P21B-01 INVITED 08:00h
Niveo-aeolian and Denivation Deposits on Mars
Hydrogen abundance data from the Gamma Ray Spectrometer on board the Mars Odyssey platform indicate that large areas of the North Polar Sand Sea have high concentrations (40-60% weight) of hydrogen molecules in the surface deposits. On Earth, cold region sand dunes often contain inter-bedded sand, snow and ice. These niveo-aeolian deposits have unique morphologies and sedimentary structures that are generally not found in warm desert dunes. An atlas of dune niveo-aeolian and denivation features was compiled from published studies of polar deserts on Earth. Features occur at a range of scales and signatures are both morphologic and stratigraphic. The atlas is used to identify similar features on Mars. Examination of high resolution Mars Orbiter Camera images of the North Polar Sand Sea and Southern Crater dune fields have identified several potential signatures of niveo-aeolian and denivation processes on Mars. These include: over steepened lee slopes, cornices, rounded slipface and/or crest, protruding ice cemented beds, alluvial meltwater channels and fans and sublimation avalanches. Other smaller-scale forms probably occur but are not detectable with current resolution data. While these findings have implications for our understanding of martian dune geomorphology, mobility and the geological evolution of the sand seas, they also highlight the potential for a significant volatile reservoir and biological habitat in sand dunes on Mars.
P21B-02 08:22h
Sand Transport by Wind on Complex Rough Surfaces: Field Studies in the McMurdo Dry Valleys, Antarctica
Although the physics of the movement of sand-sized particles by wind has been extensively studied, significant uncertainties remain in our understanding of the effects of surface roughness on aeolian transport processes. Accounting for the effects of non-erodible, isolated roughness elements on sediment transport by wind is necessary for the development of models that realistically predict rates of transport for complex surface types on Earth and Mars. Many surfaces in the McMurdo Dry Valleys of Antarctica exhibit a striking resemblance to rocky surfaces on Mars, making this area ideal as a Mars analog site in addition to providing fundamental information on sand transport processes in a cold arid environment. We conducted studies of sand transport rates (using both sand traps and Sensit saltation sensors), boundary layer winds, and surface shear stress at a site in the eastern part of the Victoria Valley for a 3-week period in January 2003, and generated a unique data set on temporal and spatial variations in boundary layer winds, surface shear stress, and sediment transport by wind in a natural setting. The surface in this area consists of a gently undulating sand sheet (median grain size 300 µm) with scattered clusters of poorly sorted angular boulder- and cobble-sized clasts, interspersed with patches of angular medium gravel- to cobble-sized sub-angular rock fragments. Overall roughness density for the area is 0.0039, with an average aerodynamic roughness of 0.0013 m. Surface shear stress was measured using Irwin Sensors with the total shear stress being derived from wind profile parameters. Approximately 20 percent of the regional wind shear stress interacts with the ground surface, indicating a significant partitioning of shear stress, even with a rather sparse roughness element density. Data were obtained for seven sand transport events ranging in duration from 271 to 1451 minutes. The threshold wind shear velocity for sand transport was determined via the time-fraction equivalence method of Stout and Zobeck (1997) and ranged between 0.30 and 0.35 m/s, equivalent to a wind speed at 6 m of 6.2 to 7.4 m/s. This compares to threshold wind shear velocity of 0.29 m/s calculated using the Bagnold formula. Sand transport intermittency as defined by Stout and Zobeck (1997) varied from 0.03 to 0.90, indicating considerable variations between events in the intensity of saltation and in the intermittency function. These variations can be characterized by: (1) a "saltation duration" curve - the cumulative percentage of the event at which different levels of intermittency occur and (2) the percentage of the event during which saltation is continuous (transport intermittency = 1). In turn, these parameters correlate with the overall wind conditions for the event as characterized by the ratio between wind shear velocity and threshold wind shear velocity and the percentage of the time wind shear velocity exceeds the threshold wind shear velocity. The saltation duration curve and percent of time during which saltation is continuous provide means to quantify aeolian activity at a site and can be used to compare levels of aeolian activity among different aeolian environments. Research supported by NSF OPP-0088136.
P21B-03 08:37h
Areal estimates of dune deposits in Kaiser Crater on Mars
Aeolian studies on Mars offer an excellent method for deciphering some of the most challenging questions regarding ongoing Martian surficial processes. A first step in understanding these processes is to produce a database of aeolian dune deposits in a global geographic context. We have begun construction of a proto-type database designed to accomplish that task using THEMIS IR images to identify, delineate (using ARCMAP), classify and archive Mars dune deposits. We have used this proto-type database to begin preliminary studies of the dune deposits in Kaiser crater on Mars. Most of the dune deposits in the crater are in one large contiguous field that is composed of several smaller areas of differing and merging morphologies. The remainder of the sand is contained in a single smaller detached field and three small craters within Kaiser crater proper. We identified 15 distinct dune deposits, drew polygons around the deposits using ARCMAP and calculated the area for each. The total areal extent of the deposits in Kaiser crater and the smaller craters within Kaiser (including dunes of unknown morphologies, sand sheets and sand bedforms) was calculated to be 2,611 km2. We then examined the dune fields within the crater and broke them into three broad categories based on dune morphology: barchan/barchanoid (BB), tranverse/complex (TC), and unknown/bedform (UB). The BB category contains seven sub-areas with a combined areal extent of 1,159 km2, the TC category has three sub-areas with a total of 1,161 km2, and the UB group has five sub-areas totaling 290 km2. The data for most of these deposits has certainly been overestimated, particularly for dune morphologies like barchans, as they tend to be isolated dunes with little to no inter-dunal sand. However, the TC category estimates are surely more accurate as the morphology allows for fewer inter-dunal areas that have little to no sand deposition. Preliminary estimates for wind direction based on observations of dune slip-face orientation for the western part of the Kaiser field are dominantly from the west with a smaller northwest, and in some cases a southwest, component as evidenced by barchan horn elongation. This elongation appears on the northern horn in the barchans in the southern deposits and on the southern horns in the barchans in the northern part of the larger contiguous Kaiser field. However, the slip-face geometry for the barchans and transverse dunes in the eastern part of the field indicate a dominant easterly wind. It is unknown at this time if the reversal is a function of topographic influences, seasonal wind change or larger climatic changes over a longer geologic period. Refinement of the slip-face orientations versus distribution and age correlations within the field are ongoing and preliminary estimates of dune deposit volume will be presented in December.
P21B-04 INVITED 08:52h
Aeolian environments observed by the Mars Exploration Rovers
Previous telescopic, orbital, and in situ exploration has shown the significance of aeolian processes on Mars. The twin MER vehicles have examined the effects of aeolian processes along geological traverses at two Mars landing sites. "Spirit" landed on a dust devil track on plains within Gusev crater, and encountered scattered low bedforms and ventifacts along its traverse to the Columbia Hills. Particle size-frequency variations between bedform crests and troughs are consistent with terrestrial ripple characteristics. Wheel disturbances in one bedform revealed the presence of a dust-covered coarse sand monolayer surface crust overlying a finer-grained, less sorted interior. Several lines of evidence are consistent with NW winds affecting the plains in this part of Gusev crater: (1) slightly asymmetric ripples sparsely distributed across the plains are oriented NNE/SSW and have slightly steeper ESE faces that are also dustier (as determined from albedo and thermal IR spectroscopy); (2) facets, flutes, and grooves on rocks interpreted as ventifacts are most abundant on NW exposures; and (3) asymmetric debris piles from Rock Abrasion Tool grinding extend to the SE. This evidence is consistent with afternoon WNW winds predicted in the area by mesoscale climate models. No dust devils have been observed yet (through 240 sols) by "Spirit." The landing site for "Opportunity" on the plains of Meridiani Planum was less dusty than the Gusev site. Hematite-enriched aeolian ripples dominate the plains, and contribute to the hematite signature detected by MGS TES. Trenching one of these bedforms revealed a surface crust of hematite-enriched spherule concretion fragments, and a substantial fraction of very fine sand in the ripple interior. Very fine sand is also found on the relatively flat areas between ripples, along with spherules and partly buried spherule fragments. Individual plains ripples are oriented about N26E but commonly are grouped en echelon into alignments along a secondary orientation of about N4E, suggesting reorientation during a clockwise change in wind direction. With a few exceptions, bedforms found inside depressions are different from ripples on the plains. A patch of ripples composed of dark, very fine sand on the floor of Eagle crater had lower abundance of finer-grained materials seen in soils elsewhere within Eagle. These ripples are oriented about N38E, consistent with transverse motion driven by the same winds responsible for the bright wind streak extending S45E downwind from Eagle crater; probably these sands are active. A complex of coalesced star dunes, ripples, and drifts about 50 m across was observed at the bottom of Endurance crater. The bright wind streak extending from Eagle crater is typical of many similar features observed from orbit on the plains of Meridiani Planum. Spectrometer results and morphological observations indicate the bright wind streak primarily is air fall dust remaining in the most protected areas immediately downwind of Eagle crater. In rock outcrops, small tails of sculpted rock extend from some spherules still partially embedded within and protruding from some rock units. These features imply erosion of rock due to saltating particles driven by strong, unidirectional wind episodes with an abundant upwind supply of saltating particles.
P21B-05 09:14h
Wind Patterns at the {\it Mars Exploration Rover} (MER) Sites Inferred from {\it Mars Express} HRSC and {\it MER} Images
Various wind-related features exist at the {\it Spirit} (Gusev) and {\it Opportunity} (Sinus Meridiani) sites, including those seen from the surface (e.g., ripples, "wind tails" associated with rocks, and ventifacts), and from orbit (e.g., linear low-albedo patterns, some of which change with time). The orientations of these features suggest that formative winds at the {\it Spirit} site are from the northwest, consistent with predictions of prevailing winds from the {\it Mars Regional Atmospheric Modeling System} (MRAMS). Most of the aeolian features seen from orbit at the {\it Spirit} site are dark tracks thought to represent the passage of dust devils. HRSC images show that many tracks formed recently, including the one on which {\it Spirit} landed; {\it MER} Microscopic Imager data show that sands within the track zone are moderately dust free, while sands outside the track are dusty. Thus, dark dust devil tracks here and perhaps elsewhere on Mars are confirmed to represent the removal of bright dust to expose a darker substrate, which, at the {\it Spirit} site, consists of coarser-grained sands and granules in the bedforms and silt- to fine-sand in the soils. MRAMS suggests that strong winds occur in the afternoon in Gusev crater, a time consistent with dust-devil formation on both Earth and Mars. At the Meridiani site, prevailing wind-related features seen from orbit are bright wind streaks associated with craters, the orientations of which suggest formative winds from the northwest. Detailed mapping of aeolian features using {\it MER} data shows that local topography (such as craters and other depressions) at both rover sites can have a strong influence on the inferred wind patterns. Thus, data from both orbit and the surface are required to determine the general wind regime.
P21B-06 09:29h
Large Ripple Bedforms on Mars: A Tale of Two Different Terrestrial Analogs
Aeolian bedforms intermediate in size between traditional dunes and small, aerodynamic ripples are common in high resolution images from orbit and at both Mars Exploration Rover sites. The bedforms show a range of ages, preservation potentials, and morphologic details. The authors have undertaken a systematic study of potential terrestrial analogs throughout the southwestern United States and elsewhere. Two general types of analogous bedforms have been found, one comprised of a ridge of loose granules overlying a substrate of loose sand. The other type comprises linear concentrations of granules capping low ridges of playa materials, with the suggestion that erosion/deflation has removed the interridge material. We have measured topographic profiles across representative examples of both types using a laser level, and we have initiated long-duration observations in order to assess migration and creation/destruction rates for the analog bedforms. At least some of the analog bedforms change very slowly under present environmental conditions, and they may be relatively impervious to damage due to burial and/or sedimentological transport, so they could be long-lived. There is some preliminary evidence that similar martian bedforms are older than would be expected for ephemeral aeolian bedforms. Other results to date include the following general observations: large ripples require a bimodal distribution of surface particle size, however, such a distribution is a necessary, but not necessarily sufficient, requirement; bedform wavelength and height is crudely proportional to the size of the particles comprising the bedform; the ripple index (wavelength/height ratio) is approximately constant (=~15) for both martian and terrestrial large ripple bedforms. Research supported by NASA Mars Data Analysis Program grant NAG5-11075.
P21B-07 09:44h
Aeolian Abrasion, a Dominant Erosion Agent in the Martian Environment
Aeolian abrasion is one of the predominant erosion mechanisms on Mars today. Martian ventifacts record the climate under which the rocks were modified (wind direction, wind speeds and particle flux) and therefore tie into the overall climatic regime of the planet. By better understanding the rates at which rocks abrade and the features diagnostic of specific climatic conditions, we can gain insight into past climates. Herein we report on numerical models, wind tunnel experiments, and field work to determine 1) Particle and kinetic fluxes on Earth and Mars, 2) the degree to which these parameters control abrasion, and 3) how, in detail, rocks of various shapes and compositions erode over time. Kinetic energy generally increases with height, whereas flux decreases, and impact angles, which affect energy transfer, and rebound effects are functions of the rock facet angle. This results in a non-linear relationship between abrasion potential and height that is a function of wind speed, planetary environment, and target geometry. We have computed the first three of these parameters numerically using a numerical saltation code, combined with published flux calculations These results have been compared to wind tunnel tests of flux vs. height, abrasion of erodible targets, and high speed video analysis under terrestrial and Martian pressures. We are also using high resolution laser scanning to characterize textures, shapes, and weathering changes for terrestrial and Martian rocks at the 100s of microns scale. We find that facet angle, texture, and rock heterogeneity are of critical importance in determining the rate and style of abrasion. Field and theoretical results demonstrate that high speed winds, not the integrated flux of lower speeds, and sand, not dust, produce most rock abrasion. On Mars, this requires sustained winds above 20-25 m/s at the near surface, a challenge in the current environment.