P33B-1440
Blueberries on Earth and Mars: Correlations Between Concretions in Navajo Sandstone and Terra Meridiani on Mars.
Concretionary Fe-Mn-rich nodular authigenic constituents of Jurassic Navajo sandstone (moki marbles) bear a certain relationship to similar concretionary forms ('blueberries') observed on Mars. Their origin on Earth is considered to invoke variable redox conditions with underground fluids penetrating porous quartz-rich sandstone leading to precipitation of hematite and goethite-rich material from solution, generally forming around a central nucleus of fine particles of quartz and orthoclase, recently verified by XRD and SEM-EDS analyses. At the outer rim/inner nucleus boundary, bulbous lobes of fine-grained quartz often invade and fracture the outer rim armored matrix. The bulbous forms are interpreted to result from fluid explusion from the inner concretionary mass, a response to pressure changes accompanying overburden loading. Moki marbles, harder than enclosing rock, often weather out of in situ sandstone outcrops that form a surface lag deposit of varnished marbles that locally resemble desert pavement. The marbles appear morphologically similar to 'blueberries' identified on the martian surface in Terra Meridiani through the MER-1 Opportunity rover. On Earth, redox fluids responsible for the genesis of marbles may have emanated from deep in the crust (often influenced by magmatic processes). These fluids, cooling to ambient temperatures, may have played a role in the genesis of the cemented outer rim of the concretions. The low frequency of fungi filaments in the marbles, contrasts with a high occurrence in Fe-encrusted sands of the Navajo formation [1], indicating that microbial content is of secondary importance in marble genesis relative to the fluctuating influx of ambient groundwater. Nevertheless, the presence of filaments in terrestrial concretions hints at the possibility of discovering fossil/extant life on Mars, and thus should be considered as prime targets for future reconnaissance missions to Mars. 1] Mahaney, W.C., et al. (2004), Icarus, 171, 39-53.
P33B-1441
Recent Results from the Mars Exploration Rover Rock Abrasion Tool
The Rock Abrasion Tool (RAT) serves as the sample preparation device on the Mars Exploration Rovers (MER) science payload. The RAT grinds a circular area 45 millimeter in diameter and to a depth of 0-15 mm into Martian rock. This is intended to remove the altered outer layers of rock as well as overlying surface fines in preparation for imaging and spectral observations. In addition to acting as a facilitator for other instruments, RAT telemetry acquired during grinding may be used to assess the physical properties of the rocks that it grinds. The most direct rock measurement extractable from the RAT grinding process is the energy expended per unit of rock volume removed. This has been termed the RAT Specific Grind Energy (SGE) and in terms of rock bulk physical properties, correlates roughly with unconfined compressive strength. Recent results from the Mars Exploration Rovers will be presented as will comparisons between Earth rocks and Martian rocks in terms of their SGEs and other physical properties. Although SGE is an uncommon metric for rock physical properties, the SGE calculated from the RAT engineering data, and linked with data from other instruments in the payload, represent the most comprehensive database yet created of the physical properties of Martian rocks. RAT SGE continues to be helpful in understanding the geologic history of Mars and will be of great value in instrument design for future Mars missions.
P33B-1442
The Infrared Spectra Study of Dehydrated and Dehydroxylated Clay Samples
The OMEGA visible and near infrared imaging spectrometer detected
phyllosilicates in ancient Noachian terrains of Mars-an observation
later confirmed by CRISM imaging spectrometer. Detailed mapping
showed that the phyllosilicates, most of which are clay minerals,
are found in ancient, heavily cratered terrains. These clay-bearing
deposits contain interbedded and buried craters, and it is possible
the clay minerals were repeatedly subjected to high temperatures
resulting from impact processes. We hypothesize that the dehydration
and dehydroxylation of clay minerals subjected to impact was a
widespread process on early Mars, and that dehydrated and
dehydroxylated clay minerals may be still abundant on the surface of
Mars.
The objective of this study is to characterize the infrared spectral
features of clay minerals when they are heated to increasingly
higher temperatures. Using a Nicolet 6700 FTIR spectrometer, we have
acquired mid-infrared reflectance (500cm-1-7000cm-1),
far-infrared reflectance (60cm-1-500cm-1) and ATR
(attenuated total reflectance)(500cm-1-4000cm-1) spectra
of 13 clay samples at different temperatures(from
100°C to 900°C). All of the clay samples have been prepared to be
very pure (<2μm size fraction) and they were heated for 24
hours at each temperature. The structures of clay minerals are well
known because of their typical layer structure and complex relations
with water and OH bonds. Our data show that the infrared spectra of
clay minerals changed as the temperature increased, which helps us
to understand the temperature-related processes including removal of
water and OH groups, change of Si-O bond, even the complete
destruction of clay layer structures. Differences among dehydration
and dehydroxylation conditions for these clay minerals are also
discussed in this work. In our future work, we will continue to
collect the infrared spectra of more clay minerals and collect
additional data for our samples, including VNIR reflectance, XRD,
NMR, and emissivity spectra, which can provide insights into the
crystal structures and help us to answer the very first question: is
it possible that dehydrated and dehydroxylated clay minerals are an
explanation for poorly crystalline or amorphous materials on Mars?
P33B-1443
Highly Comminuted Impact Ejecta on Mars: Constraints on Ballistic Emplacement
A number of Martian craters ranging from 5.5 to 220 km in diameter show concentric haloes of low-thermal inertia material that we interpret as analogous to previously-described haloes of highly comminuted impact ejecta on the Moon. Interpretation of the Martian haloes as fine-grained ejecta is supported both by their morphology and spatial distribution, and by their scaling relative to their source craters. The Martian crater halo sizes show a power law relationship relative to the source craters, consistent with scaling relationships recently derived for ~250 lunar craters. Both the Martian and lunar empirically-derived power laws are broadly consistent with theoretical predictions for ballistically emplaced ejecta. This suggests that deposition of particles comprising the Martian low-thermal inertia haloes - on the order of 100 microns to 10mm in size - is not profoundly affected by the Martian atmosphere. Thus, our results place constraints on the mechanics of Martian ejecta emplacement, and on the role of the atmosphere and regolith volatiles in mediating ejecta blanket morphology.
P33B-1444
Thermokarst Depressions on Crater Rims in the Southern Highlands of Mars
Utilizing Martian satellite images from MRO HiRISE, CTX, THEMIS VIS, and MGS MOC we have identified a meso-scale, latitude-dependent type of depression which we are calling Elevated Flat Floor Depressions (EFFDs). A detailed survey of these features was conducted across the Southern Highlands. This survey indicates that EFFDs are characterized by: 1) A restricted in geomorphic region. EFFDs are located in a latitude band of between 40° and 60° and are more prevalent in the southern hemisphere. The depressions also are situated on or near the apex of medium (30km) size crater rims. 2) A distinctive morphology. EFFDs are typically around 1000m in diameter, orientated parallel to the crater rim, have steep, well defined scarps, and are completely enclosed. 3) Appears to be a recent formation. EFFDs have a "fresh" appearance with distinctively sharp scarps even in mantled terrains and contain no superimposed features. In addition to a general survey, stereopairs of HiRISE and CTX images were used to create high resolution DEMs of the depressions to study topography and gradients produced by these depressions. We conclude that due to the latitude dependence, morphology and unaccounted volume loss that these features are periglacial in origin similar thermokarst depressions.
P33B-1445
Observational and Theoretical Assessment of Possible Pingos on Mars
Pingos are hills with cores of massive injection ice, formed when pressurized groundwater forces a layer of frozen ground upwards. They are important indicators of permafrost conditions, since they require the occurrence of liquid water. In recent years, pingos have been proposed at a number of specific sites on Mars, in a variety of settings, although many of the candidate features have other proposed origins as well. The submeter resolution of the HiRISE camera allows detailed geomorphic analysis. In particular, HiRISE is capable of resolving dilational cracks expected to occur in the overburden layer of a pingo. We assess the possibility of Martian pingos with observational and theoretical constraints. Using the diffusive ice loss equations of Hudson et al. [1], we estimate that under current conditions a mid-latitude pingo could survive on the order of 106 years, long enough to be influenced by orbital variations in climate. Flexural scaling suggests that if formation conditions were similar, pingos on Mars should be similar in size to terrestrial examples. We have surveyed 1797 HiRISE images searching for fractured mounds. Several types of fractured mound are observed, concentrated in the mid-latitudes in both hemispheres. Although not an unambiguous match, the features most similar to terrestrial pingos are a variety of fractured mounds on southern-hemisphere crater floors; these usually occur in association with gullies. Flat-topped mounds in Utopia display intriguing fractures consistent with tensile failure in many cases, but do differ from terrestrial pingos in some geomorphological aspects and could originate by a different mechanism. Other fractured mounds, while occurring in the same latitude range, may form by different processes. However, the latitudinal control suggests that many of these features are in some way related to water or ice. We conclude that of the observed fractured mounds, the strongest candidates for Martian pingos are crater-floor mounds, particularly those associated with gullies; if these are pingos, this would require past liquid water in the shallow subsurface. [1] Hudson T. L. et al. (2007) JGR 112, E05016
P33B-1446
Preliminary Investigation of Linkages Between Arctic Pingos and Subsurface Stratigraphy
This NASA-SETI supported study investigates the distribution of pingos (elliptically-shaped ice-rich topographic mounds) across 2300 square kilometers of the central coastal plain of Arctic Alaska in relation to the shallow geological framework that exists immediately beneath them. Pingos in the central North Slope of Alaska are classified as being of the closed or hydrostatic type. Their genesis is often assigned to freezing and cryogenic uplift of near-surface saturated thaw lake sediments that are exposed as lakes are drained and/or become choked with sediments. Although thaw lakes appear rather ubiquitous across the study area, pingos do not. Pingo distributions can be categorized as either clusters of elements or as relatively dispersed. Spatial statistical analysis reveals that pingo distribution is non-random and clustered. The analysis also took into account that pingo distribution is a function of preferential preservation between modern rivers channels that cross the study area. Pingo distributions and frequency were tested in relation to the location and type of stratigraphic and sedimentological features that characterized the shallow subsurface across the study area. Subsurface interpretation was derived mostly from oil well wireline logs. Gamma ray logs for more than 160 wells were used to define, correlate and assess the connectivity and conductivity of shallow and near-surface stratigraphic units between wells. Assessed also were major facies changes and the type and locations of subsurface structures such as major basement-to-surface faults and folds. The surface and near-surface truncation and subcropping of tilted, alternating units of permeable coarse-grained and confining fine- grained units were also mapped in relation to pingo locations. Preliminary and intriguing findings will be presented which contribute to the hypothesis that pingo genesis, location, and variations in morphology could be, in part, linked to a well-documented and active subsurface geohydrologic system. This system is characterized by multiple stacked hydrocarbon-, saline- and freshwater-rich reservoirs. Processes include fault reactivation and basin subsidence that drive episodic basin expulsion, and upward migration and mixing of deep basin and phreatic fluids along basin margins. Endpoints of the system include demonstrable gas hydrates deposits that occur below and within the permafrost, and documented seepage of hydrocarbons and groundwater at the surface. Our hypothesis entertains the idea that closed pingos might also be endpoints of the petroluem system as basin and phreatic fluids migrate vertically within a thick and leaky permafrost interval along faults and tilted reservoir sand-rich units, which intersect and differentially charge frozen near-surface sedimentary units. Future field-based sampling and geophysical studies may shed additional light on this model's application for pingo genesis and resource exploration on Mars.
P33B-1447
Subsurface Tectonics and Pingos of Northern Alaska
We describe preliminary results of a two-phase study that investigated links between subsurface structural and stratigraphic controls, and distribution of hydrostatic pingos on the central coastal plain of Arctic Alaska. Our 2300 km2 study area is underlain by a complete petroleum system that supports gas, oil and water production from 3 of the largest oil fields in North America. In addition, gas hydrate deposits exist in this area within and just below the permafrost interval at depths of 600 to 1800 feet below sea level. Phase 1 of the study compared locations of subsurface faults and pingos for evidence of linkages between faulting and pingo genesis and distribution. Several hundred discrete fault features were digitized from published data and georeferenced in a GIS database. Fault types were determined by geometry and sense of slip derived from well log and seismic maps. More than 200 pingos and surface sediment type associated with their locations were digitized from regional surficial geology maps within an area that included wire line and seismic data coverage. Beneath the pingos lies an assemblage of high-angle normal and transtensional faults that trend NNE and NW; subsidiary trends are EW and NNW. Quaternary fault reactivation is evidenced by faults that displaced strata at depths exceeding 3000 meters below sea level and intersect near-surface units. Unpublished seismic images and cross-section analysis support this interpretation. Kinematics and distribution of reactivated faults are linked to polyphase deformational history of the region that includes Mesozoic rift events, succeeded by crustal shortening and uplift of the Brooks Range to the south, and differential subsidence and segmentation of a related foreland basin margin beneath the study area. Upward fluid migration, a normal process in basin formation and fault reactivation, may play yet unrecognized roles in the genesis (e.g. fluid charging) of pingos and groundwater hydrology. Preliminary analysis shows that more than half the pingos occur within 150 m of the vertical projections of subsurface fault plane traces. In a previous, unpublished geostatistical study, comparison of pingo and random locations indicated a non-random NE-trending alignment of pingos. This trend in particular matches the dominant orientation of fault sets that are linked to the most recent tectonic deformation of the region. A concurrent Phase 2 of the study examines the potential role of near-surface stratigraphic units in regard to both pingos and faults. Both surface and subsurface coarse-grained deposits across the region are often controlled by fault structures; this study is the first to assess any relationship between reservoir rocks and pingo locations. Cross-sections were constructed from well log data to depths of 100 meters. Subsurface elements were compared with surface features. Although some studies have linked fine-grained surface sediments with pingo occurrence, our analysis hints that coarse-grained sediments underlie pingos and may be related to near-surface fluid transmissivity, as suggested by other researchers. We also investigated pingo occurrence in relationship to upthrown or downthrown fault blocks that vary in the degree of deformation and fluid transmission. Results will guide a proposed pingo drilling project to test linkages between pingos, subsurface geology, hydrology, and petroleum systems. Findings from this study could aid research and planning for field exploration of similar settings on Earth and Mars.
P33B-1448
Distinct Element Modelling of Landslides in Mechanical Multilayers on Mars
Mass wasting events such as landslides are an important component of the processes that have shaped the surface of Mars. Landslides are interpreted to have been active during much of the geologic history of Mars including the very recent past. The main scarp and displaced materials of landslides can tell us much about the mechanical nature of the surface and shallow subsurface of Mars. We use vertical two-dimensional distinct element models parallel with the slide direction to examine the effects of mechanical layering upon the morphology of slip surfaces and scarps that form as a result of slope failure on Mars. Bulk layer mechanical properties incorporated into the models and scaled to values likely be present on Mars include density, tensile strength, Young's modulus, Poisson's ratio, internal friction angle, cohesive strength, and unconfined compressive strength. Here we model horizontal layers with thickness range of 100 m to 500 m for a total thickness of 2500 m. Initial geometry is a 5 km long rectangle under conditions of Mars gravity where the top surface and one lateral boundary are free surfaces, and the horizontal base and opposing lateral boundary are rigid surfaces with friction coefficient of 0.5. Each layer represents one of five rock strengths, with strongest (strong basalt) to weakest (unconsolidated deposits) unconfined compressive strengths of 83, 44, 25, 8, and 2 MPa, respectively. Our models show that an initial slip surface forms some distance from the lateral free surface and subsequently migrates away from the free surface in discrete increments with concomitant decreasing slope of successive failure surfaces. Relative and absolute layer strength, thickness, and order control the morphology of the failure surfaces, the location and shape of the initial failure surface, and the kinematics of displaced material. In general, the size of coherent blocks and tendency towards sliding and spreading of displaced blocks increases with layer strength. In most cases, kinematic styles such as toppling, sliding, spreading, flow, and fall occur in some combination during development of a final kinematically stable displacement surface, although not all styles occur simultaneously nor persist throughout the modelled landslide event. Landslides are commonly observed in the Valles Marineris region on Mars, including portions of Coprates Chasma. Our models show strong similarities to landslide features observed on the north wall of Coprates Chasma, including features that are likely influenced by mechanical layering in the canyon walls. These similarities include slip surface shapes and apparent remnant layer blocks in the displaced materials. Our preliminary results demonstrate the power of distinct element modelling as an appreciable step toward a new understanding of the stratigraphy and mechanical nature of the upper crust of Mars.
P33B-1449
Source Regions of the Shergottites and Gusev Basalts
We have conducted high pressure experiments in order to test if there are conditions that could produce both the shergottite parent magmas and the Gusev basalts. We have determined that the shergottite parent magmas originated deep in the mantle (~14 GPa or ~1200 km) where majoritic garnet fractionation occurs. The Gusev basalt source region is at a shallower depth in the mantle (≤10 GPa or ≤800 km) where majoritic garnet is not stable. Several recent studies support the idea that the modern martian crust and mantle may have been derived through the differentiation and crystallization of a chondritic magma ocean in the planet's earliest history. Majoritic garnet fractionation must occur early on during magma ocean crystallization in order to impart a superchondritic CaO/Al2O3 ratio to the residual magma ocean liquid. This is required to account for the superchondritic CaO/Al2O3 ratios that are observed in the shergottite parent magmas. This ratio should remain comparatively unperturbed by subsequent crystallization of olivine and orthopyroxene. However, the Gusev basalts sampled on the surface of Mars by the Spirit Rover do not exhibit a superchondritic CaO/Al2O3 ratio. If these rocks are largely unaltered basaltic samples, then their compositions appear to be inconsistent with having formed from a deep, global magma ocean. We show that both the shergottite parent magmas and the Gusev basalts can be derived from mantle plumes or regional magma oceans that are produced through impact melting. Our experimental results demonstrate that both the shergottite parent magmas and the Gusev basalts (provided they are genuine magmatic liquids) can be derived through igneous processing of a chondritic mantle and that a deep, global magma ocean is an unlikely mechanism of formation.
P33B-1450
Constraining composition of Mars using geophysical constraints and mineral physics data
We use the observed moment of inertia, hydrostatic flattening factor and hydrostatic gravitational field to constrain the mineralogical and compositional structures of Mars. We construct 1-D density models of the interior of Mars for a series of mantle temperature and compositional models, core density and core radius. We then adopt the second-order internal theory of equilibrium of a self-gravitating and rotating planet to calculate the hydrostatic figure and flattening factor of Mars, and compare them to the observations. In the mineral physics calculations of density models in the mantle, we consider both olivine and garnet systems and the chemical interactions between them, and use the phase equilibria data to define the stable assemblages at relevant pressures and temperatures, and cation distribution data to define the chemical composition of each phase. This information, along with our current estimates of physical properties of these phases, provides a mineralogical model with volume fractions of each phase along with the aggregate velocities and density. Following this approach, we document compositional and thermal models of the interior of Mars that are consistent with the mineral physics data and the geophysical constraints of hydrostatic figure, flattening factor and moment of inertia. We also discuss the implications of these compositional and thermal models to the understanding of formation and evolution of the planet.
P33B-1451
Water, Melting, and Convection in the Martian Mantle
We present models of convection in the Martian mantle in which particular attention is paid to water and its effects on partial melting, melt extraction, and viscosity over timescales of several billion years. We have combined the mantle convection program STAG3D (e.g. Tackley 1998) with a parameterized thermodynamic model of martian mantle mineralogy and carried out calculations of convection in a two-dimensional compressible model of the planet's mantle. As the depth to the base of the mantle is known only to within a few hundred kilometers, models with core-mantle boundary depths of ~1700 and ~2000 km, which essentially cover the range of possible values, are considered. The models are heated from below by a cooling core and from within by the radioactive decay of 40K, 232Th, 235U, and 238U. Near-fractional hydrous melting is included in a parameterized form by modifying the dry solidus of martian peridotite by means of a simple method to include the solidus-lowering effect of water in low concentrations; the exhaustion of phases is also taken into account in a simple form. Melting reduces the concentration and changes the distribution of both the water and the heat-producing radionuclides in the mantle. Different initial water and radionuclide contents and distributions are assumed and allowed to evolve with time as a consequence of melting, dehydration, and the secular expansion of the ringwoodite stability field. The viscosity of the mantle is dependent on temperature, pressure, water content, and the amount of retained melt and therefore also undergoes a secular change as the mantle cools and becomes depleted. Preliminary models indicate the existence of a two-layer convection regime at least in the early stage of planetary evolution where the upper layer undergoes dehydration and depletion of radionuclides much sooner than the lower. Depending on the assumed water content, the dehydration of the zone of melt generation can lead to the formation of a high-viscosity layer in the upper part of the mantle whose thickness reaches several hundreds of kilometers. This high-viscosity layer is sandwiched between a crust/lithosphere and -- at least initially -- a lower mantle layer, both of which have higher radionuclide and volatile contents. The existence of a high-viscosity layer in the upper part of the mantle may be an alternative, non-thermal explanation for the high effective elastic lithosphere thickness beneath the northern polar deposits which was recently deduced from SHARAD observations.
P33B-1452
Impact-Induced Melting of the Martian Mantle
A large impact not only creates a giant basin on a planet but also results in considerable melting in the mantle, especially if the impact occurs in the early history of the planet. There are generally 4 stages of melting caused by a large impact: 1) Melting of a major part of the impactor and target material beneath the impact site, due to release of the kinematic energy of the impactor that is largely converted to heat energy; 2) Melting in the upper mantle due to immediate depressurization caused by excavation of crustal material from the impact site. This stage of melting is simultaneous with the excavation process and it usually occurs in the upper mantle because the pre-impact temperature is usually close to, or at, the melting temperature, and the sudden depressurization allows melting to occur; 3) Melting in the upper mantle due to its upwelling in order to achieve isostatic compensation, during which rocks from deeper parts of the upper mantle move to low- pressure upper parts. This process involves appreciable displacement of the mantle material when the resulting excavation volume is very large, and it may take up to a few thousand years to accomplish; 4) Melting in the entire mantle by convection circulations that develop in response to the temperature perturbations in the upper mantle caused by the second and third stages of melting. Depending on the size of the impactor and the pre-impact temperature condition of the mantle, this secondary convection may take a much longer time to develop and it usually results in enormous amount of melt and extensive volcanism, which are by far more important than those associated with the first three stages of melting. We study the secondary convection induced in the Martian mantle by large impacts that created giant basins such as Utopia, Acidalia, Ares, Deadalia, Hellas, Isidis, and Argyre, as well as the giant Borealis impact that likely created the major part of the northern low lands. We investigate two-flow convection in an axisymmetric spherical shell mantle model, where the mantle is allowed to melt as it crosses the solidus temperature. We consider two different models, permeable and impermeable. In the permeable model the melt is allowed to migrate inside the partially molten solid matrix which in turn convects, whereas in the impermeable model the melt does not move relative to the convecting solid matrix, but it is extracted once it exceeds a threshold of 3% rock volume. We modeled different sizes of the impactors as well as different thermal stages of the pre- impact mantle and thickness of a stagnant lithosphere. We show that depending on the size of the impactor, the pre-impact temperature conditions in the mantle, and the presence of a thick and strong lithosphere, the major melting induced by the secondary convection may take a very short time for giant impacts, such as Borealis and Utopia, but up to several hundred million years for impacts that created smaller basins such as Isidis and Argyre. It is shown that due to the long delay of partial melting in the mantle the lithosphere beneath these small basins cooled appreciable and gained enough strength to support the excess mass concentrations (mascons) that have survived for 3.5-4 Gyr.
P33B-1453
Did Tidal Deformation Power the Core Dynamo of Mars?
We first show that 7 out of the 20 giant impact basins of Mars recently reported by Frey [2008] trace a great circle on Mars. The other five basins trace another great circle and still the other three basins trace yet another great circle. The latter great circle is in good agreement with the pre-Tharsis equator of Mars that is estimated from modeling crustal magnetic anomalies and diagonalizing the moment of inertia of Mars after removing the loading effects of Tharsis bulge. It is shown in this paper that the three great circles were likely the equatorial plane of Mars at certain periods and Mars experienced appreciable polar wander. The great circles also indicate that the asteroids that created the basins were satellites of Mars whose orbits decayed in time through spin-orbit coupling with tidally deforming planet, and eventually impacted on Mars creating the giant basins at around 4 Ga. The orbital dynamics of the four largest asteroids show that any of them could have orbited Mars for several hundred million years if it were a retrograde satellite. Continual elliptical straining of otherwise circular fluid streamlines of the liquid core of Mars by tidal deformation could have exerted a strong strain, that was large enough to overcome dissipation and excite the elliptical instability inside the core. We estimate the physical properties of the Martian core that are required to allow the tidal deformation to power the core dynamo. The growth time of the elliptical instability is shorter than the dissipation time. The tidal energy dissipation rate inside Mars is found to be over two orders of magnitude greater than the magnetic energy dissipation rate in the core. It is concluded that even if only one of the 4 largest asteroids were orbiting in retrograde sense, it would have likely powered the core dynamo of Mars for several hundred million years.
P33B-1454
Mars' Paleomagnetic Field as the Result of a Single-Hemisphere Dynamo
One of the most dramatic results from the Mars Global Surveyor mission is that Mars possesses remanent crustal magnetic fields from a dynamo that was operational for a short time in Mars' early history. Several characteristics of the crustal field are not well understood, such as the field's intensity, concentration in the southern hemisphere, and lack of correlation with any surface features except for the hemispheric crustal dichotomy. Formation mechanisms for the hemispheric dichotomy, both endogenic and exogenic, can result in significant hemispheric thermal heterogeneity at Mars' core-mantle boundary (CMB). If the dynamo was active during formation of the crustal dichotomy, the heat flux variability at Mars' CMB could have a significant effect on Mars' dynamo. Here we use the Kuang-Bloxham numerical dynamo model to demonstrate that degree-one lateral variations in CMB heat flux can result in a single-hemisphere dynamo. This dynamo produces strong magnetic fields in only the southern hemisphere. The resulting magnetic field morphology can explain why Mars' crustal magnetic field intensities are significantly stronger in the southern hemisphere without relying on any post- dynamo mechanisms. It can also remedy contradictions in paleomagnetic studies with rotational stability studies, as well as the incompatible requirement that significant atmospheric loss occur during the same period as strong magnetic field generation.
P33B-1455
Dynamics of Mars and the Origin of Tharsis Constrained by Gravity and Topography
Whether the topography of Mars is supported by loading of an elastic shell at the surface or by viscous flow
in the interior has been debated. On Mars, in particular, because a significant portion of the gravity and
topography signals is manifested in the Tharsis province, distinguishing these two mechanisms is also crucial
to deciphering origin and formation of Tharsis. Here I address this issue by separating various components
of the topography related to different geological events and studying them through jointly modelling the
observed geoid on the basis of elastic loading and viscous flow. The observed topography of the major
volcanoes on Mars, Tharsis, Elysium and Olympus, can account for the residual topography (the observed
topography with the exclusion of the major impact basins and north-south dichotomy) and explain the
observed geoid by loading response of an 100-km thick surface elastic layer at the intermediate wavelengths
of spherical harmonic degrees l = 4 – 12. However, at the longest wavelengths (l = 2 -3), the volcano
topography exhibits considerably less magnitude than the residual topography, and significantly under-
predicts geoid on the basis of the elastic loading model inferred from the observations at the intermediate
wavelengths. Viscous flow calculations indicate that these unexplained long-wavelength topography and
geoid are consistent with existence of a low-density anomaly in the deep Martian mantle beneath Tharsis. A
thermo-chemical nature of the low-density anomaly explains not only its presence, but also the formation,
tectonic activities and evolution of Tharsis, suggesting compositional anomalies in the deep planetary mantle
play a key role in the planet evolution.
http://geophysics.geo.sunysb.edu/wen/
P33B-1456
True Polar Wander Due to Surface Mass Loading on Mars: Implication to Evolution of the Tharsis Province
Long-term variation of a pole location driven by evolution of a volcanic province on a terrestrial planet is investigated in order to provide implication to evolution of Tharsis on Mars. The present modeling comprehends large-scale variation. Also, an elastic lithosphere is considered here to memorize a remnant rotational bulge. For this purpose, the previous formulation for the final state of true polar wander is simply expanded into that for long-term variation. Remarkable variations are those on a true polar wander angle for the following situations. The parameter Q' (i.e. a normalized magnitude of a surface mass load) is larger than or equal to one, and also is slightly smaller than one. In addition, the initial load co-latitude is less than about ten degrees. Under the situations of such Q' and initial load co-latitude, the results are as follows. If the initial load co-latitude is close to zero, extremely large and rapid variation like as inertial interchange is possible. If not close to zero, variation is relatively large but is much more gradual. These results would give us a possibility of reconstruction of volcanic history, such as the evolution of Tharsis on Mars.
P33B-1457
A New Flexure-Dominated Lithospheric Stress Model for Mars, Driven by Pressure Differences at the Base of the Lithosphere
Two different models of lithospheric stress have been employed to explain the majority of the faulting in and around Tharsis. Banerdt and Golombek [2000] used topography and gravity as boundary conditions and a full thin shell treatment with horizontal gradient loads and both bending and membrane stresses, lithospheric deflection and a laterally varying crustal thickness determined through the system of shell equations by the two boundary conditions. They inferred that Tharsis was formed primarily by volcanic construction accommodated by lithospheric flexure. However, the faulting extending from Ceraunius Fossae north and north-east through Tantalus and Alba Fossae is not well described by this model. Either these structures formed under different conditions than we see today, or the assumptions of this model are not appropriate for this region. Dimitrova et al [2006] showed the deviatoric stress field associated with horizontal gradients of gravitational potential energy (GPE) alone provides an excellent fit to (~ 70%) of the normal faults as mapped by Knapmeyer et al [2006] in the region. This fit suggests that many of the faults were created at early times when elastic thicknesses and membrane and flexural stresses were small, a combination of brittle and ductile deformation was likely to be widespread, and GPE stresses dominated. We revisit the problem of the stress at the time of faulting by incorporating a new method for deriving the load function and the vertical displacement using the driving force of the difference between local pressure and global average pressure, inferred from the topography of Zuber et al [2000] and the crustal thickness model of Neumann et al [2004]. We build on the approach of Banerdt and Golombek [2000] by making no assumptions on the source or type of loading. The key point here is that the load that has lead to flexure involves an unknown radial displacement. In our approach, the problem of determining this radial displacement also implicitly involves determining the load function with no additional assumptions. Both our approach and that of Banerdt and Golombek [2000] satisfy topography and gravity, and in both cases, once the deflection is known, stress and strain are determined uniquely by the displacement field. In both models the range of vertical displacements is very similar (40km vs. 32km), and the results are similar in the areas of negative vertical displacements centered at (-140E, 20N), and (-110E, 0N) and in the areas of positive vertical displacement at (-45E, 15S), (-45E, 50S). Differences in the two solutions, including vertical displacements in our model that show larger variability at high spherical degree and order, will be evaluated using the strain model of Banerdt and Golombek [2000] and the surface fault data of Knapmeyer et al [2006].
P33B-1458
Martian Tectonics on a Spatially Heterogeneous Lithosphere
Together with gravity and topography analysis, the study of surface faulting is key to our understanding of the Martian crust, lithosphere and upper mantle. Extensive catalogs of surface faults are now available for Mars, based either on Viking images or on MOLA altimetry. Using forward modeling of faults under gravity and topography constraints, researchers have tried in the past to reproduce observed faults in type, orientation and associated strain amplitude. These attempts have met with mixed success in predicting the global pattern of normal faults, and little success in explaining the distribution of thrust faults or the near- absence of strike-slip faults. Part of the problem originates in our ignorance of the time evolution of gravity and topography, so that present-day values must be used in the modeling. The reconstruction of a topographic history based on geologic layering is conceivable, but the history of gravity anomalies will never be known. Some assumptions underlying the mechanical modeling of tectonics are more testable. Using flexure equations for a thin shell with variable thickness, we study the influence on fault modeling of a lithosphere having a spatially heterogeneous rigidity. We also analyze the effect of a spatially heterogeneous loading density. We apply these results to geologically-motivated scenarios for Tharsis tectonics: this volcanic province could be underlain by a thinner lithosphere or loaded with basalts of higher density than the mean crustal density.
P33B-1459
Evaluation of the Orogenic Belt Hypothesis for the Formation of Thaumasia, Mars
The Thaumasia Highlands (TH) and Solis Planum are two of the best-known examples of compressional tectonics on Mars. The TH is a region of high topography located in the southern portion of the Tharsis Province, Mars. Solis Planum is located in eastern Thaumasia. Two hypotheses for the formation of this region have been suggested: sliding on a weak horizon or thrusting analogous to orogenic wedges on Earth. Both hypotheses require a shallowly dipping to sub-horizontal weak horizon below Thaumasia. Wrinkle ridges in Solis Planum are also inferred to sole into a décollement. If Thaumasia formed by thrusting related to sliding on a décollement, then certain conditions must be met as in critical taper wedge mechanics (CTWM) theory. If the angle between the surface slope and the basal décollement is less than predicted by the critical taper equation, the 'subcritical' wedge will deform internally until critical taper is achieved. Once the critical taper has been achieved, internal deformation ceases and the wedge will slide along its base. Formation of orogenic belts on Earth (such as the Central Mountains in Taiwan) can be described using CTWM. This method is applied here to the Thaumasia region on Mars. The surface slope (alpha) was measured in three locations: Syria Planum-Thaumasia margin, Solis Planum, and the TH. Topographic slopes were compared to the results from the critical taper equation. Because the dip of the basal décollement (beta) cannot be measured directly as on Earth, the dip angle was varied at 0 - 10 degrees; these values span the range of likely values based on terrestrial wedges. Pore fluid pressure (lambda) was varied between 0 (dry) and 0.9 (overpressured); these values span the full range of this important unknown parameter. Material properties, such as the coefficients of internal friction and of the basal décollement, were varied using reasonable values. Preliminary results show that for both reasonable (such as lambda = 0, mu b = 0.85, beta = 0 deg) and extreme (such as lambda = 0.9, mu b = 0.1, beta greater than 0 deg) values of the parameters for Mars, the predicted critical taper angle was typically lower than the measured slope, rendering the orogenic belt hypothesis for the formation of the TH invalid. Comparable analysis of Solis Planum shows it also lacks a décollement.
P33B-1460
Identifying Buried Impact Structures and Understanding Their Contributions to the Relative Dating of Mars
Crater counting allows for the relative dating of regions on planetary bodies. In the past, only visible impact craters could be observed by looking at images and altimetry data. In 2004, a crustal thickness model (CTM) for Mars was derived from density and altimetry data, revealing buried impact structures. Updated MRO gravity solutions allowed for the production of an improved CTM of Mars by Neumann et al., 2008 [1]. This higher resolution dataset allows for the identification of more buried impact structures. Four rectangular ~14.5x106 km2 areas were chosen, two in the highlands (H1, H2) and two in the lowlands (L1, L2), to compare the cumulative frequency curves (CFC) produced from the old and new models. The CFCs we produced using the new model had a similar slopes to those produced by Edgar and Frey, 2008[2], however, they were shallower in all test areas compared to those from the old data set. The test areas were then broken up into four quadrants, and it was found that there was high variability in the CFCs within each test area, which was attributed to the geology. Subsequently, we analyzed two ~7.5x106 km2 geologic units, one in the highlands (H3) composed of Noachian plateau material and one in the lowlands (L3) composed of Amazonian plains material. Using the previous data set, there were not enough features in H3 and L3 to produce a working CFC, however, the improved resolution in the new CTM allowed for this analysis. The use of this new CTM allows us to refine and improve the CFCs in local regions to better constrain the relative age of those regions. Through dating all the individual regions on Mars we will be able to better reconstruct the geologic history of the planet. [1] Neumann, G. A., et al. (2008) Marscrust3- A Crustal Thickness Inversion from Recent MRO Gravity Solutions, LPI Conf. Abstracts, 39, 2167 [2] Edgar, L. A., and Frey, H. V. (2008) Buried impact basin distribution on Mars: Contributions from crustal thickness data, Geophys. Res. Lett., 35, L02201.
P33B-1461
Quantification of Crustal Strain by Faulting in the Western Hemisphere of Mars.
We quantify fault-related strains for the western hemisphere by using Kostrov's formula and the updated Knapmeyer et al. fault catalog. As a first step we match the formula and fault catalog to previously reported measurements of strain (ε) in five areas for normal faulting and two areas for thrust faulting in Tharsis. Using these comparisons, a strain and faulting depth map of the western hemisphere is generated. We obtain good agreement in strain magnitudes from each of the study areas by systematically varying the scaling parameter (γ) and fault dip angles to determine values for the depth of faulting (z), or the depth of the seismic lower stability zone (600°C paleogeotherm). We find for the northern plains NE of Alba Patera, ε = 0.2% and z = 8-11 km; Alba Patera's east flank, ε = 0.8% and z = 8-10 km; Alba Patera's south flank and southern plains, ε = 0.6% and z = 5-9 km; Tempe Terra, ε = 3% and z = 16 km; Thaumasia, ε = 0.6% and z = 10 km; Arcadia Planitia, ε = -0.06% and z = 35-45km; Lunae Planum, ε = -0.29% and z = 25-30 km by using the best-fitting range of γ = 10 -4 - 10 -2, normal fault dip angle of 55° ± 5°, and thrust fault dip angle of 30° ± 5°, all in close agreement with terrestrial values. Normal faulting depths around Tharsis range from 5 to 16 km and thrust faulting depths range from 25 to 45 km with a general west to east increase in fault depth north of the Tharsis Montes, except for the plains northeast and on the east flank of Alba Patera which are underlain by a faulted layer of comparable thickness. Interestingly, the depths of thrust faulting beneath wrinkle ridges of Arcadia Planitia and Lunae Planum are in close agreement with those previously calculated in Amenthes Rupes in the eastern hemisphere, showing in these regions that the thickness of the seismogenic crust is similar between the southern highlands and the northern plains.
P33B-1462
Geologic support for the putative Borealis basin (Mega-Impact) on Mars
A series of recent papers (all in Nature v. 453) using Martian gravity and topography [Andrews-Hanna et al., 2008], 3-D hydrodynamic simulations [Marinova et al., 2008], and 2-D hydrocode models [Nimmo et al., 2008] have eloquently reintroduced the single mega-impact hypothesis for the formation of the Martian hemispheric dichotomy boundary. Although geophysical models often return non-unique solutions, the coalition front presented by these three independent methods to test such a hypothesis lends credibility and demands further evaluation. The central tenet of these works is the proposition that an elliptical basin (long axis 10,600km, ellipticity 1.25) centered at 67N, 208E marks the pre-Tharsis crustal thickness transition and thus the real dichotomy boundary. Evaluation of this new boundary with respect to the geologic record offers new avenues, especially since geologic tests of the mega-impact hypothesis have mostly proved inconclusive because of Mars' multi-stage and multi-process geologic history. Within this survey, a slightly larger ellipse with a long axis of 12,500 km, ellipticity of 1.48, and centered at 65.3N, 250E expands the putative Borealis impact basin (which does not necessarily represent the transient or final impact cavity dimensions, but defines a potential 'affected zone') while maintaining agreement with the original observations with respect to gravity and topography. The 'affected zone' can be defined by basement structure that may become susceptible to later deformation, or it may in fact have been the paleo- topographic expression of the basin. By expanding the overall area (nearly twice the area of the original mega-impact basin proposed by Wilhelms and Squyres in 1984) several geologic features become significant in evaluating the mega-impact story. 1) Valles Marineris is concentric to the putative basin interior and parallels the ellipse margin suggesting that it is the structural manifestation of localized crustal relaxation of the Tharsis volcanic pile over pre-existing basement structure related to Borealis basin subsidence. The present day Valles Marineris may actually represent the 'missing portion' of the original crustal dichotomy trace underneath Tharsis. 2) The 'great faults' (Connerney et al., 2005) that offset the magnetic field pattern radiate from near the center of the putative basin, again suggesting basement structural control related to basin formation. 3) The mysterious Medusa Fossae Formation is completely enclosed within the basin margin and the units' southern contacts fall within 5 km of the same elliptical trace that bisects central Valles Marineris. 4) Chaos regions at the eastern end of Valles Marineris are wholly contained within the basin margin and suggest some kind of marginal control on their locations. 5) Valley network (channel) densities sharply increase outside the basin and are truncated by the Borealis ellipse. Integrating these and other geologic observations (still ongoing) with the newly formulated geophysical methods suggests that a single mega-impact is reemerging as a viable and perhaps preferred mechanism for dichotomy formation.
P33B-1463
Mineral Mapping of High Priority Landing Sites for MSL and Beyond Using Mars Express OMEGA and HRSC Data
High priority candidate landing sites for the Mars Science Laboratory (MSL) mission have been proposed by various researchers, their significance based largely on spectroscopic and geomorphic evidence for aqueous processes. Specifically, seven candidate landing sites are under consideration for MSL at the time of this writing: Mawrth Vallis, Nili Fossae, southern Meridiani Planum, Eberswalde Crater, Holden Crater, Gale Crater, and Miyamoto Crater. While only one of these sites can be visited by MSL, the other sites remain among the most compelling localities on Mars for future in-situ exploration by ESA's ExoMars mission or an international Mars sample return mission. We have produced regional scale mineral maps of these sites using data from the Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA). Visible images from the High Resolution Stereo Camera (HRSC) are used as a map base. OMEGA infrared band parameters are used to identify and map pyroxene, olivine, oxides, sulfates, phyllosilicates, and other hydrated phases. OMEGA visible channel data also provide color information, which gives an estimate of dust cover and additional insights into the mineralogy of altered deposits. The dustiest site is Gale Crater and the least dusty is Nili Fossae. The strongest signature of phyllosilicates occurs in Mawrth Vallis, followed by Nili Fossae. However, Nili Fossae also has some of the strongest olivine signatures on the planet. One fundamental difference between the Nili Fossae and Mawrth Vallis sites is that in Mawrth Vallis, phyllosilicate-bearing, light-toned rocks contain no evidence for primary phases in OMEGA data, but in the Nili Fossae area, phyllosilicates, olivine, and pyroxene are mixed at the subpixel level. South Meridiani Planum shows hydrated plains in contact with ancient, pyroxene-bearing, slightly altered, older bedrock. Patchy deposits of phyllosilicates are found in Miyamoto Crater, but their geologic context is not clear at this time. Only weak evidence for alteration is seen at Gale Crater at OMEGA resolution. While Eberswalde and Holden Craters contain fluvial landforms clearly diagnostic of aqueous processes, the surface mineralogy of each site is dominated by pyroxene. Only small, localized exposures of phyllosilicates are seen in Holden, and Eberswalde contains little evidence for alteration in OMEGA data. Based on OMEGA mineralogy and geologic context from HRSC data, we consider three sites to be the most compelling. Mawrth Vallis provides the strongest alteration signatures on the planet, with phyllosilicate minerals tied to a thick, diverse stratigraphic section of ancient bedrock. In Nili Fossae, there is the opportunity to study primary crust and altered, impact-excavated terrain in close proximity. At south Meridiani Planum, the geologic contact between sulfate-bearing terrain and ancient, phyllosilicate-bearing bedrock can be explored and sampled. At all three of these sites, it is possible to immediately address mission objectives without driving because the spacecraft can land on altered terrain using technology available for MSL.
P33B-1464
High thermal inertia surfaces and the physical nature of the upper martian crust
An investigation of martian high thermal inertia surfaces has been made using Thermal Emission Imaging System (THEMIS) one hundred meter per pixel nighttime temperature data. High thermal inertia surfaces or interpreted bedrock are defined as any pixel in a THEMIS image with a thermal inertia over 1200 J K- 1m-2s-1/2) and may refer to in situ rock exposures or rock-dominated surfaces. Three distinct morphologies, ranked from most to least common, are associated with these high inertia surfaces: 1) valley and crater walls associated with mass wasting and high surface slope angles, 2) crater floors related to melting and re-crystallization associated with large (typically >25 km), high energy impacts, 3) plains surface with compositions significantly more mafic than the surrounding regolith, possibly indicating that the martian regolith has been processed, both chemically and mechanically. Overall, Mars has very little exposed bedrock or rocky material with only 960 instances identified from 75°N to 75°S. In general, bedrock instances occur in lower albedo (<0.18), moderate thermal inertia (>350 J K-1m-2s-1/2), and relatively dust free (DCI <0.95) areas. While many locations on Mars satisfy these conditions and have expected morphologies (e.g. steep slopes in Valles Marineris), observed bedrock instances are surprisingly rare. Most instances are concentrated in the southern highlands, with very few located at high latitudes (>45°N and <58°S). The latitudinal asymmetry observed in this data indicates a process that preferentially destroys or masks bedrock at lower latitudes in the north. Several processes likely play a role in destroying or masking a majority of the bedrock on the planet, including enhanced mechanical breakdown associated with permafrost at high latitudes and chemical and/or mechanical weathering associated with the formation of regolith fines from mafic precursor material [Bandfield and Rogers, 2008]. This distinct lack of bedrock indicates that Mars has likely undergone large-scale processing and reworking of the upper crust. Bandfield, J.L. and A.D. Rogers (2008) Geology, doi:10.1130/G24724A.
P33B-1465
Martian Surface Reflectivity seen by MARSIS
Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) is a low frequency radar. The
MARSIS wavelength is about 50-100 m. These radar waves penetrate deeply in the Martian ground and the
first radar echo is due to few tens meters of the surface. The amplitude of this surface echo provides useful
information on the shallow subsurface.
Using MARSIS radar data, we extract the reflectivity of the Martian surface from the radargrams and then we
build a global radar reflectivity map. We will describe our method for the extraction and the calibration of the
reflectivity. In this calibration, we correct the absorption due to the two-way of radar waves through the
Martian ionosphere and compensate the surface roughness effect. The surface roughness effect is
estimated by simulating the radar surface returns for each MARSIS orbit.
Finally, we will present a reflectivity map without roughness effect and discuss the reflectivity variations due
to change in the dielectric constant.
P33B-1466
Surface and Subsurface Radar Backscattering Coefficient Over the Martian South Polar Layered Deposits From MARSIS Data
Planum Australe was probed for the first time with the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the European Space Agency's Mars Express. In most cases, a strong reflection is seen at a time delay consistent with the expected depth of the contact of the south polar layered deposits (SPLD) materials with the substrate, to a depth of more than 3000 meters for the case of the SPLD maximum thickness. We extend the analysis of MARSIS data over the SPLD, extracting quantitative information from radar echoes on the strength of surface and subsurface reflections, characterized through the radar backscattering coefficient. We recover the backscattering coefficient only in a relative sense, starting from measured echo amplitudes and correcting for geometric terms in the radar equation such as the altitude. We are capable of discerning changes in the backscattering coefficient from place to place, but we cannot attach an absolute physical quantity to measured values. We produced a database of backscattering coefficient values for reflections coming from both surface and subsurface (the base of the SPLD), for all observations of MARSIS over Planum Australe. Other relevant parameters are also reported, such as latitude, longitude, spacecraft altitude, frequency and operative mode. We produce maps of the surface and subsurface backscattering coefficients for homogeneous observations. We correlate them with SPLD thickness, to find significant departures from the expected decrease of the subsurface echo strength with depth.
P33B-1467
Organized Stone Stripes in the Northern Plains of Mars
Polygonally-patterned ground at scales of meters to tens of meters is nearly ubiquitous in the northern plains of Mars. Collections of cobble to boulder-sized clasts are commonly found superimposed on polygonal terrain in a regular pattern that resembles the surface of a basketball [Mellon et al., 2008; Malin and Edgett, 2001]. Here we describe a variation of this basketball terrain, dubbed "stone stripes," in which piles of clastic debris are arranged into a series of parallel to subparallel ridges spaced at intervals of ~40 m. Stone stripes appear to be continuous or nearly continuous over areas of tens to hundreds of square kilometers. This type of landform is most prevalent poleward of ~70°N latitude, at the northern margin of Utopia Planitia . We hypothesize that the orientation of stone stripes is controlled by regional structures, such as wrinkle ridges. First results from a survey of THEMIS VIS images within the area from 60°N-80°N and from 70°E-130°E show no obvious correlation between wrinkle ridge orientation and the orientation of stone stripes. Work is ongoing to further characterize the orientation of stone stripes in relation to regional structures and local and regional topographic slopes. Mellon et al., (2008), Periglacial landforms at the Phoenix landing site and the northern plains of Mars, Journal of Geophysical Research, doi:10.1029/2007JE003039, in press. Malin and Edgett, (2001), Mars global surveyor mars orbiter camera: Interplanetary cruise through primary mission, Journal of Geophysical Research, 106, 23429-23570.