P23D-01 INVITED
Impact and Aqueous Strata in Holden Crater, Mars
Images from the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) at 2652 cm/pixel scales reveal a sequence of exposed impact megabreccia and sedimentary units in the Noachian-aged Holden crater in Margaritifer Terra, Mars (26S, 326E, 154 km diameter). Formation of Holden crater interrupted the previously through-flowing Uzboi-Ladon-Margaritifer (ULM) outflow channel system and excavated sediments deposited by ULM within the pre-existing (Early Noachian) Holden impact basin. Crater walls display variably rounded, poorly sorted, chaotically arranged, and variably bright blocks up to 50 m across residing within a finer matrix. These characteristics suggest a possible origin for many blocks as sedimentary materials excavated from the pre-impact Holden basin and these materials are interpreted as coarse, impact- fragmented megabreccia. At least 150 m of sedimentary facies partially fill the crater and were emplaced during two wet phases during the Noachian. Early prolonged erosion of crater walls and basin deposition in a quiescent distal alluvial or lacustrine setting resulted in a lower light-toned unit displaying meter- to submeter-scale bedding traceable for up to kilometers and containing phyllosilicates, but with few resolvable blocks. The lower unit is topographically restricted and capped by a thin, dark-toned layer commonly exhibiting 4–5-m-diameter polygonal fractures that may record a terminal playa phase. Later-Noachian high-magnitude flooding of the crater occurred as an impounded Uzboi Vallis lake overtopped the crater rim and rapidly emplaced a topographically restricted darker- toned, more crudely bedded deposit that drapes unconformably over antecedent relief and envelops large blocks (up to 100 m across) of material eroded from the lower unit. The upper unit exhibits alluvial morphology near the crater rim breach, but grades distally an upwards into more continuously bedded, possible lacustrine facies that were deposited during a relatively short second wet phase. The aqueous strata in Holden crater indicates a more clement, wet climate characterized some of the Noachian Period and provide the first clear context for phyllosilicates in an alluvial/lacustrine environment. Emplacement of the beds comprising the lower unit likely requires stable wet conditions and a setting likely to preserve geochemical or lithological signatures related to habitability. Hence, the possibility of evaluating the changing potential for habitability over part of Noachian Mars makes Holden crater a priority for future landed missions.
P23D-02 INVITED
Recent bright gully deposits on Mars: Wet or dry flow?
Bright gully sediments attributed to liquid water flow have been deposited on Mars within the last several years. To test the liquid-water-flow hypothesis, we constructed a high-resolution (1 m/pixel) photogrammetric Digital Elevation Model of a crater in the Centauri Montes region where a bright gully deposit formed between 2001 and 2005. We conducted 1D and 2D numerical flow modeling to test whether the deposit morphology is most consistent with liquid-water or dry-granular flow. Liquid-water-flow models that incorporate freezing can match the runout distance of the flow for certain freezing rates but fail to reconstruct the distributary lobe morphology of the distal end of the deposit. Dry-granular-flow models can match both the observed runout distance and the distal morphology, where distributary lobes are formed by viscous fingering. Wet debris flows with high sediment concentrations are also consistent with the observed morphology because their rheologies are often similar to that of dry granular flows. As such, the presence of liquid water in this flow event cannot be ruled out, but the available evidence is consistent with dry landsliding.
P23D-03 INVITED
Current and Past Aeolian Processes as Seen by the HiRISE Camera
With more than a year of observations and almost 3000 images, the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) has provided unprecedented details of aeolian features that give insight into current and past processes on the planet. Major finding include: 1) Bedform texture and relation to current sand movement: Dunes and ripples generally have superposed bedforms of smaller scale, as is commonly seen on Earth. These are visible down to the 3rd order (bedforms upon bedforms upon bedforms). Mapping of wind tails seen in the lee of rocks and, at the MER Spirit site, ventifact facets and textures, shows a co-alignment with the 2nd order, not 1st order, bedforms. This implies that recent or high energy wind activity is confined to directions orthogonal to the 1st order in these cases, probably because the major bedforms are relatively immobile due to cohesion or surface granule lags and also act to funnel near surface winds in the 2nd order direction. 2) Young bedforms: Many "young" features such as gullies and some (but not all) fresh craters contain bedforms, indicating fairly recent wind modification of the surface. 3) Bedforms in high elevation and low thermal inertia surfaces: The mantled surfaces of the Martian volcanoes (except Albor Tholus) display meter-scale reticulate ridges that probably formed from multi-directional winds. The reticulate ridges are clustered in sets at the ~20 m scale bordered by larger ridges clustered at the ~50 m scale. The larger-scale clusters may represent topography of an underlying cratered surface. The reticulate texture is generally confined to areas of thermal inertias less than 100 J/m2/K/s0.5, consistent with dust. How such material would be mobilized under the very low pressure (<1 to 2 mb) conditions is unknown. 4) Active sand migration at Victoria Crater: At the current site of the MER Opportunity rover, HiRISE sees a clear association between dark wind streaks extending northward from Victoria and bedforms within adjacent crater alcoves, suggesting a depositional origin for the streaks. HiRISE is continuing a monitoring campaign to assess changes in the bedforms and streaks over time
P23D-04
MRO Observations of Light-toned Layered Deposits along the Plains Adjacent to Valles Marineris
We report on light-toned layered deposits (LTLD) that are located in several locations along the Hesperian-aged plains adjacent to Valles Marineris, including (1) southwest of Melas Chasma, (2) south of Ius Chasma, (3) south of West Candor Chasma, (4) west of Ganges Chasma, and (5) west of Juventae Chasma. The LTLD on the plains are only seen in locations where there has been erosion of an overlying meters-thick darker-toned unit or along crater and trough walls so they are likely more extensive in area than seen by outcrop exposures. Based upon exposures along crater and trough walls, thicknesses for the deposits can be up to 200 meters. The LTLD along the plains that we have analyzed with HiRISE data show complex stratigraphic sequences where beds can be divided based upon variations in their color, albedo, thickness, and erosional style. Polygonal fractures seen along some bedding surfaces can vary in size and shape. Color differences seen in HiRISE images appear to reflect compositional variations rather than surficial contamination (i.e., dust or basalt sand). Pitted and rounded features are common to some layers along the plains but not seen in LTLD within troughs. We have noticed that similar beds can repeat throughout an exposed sequence, suggesting a cyclic process for deposition. The layered beds appear alike in morphology for all five locations, although the deposit south of West Candor has less evidence for layering compared to the other four locations. The morphology and color variations seen in the LTLD along the plains appear distinct from those seen for LTLD inside the troughs, suggesting different depositional processes. Proposed origins for the LTLD along the plains include eolian, fluvio-lacustrine, and pyroclastic volcanism. A pyroclastic origin is likely based upon their association with the Hesperian-aged lava plains, their cyclic nature and lithologies that are consistent with pyroclastic deposits on Earth, and the identification of hydrated silica-rich phases in CRISM data. However, at Ganges and Juventae Chasmata the LTLD are adjacent to a valley (Ganges) or postulated inverted channels (Juventae and Ganges) that could support a fluvial origin at these two sites. There is no evidence for fluvial activity at the other three locations, though. If some of the deposits are pyroclastic in origin, their limited distribution implies eruptions were localized and probably associated with emplacement of the lava plains that surround the canyons, rather than in association with Tharsis volcanism. In the case of a fluvio-lacustrine origin for the deposits at Ganges and Juventae Chasmata, water activity could have resulted in the formation of depositional fans composed of the light-toned beds.
P23D-05
Stratigraphy and Tectonics of Arabia Terra Layered Deposits from HiRISE Imagery and Derived Meter Scale Topography
Meter scale topography from HiRISE stereo imagery allows unprecedented geometrically controlled stratigraphic studies of martian layered deposits. The widespread occurrence of light toned layered deposits within craters in western Arabia Terra was first recognized from Mars Orbiter Camera (MOC) imagery. These isolated exposures occur only within craters, although they are found over a range of hundreds of kilometers. Typically, these layered rocks exist as mounds on the crater floors which have been greatly eroded back from their original extent. In some cases, they are hundreds of meters in thickness, and composed of hundreds of parallel, meter-scale beds. The occurrence of similar layered deposits over such a large region of the planet implicates a common depositional process with relevance to planetary scale surface evolution. Using HiRISE stereo images we have derived one meter scale topographic maps of several of the western Arabia Terra layered deposits. These datasets allow a stratigraphic investigation at the scale of the observed bedding. We will discuss results of the three-dimensional mapping of these structures, focusing on tectonic and stratigraphic observations. Topographic analysis of widespread faulting within the bedrock reveals tens of meters of displacement accommodated along the larger faults, which can extend for several kilometers. Calculations of bedding orientations show large rotations up to ~30 degrees for some layered sections, indicating significant tectonic activity throughout the western Arabia region. Through precise measurement of the bedding structure the effects of deformation can be accounted for, allowing accurate reconstruction of the stratigraphic column. This process also removes the effect of the modern erosional topography, which can mask true stratigraphic thicknesses. In particular, we examine erosion resistant cliff-forming beds which occur repeatedly through some sections of the layered deposits. Our measurements allow us to accurately place these marker beds within the stratigraphic sequence, to evaluate whether they record a cyclic or stochastic depositional process. These observations will be discussed in a regional context, via comparison among various intracrater deposits for which stereo data is recently available.
P23D-06
High-resolution structural mapping in Southwest Candor Chasma
We report initial results of high-resolution structural mapping of layered deposits in southwest Candor Chasma, near the contact with the surrounding wall rock. Mapping is accomplished on a digital terrain model created from stereo HiRISE imagery, with postings at every one meter. A prominent stratigraphic package, herein referred to as the ‘Slickrock Member', is present throughout the study area and consists of a characteristic sequence of massive and friable layers. Mapping of the Slickrock Member reveals both the stratigraphic continuity between faults and folds, as well as the younging direction throughout the study area. Numerous synclines, anticlines and monoclines generally trend NW-SE and have wavelengths of ca. 0.5-0.7 km. Present-day exposures through these folds indicate that at least several hundred meters of layered deposits have been eroded and removed. Normal and thrust faults are identified from the dip direction of the fault plane and sense of bedding offset. These faults generally strike NE-SW to E-W. Fault dip angles are consistent with an effective rock friction of ca. 30 degrees. Thrust faults have lengths that are 1-2 km or more, while normal faults are generally less than 0.5 km in length. Damage zones occur at several fault stopovers and bends, and fault propagation folds are also observed. Thrust fault vergence directions appear non-systematic, consistent with thick-skinned deformation. Fault planes have a ridge-like erosional morphology that rises up to 5 m above the surrounding terrain, consistent with an origin through deformation band processes. Faults crosscut the NW-SE- trending folds and these faults do not appear to be folded, indicating that brittle deformation occurred after formation of the folds. These overlapping styles of deformation, and senses of faulting, reveal multiple changes in the orientations and magnitudes of the principal stresses that drove deformation in this area. Further, large-scale normal faulting associated with opening of the chasma is not evident in the study area, suggesting that chasma formation occurred prior to deposition of the layered deposits. http://hirise.lpl.arizona.edu
P23D-07
New Insights on Ejecta Emplacement at Martian Impact Craters from HiRISE Images
New insights into the process of fluidized ejecta emplacement for impact craters on Mars are being derived from the analysis of the publicly-released HiRISE images. Fresh imaged craters have diameters from 1.1 km (Winslow crater) to 29.0 km (Tooting crater), and formed on a variety of target materials including Amazonian-age lava flows, Chryse Planitia, the S. Highlands, and the flanks of the Tharsis volcanoes. These data help to resolve some of the long-standing issues pertaining to the fluidization of single layered ejecta (SLE), double-layered ejecta (DLE) and multi-layered ejecta (MLE) craters on Mars. For example, at a SLE crater in the S. highlands, we see a wide range of polygonal deposits on top of the ejecta layer that are absent from the surroundings, suggestive of desiccation of a previously wet ejecta layer. Kilometer-long flows, either of impact melt or of mud, have been identified on the outer rims of both Zumba (a SLE crater) and Tooting (a MLE crater). Also at Tooting crater, we see numerous 15 to 20 m diameter pits concentrated on the up-slope sides of the distal ramparts. These pits formed in broadly linear chains, and may be related to the loss of volatiles as the ejecta came to a rest to form the rampart. In a few rare places at Tooting crater, we have found valleys on the outer scarps of the ramparts, suggestive of dewatering of the ejecta once it came to rest. The idea that recent impacts within the S. hemisphere were associated with fragmented targets is not supported by an unnamed crater (at 49.3oS, 18.5oE) that displays numerous angular boulders as much as 20 m in diameter on top of the ejecta layers. A fresh 6 km diameter MLE crater in Chryse Planitia (at 26.1oN, 316.7oE) shows that individual ejecta layers are more numerous than had previously been identified from THEMIS VIS images; these layers display considerable diversity in the number and spatial distribution of pits and are suggestive of different volatile contents and/or resistance to erosion of the layers.
P23D-08
The Effect of Crater Obliteration on Inferred Surface Ages on Mars
The density of impact craters is currently the only measure of numerical age for terrestrial planets, after conversion with lunar calibration curves. Unlike the Moon, however, the martian surface has been eroded, obliterating some small (<1 km) craters. Recent images from high-resolution satellite-based cameras, such as the Mars Orbiter Camera (MOC) and the Thermal Emissivity Imaging Spectrometer (THEMIS), have identified many small, geologically distinct units, some of which may hold large implications for the geological and climatic history of Mars. Some of these units, such as the light-toned layered deposits, have shown greater susceptibility to erosion than the rest of the martian surface and may not retain small craters over geologic time. We have built upon early attempts to quantitatively account for rates of crater obliteration (combined erosion and deposition) on crater-frequency distributions by developing a model to measure its effect on inferred surface ages over the full range of orbitally observable crater sizes and reported erosion and deposition rates on Mars (0.01 - 10,000 nm/yr). For low long-term rates of obliteration (less than 10 nm/yr), like those observed by the Spirit and Pathfinder rovers, we find that surface modification has a minimal effect on the calculated surface age, even when only small craters are counted. However, for intermediate to high obliteration rates (greater than 50 nm/yr), like those observed by Viking Lander 2 and the Opportunity rover, the change of the calculated surface age can be large, especially when counting only small craters. This sensitivity likely precludes accurate determination of deposit ages by crater counting alone, without considering infilling and erosion. The model, when applied to previous studies using only small craters to date the surfaces of erodable deposits reported as geologically young, can yield surface ages that are older by orders of magnitude than those originally estimated. Consequently, conventional crater counting should be adjusted for obliteration before being construed as evidence for surface ages, especially for surfaces with low densities of small craters.