P13A-0969 1340h
Mars 2007 Phoenix Lander: Site Selection and Terrain Analysis
The Phoenix Lander will touch down on the northern plains of Mars in the summer of 2008 to characterize surface and near subsurface materials hypothesized to be enriched in water ice. The landing site will be between the latitudes of 65 and 72 degrees north and below -3500 m elevation to meet entry constraints. In addition, the site will be located where Mars Odyssey Gamma Ray Spectrometer (GRS) data and modeling suggest a relatively thin (less than 20 g/cm$^{2}$) soil cover over ice. The site should also have slopes at the 10 to 1000 meter length scale consistent with favorable radar return during Entry, Decent, and Landing (EDL), modest slopes at the lander scale to avoid adverse spacecraft tilts, and rock abundances comparable to or less that those found at the Viking Lander 2 site in Utopia Planitia. Further, the site should show geomorphic evidence of periglacial activity (e.g. patterned ground). Four broad regions (A: 250 to 270E, B: 120 to 140E, C: 65 to 85E, D: 230 to 250E) have been identified to focus coverage of Mars Orbital Camera (MOC), Thermal Emission Imaging System (THEMIS), and OMEGA instruments. Geomorphic maps and quantitative analyses of landforms and hazards will be presented for these regions, including slope distributions, rock abundances, and mineralogy based on data acquired during the current northern summer season.
P13A-0970 1340h
The Laser Dust Detector - a strategy for in-situ dust detection in planetary ice sheets
Laser Light Scattering (LLS) is a powerful, fast, and non-destructive way of optically interrogating embedded microparticles in solid ice. This is the basis for the design of the Laser Dust Detector (LDD) where laser light is directed via fiber optics into the ice surrounding the probe. If the ice is bubble and dust free the laser light will penetrate meters of ice before the light is attenuated completely. If, however, the laser light encounters dust particles the light will be scattered in all directions, according to the theory of Rayleigh and Mie scattering. Some of the scattered light will make its way into the aperture of the CCD camera and be detected in a digital image. A series of these images are taken as the Cryobot descends into the ice sheet. The images will be acquired in a way to ensure enough overlap between neighboring images to be able to fit them together to get a continuous in-situ mapping of the dust in this local environment of the ice sheet. In the laboratory, this method has been successful, in part, because the particle size distributions in archived ice cores have been very constant in time. Even during a change by a factor of 100 in concentration the size distribution is only slightly changed. Another reason the optical interrogation of dust has been successful is that the overall mineralogy of the dust particles appear to be more or less constant, i.e. the index of refraction of the particles doesn't change much with depth or particle size, an important factor when analyzing the experimental data. The key to obtaining suitable data on the properties of Mars ice caps is in the design of an exploratory polar cap in-situ mission and its instrumentation, of which the optical systems for the visible stratigraphy and dust character are significant. In taking steps to obtain the optical data we begin by noting that the polar cap ice may be clear in the fashion of deep ice sheet ice from Greenland or Antarctica, or it may have sufficient dust and bubbles to be opaque, or somewhere in between. A Cryobot instrumented with a Laser Dust Detector and an additional light source would be able to profile the ice column either by penetrating a rather clear ice sheet or by imaging the wall of the melt hole of an opaque ice sheet. Results from analysis of dust embedded in archived ice cores as well as `artificial ice' made in the laboratory will be shown. Applications for planetary in-situ dust detection will be discussed.
P13A-0971 1340h
Basal Topography of the South Polar Layered Deposits
The ice inferred to comprise the south polar layered deposits (SPLD) represents a significant fraction of the total water reservoir of the planet. The basal topography of these deposits is currently unconstrained but may be expected to contain considerable relief based on the heavily cratered nature of the surrounding terrain. In this work we report on our efforts to characterize the overall nature of this basal topography and in so doing better constrain the volume of this important volatile reservoir. Our approach has been to measure elevations at the periphery of the SPLD (defined by [1]) and use various interpolation techniques to estimate the basal topography. We used 1300 control points from the edges and areas surrounding the SPLD and included extensive control points from within the Chasmae and other features to fit a surface beneath the SPLD. No assumptions were made about any lithospheric flexure, nor did the results suggest that possibility. We first tested a variety of surface interpolation routines on a comparable area of cratered terrain immediately adjacent to the SPLD, using the same spatial distribution of 1300 height control points as we used for the SPLD itself, and found that the topography was broadly reproducible (ignoring craters) to within a few hundred meters. The SPLD basal topography we derive can be subtracted from the current spatial topography to produce isopach maps of the layered deposits. All interpolation methods we tested (within the ArcMap 8.3) indicate a lower total SPLD volume than that previous published [Smith et al., 2001]. Our best estimate for the SPLD volume is ~1 million km$^3$, with a formal error in volume of ~5%, corresponding to an average thickness of ~950 meters. In comparison, [2] estimated this volume to be ~1.2-1.7 million km$^3$. The Prometheus impact basin is present as a rimmed depression, consistent with the inference by [3]. More unexpected is the presence of a broad ridge underlying nearly the entire eastern half of the SPLD, which makes those deposits relatively thin. Our isopach maps show the northwestern portion of the Ultimi lobe to be an isolated thick region, in agreement with [1]. [1] Kolb, E. J., and K. L. Tanaka (2001), Icarus, 154, 22-39. [2] Smith, D. E., et al. (2001), J. Geophys. Res., 106(E10), 23,689-23,722. [3] Byrne, S., and A. B. Ivanov (2004), J. Geophys. Res., In press.
P13A-0972 1340h
Some Mars South Polar Swiss CheeseTerrain has Warm Walls
Much of the surface of the Mars south polar residual cap (SPRC) consists of quasi-circular depressions with steep walls that have been named "Swiss Cheese" terrain. High-resolution Mars Observer Camera (MOC) images have shown that the "Swiss Cheese" mesas, consisting of CO$_2$ ice, are retreating on the order of 1-4 meters per martian year. Many of these "Swiss Cheese" mesas are surrounded by dark moats that appear warm in mid summer Thermal Emission Imaging System (THEMIS) IR images. These warm moats are thought to be the exposed substrate of the underlying water ice rich polar layered deposits. In addition to the thermal signatures from the warm moats, THEMIS also detects a thermal signature from the walls of some of these mesas. By combining THEMIS IR and VIS imaging, we are able to spectrally deconvolve warm and cold temperatures at a higher spatial resolution than the nominal THEMIS resolution of 100m/pix. We take into account that each THEMIS band has a slightly offset footprint from the other bands and assume that the thermal signature is coming from the darkest regions identified by the THEMIS VIS image. Preliminary analysis of one of these warm walled mesas suggests the wall consists of a low thermal inertia dust lag, which would have significant implications for the volatile content and evolution of the SPRC.
P13A-0973 1340h
Pitting within the Martian South Polar Swiss Cheese Terrain
The morphology of the Martian South Permanent Residual Cap is dominated by enigmatic quasi-circular landforms commonly referred to as "Swiss cheese" terrain. These large Swiss cheese depressions, which typically have widths of more than 100 m and extend down to the base of the layer, are expanding at rates of a few meters per Martian year due to CO2 sublimation. We present high-resolution Mars Orbiter Camera (MOC) images detailing extensive "pits," by which we mean small cavities generally less than 10 m in diameter that do not penetrate completely through the Swiss cheese terrain. This pitting is only observed upon the thickest (~10 m) Swiss cheese mesas ("Unit A" as classified by Thomas et al. 2004), and moreover only occurs within 50 meters of the edges of these deposits. We argue that the pits are collapse features caused by the release of CO2 gas from a pressurized layer several meters below the mesa top. As the walls of the mesa retreat due to radiation imbalance, the pressurized layer is exposed, and CO2 vents out laterally, weakening the layer and causing the collapse. We can think of no other process that communicates laterally over distances of 50 meters in one Martian year, which is the time scale over which the pits form. For a layer 6 meters thick, the hydrostatic head is ~200 mbar, which provides an upper bound to the gas pressure in the sealed lower layer. However, for that maximum pressure to be attained, the CO2 in the lower layer must be approximately 30 K warmer than CO2 on the surface. Such a temperature differential is difficult to maintain, though, given that 6 meters is also the thermal skin depth for CO2 over 1 Martian year. We are exploring a number of mechanisms that might continually or cyclically warm this layer and enable rapid venting when the seal is broken. The persistence of polygonal cracks on the mesa tops could be further evidence of subsurface thermal variations.
P13A-0974 1340h
Martian residual-ice cap albedos from MOLA radiometry
The retreat of the Martian seasonal CO$_2$ ice caps uncovers bright residual ice caps that persist throughout the rest of the year and partially cover the underlying polar layered deposits. On the north polar layered deposits this residual ice is composed of dirty H$_2$O ice [Kieffer et al., Science, 1976]. Its thickness is unknown; however, it is thin enough to vary in lateral extent from year to year [Malin and Edgett, JGR, 2001]. On the south polar layered deposits this residual ice is composed of high-albedo solid CO$_2$ [Kieffer, JGR, 1979]. It is on the order of a few meters thick [Byrne and Ingersoll, Science, 2003]. MOLA has been operating as a high resolution (both spectrally and spatially) radiometer since the end of altimetry operations in mid-2001. The convergence of orbital tracks at the poles means that complete albedo maps of both residual caps can be generated with MOLA radiometry data spanning a short range of season and with little or no interpolation. The high resolution nature of these data presents an opportunity to attempt new measurements and revisit old problems. The albedo of the southern residual cap has been observed to increase during the summer season in response to increased insolation [Paige, Caltech thesis 1985; James et al., Mars book, 1992, Kieffer et al., JGR, 2000]. The reasons behind this phenomenon are unclear but may be related to heating of surface dust grains which can then sublimate ice beneath them allowing the grains to burrow into the ice or microphysical changes in the ice in response to the increased incident energy. Initial examination of MOLA radiometry indicates the same effect can be observed with this dataset. The high spatial resolution of these data will resolve which areas of the southern residual cap are responsible for this brightening, as well as the true magnitude of the effect i.e. unaffected by averaging with areas where this effect does not operate. Thomas et al. [Nature, 2000] reported a wide range of morphologies on the southern residual CO$_2$ ice cap. We will report on correlations between geomorphology and this brightening behavior. High-resolution imagery [Thomas et al., Nature, 2000] shows that the surface of the northern residual cap has an extremely rough but homogenous texture at length scales of a few meters. This contrasts sharply with its smooth appearance at longer length scales. The variation in the reflectance of this surface at various incidence angles indicates the relief of this texture as the fraction of the field of view that is shadowed varies with incidence angle. The thickness of the northern residual cap is unconstrained and a measurement of the relief of these features would provide a minimum bound. We will report on how this limit on the northern residual cap thickness varies with location.
http://www.gps.caltech.edu/~shane
P13A-0975 1340h
Modeling the Evolution of Snowpacks on Mars
A radiative-convective atmosphere model has been coupled with a numerical model of snowpack evolution in order to evaluate the factors that can lead to snowpack melting on Mars. The model accounts for snowpack temperature, melting, transport of liquid and water vapor, ablation, densification and grain metamorphosis. The model allows us to explore the importance of latitude, ambient relative humidity, hillside slope and azimuth, variations in the optical properties and abundance of admixed martian dust, and wind speed. The model also allows us to vary the orbital parameters, atmospheric pressure, and solar irradiance to simulate snowpack evolution over geologic history. Although snowmelt has been argued to account for Mars gully formation (Christensen, 2002), it has been argued by Heldmann and Mellon (2004) that melting is an unlikely origin, based on the form and distribution of gullies. The model presented here will be used to determine the minimum conditions necessary for melting in a variety of gully configurations, and those requirements can be compared to likely climate scenarios to evaluate the plausibility of a melt origin. Particular attention will be focused on determining the dust abundance and distribution required for melting, based upon realistic optical constants for Mars surficial dust. In addition, the potential role of topography in shielding the snowpack from radiation to the Martian sky will be addressed. Initial dust-free, planar geometry simulations have yet to identify any circumstances in which melting is likely, as expected. Sublimation, ablation is the mechanism by which snowpacks disappear
P13A-0976 1340h
Evolution of the Martian Northern cap Over the Last 10 Myr Inferred From GCM LMD Water Cycle Simulations: Implications for Layered Deposits and Cap Formation.
Layered deposits are exposed in the walls of the troughs cutting the north polar cap of Mars. They consist of alternating ice and dust layers or layers of an ice-dust mixture with varying proportions and are found throughout the cap. However, the details of their formation process remain unknown. Extensive simulations of the LMD GCM water cycle (Forget et al., 1999; Montmessin et al., 2004) have been performed at various obliquities (15$^{\circ}$ to 45$^{\circ}$ with a 5$^{\circ}$ step), eccentricities (0 to 0.12) and longitude of perihelion, to investigate the rates of water ice exchange between the northern cap and potential equatorial ice reservoirs. The corresponding rates were propagated over the last 10 Myr martian orbital and axial history (Laskar et al, 2002; 2004) in order to track the evolution of polar and equatorial ice thicknesses and test simple models for layers formation over orbital cycles. We found that the annual cap stability mainly depends on the summer solstice insolation. Above a critical insolation, the annual loss rates appear to be a marked exponential function of the insolation. Conversely, beyond this value, polar accumulation rates estimated from various sizes and locations of equatorial sources have found to be nearly unsensitive both to insolation values and remain close to 2 mm/yr. The evolution of the northern cap thickness was then tested for three scenarii.: (1) an infinite equatorial reservoir (2) The formation of a protecting dust lag reducing the further ice sublimation by a variable factor since ~ 10 meters of ice are sublimed (assuming that ice deposited at low obliquity contains about 10% of dust and that a ~1 m dust thickness is sufficient to strongly reduce the ice sublimation) (3) The previous scenario coupled with a realistic history of the equatorial reservoir. The exchange of ice between high-latitude deposits and equatorial reservoir (but not between the polar cap and high-latitudes reservoir) was thus simulated to ensure the global ice mass conservation. Our three-box model is then able to track the evolution of polar, high-latitudes and tropical ice reservoirs for arbitrary initial ice distributions. In the three scenarios, ice rapidly goes to the equator in the high mean obliquity regime (5-10 Myr) and the onset of the polar cap formation begins around 4 Myr during the transition towards the low-mean obliquity period. In the third scenario, more than 600 meters of ice could have accumulated at the north pole from the equator since 4 Myr, without major erosionnal episodes, allowing the formation of ~27 ice layers (or 54 dust-ice layers) with an averaged 22,0 m thickness and a 13.8 m standard deviation, consistent with current thickness observations (Milkovich and Head, 2004). These properties are not modified for moderate variations of model parameters. Our results implies that most of the current 3-km polar ice thickness must originate from other subsurface or surface reservoirs (south cap, high-latitudes ice deposits) acting when the equatorial reservoir disappears. Our model predicts that such periods could have occured recently (0-350 kyr and around 2.4 Myr) corresponding to periods of mininal obliquity variations.Conversely, we discuss the possibility of transient equatorial ice reservoirs during recent large insolation excursions and erosionnal periods for the northern cap.
P13A-0977 1340h
The Long-Term Evolution of Transient Liquid Water on Mars
Liquid water is not currently stable on the surface of Mars but transient liquid water, generated by the melting of ice, may occur if surface temperatures are between the melting and boiling points and the surface pressure exceeds the triple point. Such conditions can be met on Mars with current-day surface pressures and obliquity due to the large diurnal range of surface temperatures, yielding the potential for liquid water. A general circulation model is used to undertake an initial exploration of the variation of this ``liquid water potential'' (LWP) for different obliquities and over a range of increased atmospheric CO$_{2}$ abundances representing progressively earlier phases of Martian geological history. At higher obliquities and slightly higher surface pressures ($<$50 mb) possible in the relatively recent past ($<$10$^{8}$ yr), the LWP conditions are met over a very large fraction of the planet. However, as the surface pressure is increased above about 50--100 mb, the increased atmospheric heat capacity and greenhouse effect reduce the diurnal surface temperature range, resulting in daytime temperatures rarely exceeding the melting point. This reduction of peak daytime temperatures below the melting point greatly reduces the possibility of even transient liquid water. The modeling presented here does not extend to a state of stable liquid water for early Mars---how Mars may have yielded a ``warm, wet'' early climate is currently an open research question. However, if Mars had an early ``warm, wet'' stage, then the potential for liquid water on Mars has not decreased monotonically from that state to the present day, as the atmosphere was lost. Instead, a distinct minimum in LWP will have occurred during the extended period for which pressures were in the middle range of about 0.1 and 1 bar. These results suggest that the current climate and recent paleoclimate may be more conducive for liquid water than paleoclimate states corresponding to much thicker atmospheres. The existence of this ``dead zone'' for liquid water, likely extending over a large fraction of Martian history has direct and restrictive implications for chemical weathering and life. The fundamental conclusion of this study is insensitive to invocation of brines and to more detailed treatment of atmospheric radiative processes.
P13A-0978 1340h
Tracking seasonal changes in the Martian polar regions.
In this work we use the THermal Emission Imaging System (THEMIS) instrument to analyze seasonal changes of the Martian polar regions. THEMIS instrument is operating onboard of the Mars Odyssey spacecraft and has been acquiring data in Thermal IR and Visible wavelengths since February of 2002. Data now spans one full Martian year. We will concentrate our analysis on differences observed by THEMIS in winter/early spring and summer periods in the South Polar Layered Deposits (SPLD) region and illustrate some year to year changes in the North Polar Layered Deposits (NPLD) region. We have successfully obtained a spring time mosaic (VIS subsystem) of South Polar Layered Deposits area ($L_s=170-200$), when it was completely covered by seasonal ice. In order to gain coverage within allocated data volume, images are downsampled to a resolution of 36m/pixel. We were also able to obtain a partial coverage of the summertime SPLD (starting $L_s=300$) at full THEMIS resolution in one color (18m/pixel, 650nm) as well as almost total coverage at 36 m/pixel. In this work we track the process of defrosting of various areas in SPLD region from early spring to summer. Data from the North polar region includes Visible images from two consecutive Martian years, which was taken at 18 and 36 m/pixel resolution. The most interesting phenomena have been so far observed in the SPLD region, during defrosting of the seasonal ice cap. THEMIS Visible images show small variations in the albedo of the $CO_2$ ice covering SPLD. We will speculate on kinds of physical processes responsible for observed variations in albedo. Observations of the $CO_2$ ice cover in the North are problematic due to presence of the north polar hood. However, we still have some examples of the ice-covered terrain in the NPLD region. Boundaries of regional storms are evident in the images. Analysis of polar visible images is often aided by unique high resolution (100m/pixel) Thermal IR images, taken simultaneously with the Visible images. THEMIS platform is an ideal tool for observing seasonal and yearly changes due to 2 hour sun-synchronous polar orbit and consistent performance of the instrument.
P13A-0979 1340h
Northern Polar Layered Deposits on Mars: Identification and Characterization of Signals of Possible Climatic Origin
The record of recent climate change on Mars is encoded in the polar layered deposits (PLD). Individual MOC images of exposed layer sequences in troughs provide the equivalent of cores through the PLD. Techniques employed in the study of terrestrial sediment cores to search for patterns in layer sequences (Fourier analysis) and to determine correlations between sequences (curve-shape matching algorithms) were applied to the north PLD in an attempt to decode this record by 1) characterizing quantitatively the layers in individual locations and 2) assessing possible correlations between locations. Several properties of north polar cap stratigraphy were revealed: 1) A characteristic wavelength of ~30 m thickness exists throughout the upper part of the cap. 2) We have thus far been able to correlate layers across at least three quarters of the cap. We tentatively interpret these results as follows: 1) the ~30 m signal is a climate signal that may correspond to a 51 kyr insolation cycle. 2) Net layer accumulation processes occur across the cap, not confined within a single trough. Analysis of a composite stratigraphic column reveals that the ~300 m thick upper unit (Zone 1) has a dominant wavelength of ~30 m, and overlies Zone 2, a ~100 m thick unit with no dominant signal. We interpret Zone 2 as a lag deposit formed during a recent high-obliquity phase, at which time polar volatiles underwent mobilization and transport equatorward. Below these units lies Zone 3 (~200m) containing a dominant wavelength of ~35 m, and Zone 4 (~200m), characterized by multiple signals but no dominant signal.
P13A-0980 1340h
Evidence for Ancient Equatorial Ice Sheets on Mars?
During August 2004, a survey of available high-resolution MOLA gridded topography and THEMIS VIS imagery in the Equatorial Transition Zone of Mars was carried out. Other data sets, paticurlarly THEMIS IR and MOC NA, were exploited to study areas of interest. Although ~100 metres-per-pixel THEMIS daytime IR coverage is almost complete at the equator, ~18 metres-per-pixel THEMIS VIS coverage was patchy at the time of the survey, and repeat observations are lacking. Therefore, the THEMIS VIS survey could only capture a subset of the geomorphology of the Equatorial Transition Zone. Nevertheless, a suite of features were catalogued: some may be of relevance to the problem of the genesis and postdepositional history of the Medusae Fossae Formation. At the THEMIS scale, the features include eskers, subparallel hummocky ridge packages, ridge-bounded hummocky terrain, metre-scale layering, small-scale chaos terrain / outflow channel landsystems, dissected terrain, rim and central mound crater-interior deposits, polygonally fractured and channelized mesa tops, "wirebrush," "eggbox/bullseye," outcrops of a pasty lithology, and apparent cwms and aretes. At MOLA scale (as noted by other workers) they include rampart craters and trough-and-lobe landscapes. One possible framework for an initial synthesis of these early results will be adumbrated, exploiting recent progress in numerical modelling of the Martian water cycle at high obliquity, and the chaotic diffusion of Mars' obliquity over geological time. Finally, the relationship of these initial results to those of other workers will be described, and some future research directions will be sketched out.
P13A-0981 1340h
Is the Medusae Fossae Formation in southeastern Elysium, Mars, an ice rich deposit, superposing a frozen lake with a polygonal patterned ice crust and adjacent pingos?
The Elysium region near the martian equator is thought to be an area associated with the youngest volcanic, fluvial and glacial activity seen so far on Mars. The relevant geological units are located in southeastern Elysium Planitia and include Aeolis (A), the Cerberus Plains (CP) and the Medusae Fossae Formation (MFF). Parts of the CP are situated at the end of the Athabasca Valles outflow channel and have been previously interpreted as either flood lava or outflow channel deposits. Bright platy material in the CP shows small scale, polygonal ground (MOC m0304228) and pingo-like cones, both are common in terrestrial permafrost. The surface of the platy material is broken up in plates, which sometimes show zoning features features (MOC e1201728), possibly from a sublimation process. The fill of any basin in Elysium would result in a magma or water lake. Clusters of cones are present on the CP, whereas many of these cones are near or eroding out from beneath the MFF or impact crater ejecta (MOC e1200416, m0800090, e1701551). Cones in Elysium planitia have been interpreted as Pseudovolcanoes or pingos. Pseudovolcanoes are formed by phreatomagmatic explosions owing to the flow of lava over wet ground. Terrestrial pingos (conical mounds in permafrost areas) are caused by the freezing and growing of water lenses in the ground. Ice lenses form either from former water bodies and lakes or as abandoned ice from sublimating glaciers. Scars of terrestrial pingos have been found next to former pleistocene ice sheets. The proximity of pingo-like cones to the MFF suggest that the cones accumulate subsequent to the erosion and/or sublimation of the MFF, a process likely related to the presence of permafrost soil. The MFF, previously interpreted as a volcanic airfall deposit, is superposed on the CP and shows drumlinoid, layered, sometimes triangular blade-like features (MOC m0305363, e0400568), interpreted as yardangs. Other explanations include penitents or ablation hollows. Terrestrial penitents are triangular blades, common on glaciers in the dry andes and can get up to 10 m high. They usually point in one single direction and are formed by strong solar rariation. Longitudinal, streamlined and parallel aligned accumulations are concentrated in fields (MOC m2000022) and seem to be the erosional remains of both impact crater ejecta and MFF. The similarity of surficial deposits on both MFF and impact crater ejecta suggests similar formation mechanism. The enhanced ice content proposed here for the MFF is in agreement with the former work of previous authors.
P13A-0982 1340h
Young (late Amazonian), near surface, ground ice features near the equator, Athabasca Valles, Mars
A suite of four feature types near 10N 204W in Athabasca Valles are interpreted to have resulted from near-surface ground ice. These features include mounds, conical forms with rimmed summit depressions, flatter irregularly-shaped forms with raised rims, and polygonal terrain. Based on morphology, size, and analogy to terrestrial ground ice forms, these Athabascan features are interpreted as pingos, collapsing pingos, pingo scars, and thermal contraction polygons, respectively. THEMIS night IR data and surficial geological features in the area indicate that a sedimentary substrate comprises these features. In conjunction with morphology, this supports a ground ice interpretation over alternative (volcanic) hypotheses. The ground ice that formed the mounds and rimmed features may have derived from the deposition of saturated sediment during flooding; alternatively, the ground ice may have derived from magmatically cycled groundwater. The ground ice implicit in the hypothesized thermal contraction polygons may have derived either from flooding or from atmospheric water. The lack of flood modification of the mounds and rimmed features indicates that their formation took place more recently than the last flood inundated the area. Analogy with terrestrial pingos suggests that ground ice is still extant within the positive relief mounds beneath less than a few meters of overburden. As the water that flooded down Athabasca Valles emerged via a volcanotectonic fissure from a deep aquifer, this pingo ice may contain evidence of a deep subsurface biosphere.
P13A-0983 1340h
Distribution, Exchange, and Topographic Control of Subsurface Ice on Mars
Spacecraft observations of a vast reservoir of subsurface ice on Mars raise an important question: does the distribution represent a record from a past climate with a different obliquity and humidity from today, or has it adjusted to, and is in equilibrium with, the present-day climate? We consider physical models, experiments, and observations in addressing this question of long-term thermal stability, as well as dynamical phenomena including seasonal accumulations and the response to surficial and environmental changes in the presence of adsorption. The effect of surface slope and slope distribution is investigated, and we find that depending on the length-scale, these slopes may have profound implications for the equilibrium ice table.
P13A-0984 1340h
Nonequilibrium melting of icy soil in confined geometries on Mars
While applicable to natural phenomena such as landslides, the study reported here was motivated by concerns about radioactive power sources (RPS) that might be emplaced just below the martian surface as a result of a landing "anomaly." Mars is best described as a cold, dry desert in the sense that water (in any form) is not circulated by precipitation or other means. Thus, while the phase diagram supports liquid water in many places, over time it would all migrate to the coldest locations, primarily at the poles or just beneath the surface at high latitudes. Transient water can be formed, however, when this equilibrium is disturbed by introduction of a heat source. Since the crash of a spacecraft might deposit a halo of microbial contamination in the vicinity of an RPS, such transient sources of water could provide breeding areas that would violate planetary protection treaties. The Mars Odyssey spacecraft has identified vast stretches of high latitude terrain as zones where liquid water could easily be formed in this scenario. To address both the extent and the duration of wet soil, certain scenarios have been modeled in two dimensions using both a finite difference time-marching method and by one dimensional analytical approximation. Considered in the analysis are the diffusion of both heat and water vapor, capillary forces on liquid water, latent heat exchange, surface processes such as radiation, evaporation, and convection, and continuous equilibration between the liquid, vapor, and solid phases. Results indicate that the initial ice content of the soil, a proxy for thermal conductivity, exerts the greatest influence on the progress of the wetting and drying cycle. Ice can be melted at distances of almost a meter from a 250W power source and may, under certain circumstances, persist for months. It is not yet clear, however, whether the result suggests a planetary protection risk.
P13A-0985 1340h
Numerical Methods in Understanding the Performances of Radar Sounding Techniques: Multiple low Frequency Approach for Unambiguous Identification of Subsurface Water Saturated Interfaces on Mars
Low frequency sounding radars can probes the subsurface layers of a planetary surface down to varying depths depending on the sounding frequency, geometry, surface topography, geoelectrical and geomagnetic properties of the sounded terrains. Hence a good understanding of the electric and magnetic wave interaction mechanisms between the radar waves and the rocks and sediments constituting the investigated media is crucial for any future data analysis and interpretation. In this presentation we validate the use of a Finite Difference Time Domain algorithm adapted to simulate the radar wave interaction with complex geological model that take into account real topographic data and layer heterogeneities. The radar-backscattered echoes from some volcanic (cones, ashes and faults) and sedimentary features (dunes, fluvial and dry lake deposits) have been investigated at the frequency band from 1 to 100 MHz and compared to Ground Penetrating Radar field collected data at the same frequency band. In a first step the algorithm have been adapted to simulate the response of orbital sounding instruments (i.e.: MARSIS and SHARAD) and a monostatic GPR all dedicated to map the possible presence of subsurface water in the Martian permafrost. We also suggest some solutions for the clutter problems for orbital and future landing Ground Penetrating Radar on Mars. In a second step the algorithm is being validated for the case of Europe and the Moon. Results show that combining multiple low frequency sounding frequencies strongly reduces the ambiguities on the identification of subsurface water saturated interfaces and stratigraphy at different depths.
P13A-0986 1340h
Formation of Valles Marineris and Associated Outlfow Channels by Catastrophic Dewatering of Evaporite Deposits
Geological mapping based on topographic analysis of Mars Orbiter Laser Altimeter (MOLA) data, together with photointerpretation of Mars Orbiter Camera (MOC) images, and thermodynamic and heat-flow considerations lead to a new hypothesis for the formation of Valles Marineris and the associated outflow channels through catastrophic dewatering of ancient evaporite deposits. MOLA transects across Valles Marineris reveal that the valley is located at the crest of a 2-km-high topographic bulge on the flank of the much larger Tharsis Rise. The Interior Layered Deposits (ILDs) within Valles Marineris unconformably underlie, and therefore would have been heated by, Hesperian age lava flows, as well as from below by insulation by the Hesperian flows and an increased geothermal gradient due to development of the Tharis Rise. The ILDs are now thought on the basis of spectroscopic data to contain hydrous sulfate salts. The estimated range of increased temperatures predicts the potential to dehydrate $>$km-thick sections of gypsum or epsomite in reactions that would trigger volumetric expansion sufficient to account for the topographic bulge and catastrophically release tremendous amounts of over-pressured water.
P13A-0987 1340h
Tharsis Recharge: Analysis of Groundwater Flow to the Martian Outflow Channels
The large Martian outflow channels terminating in Chryse Planitia are remnants of Hesperian flooding events involving the localized discharge of millions of cubic kilometers of subsurface water. Given reasonable crustal porosities, such volumes cannot be stored in the regional aquifer and recharge is required. Initial results of dynamic groundwater models [1] demonstrated that snowpack or glaciers on the Tharsis rise, formed during periods of high obliquity when ice was stable at low latitudes, may have provided an efficient source of recharge and hydraulic head for the circum-Chryse outflow channels. Comparison of model results with Martian South Pole recharge simulations (based on the hypotheses of Clifford and Parker [2]) show that Tharsis recharge produces up to four times more discharge through the circum-Chryse outflow channels in a given time period. The Medusae Fossae formation may support the past existence of large low-latitude ice sheets as early as the Hesperian [3]. Ground ice may have been involved in the formation of rootless cones, the Olympus Mons aureoles, and glacial features on volcanic edifices. The U.S. Geological Survey MODFLOW-2000 groundwater code, which we have modified to simulate spherical geometry, is used to explore further two key aspects of Tharsis recharge: infiltration area and initial water table. In previous models we assumed an infiltration area equal to that estimated by other authors for recharge over the Martian South Pole. We present new models with the infiltration area extended to regions of Tharsis above a range of threshold elevations. Early Hesperian Tharsis recharge also depends on initial water table elevations determined by hydrologic conditions in the late Noachian. In previous models, we assumed that Noachian precipitation rates maintained a shallow water table that could be approximated by the Martian topography. The transition from unconfined to confined conditions in the late Noachian may, however, have resulted in some relaxation of the water table before Tharsis recharge commenced. We thus present models with alternative initial water tables representing varying degrees of relaxation, including the extreme case of constant elevation. [1] Harrison and Grimm, GRL, 31, DOI:10.1029/2004GL020502, 2004. [2] Clifford and Parker, Icarus, 154, 40-79, 2001. [3] Head and Kreslavsky, 35th LPSC, 1635, 2004.
P13A-0988 1340h
Lobate Debris Aprons in the Hellas Planitia Region, Mars: Insights from High-Resolution Data from the HRSC Experiment on Mars Express
A variety of landforms indicates the possible existence of past or present ice in the near subsurface of Mars. Among the most spectacular ice-related features are lobate debris aprons (LDA). They have been interpreted to be a mixture of rock particles and interstitial ice analogous to terrestrial rock glaciers (debris transport systems comprising a creeping mixture of rock and segregational or interstitial ice). Rock glaciers are indicators for the climatic environment during their formation. The analogy between terrestrial rock glaciers and Martian LDA is predominantly based upon the shape of debris aprons, their surface texture, their relationship to adjacent regions with permafrost-related morphologies, and the correlation of their global distribution with the predicted stability of Martian ground ice. LDA are known from the dichotomy boundary and from regions surrounding the large impact basins of the southern hemisphere. This work focusses on the distribution of LDA in the circum-Hellas Planitia region and compares morphometric results as well as mapping results of the textural context between MOLA/Viking/MOC data and the data obtained by the High Resolution Stereo Camera (HRSC) on Mars Express. The HRSC has obtained colour and stereo information with a resolution of 11 metres at its best from 35 orbits over the northern and eastern impact basin region (01/09/2004 - 06/28/2004). The vertical accuracy of the derived digital terrain data is comparable to MOLA-derived products but several regions are of such a good photogrammetric quality that elevation model of at least 100~m per pixels could have been obtained. First results of morphometry in the Hellas Planitia region fit perfectly into the MOLA/Viking derived area and volume ratios and are consistent with terrestrial measurements of mountainous debris transport systems. The data obtained so far fill the morphometric gap in the smaller-sized distribution of remnants and debris aprons.
P13A-0989 1340h
An Unusual Lobate Deposit in Coprates Catena, Mars
We have studied a lobate deposit centered at -15.2 S, 299.7 E within one of the troughs of Coprates Catena in order to investigate the role that water may have played in its formation. The deposit, which is fan-like in appearance, is 7.1 km in width and 8.3 km in length, and it emanates from a 45 km long sinuous channel that cuts through the plains before terminating in the trough system. The channel is one of several in the area that flow northeast, roughly perpendicular to the NW-SE trough system of Coprates Catena. The channel increases in depth and width downvalley, with a maximum depth of 1.7 km and width of 4.0 km measured where it enters the trough. The channel has no obvious source visible in the MOC or THEMIS images. Although the main channel terminates at the intersection of the trough, a much smaller channel has incised the uppermost portion of the deposit and is oriented along the same northeast trend. There is no terminal deposit associated with this smaller channel. The lobate deposit is approximately 1.2 km thick based upon a MOLA profile that crosses it, and the top of the deposit lies 1.7 km below the top of the trough's southern end. There are concentric steps along the sides of the deposit, suggesting either multiple periods of deposition with each subsequent deposition episode smaller in size or modification of the unit post-deposition. The clear association between the channel and lobate deposit indicates that material was transported along the channel and deposited on the trough floor once the channel entered into the trough. Although both the channel and lobate deposit are younger than this particular trough, to the south another NW-SE system of troughs has disrupted the channel by cutting though it. There are also several small impact craters on the lobate deposit, suggesting that it is not much younger than the adjacent plains or the trough system where it is located. A light-toned unit in the eastern portion of the trough may represent another water-lain deposit, perhaps an evaporitic unit formed when water from the channel partially filled the trough. Thus far, we have not seen any other similar lobate deposits within Valles Marineris.
P13A-0990 1340h
New Results from Topographic Studies of Martian Debris Aprons
Lobate debris aprons in the mid- to high latitudes for Mars' northern and southern hemispheres have been interpreted as ice-related features [e.g. 1, 2, 3]. Using MOLA topographic profiles perpendicular to apron flow fronts, we surveyed 45 debris aprons in the 35-$55\deg$N>| latitude range of both the northern and hemispheres, specifically Mareotis, Protonilus, and Deuteronilus Mensae and Acheron Fossae, Argyre and eastern Hellas Basin. The profiles of these aprons were compared with predictions from idealized simple plastic and viscous power law models for ice-rock mixtures. All aprons studied exhibit convex profiles that closely match or follow the overall trend of a simple plastic model. This result is consistent with previous interpretations [1, 2, 3, 4] that debris aprons are ice-rich mixtures with rheologies similar to stagnant ice sheets and furthermore requires high ice concentration ($>$40 percent by volume) in apron deposits. About 60 percent of the surveyed debris apron population deviates from the idealized simple plastic model profile, which may be due to locally reduced ice content, with ice content likely being the primary control on apron topography. Although post-emplacement modification due to near-surface ice sublimation may play a secondary role in defining the overall shape of aprons, it causes conspicuous surface textures. Degradation by ice sublimation results in pitted and ridge-and-furrow surface textures revealed by high resolution MOC images. Such textures may indicate decreased near-surface ice stability since the formation of the aprons, consistent with a recently proposed interglacial period after their emplacement [5]. Despite their elevation difference, northern and southern hemisphere debris aprons have essentially identical profile shape and exhibit similar surface texture and surface age. These similarities suggest two groups of aprons share same origin and degradation processes and their most recent reactivation likely occurred around the same time. Our crater counts(limited areal extent) suggest that debris aprons are young features with an age constrained to be $<$100 Myr. Since current climate at mid to high latitudes of Mars is not conducive to the formation of ice-related flows the climate ~100 Myr ago must have been wetter than today, and was probably similar to that of the terrestrial periglacial regions. High ice content, inferred from morphology, suggests some debris aprons have ice cores, which are potentially exploitable water resources for future robotic/human operations that could prove invaluable for missions remote from polar regions. {[1]} Squyres, S.W., Icarus, 34, 600-613, 1978. {[2]} Lucchitta, J. Geophys. Res, 89, 409-418, 1984. {[3]} Crown, D.A. et al., Icarus, 100, 1-25, 1992. {[4]} Mangold, N. and Allemand, P., Geophys. Res. Lett., 28, 3,407-3,410, 2001. {[5]} Head, J.W. et al., Nature, 426, 797-802, 2003.
P13A-0991 1340h
Survey of Martian Alluvial Fans
Recent higher resolution images of the Martian surface reveal another complex geomorphic surface process - alluvial fans. Alluvial fans composed of water-transported, loose sediment deposited as the flow moves from steep, eroding terrain to low-gradient depositional basins. Martian alluvial fans are concentrated along crater rims where they erode into the higher surround topography depositing sediment into the crater interior. Initial work by Moore and Howard [2004] examined some of these fans in a band from 0-30 S. Their initial results included fan statistics, such as area and slope, and indicated that the fans were clustered in discrete areas. However, this survey was hampered by incomplete image coverage. We present the progress of our continuing, planet wide survey of alluvial fans. Specifically we focus on understanding if alluvial fans are indeed clustered in particular areas and if so, why. In addition, we expand fan statistics to include a comparison of fan volume to eroded area.