images give us a good opportunity to examine it. In this study, we propose that wind could play an important role in some of the gullies formation. Wind is the most important agent acting on Mars surface (Fenton, 2003) and produced many features on Mars surface, including ubiquitous dunes, yardangs, deflation pits, dust storms, and dust deposits. Similarly, wind can also affect the inner edge of craters and valleys, where the gullies have been found mostly. Under the erosion of wind, the small channel will turn to a big gully. Wind could be a major reason to explain (1) why gullies formed in one side of a crater wall, while small wind-blown sand deposits in the opposite side of the same crater, as found in the crater of this HiRISE image (PSP_001697_1390_RED.JP2) and (2) why two craters next to each other, but only the big one has gullies developed. The reason for this is that big crater can form a strong wind circulation. In another HiRISE image (PSP_001330_1395_RED.JP2), we found a rock in the lower end of a gully course in a crater wall, for which we explain this gully is in the process of formation. This rock could be moving down and carving in to form the course due to the wind force. Based on the common characteristics of wind abrasion mechanics (Greeley and Iversen, 1985), we propose the following processes of gully formation by wind: (1) Embryonic stage: one side of a crater wall or valley wall was "softened" by the wind storm and formed some irregular and V-shaped fractured channels. (2) Youthful stage: small impact pits formed due to wind storm increase in numbers and coalesce due to lateral growth of the pits. In the same time, the abrasion channels grew longer down the wall. (3) Advanced stage: under the influence of gravity and wind abrasion, mass materials on the upper wall begin to avalanche in the downwind side, while wind-blown sands deposit in the upwind side of the valley or crater under the circulation of wind. (4) Mature stage: complete removal of the sands in the channels and the gullies are completely exposed. Sands continue to deposit on to the upwind side of the crater or valley.
P31B-0437
Image Differencing for Detecting Surface Changes on Mars
Visual observation methods to compare images for detecting Martian surface changes have been effective and efficient for images with different spatial resolutions and varying temporal windows. Large data streams from the Mars Reconnaissance Orbiter and future missions require automated detection techniques to detect changes that might reveal surficial processes. This study investigates the use of image differencing techniques to assist with identifying new slope streaks on the Martian surface. Two separate differencing methods were used to generate difference maps - a simple difference method, in which the initial state pixel values are subtracted from the final state pixel values, and a percentage difference method, in which a simple difference pixel value is divided by the initial state pixel value. Difference maps were normalized by subtracting the minimum pixel value and dividing by the range of pixel values across the image. The difference maps were then overlaid over the original image to emphasize new slope streaks. The simple pixel differencing results improved when 11 equal classification thresholds between -1 and +1 were defined, however the percentage differencing technique yielded a difference map that better emphasized slope streaks. The image differencing technique showed other changes in the computed difference maps, though these differences may be attributed to changes in lighting conditions, perspective, or image calibration.
P31B-0438
Analysis of the Argyre Planitia (Mars) Sinuous Ridges Using Early MRO Images
Southeastern Argyre Planitia hosts a suite of dendritic and braided sinuous ridges. Here we use imagery from the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) camera and Context Camera (CTX) to constrain formation processes of the ridges. HiRISE images resolve large boulders, up to ~8 m in diameter, on the ridges; however, it is unclear if the boulders, particularly those that appear more angular, may be eroding out of layers of indurated material or represent transported particles. Well defined layers observed in one of the ridges are quasi-horizontal and, in general, longitudinally continuous. Frequent transitions in ridge crest morphology from sharp-crested to rounded or flat-topped are evident. In several locations, ridges also appear to lie in shallow troughs that are several kilometers in width and mantled with finer sediments. Topographic profiles across and along the three ridges imaged by HiRISE and CTX are derived from Mars Orbiter Laser Altimeter (MOLA) data and reveal that the ridges generally trend in a slope-parallel direction, but cross over low intrabasinal topographic divides. If the regional topography has remained stable, the tendency for the ridges to cross topographic highs suggests formation by pressurized flow rather than gravity driven flow (open river channels). Altogether, the Argyre ridge characteristics are consistent with sub-ice fluvial processes and most likely represent terrestrial esker-like features. Terrestrial eskers have similar morphologies and distributions and may cross topography, contain layers, and lie within troughs, often referred to as Nye channels, which form from subglacial meltwater erosion. The nature of some eroding beds within the Argyre ridges suggests a possible induration process. We present a further analysis of the characteristics of the Argyre ridges and the potential environment in which the ridges formed.
P31B-0439
A Closer Look at Gully Morphology and Formation on Mars with HiRISE
As of early September 2007, HiRISE has returned over 3,200 images of the surface of Mars, including over 400 images of gullied locations. The HiRISE images (with resolution as high as 25 cm/pixel) provide an opportunity to test the current MOC-derived understanding of gully provenance and various suggested formation mechanisms. The HiRISE images show that the gullies exhibit a great deal of morphological diversity. They range in size with widths as large as several tens of meters down to the HiRISE resolution. Lengths range from several tens of meters to several kilometers. Some form tributaries that coalesce into gully networks, while other gullies exhibit the canonical single source alcove, incised middle reach and terminal debris fan. Some gully sources blend in gradually with the surrounding uplands, while others start full-borne from blunt, theater heads. Gully systems displaying different morphologic patterns can be located physically adjacent to each other. The most morphologically complex gully systems exhibit point bars, cut banks, undercutting of walls and source regions, erosion into underlying surfaces, braided and anastomosing reaches, multiple terraces located along gully margins, and erosion and deposition of materials along the gully and overlapping adjacent systems. This complex suite of morphological features suggests formation by fluvial processes. Other gully-like systems, such as those located on dunes, lack key morphological indicators and are little more than sets of parallel troughs without apparent debris fans. Other simple forms have distinct source regions and debris fans, but lack incised middle reaches. These particular features are on steep slopes, such as the inner walls of several volcano calderas, as well as on some crater, valley and canyon walls and may be more akin to debris chutes where material is transported down steep slopes mostly by gravity alone. We conclude from these observations that there may be a continuum of processes involved in the formation of gullies and gully-like forms, ranging from fluvial erosion to mass movement processes involving dry flows and slides. HiRISE imaging also shows that gullies in a single locale may emerge at a variety of elevations and may display strikingly different morphologies. For example, miniature gully systems, some less than a kilometer long, are found along a crater wall in the Terra Sirenum region. These small gullies are adjacent to larger ones and exhibit typical gully morphologic characteristics, however they emerge much further downslope than their nearby counterparts. In another example, along a crater wall in the Terra Cimmeria region, adjacent gully systems emerge from source regions at a variety of elevations. Some gullies have characteristics typically associated with runoff-dominated fluvial processes while others have characteristics of terrestrial sapping-dominated fluvial systems. Other adjacent gullies have transitional morphologies. Another intriguing set of gullies is found in Hale Crater, where pristine gullies are located on two sides of the eroded crater rim, flowing in opposite directions. In one location, only a narrow ridge separates eastward and westward oriented gullies. Associations such as these may challenge any single gully formation mechanism.