P41B-1357
Growth and Decay of the South Polar Residual Cap of Mars
The southern residual ice cap (SRC) is composed of high-albedo solid CO2, is a few meters thick and has areas at its margins and in its interior where the underlying water ice of the polar layered deposits shows through. Previous observations show that pits within the SRC expose sections of layering in the CO2 ice (up to 10m thick) and are expanding by meters/year (Thomas et al., Icarus, 2005). The expansion of these pits is a problem as their spatial density and expansion rates indicate that there should be nothing left of the SRC within a century or so. Our analysis of pit expansion rates indicates that they increase during periods of dust-storm activity. The sensitivity of SRC evolution to climate indicates that its stratigraphy is likely also a valuable source of information about recent climate variability. We are using HiRISE data to constrain landscape evolution models of CO2 ice landscapes to quantify the history of the SRC and the connection of its stratigraphy to martian climate. Here we report on two investigations utilizing different models. 1. We use a landscape evolution model to simulate growth of an accumulating CO2 deposit. This model shows pits naturally form in this landscape, without any associated change in climate, due to increases in surface roughness with time and the feedback between slope and ice ablation. Comparison of model results with the current SRC indicates the thickest sections of stratigraphy likely represent up to 100 martian years of accumulation. 2. HiRISE data shows that some pits expand by faster retreat of the lower ice layers, undercutting of the upper bright ice and subsequent mass-wasting. A separate model that can handle topographic overhangs has been developed and is being used to investigate this behavior. Using this model to reproduce the non- circular aspects of the pit-shapes and their expansion rates (which HiRISE can measure as a function of azimuth) constrains properties of the lower CO2 ice layers such as density and albedo. HiRISE observations combined with these models allow us to start interpreting the SRC stratigraphy and history, both by assigning timescales and deducing how the lower strata differ from the upper. The very short timescales involved in the evolution of this landscape indicate that the SRC stratigraphy is an important record of the variability of the current climate rather than a record of climate change.
P41B-1358
HiRISE Images of the Sublimation of Mars Northern Seasonal Polar Cap
The High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter (MRO) has completed a campaign to image the sublimation of Mars" northern seasonal polar cap. The images show a number of interesting phenomena associated with the dunes in the north polar erg, including slope streaks, circular defrosting features, and evidence for explosive gas release. Time-lapse sequences show these features evolving through the spring as the seasonal frost disappears. Slope streaks appear to be mobilized by the vigorous activity associated with sublimation of the CO2 ice. Streaks are observed to lengthen as the season progresses, and mass wasting from the crests of the dunes is implicated. Oblong features associated with defrosting show alternating bright and dark rings. They are found at the crests of dunes, and at the interface of the bottom of the dunes with the substrate. The contrast is most pronounced early in the spring and gradually fades as more and more ice is sublimated. This is a surficial albedo effect, not due to underlying surface morphology. The degree of cementation of the substrate below the ice layer is important and sublimation features which show mobilization of material are found on dunes, not on the surface beneath the dunes. Evidence for translucent CO2 ice similar to the seasonal ice found at southern high latitudes has been found at northern sites suggested by Piqueux and Christensen [1]. Dark and bright streaks are observed with orientations determined by the prevailing wind. We have not observed the araneiform terrain with its spider- like channels found in many locations in the south polar region. Frost-filled polygons are observed however. This work was partially supported by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. [1] Piqueux, S., P. Christensen, JGR 113 E06005 (2008).
P41B-1359
Density Variations within the South Polar Layered Deposits of Mars
The south polar layered deposits (SPLD) constitute the largest known reservoir of surface water on Mars. Previous studies solved for the best-fit uniform density of the deposit using a forward approach. We invert for the lateral density variations in the layered deposit using gravity data from radio tracking of Mars Reconnaissance Orbiter, topography from MOLA on board Mars Global Surveyor, and radar sounding data from MARSIS on board Mars Express. The gravity was first filtered to remove the effects of Tharsis loading and deformation. The residual gravity anomalies over the SPLD result from a combination of topography along the surface of the crust and Moho, density variations within the crust and mantle, and density variations within the polar layered deposits. We cannot uniquely solve for all of the unknowns, and thus must make assumptions regarding the nature of the gravity anomalies arising from the crust and mantle beneath the SPLD. The gravity and topography outside the cap were used to calculate the best-fit degree of compensation of the surface topography, which was then applied to the crust beneath the cap. Residual gravity anomalies outside the cap that are uncorrelated with the surface topography can arise from within the crust or mantle. We use the gravity anomalies outside the polar cap to construct a Wiener filter, which is applied to the gravitational signature of the cap to remove the short-wavelength anomalies over the SPLD that are spectrally consistent with an origin in the crust or mantle. These corrections attempt to isolate the gravity anomaly associated with the mass of the SPLD. We then use a constrained inversion for the vertically-averaged density within the SPLD as a function of position. The inversion smoothes the density variations and constrains the absolute deviation from the mean reference density. A nominal inversion with a mean density of 1300 kg/m3, yields a total range of approximately 700-1700 kg/m3. Apparent relationships between density and thickness suggest that the density variations within the SPLD are real, and may reflect spatial variations in the dust or CO2 content of the deposits.
P41B-1360
Craters and Resurfacing of the Martian North Polar Cap
The north polar residual cap (NRC), which covers the north polar layered deposits (NPLD), is about 1m thick and composed of large-grained, dust-poor water ice. The lack of dust accumulation indicates that this material accumulated recently; however, paradoxically, the exposure of large-grained (old) ice indicates a current state of net ablation. The NRC is often considered to be new NPLD material, thus understanding its mass-balance and relationship to the current climate could provide a key to reading the climatic record of the NPLD. Previous studies of the NRC cratering record (Herkenhoff and Plaut, Icarus, 2000; Tanaka, Nature, 2005) concluded a resurfacing age of only 10-100 Kyr; however, these conclusions were based on 0-2 craters. In order to constrain the processes and rates of NRC resurfacing we undertook a search for craters within the CTX (MRO's context camera) dataset. CTX data released to date provide almost complete coverage of the NRC with pixel scales of ~6m. This search increased the number of known craters on the NRC and NPLD from 4 to ~100 with crater diameters ranging from ~20 to ~400m. We then followed up on specific craters with observations by HiRISE (MRO's High Resolution Imaging Science Experiment) with pixel scales of ~30cm, acquiring ~50 of these targets. HiRISE observations reveal that craters are the sites of preferential ice accumulation (which gradually infills the crater cavity), while ablation and wind action combine to remove the crater rim. A morphologic sequence of degradation states (including depths from shadows) allows for a qualitative understanding of the processes involved in converting fresh craters (measured depth/diameter ~0.23) to craters currently almost unidentifiable. Variation in the argument of perihelion affects polar climate over the lifetime of these craters. We are combining recent orbital solutions (Laskar et al., Icarus, 2004) with the processes described above to create landscape evolution models that quantifiably investigate the recent (10-100 Kyr) mass-balance history of the NRC and which are constrained by the size-frequency and degradation of the observed crater population.
P41B-1361
Young Populations of Small Impact Craters on Mars as Observed in HiRISE Images
High resolution and quality of images obtained by HiRISE camera onboard MRO gives a unique opportunity to study systematically the smallest impact craters on Mars. Very young crater populations are unaffected by distal secondaries, because they postdate the latest secondary-forming impact, therefore, crater populations can potentially be used to study resurfacing histories of the youngest terrains. I studied a population of small impact craters on the proximal ejecta lobes of crater Zunil. The ejecta material has a uniform age. About ~1000 small impact craters were registered. Crater morphology suggests minor eolian modification of the craters, total obliteration of craters seeming insignificant. Some craters in the population form clusters, which is caused by break-up of projectiles in the atmosphere. For the purposes of age estimates each cluster was considered as a single impact event. Statistical tests do not reject spatial randomness of the inferred independent events. The cumulative size-frequency distribution (SFD) fits well a power law with an exponent of 3.16 - 3.20 for craters larger than 4.9~m. Thus, the SFD is steep and perfectly extrapolates the Neukum - Hartmann production function (NPF) beyond its lower diameter limit. Absolute crater retention age estimates can be obtained from the NPF extrapolation (about 180~ka) and from the present-day cratering rate (<540~ka). Remembering that both estimates are very far extrapolations, their consistency is wonderful. The ages are also consistent with Zunil being the youngest 10~km-size crater on the planet. The studied properties of the accumulation population on Zunil ejecta allow addressing interesting problems with analysis of small crater populations. For example, the observed extreme scarcity of small craters at high latitudes indicates that the high-latitude patterned ground is a very active landscape under the present climate conditions. Another example is a population of craters in Olympus Mons caldera, which clearly indicates significant eolian resurfacing at time scales consistent with the recent obliquity changes, which suggests noticeably higher atmospheric pressure in the recent past.
P41B-1362
Fourier Shape Analysis of Simple Crater Morphology: Relating Crater Shape and Target Properties.
We examine the dependence of planimetric crater shape upon geological target materials for > 500 simple impact craters on Mars. In particular, we have applied Fourier shape analysis to characterize the rim outlines of comparatively fresh craters in MOC and HiRISE imagery. This was used to extract a small set of morphometric parameters which capture most of the variation in shape. Target materials for all craters examined were queried from recently-digitized global geologic maps of the Martian surface. In this way, we determine whether craters were formed in lavas, sediments, or other materials. We obtain from this a range and distribution of morphometric parameter values in targets having different patterns of strength heterogeneities. We distinguish primarily between target materials likely to have highly disordered fractures with no directional correlation over large distances (polygonal or columnar joints) and materials likely to have highly ordered fractures with strong correlations in direction spanning large distances (systematic and orthogonal joints related to regional tectonics). We also propose a suite of kinematic models of crater excavation which illustrate possible relationships between the pattern of strength heterogeneities and resulting crater shape, where these exhibit similar distributions for the aforesaid morphometric parameters to what is observed in our data set. We have also mapped the locations of crater populations having shape characteristics that can be associated with structural features clearly expressed in MOC and HiRISE images.
P41B-1363
Size and Spatial Distribution of Rocks in Polygonal Patterned Ground on Mars
Many HiRISE images have resolution approaching 10 cm/pixel. At this resolution, individual rocks on the surface are clearly observable, and we are examining these rock size- and spatial distributions for evidence of soil convection and rock migration, in our pursuit of understanding Martian atmospheric/soil interactions in the present and most recent epoch. Our hypothesis is that in many of the plains on Mars there exists an active layer a few m to 10's of m thick that causes convective patterning and migration of rocks of various size. While the cause of pattern formation and rock migration is widely open to debate (Noe Dobrea et al. Mars Caltech 2007; Mellon et al. JGR in press 2008) and likely involves multiple mechanisms separately or in combination, an important initial task is to obtain data for rock size and spatial distributions in patterned ground, for the purpose of establishing and testing the varying hypotheses. In certain terrain in the northern hemisphere of Mars, rocks are observed to arrange themselves into geometric patterns, in association with topographical deformations reminiscent to patterned ground on Earth. Patterned ground in arctic tundra is known to form when growth and loss of interstitial ice in the active layer of the surface (freeze-thaw) cause net displacements of the soil. These displacements, aggregated over time, transport rocks to and along the surface into the patterns we see (Kessler et al. Science 2003). While freeze-thaw is difficult to sustain on Mars in the present epoch, and seemingly impossible below a few millimeters, it appears that some process is active in the upper meters which creates decameter-scale patterns and moves meter scale rocks and boulders. Small, recently-formed craters are the obvious candidate in our search for clues. Polygonal patterned ground is found to cover and consume many small craters in the northern hemisphere of Mars. HiRISE images show polygonal patterned craters with much higher rock count density than polygonal patterned terrain in non- cratered regions. Impacts may be responsible for the source and initial distribution of rocks that polygonal patterned ground now act upon. Furthermore, the geometric patterns themselves may be influenced due to the quantity of rocks on the surface. Here we report two findings: (1) the distribution of rocks on the surface with distance from the center of each crater identified that is in the process of being consumed by patterning, and (2) the smallest craters identified to date in various types of patterned ground.
P41B-1364
Rock Abundances at MSL Candidate Landing Sites using HRSC Stereo and HiRISE Images
Landing spacecraft safely on Mars is a fundamental concern for mission planning. One recognized hazard is the presence of boulders large enough to tilt or damage the lander. We investigate the safety of the candidate landing sites for the MSL rover, set to launch in 2009, by calculating the abundance and spatial distribution of large (>1.5 m) rocks using Mars Explorer HRSC stereo imagery. Most satellite-based cameras are unable to resolve rocks down to 1.5 m diameter, except for the HiRISE camera (aboard the Mars Reconnaissance Orbiter), and HiRISE has not provided complete coverage of the candidate sites. We utilize HRSC stereo pairs which do offer complete coverage of landing sites. Because of the lower resolution (approx. 20 m/pixel) of HRSC, it is not possible to map all dangerous boulders directly. Therefore, we use an indirect method of measuring surface roughness, based on an optical two-look system and calibrated by HiRISE boulder counts where available. The two-look approach has been previously used and validated at the Phoenix and Spirit rover landing sites. It assesses surface roughness at sub-pixel scales by ratioing HRSC images taken at different angles relative to the sun and ground. The brightness of rough surfaces is greater when looking away from the sun, because even unresolved shadows from rocks are all or partly masked by the rocks themselves. Smooth surfaces show much less brightness difference. The ratio values are converted to rock abundances by calibration with automated rock counts from co-registered HiRISE images. From this, we are able to produce rock abundance maps and rank the candidate sites according to relative safety and probability of encountering large and hazardous rocks. In doing so, we hope to aid in the decision to choose the safest possible landing site for MSL.
P41B-1365
Signs of aqueous activity in Gale Crater, Mars as viewed by Mars Reconnaissance Orbiter
Gale Crater represents a unique environment with one of the largest free-standing exposures of layered deposits in the equatorial region of Mars. Thick, laterally extensive sedimentary sequences with fine layering generally require an extended period of time to form, and therefore they capture rhythmic, cyclical, or episodic environmental changes that occurred in their depositional settings while they were forming. Using High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) images, we have mapped and characterized the attitude and bedding geometry of layers exposed in the central mound in order to place constraints upon their mode of origin and evolution. Measurements of layers in the lower mound sequence from stereo pairs of HiRISE images reveal meter-scale to decameter scale layers that are near- horizontal. Layers in the upper mound, in contrast, have inclinations up to 15° and appear to have been deposited on an inclined erosional unconformity (previously identified in MOC images [1]). CRISM spectra indicate the presence of Mg and Fe phyllosilicates in some of the lower mound layers, and a sulfate- bearing layer has been identified immediately above the phyllosilicate-bearing layers. Gale Crater may represent an ancient lacustrine setting, although one that is capped off by eolian-deposited sediments that may be genetically related to the Medusae Fossae Formation (MFF). Fluvial channels that are sourced from the upper mound layers indicate a possible volatile-component in these presumably eolian deposits. Upcoming observations by the SHARAD radar instrument on MRO may provide further insight into the internal structure of the layered deposits and assess their possible connection to the MFF. [1] Malin, M.C. and K.E. Edgett (2000) Science, 290, 1927-1937.
P41B-1366
Current Results From High-Resolution Structural Mapping in Southwest Candor Chasma
High-resolution topography generated from stereo HiRISE imagery reveals the 10- to 100-meter-scale structure of Light-Toned Layered Deposits (LTLD) in southwest Candor Chasma. The study area is located near the contact of the LTLD and the wall rock and is in an area where chasma-forming normal faults have been previously proposed. Structural mapping is conducted using digital terrain models (DTMs) and orthorectified imagery constructed from two adjacent HiRISE stereo pairs (PSP_001918_1735 / PSP_001984_1735 and PSP_003474_1735 / PSP_003540_1735). These two DTMs are offset in longitude by roughly 3 km. CTX imagery is used to correlate interpretations between DTMs and to place these structural observations in the regional geologic context established by previous studies. Stratigraphic members, consisting of massive to weakly bedded, ~100m thick, layers within the LTLD, are used to draw these correlations between datasets. Populations of crosscutting thrust faults, normal faults and folds are observed in the mapped area. These folds are generally the oldest deformational structures in the study area, followed in age by the normal faults and then the thrust faults. The normal faults measure less than a few km in map length and thus are not sufficiently large, and are also not appropriately located nor oriented, to accommodate the roughly northeast-southwest extension that is required for normal faults that can be attributed to chasma formation. Further, bedding exposed in the local LTLD generally dips toward the center of Candor Chasma, consistent with sediment deposition in a preexisting basin. Independent lines of evidence such as these support a post-chasma age for these LTLD exposed at the surface. Approximately 2 km of conformable stratigraphy is exposed in the study area, and therefore at least several kilometers of the local LTLD were deposited subsequent to any chasma-related normal faulting that may have occurred in this part of Candor Chasma. Rather than being attributed to regional tectonism, the observed deformation appears to be endogenic in nature and may reflect local slumping prior to pervasive erosion and exposure of these LTLD.
P41B-1367
Formation and Tectonic Evolution of Sedimentary Sequences on Mars from HiRISE Stereo Topography
Stereo topography from the High Resolution Imaging Science Experiment (HiRISE) onboard the Mars Reconnaissance Orbiter (MRO) provides a new tool for investigating the stratigraphy of widespread sedimentary deposits, including both their formation and subsequent tectonic evolution. Digital Terrain Models from HiRISE images with a post spacing of 1 meter have been created which enable resolving the topographic signature of sub-meter thickness bedding in layered outcrops. By accounting for tectonic deformation and erosional topography, an accurate stratigraphic record can be recovered. By stratigraphically correcting profiles of either topographic slope or image brightness, we are able to obtain a continuous stratigraphy with a mean sampling interval of order 10 cm. We will discuss the applications of such reconstructions for understanding the formation and evolution of sedimentary rocks on Mars. Reconstructed stratigraphic records are used to compare sedimentary records between adjacent outcrops to examine the lateral persistence of individual beds across layered deposits. In Arabia Terra, bedding is remarkably parallel, and nearly identical stratigraphic columns can be extracted over several kilometers. In other locations such as the deltaic deposits in Eberswalde crater, the stratigraphy is highly variable and difficult to correlate over similar scales. This observation suggests a fundamental difference in lateral depositional scales and formative mechanisms. Time series analysis has been performed to assess potential signals recorded in the sedimentary rocks at several locations around the planet. Through statistical comparison to a red noise spectral background, this technique has confirmed the presence of quasi-periodic stratigraphy at several locations in Arabia Terra, where repetitive bedding scales range from meters to tens of meters. Further, time series analysis has been used to demonstrate the apparent stochastic nature of layering at several sites in other regions of the planet. Where rhythmic sequences have been identified, we consider a connection to periodic orbital variations. HiRISE stereo observations have also revealed the pervasive influence of tectonics in some layered deposits, particularly in Arabia Terra. Correlation of the stratigraphy across zones of faulting has allowed us to quantify sense and magnitude of offset, accommodation by both brittle and ductile modes of deformation, as well as syn- vs. post-depositional timing.
P41B-1368
Comparison of Martian Dust Devil Track Morphologies in Gusev and Russell Craters
Detailed HiRISE images were used to observe the seasonal changes of dust devil tracks in Gusev and Russell craters, focusing on the temporal and morphological differences between the two locations. Seasonal variations in dust devil activity are influenced by topography, sediment supply, altitude, as well as latitudinal variations in the atmospheric dust cycle and local winds. Topographical features, such as the dunes in Russell, enhance convective circulation, thereby playing a key role in dust devil formation. The greater the contrast between surface and air temperatures, the greater the surface heat flux and potential for dust devil activity. The NASA Ames General Circulation Model (GCM) was used to compare predicted wind directions to those determined from inferred scallops of tracks mapped using ArcMap. Observations indicate distinct variations in the density, shape, and size of the tracks during specific seasons. Russell crater tracks are curvilinear and highly sinuous, with widths and lengths ranging from 20-40 m and 340 m to 9 km respectively. Gusev crater tracks are less sinuous, measuring tens of meters wide and 2-4 km long. Tracks in Russell crater are aligned with the northwesterly oriented prevailing wind as predicted by the GCM. The dust devil season in Gusev Crater is much shorter (Ls= 160° - 340°, with only minor activity afterwards) than that of Russell Crater (Ls= 172° - 40°). Peak dust devil frequencies occur sooner at Gusev (Ls 250°) than at Russell crater (Ls 288°). Track densities are greater and more consistent in Gusev crater and are more variable at Russell, particularly during the early part of the season. Possible explanations for the differences in seasonal behavior between the study sites include: (1) average altitudes up to 2000 m higher at Russell crater than at Gusev, resulting in enhanced convective circulation; (2) increased insolation at higher southern latitudes during perihelion; and (3) frost on the dunes delays the start of the dust devil season in Russell crater.
P41B-1369
An MRO View of Transverse Aeolian Ridges on Mars
Transverse Aeolian Ridge (TAR) is the nongenetic term for linear to curvilinear aeolian features on Mars that could result from either dune or ripple formation processes. These features are typically oriented with their crests perpendicular to the dominant wind direction in the area, as indicated by other wind-related features or as would be confined by surrounding topography. Three High Resolution Imaging Science Experiment (HiRISE) images provide important new information about TARs through the increase in spatial resolution of HiRISE as compared to Viking or Mars Orbiter Camera images. TARs on the floor of Ius Chasma show remarkably symmetric profiles with average slopes of about 15 degrees. On the floor of the Gamboa impact crater, in the northern lowlands, dark dunes are surrounded by small TARs that appear to be large ripples comparable to granule ripples on Earth. TARs in the cratered highlands of Terra Sirenum have thin tapered extensions and textured surfaces that suggest these features could be erosional remnants rather than pristine depositional landforms. Context Imager (CTX) views usually fail to resolve details of individual TARs but they do document the local distribution of these features. The new observations generally support a reversing dune origin for TARs with wavelengths greater than 15 m and a granule ripple origin for TARs in the wavelength range of 3 to 15 m. TARs with a wavelength around 15 m can display attributes suggesting either dune or ripple origins. A recently funded Mars Data Analysis Program grant will allow the capabilities of the instruments on the Mars Reconnaissance Orbiter to be applied to many examples of these intriguing features.
P41B-1370
Valles Marineris Dune Fields as Seen From the HiRISE, CTX and THEMIS Cameras
Dune fields on Mars offer an opportunity to investigate the nature of eroded sediments and their interactions with the atmosphere. We examined 20 dune fields in Valles Marineris (VM) from the Mars Global Digital Dune Database [Hayward et al., 2007] to identify significant trends in composition, thermophysical properties, morphology and origin. Dune fields were examined in terms of: slopes, albedo, dust index, thermal inertia and the corresponding derived particle size. We have used image data from the Mars Reconnaissance Orbiter (MRO) instruments CTX [McEwen et al., 2006] and HiRISE [Malin et al., 2007] to establish geologic context for the dune fields, and in particular, to examine their relationships to neighboring geologic units. In general, VM dune fields display greater topographic relief and closer proximity to their inferred source regions than is typical for dune fields elsewhere on Mars. These dunes have a relatively high TES-derived thermal inertia mean value (394 Jm-2K-1 s-1/2, units hereafter assumed), which corresponds to ~1000 μm grains [Pelkey et al., 2001] or very coarse sand sizes. In contrast, typical non-VM dunes have a lower thermal inertia value of ~250, corresponding to ~350 μm grains. To investigate this more closely, high-resolution THEMIS-derived thermal inertia maps were created [Putzig et al., 2004]. CTX and HiRISE visible images revealed that bedrock outcrops are commonly found within dune fields, erroneously elevating the TES thermal inertia values over the ~3x5-km TES footprint. However, even after excluding intra-dune outcrop areas using higher-resolution THEMIS data, several VM dune fields have anomalously high thermal inertia values (>500) compared with non-VM dune fields. It is possible that the high thermal inertia values are indicative of indurated (fossilized) dune surfaces, rather than large individual grain sizes. Coprates Chasma contains a concentration of 6 dune fields both within the main chasm and in depressions to the south. The southern fields are comprised of isolated barchanoid dunes, in close proximity to or atop wall material that has been deposited by mass wasting. In the main chasm, previously unidentified barchans composed of large grain sizes, as inferred from THEMIS thermal inertia, are found in CTX images within spur and gully wall units 2-3 km above the canyon floor. TES spectrum of these dunes indicates a basaltic composition, suggesting that the nearby wall units, also thought to be of a basaltic composition [McEwen et al., 1999], could be the source of the dune sediments. Future MRO observations of this area may resolve whether these dune sediments are locally derived. Ganges Chasma has the highest concentration of dunes in VM, including the largest (~6000 km2) non-polar dune field on Mars. These dunes are found surrounding the sulfate-bearing Ganges Mensa and other layered deposits. In one example, a light-toned yardang containing CRISM-detected hydrated sulfates [Pelkey et al., 2007] has shed fans of fine-grained material, contributing sediment to the area. Dune slipface orientation would suggest a dominant wind direction blowing to the west at the last time of dunes activity. This corresponds with the more recent deposit of lighter-toned material down-wind and atop the dark-toned sand sheets, as observed in HiRISE and THEMIS thermal inertia images. These lighter-toned materials, inferred to be composed of sulfate grains (~350 μm), form bright ripples which gradually disappear away from the yardang. Whether these sulfates constitute a significant percentage of the dune composition is currently under investigation.
P41B-1371
Multi-spacecraft synergy with MEX HRSC and MRO SHARAD: Light-Toned Deposits in crater bulges
Light-toned deposits (LTDs) occur extensively on Mars. Several of them have been imaged by the Mars Express (MEX) High Resolution Stereo Camera (HRSC), adding 3-D information to the available large extent of datasets from several missions. (e.g. MGS, Odyssey, MRO). Although LTDs are often located in steep and rough areas (e.g. Valles Marineris, chaotic terrains), the use of HRSC high-resolution Digital Elevation Models (DEMs) greatly helps the analysis of ground penetrating radars such as Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD). We are concentrating on various craters with bulges in Arabia Terra (e.g. Crommelin) We built a Digital Elevation Model (175 m/pixel) covering the entire area or Crommelin crater and its neighbor in order to better simulate the surface clutter. LTDs in Crommelin crater have a thickness ranging from 1.5 to 2 km, and the crater just south of Crommelin shows comparable thickness. The deposits are finely stratified and they are affected by a variable amount of erosion. The morphology and structure of the deposits cropping out within the craters show a complex interplay between primary deposition, possible deformation and erosion, increasing the spatial variability of LTDs surface expression. Therefore the surface geology of Crommelin is rather complex, which suggests that its subsurface could be comparably complex. The relatively close distance between these LTDs and Meridiani Planum, which is characterized by the presence of LTDs that share several features with those in Crommelin offers the possibility of a comparison, not only on the surface geology and geomorphology of the two areas, but also on their subsurface radar expression, possibly useful to draw comparisons and distinctions between them. We plan to extend the synergic observation campaign to further areas where extensive (and relatively rough/steep) LTDs are present: this include other crater bulges, such as Gale, close to the dichotomy boundary, and few case studies of LTDs in theValles Marineris canyon system. Although the choice of targets is challenging, the use of high resolution DEMs will hopefully provide better chances to detect subsurface echoes.
P41B-1372
Dielectric Mapping of Bulk Polar Ices on Mars With SHARAD Radar Data
The SHAllow RADar (SHARAD) is a subsurface sounding instrument aboard the NASA's Mars Reconnaissance Orbiter (MRO) spacecraft. The main SHARAD scientific objectives are to map the underground distribution of water over the planet as well as to seek buried geological structures in order to understand the formation of the superficial Martian landscape. SHARAD is working at a 20 MHz central frequency with a 10 MHz bandwidth. Its penetration depth (i.e. the attenuation of the signal) depends of the dielectric properties of the sounded material; typically more than 1 km in water ice with ~7 m of vertical resolution. The Martian polar layered deposits (NPLD) are the largest reservoir of water on the surface. The physical properties of the polar ices is one the main unresolved questions in Martian polar science. In particular, accurate estimation of the dielectric properties is important since it is interrelated to the impurities contamination of the ice and consequently to its rheology; whereas the spatial distribution of the impurities is linked to the interaction of the NPLD with the Martian climate. Moreover, it is also a significant contribution for comparative planetology with terrestrial caps since the Martian ice has the same structure than on Earth (Ice Ih), but accumulated under an extreme planetary environment (mean surface temperature and pressure of 155 K and 0.008 bar respectively). We will present a study over the Gemina Lingula region where the bedrock of the NPLD is imaged by SHARAD. From this, it has been possible to map the dielectric properties of the bulk ice. A data set of 140,000 SHARAD pulses was used. The maps of the dielectric constant and the loss tangent of the ice will be presented. Both properties have a Gaussian distribution giving accurate results. The pick values are consistent with an extremely pure ice. The loss tangent map highlights the spatial distribution of the impurities. A drop of the dielectric constant along Chasma Boreale could be explained by a brutal 250-meter uplift of the base of the NPLD corresponding to an extent of the basal unit below Gemina Lingula.
P41B-1373
Using SHARAD radar soundings to evaluate the origin of martian gullies and pingos
Gullies are some of the geologically youngest features on Mars (< 10 Myr), and their morphology, defined normally by alcove-channel-aprons, led instantly to the hypothesis of formation by the action of fluid water. Hence, gullies might offer an opportunity to constrain the recent history of water on the planet. Different models propose a variety of mechanisms to produce the liquid, with sources generally falling into either surficial (mantling) deposits or ground water/ice. The SHARAD sounding radar aboard the Mars Reconnaissance Orbiter, with a ~10 m subsurface vertical resolution, may have a strong role to play in testing the water hypothesis for the origin of gullies and in characterizing the source of the fluid. We have surveyed radar data for nearly 50 gullies, which occur predominantly at mid-latitudes on both hemispheres of Mars. In most of these cases, strong subsurface reflectors are not observed. In the few cases where strong subsurface reflections are observed, they possibly relate to volcanic units (e.g. Amazonis and Arcadia Planitiae). Weak, near-surface (< 200 m) reflectors appear to be present at some but not all gullies. We are also examining another type of feature postulated to arise from subsurface water/ice activity: pingos. These are nearly circular mounds hundreds of meters across and tens of meters high that are characterized by fractures or collapsed pits on their crests. On Earth, pressurization at depth leads to injection and subsequent freezing of water into the shallow subsurface. The uplifted layer of permafrost cracks as the mound grows and collapse pits form over time due to ice loss. As in the case of gullies, weak shallow subsurface reflections are present in some of the pingo occurrences. Up to this point, however, our limited survey of gullies and pingos is not supportive of shallow aquifers, as the high dielectric constant of liquid water would produce strong reflections, according to simple propagation models. The weak reflections observed are more consistent with variations in lithic compositions and perhaps an ice component. Interpretation of reflections is non-unique because radar is susceptible to changes in dielectric properties that are not due exclusively to the presence of water/ice. The most robust inferences will be derived from the analysis of datasets from multiple remote sensing techniques and geologic settings. This limited survey represents our initial effort in utilizing SHARAD data to address the question on the origin of gullies and pingos, as radar coverage of these small features is still growing.
P41B-1374
Subsurface Structure of Planum Boreum on Mars from Shallow Radar (SHARAD) Soundings
We have mapped the subsurface structure beneath Planum Boreum using results from the Shallow Radar
(SHARAD) instrument, which has acquired sounding observations on more than 1000 orbital passes across
the north polar region of Mars since the beginning of its primary science mission in November of 2006. Two-
dimensional profiles beneath the instrument's ground track show a series of returns corresponding to
dielectric contrasts in the subsurface to depths of 2 to 3 km. Using interactive subsurface-data interpretation
software, we have mapped packets of layers within the North Polar Layered Deposits (NPLD) in three
dimensions, from the surface down to returns from underlying materials, which are seen as either a diffusely
reflective zone (DRZ) or a more coherent basal reflection. The latter presumably represents an extension of
the Early Amazonian Vastitas Borealis Interior Unit (Tanaka et al. 2008, Icarus 196, 318) under the NPLD.
The DRZ likely corresponds to a Basal Unit identified previously using surface imagery (Byrne and Murray
2002, JGR 107 E6, 5044) and later mapped as the Rupes Tenuis and Planum Boreum cavi units (Tanaka et
al. 2008). This radar unit extends under most---but not all---of the main lobe of the NPLD, into Olympia
Planum, and also across Chasma Boreale and partly under the Gemina Lingula lobe. These radar results
suggest a revised boundary for the Basal Unit that has important implications for its association with the
emplacement of Chasma Boreale.
Within the NPLD, four radar units, consisting of alternating packets of strongly reflective layers and quiescent
zones that may represent nearly pure water ice, extend into both lobes of the deposits. A fifth radar unit is
isolated to eastern Gemina Lingula and occurs between the lower two of the regional units, pinching out
below the topographic saddle between the two lobes. The layering associated with the radar units is thought
to be the result of variations in dust content within water-ice deposits that are driven by climate cycles
(Phillips et al. 2008, Science 320, 1182). With the exception of areas immediately below crosscutting
troughs, the thickness of the NPLD is remarkably uniform across both lobes, tapering toward the edges, with
most of the topographic difference between the two lobes explained by the Basal Unit residing predominantly
under only the main lobe. Also, while there are indications of a few angular unconformities, the layering
within and between radar units is typically smoothly varying and quasi-parallel, which contrasts with the more
heterogeneous layering seen in the South Polar Layered Deposits (e.g., Seu et al. 2007, Science 317,
1715). These characteristics suggest relatively uniform rates of deposition and erosion across Planum
Boreum throughout the history of the NPLD. SHARAD was provided by the Italian Space Agency (ASI) to
NASA for the Mars Reconnaissance Orbiter mission.
http://nathaniel.putzig.com/research/agu2008
P41B-1375
Implications of Large Elastic Thicknesses for the Composition and Current Thermal State of Mars
The elastic lithosphere thickness at the Martian north polar cap has recently been constrained using radar sounding data obtained by SHARAD, the shallow radar onboard the Mars Reconnaissance Orbiter. Analysis of the SHARAD radargrams showed that the amount of deflection caused by ice loading at the polar caps is negligible - less than 100 m. Quantitative analysis yielded a lower bound on the elastic lithosphere thickness Te of 300 km, a value twice as large as previous estimates from theoretical considerations and flexure studies. Such large elastic thicknesses are only compatible with the planet's thermal evolution if the planetary interior is relatively cold and this could have direct bearing on the admissible amount of radioactive elements in the Martian interior. On the other hand, if the concentration of heat producing elements in the Martian interior is indeed reduced, the resulting low interior temperatures could possibly inhibit partial mantle melting and magmatism. However, geological evidence suggests that Mars has been volcanically active in the recent past. We have investigated the Martian thermal evolution and identified models which are consistent with a present day elastic thickness in excess of 300 km. We find that a wet mantle rheology is best compatible with the observed elastic thicknesses, but in this case the bulk concentration of heat producing elements in the silicate fraction cannot exceed 50 % of the chondritic concentration if 50 % of the radioacitve elements are concentrated in the crust. Furthermore, due to the efficient cooling of the planet for a wet mantle rheology, recent volcanism can only be explained by hydrous mantle melting. This requires the mantle water content to exceed 1500 ppm and although this is within the range reported for the shergottite parent magmas, it is certainly on the boundary of the plausible parameter range. If a dry mantle rheology is assumed, bulk Mars does not need to be sub-chondritic, but at least 70 % of the radiogenic elements need to be concentrated in the crust to be consistent with the large elastic thicknesses. For a dry mantle, recent volcanism could be driven by decompression melting in the heads of strong mantle plumes which are present in numerical simulations of mantle convection if the viscosity is strongly pressure dependent or endothermic phase transitions are present near the core-mantle boundary.
P41B-1376
New Solutions for the Mars Static and Temporal Gravity Field using the Mars Reconnaissance Orbiter
The Mars Reconnaissance Orbiter entered low-altitude orbit about Mars in August 2006, and has since completed one Mars year in the mapping orbit. The orbit, with a periapsis of 255 km and and apoapsis at 320 km, allows for a higher resolution of the Mars gravity field, compared to earlier models based on data from combinations of Mars Global Surveyor and Mars Odyssey, which had periapsis altitudes of 370 to 390 km. In this paper, we present the new solutions developed at NASA GSFC up to 100x100 in spherical harmonics using X band tracking data from NASA's Deep Space Network, combining data from all three missions. The new solutions extend the resolution of the static field –to about l=85 (compared with l=65 to 72 based on MGS or MGS+Odyssey alone), and the correlation with topography is also improved compared to previous solutions, with values of 0.6 at l=80. We note that the resolution is not spatially uniform due to the eccentricity of the MRO orbit, and the resolution of the static field is slightly greater over the Southern Hemisphere, than the Northern Hemisphere. Using data from Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter, we are also able to construct a time history of the temporal variations in the low degree Mars gravity field for over five Mars - from March 1998 to the present, reflecting mass transport in the Mars system.
P41B-1377
Improvement of the Mass Determination of Both Martian Moons Using MEX, MGS, ODY and MRO Tracking Data.
Two recent studies using X-band Doppler and range tracking data of Mars Global Surveyor (MGS), Odyssey (ODY) and Mars Express (MEX) have provided new solutions of the mass of both Martian moons. In this present study, we re-estimate these solutions by combining MGS, ODY and MEX tracking data over a longer time span, including the MEX-Phobos flyby at the closest approach distance of 273 km occurred on July 17th 2008 (see also the AGU presentation by Andert et al.). We used the GINS software, developed by CNES and further adapted at ROB for planetary geodesy applications, to compute the Precise Orbit Determination (POD) of each spacecraft, and to estimate a new GM value for both moons. We found a value of 7.15 +/- 0.02 105 m3/s2 and of 1.12 +/- 0.5 105 m3/s2 for Phobos and Deimos, respectively (uncertainty corresponds to 10 times the formal error). Our new values are in agreement with those estimated recently from the re- processing of the Viking flybys tracking data, at a comparable precision (considering formal error) for Phobos, but at a poorer precision for Deimos, caused by the weaker signal-to-noise-ratio from this more distant moon. We also computed the bulk density of both moons using their volume estimate and associated uncertainty (Thomas, et al., Icarus, 105, p. 326, 1993). We found values of 1860 +/- 60 kg/m3 and 1650 +/- 300 kg/m3 for Phobos and Deimos, respectively, which suggest substantial porosity inside these bodies (10 to 50 percent of the volume, depending on the chosen meteorite analog). New data sets from Mars Reconnaissance Orbiter (MRO) are now available, and may be used to further improve the mass determination. In addition, only MEX is able to perform close encounters with Phobos. Future encounters at distances as close as 100 km will allow the estimation of the second-order gravity field coefficients of Phobos, which may be used as a constraint on the internal mass distribution.
P41B-1378
Characterizing Mafic and Clay Components in Libya Montes, Mars, using Automated Gaussian Modeling of Spectral Features found in MRO/CRISM Images
Aqueous processes have been inferred at the Libya Montes rim/terrace complex of the southern Isidis Basin due to the dense concentration of valley networks [1]. Coordinated CRISM-HiRISE investigations of this region characterized discrete units of ancient phyllosilicate deposits covered by an olivine-rich material and a pyroxene caprock [2]. CRISM mapping data show minor phyllosilicate abundances widespread throughout the Southern Highlands [3], which are dominated by low-Ca pyroxene bearing material [4,5]. The layered capping materials have been suggested to be remnant eroded lavas that infiltrated the Libya Montes and Isidis Basin from Syrtis Major [6]. Our current study involves detailed characterization of the minerals present at Libya Montes through implementation of newly created data analysis tools. We have developed an automated procedure for modeling spectral features using Gaussians. This study builds on the modified Gaussian model (MGM) technique [7] that has been successfully applied to hyperspectral analyses of Mars [8]. However, these models require user-selected initial parameters. Initial efforts in automating this modeling process for CRISM hyperspectral images has been successful, but is limited to the study of pyroxene bands at 2 μm. We have extended this technique to model spectral features using Gaussians from 0.5-2.6 μm. We have developed an automatic parameter initialization step based on the features of the spectrum being modeled in order to further automate MGM modeling. Initial results have successfully yielded automatically generated starting parameters for the MGM with low residuals following optimization. This new algorithm is currently being applied towards analyses of hyperspectral images in Libya Montes. Specifically, olivine, pyroxene, and phyllosilicate deposits are being modeled and classified by composition in 13 CRISM images using the automatic parameter initialization with the MGM. We are investigating and mapping mineral compositions in this region and comparing them to those observed for the greater Isidis region. References [1]Crumpler, L. S., and K. L. Tanaka (2003) J. Geophys. Res., 108, DOI: 8010.1029/2002JE002040. [2]Bishop, J. L., et al. (2007) 7th Int'l Mars Conf. [3]Mustard, J. F., et al. (2008) Nature, 454, 07305. [4]Bibring, J.-P., et al. (2005) Science, 307,1576. [5]Mustard, J. F., et al.(2005) Science, 307, 1594. [6]Tornabene, et al. (2008) J. Geophys. Res., DOI:10.1029/2007JE002988, in press. [7]Sunshine, J. M., et al. (1990) J. Geophys. Res., 95, 6955. [8]Mustard, J., and J. M. Sunshine (1995) Science, 267, 1623.
P41B-1379
Mars Polar Cold Spots: Modeling and Observation With MRO/Mars Climate Sounder
Using radiometric observations from the Mars Climate Sounder (MCS) during southern winter, we report evidence that polar cold spots (Tb < 148 K) at Mars' south pole are correlated with tropospheric, optically thick clouds. In limb observations coincident with nadir measurements of low brightness temperature regions, clouds appear as radiation sources at typical altitudes of 20 - 30 km. Using statistical methods, we confirm that transient cold spots appearing during polar night are correlated with the presence of clouds above the surface, which show spectral features consistent with CO2 ice. Areas where cold spots persist over the entire winter season are also characterized by persistent clouds, the thickness of which is inversely correlated with nadir brightness temperature. We model the Mars south polar night atmosphere with a δ-Eddington radiative transfer code, including spherical geometry for limb and nadir viewing. By forward-modeling a multi-layered atmosphere including clouds, we attempt to reproduce the MCS radiance data. Emission, absorption, and scattering by gas, dust, and ice (CO2 and H2O) clouds are all included in the model. Surface emissivity, as well as cloud optical thickness, composition, and altitude are constrained by comparison of the MCS data with the model output. Theories on Cold Spot Origins: Three primary explanations for "cold spots" have been explored previously: (1) Localized depletion of atmospheric CO2, (2) Non-unit emissivity of surface CO2, and (3) Carbon dioxide clouds. Hess (JGR, 1979) demonstrated that (1) is feasible only under special circumstances due to dynamical instability. The spectral properties of CO2 frost and clouds are similar such that it has been difficult to discriminate between (2) and (3) as viable hypotheses. Our novel approach uses radiance measurements of both the atmospheric limb and nadir to better constrain the origin of polar cold spots. Observations of Cold Spots: The Mars Climate Sounder onboard the Mars Reconnaissance Orbiter records nadir and limb radiance measurements in nine spectral channels, from ~1-40 μm wavelength. The observations used in this study spanned a range of solar longitude, Ls = 111-148°, i.e., southern winter. Radiance is converted to equivalent brightness temperature using the radiometric response of each filter and the Planck function. Observations are binned by latitude and longitude (1-5° bins) and Ls, and nadir brightness temperatures are compared to measured limb brightness temperature profiles. MCS Results: We find a statistical correlation (R ~0.55, p < 10-14) between cold spots and clouds (defined by their high T22). Spectral features in the infrared, specifically absorption at the wings of the 15-μm (667 cm-1) CO2 bending fundamental and a broad feature from 20 - 35 μm (300 - 500 cm-1) attributable to scattering by CO2 ice grains, suggest the clouds are composed of carbon dioxide ice. Modeling: Monochromatic fluxes are calculated using a custom δ-Eddington code for an N- layer atmosphere and convolved to the MCS filter response. Cloud height and composition are varied to produce an emission spectrum comparable to the MCS observations. We forward-model the polar atmosphere iteratively to constrain cloud properties, surface emissivity, and dust content. Our results may have important implications for Mars' polar heat balance and climate.
P41B-1380
Martian Airborne Dust during the Clear Season: The View from Mars Climate Sounder
Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO) has observed the atmosphere of Mars for more than one martian year, providing data sufficient to generate over seven hundred thousand vertical profile retrievals of pressure, temperature, dust opacity, and water ice opacity, using the current retrieval algorithm. Most of these retrievals come from "the clear season" of Mars, during which regional and global dust storms do not occur and the general weather patterns of the planet are thought to vary little interannually. From these retrievals, we have compiled a mostly complete record of the meridional and vertical variability of dust during the clear season (Ls=0°-170°) that has some information about zonal variability as well. Three unusual structures have been identified in the vertical distribution of dust: (1) an enhancement in the apparent mass mixing ratio of dust at 10-25 km in the tropics; (2) a similar region of mass mixing ratio enhancement in the northern extratropics and near the pole observed during Ls=85°- 150°; (3) a region of extremely dust-clear air in the southern extratropics during almost the entirety of the period. This presentation will describe ongoing work to understand the implications of these unusual features for the lifting and transport of dust and the forcing and behavior of large-scale meridional circulations.