P22A-01
Debris-Covered Glaciers in Antarctica: Analogs for Viscous-Flow Features on Mars
The McMurdo Dry Valleys (MDV) are generally classified as a hyper-arid, cold-polar desert. Subtle variations in climate parameters throughout the region result in considerable differences in the distribution, origin, and morphology of buried ice. In the coastal thaw zone, near-surface buried ice experiences seasonal melt and may have formed where pore water from surface snowmelt freezes underground (segregation ice). Characteristic landforms associated with this type of buried ice include thermokarst, shallow planar slides, and solifluction. In contrast, in the coldest and driest regions of the MDV, the stable upland zone, there is insufficient meltwater to produce extensive segregation ice. Rather, widespread buried ice in this zone is typically glacier ice. Temperature data indicate that ice remains frozen in this zone if buried beneath ~15 cm of debris. The Mullins-valley debris-covered glacier, which lies within the stable upland zone, contains ancient glacier ice beneath a thin layer of sublimation till. Four independent dating techniques confirm that the glacier age ranges from ~10 ka near the valley head, to >8 Ma at its diffuse terminus in central Beacon Valley. The dating methods include cosmogenic-nuclide analyses of surface boulders; horizontal ice-flow velocities as determined from synthetic aperture radar interferometry; 40Ar/39Ar analyses of in-situ ash fall in relict polygon troughs at the till surface; and numerical ice-flow models. Age results so derived are in accord with measured variations in ancient community DNA extracted from pristine ice samples along the length of the glacier. Multi- channel seismic and ground-penetrating radar surveys demonstrate that the ice is relatively clean and that it averages from ~45 m to ~150 m thick. Morphologic comparisons of the Mullins Valley debris-covered glacier are used to shed light on the origin and modification of near-surface ice on Mars.
P22A-02 INVITED
Quantifying Vapor Flow Within Sublimation Tills Over Buried Glacier ice in Antarctica: Implications for the Origin and Modification of Near-Surface ice on Mars
Buried glacier ice rests beneath a 40–80 cm thick layer of sublimation till in central Beacon Valley, Antarctica. We applied a diffusion model to track vapor flux within the till to assess long-term stability of subsurface ice. As input, we used meteorological data from HOBO data loggers that captured changes in solar radiance, atmospheric temperature, relative humidity, soil temperature, soil moisture, and wind speed and direction from November 2004 to January 2007. Additional factors influencing ice stability that are not addressed in the present model include surface roughness, turbulence, and the growth and mobilization of secondary ices and salts. Results show that ice loss is extremely sensitive to minor perturbations in air temperature and relative humidity, and that vapor flows into and out of the sublimation till at rates dependent on the non-linear variation of soil temperature with depth. Current annual ice loss is calculated at 0.07 mm in central Beacon Valley. Net loss of buried glacier ice during summer months (equaling >90% of total annual ice loss) is reduced to zero if average summertime air temperatures decrease from -7°C to -12°C, or average summertime RH increases from 36% to 58%, or infiltration of minor snowmelt increases to ~0.002 mm per day. Such changes in summertime conditions could be achieved with an increase in average cloud cover. An understanding of the range of physical processes affecting buried ice in the Antarctic Dry Valleys, Earth's closest terrestrial analog for Mars, is helpful in defining factors that could potentially alter the stability and evolution of near-surface ice on Mars.
P22A-03
Laboratory Characterization of the Structural Properties Controlling Dynamical Gas Transport in Mars-Analog Soils
Dynamical transport of gases with in the martian regolith controls many climatic processes, and is particularly important in the deposition and/or mobilization of shallow ground ice, as well as exchange of other volatiles between the martian regolith and atmosphere. A variety of theoretical studies have addressed issues related to ground ice dynamics on Mars and in the terrestrial analog environment of the Antarctic Dry Valleys. These theoretical studies have drawn on a limited set of empirical measurements to constrain the structural parameters controlling diffusion and flow in soils. Here, we investigate five groups of Mars-analog soils: glass spheres, JSC Mars-1, aeolian dune sand, Antarctic Dry Valley soils, and arctic loess. We present laboratory measurements of the structural properties most relevant to gas transport in these soils: porosity, tortuosity, permeability, bulk and intrinsic density, grain size distribution, pore size distribution and BET surface area. Our results bear directly both on the appropriateness of assumptions made in theoretical studies and on current outstanding issues in the study of shallow ground ice on Mars and the Dry Valleys. Specifically, we find that 1) measured values of tortuosity are lower than commonly assumed values by a factor of two to three; 2) diffusive loss of ground ice on Mars can likely proceed up to four times faster than predicted by theoretical studies; 3) soil permeabilities are sufficiently high that flushing of the soil column by bulk flow may further speed loss or deposition of shallow ground ice; 4) the pore volume in some Mars-analog soils is adequate to account for high volumetric ice abundances inferred from Mars Odyssey Gamma Ray Spectrometer data; and 5) superlative soil properties cannot resolve the on-going debate concerning the age of shallowly buried ice in Beacon Valley, Antarctica.
P22A-04 INVITED
Thermal Contraction Crack Polygons on Earth and Mars: Morphological Analysis and Climate Implications
The generation of thermal contraction crack polygons on Earth and Mars is a complex process, governed by the interplay between climate or microclimate conditions and the physical properties of the cracking medium. The morphological evolution of thermal contraction crack polygons is controlled in large part by local climatic and geological conditions: principally, the presence or absence of a seasonally wet active layer; the abundance of windblown, crack-filling material (snow or sediment); and the temperature and humidity conditions controlling the transport of water vapor between the ice-rich substrate and the atmosphere. Thermal contraction cracking of ice- rich soils is expected under present Mars conditions for latitudes polewards of 30 degrees, and a correlation between polygon size and latitude is predicted: larger stresses generated polewards will produce finer polygon networks. Observations confirm this general prediction, and a latitude dependence of polygon morphological type is observed, suggesting that the latitudinal range of current climate conditions is partially responsible for the difference in morphology. In particular, some hypothesize that localized melting and freeze-thaw processing are a significant source of the variety in polygon morphologies observed. A suite of thermal contraction crack polygon morphologies has been documented in the Upland Stable Zone (USZ) and Intermediate Mixed Zone (IMZ) of the Antarctic Dry Valleys, forming under known microclimate and subsurface rheological conditions, informing a comparison with polygonally patterned ground observed on Mars. We address the questions. What do current climate conditions suggest about the equilibrium conditions under which polygons are evolving on the martian surface? Is the decreasing spacing of thermal contract crack polygon networks polewards entirely a climate signal, or is there also a substrate rheology signal? What polygon features require freeze-thaw processing, and what features compare favorably to morphologies observed in the Upland Stable Zone? Does Mars have a latitude-dependent transitional Intermediate Mixed Zone? What do fossil sand/ice wedges look like on Earth as compared to Mars, and how do they change our understanding of the geomorphic effects of current Mars climate conditions?
P22A-05
Testing the Snowpack Hypothesis for Gully Formation on Mars: Utilization of the Antarctic Dry Valleys (ADV) as a Terrestrial Analog
The identification of young gullies on Mars suggests that liquid water has flowed across the martian surface during the recent climatic regime which has otherwise been considered to have been cold and dry. Research into the martian gullies suggest that water flow was concurrent with periods of higher obliquity, yet, no consensus has been reached regarding whether the water which eroded the gullies originated within internal confined aquifers or was sourced from surface/near-surface snow and ice deposits. We undertook research into gully formation in the ADV, a hyper-arid very cold polar desert which is considered the closest terrestrial analog to current Martian conditions. Our research identified two water sources: 1) perennial snow/ice deposits within the gully alcoves. 2). Annual accumulations of windblown snow trapped within the channels themselves. The melt produced by each source was found to be a function of: the local microclimatic zone, lithology, slopes and elevation. We also classified and mapped a range of meso-scale features (m to 10s of m scale) that can be compared to landforms identifiable within HiRISE images in order to further constrain gully formation processes and potential levels of recent activity on Mars. The exchange of salts between the runoff within the gullies and the surrounding ADV soils may also provide further insights into the generation of brines within polar deserts; this has important ramification regarding their development on Mars and the extent to which the freezing point can be depressed. Our findings demonstrate how gully erosion can take place in the absence of aquifer-fed sapping and within a region of low precipitation and thus provides further support for a surface source of water for the martian gullies. These results also underline the significance of snowmelt as a source of water for both ADV hydrological systems and ecosystems.
P22A-06 INVITED
Glacial Meltwater Streams of the McMurdo Dry Valleys, Antarctica: Ecosystems Waiting for Water
The McMurdo Dry Valleys of Antarctica contain many glacial meltwater streams that flow for 6 to 12 weeks during the austral summer and link the glaciers to the lakes on the valley floors. Dry valley streams gain solutes longitudinally through weathering reactions and microbial processes occurring in the hyporheic zone, evident as a damp area underneath and adjacent to the stream. The lower boundary of the hyporheic zone is determined by the depth to permafrost. On sunny days, stream temperatures can reach 15 °C, and advection of this warm water can erode the frozen lower boundary of the hyporheic zone. In cold summers, streamflow is fed mostly by melt from the faces of the source glaciers and a large portion of this meltwater may be stored in the hyporheic zone and then lost through sublimation, rather than discharged to the lakes. Some streams have thriving microbial mats composed of cyanobacteria and diatoms. These mats are freeze-dried through the winter and begin photosynthesizing with the onset of flow. To evaluate the longer term persistence of cynaobacterial mats, we diverted flow to an abandoned channel, which had not received substantial flow for approximately two decades. We observed that cyanobacterial mats became abundant in the reactivated channel within a week, indicating that the mats had been preserved in a cryptobiotic state in the channel. Over the next several years, these mats had high rates of productivity and nitrogen fixation compared to mats from other streams. These stream-scale experimental results indicate that the cryptobiotic preservation of cyanobacterial mats in abandoned channels in the dry valleys allows for rapid response of stream ecosystems to climatic and geomorphological change.
P22A-07 INVITED
Low elevation hydrogeological features in the McMurdo Dry Valleys as analogs to recent Mars
Low elevation regions of the dry valleys in East Antarctica are often used as analogs for Mars at some time in the past when surface water was readily available. Numerous perennially ice covered lakes in particular have been touted as excellent examples of the last vestige of aquatic ecosystems on Mars during a planetary climatic deterioration [e.g. Doran, et al., 1998; Wharton, et al., 1995]. Recent climate on Mars though is too cold (and pressures too low) to support standing water at or near the surface. Yet features reminiscent of water flow, even in recent times, have been identified [Malin and Edgett, 2000; Malin, et al., 2006]. We discuss phenomena in the dry valleys which may provide clues to the nature and origin of the recent flow features on Mars. In this paper we will discuss two main phenomena: 1. Irregular spring flow derived from melting ground ice, snow patches, refrozen precipitation or buried glacier ice. Some of these seeps flow only during the warmest summers. The majority of the seeps observed flow directly out of the ground with no nearby glaciers to supply the water. Both solute chemistry and isotopic signatures are distinct from nearby streams an glaciers indicating that seep waters have been substantially modified if they originated from the same meteoric water. The geochemical data support a subsurface origin for these waters with a relatively long residence time. 2. Persistent saline groundwater discharge in a region which has permafrost to depths from 240 to 970 m [Decker and Bucker, 1977]. Don Juan Pond in Wright Valley is a CaCl-rich groundwater outcrop which is liquid year-round. Evidence of the existence of saline groundwater is also provided by flows into the bottom of Lake Vanda, and well measurements in Victoria Valley References Decker, E. R., and G. J. Bucker (1977), Geothermal studies in Antarctica, Antarc. J. US, XII, 102-104. Doran, P. T., et al. (1998), Antarctic paleolake sediments and the search for extinct life on Mars, Journal of Geophysical Research, 103, 28481-28493. Malin, M. C., and K. S. Edgett (2000), Evidence for recent groundwater seepage and surface runoff on Mars, Science, 288, 2330-2335. Malin, M. C., et al. (2006), Present-day impact cratering rate and contemporary gully activity on Mars, Science, 314, 1573-1577. Wharton, R. A., et al. (1995), Paleolakes on Mars, Journal of Paleolimnology, 13, 267-283.
P22A-08
Slope streaks in the Antarctic Dry Valleys: Characteristics, candidate formation mechanisms, and implications for slope streak formation on Mars
Slope streaks on Mars are typically dark, extend downslope for up to ~2 km, are <200 m in width, show no detectable relief, and have been observed to form and change over less than decadal time periods. Mars slope streaks occur exclusively in regions of low thermal inertia, steep slopes, and only where peak temperatures exceed 275 K; changes are observed only if the interval includes the warm season. Mechanisms proposed for Mars slope streaks include dry dust avalanches, dust avalanches controlled by wind, wet debris flows, both wet and dry debris flows, and erosive fluvial processes from spring discharge, where melting is aided by hydrothermal activity or hypersaline aquifers. Although the ADV represent one of the most Mars-like terrestrial environments, there are also substantial differences (e.g., atmospheric pressure and composition; abundance of water, etc.) and thus analogs must be assessed cautiously. We investigated very similar slope streaks in upper Wright Valley of the Antarctic Dry Valleys and interpret their formation to be due to snowpack and near-surface melting-derived saline water traveling downslope along the top of the ice table, wicking upward, and dampening the surface to cause the streak. Among the observations of Mars streaks that suggest that this mechanism should be seriously considered are: 1) similarities in characteristics, brightness, scales, slopes, aspect ratio, temporal behavior, and modes of occurrence; 2) distribution and geometry of occurrence suggesting a relation to solar insolation (low latitudes and northernmost streaks occur preferentially on warmer south-facing slopes); 3) the observation that they occur only where peak temperatures exceed 275 K, and that changes occur only where there has been an intervening warm season, suggesting a potential role for the melting of surface snow and ice. We thus conclude that the saline-assisted surface-near surface melting and water migration origin of slope streaks interpreted from the ADV should be further assessed as a candidate mechanism for the origin of slope streaks on Mars. If this origin proves to be applicable to Mars, then the high frequency, continuity, and renewal of occurrence of these features in specific areas of Mars suggests an important role for liquid water and brines under current seasonal conditions.