P33A-1011
HiRISE Observations of the Mars North Polar Layered Deposits
We present the latest results of our observations of the martian north polar layered deposits at high resolution using MRO HiRISE. The NPLD can be divided into two different sections, based on the presence or absence of fracturing. The lower section is ubiquitously fractured, for unknown reasons, on the portions exposed in marginal scarps. The upper section lacks this extensive fracturing and comprises most of what is exposed in trough walls. Fracturing of the lower section may be pervasive, given the change in radar characteristics observed by SHARAD at approximately the boundary between the upper and lower sections. Preliminary analysis of HiRISE color data within the upper section shows that color differences between layers may exist. However, most images show these layers to be covered by frost and dust, masking their inherent albedo and making it difficult to distinguish individual layers from one another. This dust and frost cover hides thin layers; HiRISE has only observed layers at scales down to the limit of resolution in places where the slopes are relatively steep, indicating that all layers are being resolved on more typical slopes. The texture and morphology of a layer appears highly dependant on the style and extent of erosion, which, in turn, depends on the location of the NPLD outcrop. For example, by comparison with nearby MOC images used for previous layer correlations, we have identified the well-known "marker bed" in several HiRISE images; its only defining characteristic appears to be its resistance to erosion. Within one HiRISE image, we have identified several other layers that closely resemble the appearance of the original marker bed in that image, possibly indicating a major, repeating climate signal. No definitive evidence of ductile layer deformation has been discovered, other than possible soft-sediment deformation of the overlying dust lag. Examination of HIRISE images containing previously mapped faults confirms that these features are the result of brittle deformation. HiRISE observations of the north polar basal unit show that deposit to be interbedded sand and fractured ice. We suggest that the lower, fractured section of the NPLD was deposited when the sand supply feeding parts of the basal unit was exhausted (and/or average obliquity decreased) and that the upper section was emplaced during mid-range average obliquity conditions. An unconformity likely exists between the two sections, and the timing of the fracturing is unknown
P33A-1012
A Test for Past Ice Flow in the Martian North Polar Layered Deposits Based on Observed Radar Stratigraphy
Recent analysis of surface topography on the North Polar Layered Deposits (NPLD), specifically Gemina Lingula, has yielded evidence for a period of near-balance between ice flow and surface mass fluxes, followed by a period of trough formation. In this scenario, ice flow during the period of balance acted to equilibrate accumulation (positive surface mass balance) at higher elevations with ablation (negative surface mass balance) at lower elevations. Locations where the surface mass balance changed sign are predicted by fitting an ice-flow model to present-day, inter-trough topography (Winebrenner et al., submitted). Assuming that internal layers are isochrones, this mass-balance pattern implies that layers will intersect the surface only in the ablation zone. Here we test this prediction using observations of radar stratigraphy within Gemina Lingula made with the Shallow Subsurface Radar (SHARAD) aboard the Mars Reconnaissance Orbiter (MRO). More than 100 profiles of radar data over Gemina Lingula have thus far been acquired, along ground tracks determined by orbital parameters rather than possible ice dynamics in the area. We therefore trace layers on many tracks and fit surfaces to layer locations, so as to infer layering along the prospective paths of past flow. We focus initially on the area of Gemina Lingula between its central ridge and Chasma Boreale, where radar observation of internal layering is simpler because high-elevation troughs are rare, and trough geometries at lower elevations are simple relative to those on the equatorward side of the ridge. Our ice-flow model accurately fits high-elevation topography, but places the ice margin during the period of balance well inside what now is Chasma Boreale. We thus far observe no radar layers near the ridge that intersect the ice surface, consistent with our inference of accumulation in that region during the period of balance.
P33A-1013
Exposed Ice in the Northern Mid-Latitudes of Mars
Ice-Rich Layer: Polygonal features with dimensions of approximately 100 meters, bounded by cracks, are commonly observed on the martian northern plains. These features are generally attributed to thermal cracking of ice-rich sediments, in direct analogy to polygons in terrestrial polar regions [1,2]. We mapped polygons in the northern mid-latitudes (30 to 65 N) using MOC and HiRISE images [3]. Polygons are scattered across the northern plains, with a particular concentration in western Utopia Planitia. This region largely overlaps the Late Amazonian Astapus Colles unit, characterized by polygonal terrain and nested pits consistent with periglacial and thermokarst origins [4]. Bright and Dark Polygonal Cracks: An examination of all MOC images (1997 through 2003) covering the study area demonstrated that, at latitudes of 55 to 65 N, most of the imaged polygons show bright bounding cracks. We interpret these bright cracks as exposed ice. Between 40 and 55 N, most of the imaged polygons show dark bounding cracks [5]. These are interpreted as polygons from which the exposed ice has been removed by sublimation. The long-term stability limit for exposed ice, even in deep cracks, apparently lies near 55 N. Bright and Dark Spots: Many HiRISE and MOC frames showing polygons in the northern plains also show small numbers of bright and dark spots, particularly in western Utopia Planitia. Many of the spots are closely associated with collapse features suggestive of thermokarst. The spots range from tens to approximately 100 meters in diameter. The bright spots are interpreted as exposed ice, due to their prevalence on terrain mapped as ice rich. The dark spots are interpreted as former bright spots, which have darkened as the exposed ice is lost by sublimation. The bright spots may be the martian equivalents of pingos, ice-cored mounds found in periglacial regions on Earth [6,7,8,9, 10]. Terrestrial pingos from which the ice core has melted often collapse to form depressions similar to the martian dark spots. Future Observations: The SHARAD radar should be able to confirm the presence and measure the depth of the interpreted ice-rich layer that forms the Astapus Colles unit. If this layer is confirmed it will strengthen the interpretation of bright polygon cracks and bright spots as exposed ice. HiRISE images of the northern plains are showing unprecedented details of the polygonal cracks. Future HiRISE images that include bright spots, compared to MOC images taken years earlier, will illustrate the temporal stability of the spots. The CRISM spectrometer, with multiple spectral bands and a spatial resolution around 20 meters, should allow mineralogical identification of the material exposed in the polygonal bounding cracks and in the bright spots. References: [1] Seibert N.M. and Kargel J.S. (2001) GRL, 28, 899-902. [2] Mangold N. et al. (2004) JGR, 109, E08001. [3] Kanner L.C. et al. (2004) LPS XXXV, Abstract #1982. [4] Tanaka K.L. et al. (2005) Map 2888, USGS. [5] Kanner L. (2004) Geology Comprehensives Paper, Carleton College (unpublished). [6] Soare R.J. et al. (2005) Icarus, 174, 373-382. [7] Osinski G.R. and Soare R.J. (2007) LPS XXXVIII, Abstract #1609. [8] Ferrand W.H. and Lane M.D. (2007) LPS XXXVIII, Abstract #1972. [9] Dundas C.M. et al. (2007) 7th Intl. Conf. Mars, Abstract #3214. [10] Allen C.C. and Kanner L.C. (2007) 7th Int. Conf. Mars, Abstract #3065.
P33A-1014
Dry avalanches at seasonal defrosting as a process for current gullies activity
Recent gullies on Mars, discovered by the MOC/MGS instrument are observed on the wallslopes of the mid latitude regions with a preferential orientations on poleward facing slopes. They might sign the presence of fluid flows, likely involving liquid water, in a recent past, or even currently as shown by the recent gullies activity. Our study is an hirise image that was obtained at Ls=145° and it is located at 38.9°S, 196E. The image shows some recent gullies that erode inside the wallslopes of a fresh impact crater. A close-up on the alcoves is possible thanks to the very good spatial resolution (25 cm/pixel). Frost, visible by the very bright tone, is locally present on the image, especially inside the gullied alcoves and on the steepest part of the wallslopes. Our key observation consists of the presence of new streaks formed within the frost blanket. These streaks are visible from their lower albedo in a strong contrast with the surrounding frost layers. We infer that these streaks form over the frost, or cross this frost. Streaks could be highlighted either because the frost is transparent, or because the frost has been sublimated on the streaks only but this is not consistent with most observations, especially because these streaks are also visible on the part of terrain not covered by frost. This observation of streaks, formed over seasonal frost, signs a current activity of mass wasting inside gullies alcove. The streaks shapes appear as typical of granular flows, different from viscous or liquid flows involving liquid water. Thus, the observed flows are small avalanches that are likely dry. They form currently as a consequence of the defrosting by destabilization of the debris aprons material. We exclude liquid water as being responsible of the current gullies activity due to lack of adequate thermodynamic conditions shortly after defrosting and lack of geomorphic landforms typical of water-rich material. Nevertheless, liquid water is not excluded for a past activity that created the large channelled flows, different from the small avalanches observed.
P33A-1015
Late Amazonian Glaciation at the Dichotomy Boundary on Mars: New Evidence from MRO for Glacial Thickness Maxima and Multiple Glacial Phases
Lineated valley fill (LVF) in fretted valleys at the dichotomy boundary has been interpreted as glacial in origin. Unknown are 1) the original thickness of the glacier ice, 2) the amount of ice-surface lowering, through sublimation, retreat and ice loss, to its presently observed level, and 3) whether there were multiple periods of glaciation. We address these questions through analysis of an integrated LVF glacial landsystem along the dichotomy boundary at the contact of Protonilus Mensae and Coloe Fossae (41°N, 54.5°E). Analysis of MRO CTX and HiRISE data confirm the presence of a 5-km wide loop-like lobe emanating from LVF that terminates in an elevated box canyon, and the remnant flow patterns do not correlate with present-day topography, indicating that a substantially increased volume of ice was necessary to alter the flow regime and deposit the observed features. Additionally, the upper limit of glacial deposits at this site is topographically consistent with other regional glacial deposits in Coloe Fossae and Protonilus Mensae. The elevation difference between the upper limit of glacial deposits and the current surface of the LVF at the study site is ~920 meters. We interpret this difference to reflect the minimum amount of ice-surface lowering of the valley glacier system during retreat. Consistent with a general lowering of the ice surface are multiple moraines and/or trimlines, and changes in LVF flow patterns, including local flow reversals, as the ice retreated and decreased in thickness. The clear superposition of several lobes out onto the current surface of the LVF indicates that a less extensive phase of glaciation followed the lowering of the valley glacial landsystem. These data suggest that the major Late Amazonian glaciation that produced LVF in this region involved significantly larger amounts of glacier ice than previously thought, and that subsequent, less extensive local tributary glaciation followed.
P33A-1016
Mars: South Polar Spring Recession as observed by CRISM
We are creating a seasonal mosaic of CRISM multispectral data for each MSP (multispectral) image obtained in the south polar region (all regions poleward of 55 deg S) since the start of the MRO primary mission. This effort compliments high resolution targeted observations of the South Polar Cryptic Region (Titus et al., 2007) and OMEGA observations of the south polar cap during spring recession (Langevin et al., 2007). Langevin et al. presented similar maps at 1-10km/pixel - CRISM multispectral observations used here have a nominal resolution of 200m (Seelos, 2007). A similar CRISM multispectral mosaic has been constructed for the north polar region by Seelos et al. (Seelos et al., 2007). Science objectives of this investigation include: 1. High resolution (200m/pixel) multispectral observations of CO2 and H2O seasonal distribution, 2. Observing the point where Cap Recession Observations indicate CO2 has Ultimately Sublimated (CROCUS) line (Kieffer et al., 2000) as a function of solar longitude (ls), 3. Observations of changes in CO2 grain size linked to seasonal brightening of the polar high latitude areas (Langevin et al., 2007), and 200m/pixel observations of dust cover linked to cold jets in the Cyptic Region (Kieffer et al., 2000; Langevin et al., 2007), 4. Observations of small water ice patches in late southern spring (Langevin et al., 2007), 5. Development of climate-based atmospheric correction for use with multispectral data based on TES climatology and DISORT. We will report on the progress of our investigation and present high resolution multispectral mosaics of the south polar cap during spring recession and compare them to data from previous years. References Kieffer, H., et al., 2000. Mars south polar spring and summer behavior observed by TES: Seasonal cap evolution controlled by frost grain size. JGR 105, 9653-9700. Langevin, Y., et al., 2007. Observations of the south seasonal cap of Mars during recession in 2004-2006 by the OMEGA visible/near-infrared imaging spectrometer on board Mars Express. JGR 112, 10.1029/2006JE002841. Seelos, F. P., et al., 2007. CRISM Multispectral Survey Campaign - Status and Initial Mosaics. LPSC XXXVIII, Abstract 2336. Titus, T. N., et al., 2007. MRO CRISM observations of the South Polar Cryptic Region. 7th International Conference on Mars, Abstract 3275. http://abrown.seti.org
P33A-1017
Constraints on the composition of the Martian south polar cap from gravity and topography
The polar caps of Mars have long been acknowledged to be composed of unknown proportions of water ice, solid CO2 (dry ice), and dust. Gravity and topography data are here analyzed over the southern cap to place constraints on its density, and hence composition. Using a localized spectral analysis and the assumption that the polar cap is uncompensated (as attested by data obtained from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) experiment), the density of the volatile-rich south polar layered deposits is constrained to be 1175±55~kg~m-3. A maximum of about 40% dry ice by volume could be sequestered in these deposits if they were completely dust free. Alternatively, if these deposits were completely free of solid CO2, the dust content would be constrained to lie between about 10 and 20% by volume. The bulk thermal conductivity of the polar cap is not significantly affected by these maximum allowable concentrations of dust. However, even if a moderate quantity of solid CO2 were present as horizontal layers, the bulk thermal conductivity of the polar cap would be significantly reduced. Reasonable estimates of the present day heat flow of Mars predict that dry ice beneath the thicker portions of the south polar cap would have melted. Depending on the quantity of solid CO2 in these deposits today, it is even possible that water ice could melt where the cap is thickest. If independent estimates for either the dust or CO2 content of the south polar cap could be obtained, and if MARSIS data could determine whether this polar cap is presently experiencing basal melting or not, it would be possible to use these observations to place tight constraints on the present day heat flow of Mars.
P33A-1018
South Polar Residual Cap of Mars: Features Within, and Models of, MRO HiRISE Data
We report on observations by the High Resolution Imaging Science Experiment (HiRISE) of the south polar residual CO2 cap of Mars and our related modeling efforts. HiRISE is currently acquiring continuous coverage of this deposit at spatial resolutions of up to 25 cm/px in three separate bands. Several previously undetected features in this CO2 landscape have been noted such as networks of linear ridges and some examples of mass wasting at the rims of pits within the ice. The thicker portions of the CO2 ice appear to be composed of roughly 10 layers. Previous MOC observations have shown that features within this landscape are evolving at meters per year. HiRISE observations will resolve the seasonal dependence of this expansion by the end of the Martian year. High resolution color data are currently showing that the seasonal frost is in the process of disappearing over exposures of the layers on the walls of the CO2 mesas. Although their ablation rates determine that they must be mostly composed of CO2 ice their color is much redder than expected which could possibly be due to dust contamination or grain-size effects. We have modeled the evolution of this CO2 landscape and reproduced its evolution as observed by previous spacecraft. These models have also successfully predicted some features visible at HiRISE resolution such as the linear ridges mentioned above. We report on these new observations and how they extend our current models. Model results have already suggested that the thick portions of this CO2 deposit are likely to be 50- 100 Martian year old, implying that the individual layers observed by HiRISE represent periods of time of order a few Martian years. Global dust storms occur with a similar frequency and may be modulating the behavior of the ice cap. http://www.lpl.arizona.edu/people/faculty/byrne.html
P33A-1019
North-South Asymmetry in Martian Crater Slopes
The presence of an extensive ice-rich layer in the near subsurface of the Martian regolith can result in viscous creep responsible for softening craters at mid and high latitudes. The temperature of ground ice will vary spatially within a crater due the effect of slope on the angle of insolation. The temperature at a particular latitude will also vary due to changes in Mars' obliquity. Results from numerical simulations of viscous flow indicate that these temperature variations cause the pole-facing slopes of craters to be systematically steeper than those of equator- facing slopes. Crater slopes should be most asymmetric between 25 and 40 degrees latitude, depending on the thickness of the creeping layer. This slope asymmetry predicted from theoretical simulations of regolith creep is not well developed in observed Martian crater topography. MOLA topography of craters 16 to 40~km in diameter was analyzed for north-south slope asymmetry within five latitude regions ranging from 60\rm{°}S to 60\rm{°}N. Based on the lack of any systematic slope asymmetry observed in the craters, we can place an upper limit of ~1.2~km on the thickness of the creeping layer assuming a volumetric dust content of 70% and an exponentially increasing soil viscosity with depth. If the creeping layer contains relatively clean ice, then the thickness of ice-rich material is limited to ~600~m or less based on our results. The observations also suggest that the thickness of this creeping layer is reduced by a factor of ~2 towards the equator.
P33A-1020
Surface Change Detection From Mars Orbital Imagery
Recently identified surface changes on Mars have provided evidence of water present on the surface within the past decade. These and previously studied changes such as the appearance of new dark slope streaks and dust devil tracks have been identified through the manual process of visually comparing two images taken of a region at different times. This tedious and time-consuming process does not scale to the vast amount of historical Mars data currently available from Viking, Mars Global Surveyor, Mars Odyssey, and Mars Express, and continuing to be collected by Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. Automated change detection methods have yet to be effectively applied on a wide scale to robustly identify significant changes on Mars. One important challenge is how to reliably remove changes in appearance that do not represent actual surface alteration, such as changes in viewing geometry, lighting, and atmospheric effects. We present work on the development of methods to provide rapid identification of surface changes on Mars from orbital imagery. We have focused on methods and image matching techniques which, for the most part, do not require the computation of a pixel-based correspondence between images. Our approach is to identify landmarks and changes based on descriptive statistical models of surface regions. We show examples from two methods. The first method uses an intensity histogram to describe a region. Interesting features are identified using the KL-divergence between regions. We applied this method to 12 MOC (Mars Orbiter Camera) image pairs, in which the later image is known to contain evidence of one or more new dark slope streaks. The dark slope streaks, as well as a variety of other surface features, were identified as landmarks. The second method employs compact covariance descriptors to describe regions. It has been shown to effectively identify regions of surface change while being relatively robust to viewing conditions. We applied this method to 5 MOC images in which new gullies have previously been discovered. In each case, the gully was quickly detected, providing a demonstration that automated change detection algorithms could in principle make similar discoveries, while covering many thousands of images in the time it would take a human to examine only a few. Using automated change detection methods, we can identify surface changes that may otherwise be overlooked in the large Mars data sets that have been archived and are continuing to be collected.