The rate of granule ripple movement on Earth and Mars

The rate of movement for 3- and 10-cm-high granule ripples was documented in September of 2006 at Great Sand Dunes National Park and Preserve during a particularly strong wind event. Impact creep induced by saltating sand caused ∼24 granules min⁻¹ to cross each cm of crest length during wind that averaged ∼9 m s⁻¹ (at a height well above 1 m), which is substantially larger than the threshold for saltation of sand. Extension of this documented granule movement rate to Mars suggests that a 25-cm-high granule ripple should require from hundreds to thousands of Earth-years to move 1 cm under present atmospheric conditions.     

Determination of the wind induced detachment threshold for granular material on Mars using wind tunnel simulations

Experimental results are presented of wind induced grain detachment under Mars simulation conditions. A simple force balance equation is applied to quantify the wind shear stress required for removal of glass spheres from a sand bed. The transport of fine grained martian dust is simulated by the detachment of hollow glass spheres which resemble low mass density dust aggregates observed to form during simulations when using Mars analogue material. The results agree well with observations of dust removal and wind speed measurements made by the NASA Viking landers at the martian surface. This work supports the suggestion that dust aggregate fragmentation allows wind induced dust entrainment at substantially lower wind shear than that of solid sand grains and has direct application to the study of global dust transport and martian climatology.     

The electrical properties of Mars analogue dust

Dust is a major environmental factor on the surface and in the atmosphere of Mars. Knowing the electrical charge state of this dust would be of both scientific interest and important for the safety of instruments on the Martian surface. In this study the first measurements have been performed of dust electrification using suspended Mars analogue material. This has been achieved by attracting suspended dust onto electrodes placed inside a Mars simulation wind tunnel. The Mars analogue used was from Salten Skov in Denmark, this contained a high concentration of ferric oxide precipitate. Once suspended, this dust was found to consist of almost equal quantities of negatively (46±6%) and positively (44±15%) charged grains.These grains were estimated to typically carry a net charge of around 10⁵e, this is sufficient to dominate the processes of adhesion and cohesion of this suspended dust. Evidence is presented for electrostatic aggregation of the dust while in suspension. Development of a simple instrument for measuring electrical charging of the suspended dust on Mars will be discussed.     

On possible release of microbe-containing particulates from a Mars lander spacecraft

Due to possible planet contamination, before Earth-departure, Mars landers and/or rovers are subject to strict requirements on the maximum number of attached spores or particles that carry viable microbes. Estimates of the release rates of these particles on Mars are made considering the three mechanisms of wind shear, collision with suspended dust, and collision with saltating sand particles. The first mechanism is found to apply only to particles of size greater than , the second mechanism has a characteristic particle adhesion half life that is so long as to be of no concern, and the third mechanism is deemed of possible importance, vitally depending on attached particle size and detailed surface characteristics of sand and spacecraft. While not investigated in detail, dust devils are shown to be possible contributors to release of microbe-containing particles.     

Venusian saltation

Estimates of the trajectories of saltating particles on Venus show the level of saltation on Ve low when compared to either Earth or Mars. Particles in saltation on Venus obtain maximum heights of only 1 cm over a wide range in particle size and surface wind speeds. Their path lenghts are only a few centimeters at the wind speed of 1 and 2 m/sec. The entire saltation process and particle trajectories are insensitive to changes in surface pressure over the range from 70 to 100 bars and to changes in surface temperature over the range from 600 to 900°K. Secondly, the net rate of surface material transport due to saltation on Venus is small when compared to Earth or Mars. This result is due to the dense Venusian atmosphere. It is estimated that approximately 10 times more surface materials is transported by saltation on Earth than on Venus for dynamically similar conditions. And approximately 250 times more material is moved by the saltation process on Mars than on Venus, again for dynamically similar conditions. Both these estimates apply over a wide range of particle diameter, from 0.01 to 7 mm. Thirdly, the ripple wavelenghts may be small, such that thay may not be detected by the high-resolution radar images of the surface of Venus.     

Fluid transport on Earth and aeolian transport on Mars

Experimental data on cohesion-free particle transport in fluid beds are applied, via a universal scaling relation, to atmospheric transport of fine grains on Mars. It may be that cohesion due to impact vitrification, vacuum sintering, and adsorbed thin films of water are absent on Mars—in which case the curve of threshold velocity versus grain size may show no turnup to small particle size, and one micron diameter grains may be injected directly by saltation into the Martian atmosphere more readily than 100 micron diameter grains. Curves for threshold and terminal velocities are presented for the full range of Martian pressures and temperatures. Suspension of fine grains is significantly easier at low temperatures and high pressures; late afternoon brightenings of many areas of Mars, and the generation of dust storms in such deep basins as Hellas, may be due to this effect.     

Aeolian bedforms, yardangs, and indurated surfaces in the Tharsis Montes as seen by the HiRISE Camera: Evidence for dust aggregates

HiRISE images of Mars with ground sampling down to 25 cm/pixel show that the dust-rich mantle covering the surfaces of the Tharsis Montes is organized into ridges whose form and distribution are consistent with formation by aeolian saltation. Other dusty areas near the volcanoes and elsewhere on the planet exhibit a similar morphology. The material composing these “reticulate” bedforms is constrained by their remote sensing properties and the threshold curve combined with the saltation/suspension boundary, both of which vary as a function of elevation (atmospheric pressure), particle size, and particle composition. Considering all of these factors, dust aggregates are the most likely material composing these bedforms. We propose that airfall dust on and near the volcanoes aggregates in situ over time, maybe due to electrostatic charging followed by cementation by salts. The aggregates eventually reach a particle size at which saltation is possible. Aggregates on the flanks are transported downslope by katabatic winds and form linear and “accordion” morphologies. Materials within the calderas and other depressions remain trapped and are subjected to multidirectional winds, forming an interlinked “honeycomb” texture. In many places on and near the volcanoes, light-toned, low thermal inertia yardangs and indurated surfaces are present. These may represent “duststone” formed when aggregates reach a particle size below the threshold curve, such that they become stabilized and subsequently undergo cementation.     

An integrated laser anemometer and dust accumulator for studying wind-induced dust transport on Mars

Windborne dust is one of the most important and dynamic factors affecting the Martian surface and its atmosphere, yet there lacks a detailed physical understanding how it is transported. We present a miniature laser-based optoelectronic instrument for use on a Mars lander. It integrates sensors capable of quantifying important parameters needed for the understanding and modeling of dust transport on Mars, these include wind speed, wind direction, suspended dust concentration, dust deposition and removal rates as well as the electrification of the Martian dust. Dust electrification has been seen from experimental simulations to be of considerable importance to the processes of adhesion and cohesion, specifically prompting the formation of low mass density dust aggregates. Testing of this prototype instrument has been performed under simulated Martian conditions in a wind tunnel facility. The results and analysis of its functionality will be presented.     

Mineral alteration induced by sand transport: A source for the reddish color of martian dust

The iron mineral thought to give the characteristic reddish color to martian dust could have formed through mechanical activation during sand transport. This has been demonstrated experimentally under conditions which are known to occur on Mars.     

Aeolian processes and dune morphology in Gale Crater

Datasets at resolutions many times greater than previously available were used to study aeolian features within Gale Crater. High resolution thermal inertia data allowed for detailed particle size estimation, with the data sufficient to resolve dunefields. A wide range of grain sizes have now been identified in the Gale Crater dunefields, ranging from medium to very coarse sand. High Resolution Imaging Science Experiment (HiRISE) and THEMIS VIS data allowed for detailed analysis of the dune morphology and slip-faces, which shows that the dunes have responded to topographic influences on prevailing wind directions under a present day wind regime. This result was corroborated by a regional mesoscale model for the crater under dust storm conditions. The central mound and smaller scale crater floor topography has altered the prevailing wind regime and dune patterns. Aeolian activity has thus played, and continues to play, an important role in shaping many of the present surface features of Gale Crater. The arrival of a future lander mission such as the Mars Science Laboratory would be able to sample these surface features directly and add a wealth of data to the understanding of Gale Crater.     

Interannual variability of Mars global dust storms: an example of self-organized criticality?

Previous simulations of martian global dust storms with a simple low-order model showed the desired interannual variability of storms if one of the model parameters—the threshold wind speed for starting saltation and lifting dust from the surface—was finely tuned. In this paper we show that the fine-tuning of this parameter could be the result of negative feedback in which processes associated with global dust storms raise the threshold and small-scale processes like dust devils, which are active in years between the storms, lower the threshold.     

Modification of martian slope streaks by eolian processes

Recent images from the High Resolution Imaging Science Experiment (HiRISE) camera have shown that slope streaks have relief on the order of a meter or less. This study presents observations of transverse bedforms and infill deposits within slope streak beds that were not previously identified or were uncommon from earlier analyses of HiRISE images. Transverse bedforms are linear to slightly arcuate features oriented transverse to the slope streak bed which may be analogous to terrestrial splash or coarse-grained ripples based on their morphology, wavelength, and amplitude. In addition to the bedforms, there is also evidence that slope streak beds gradually shallow over time by infilling of material. The presence of ripples within slope streaks implies that saltation-capable material is available on the surface today and/or was available in the recent past. Although airfall dust is not a capable saltation source material, aggregates of electrostatically-bound dust that are possibly later cemented by salts may serve as a source. From the results of this study, we hypothesize a sequence of events in a slope streak formation and modification cycle where grains saltate to form ripples along the bed of a slope streak, airfall dust mantling causes gradual fading of the streak, and infill material buries the ripples, eventually reaching the pre-avalanche surface that removes all traces of relief.     

A miniature laser anemometer for measurement of wind speed and dust suspension on Mars

Suspended dust is a dominant component of the Martian environment. It has a major influence on atmospheric circulation and it is deposited widely over the planetary surface causing a serious hazard to instrumentation. In order to study dust transport, quantification of the wind flow and dust concentration are vital. A simple laser-based anemometer system is presented that is able to measure suspended dust grain velocity and turbulence from a landed spacecraft. This system has advantages over other techniques of wind speed determination in being insensitive to contamination or atmospheric conditions such as temperature, pressure or composition. For the first time it would allow direct measurement of the suspended dust concentration on Mars. A prototype instrument has been constructed and successfully tested in a wind tunnel facility under simulated Martian conditions. The optics are simple in design, light weight and the instrument has low power consumption. It is also robust and the output signal is easily interpreted, producing only a small data volume. Future improvements will be discussed, specifically modification to measure wind direction, the possibility of obtaining information about dust grain size and the construction of a flight model.     

Interannual Variability of Martian Global Dust Storms: Simulations with a Low-Order Model of the General Circulation

We present simulations of the interannual variability of martian global dust storms (GDSs) with a simplified low-order model (LOM) of the general circulation. The simplified model allows one to conduct computationally fast long-term simulations of the martian climate system. The LOM is constructed by Galerkin projection of a 2D (zonally averaged) general circulation model (GCM) onto a truncated set of basis functions. The resulting LOM consists of 12 coupled nonlinear ordinary differential equations describing atmospheric dynamics and dust transport within the Hadley cell. The forcing of the model is described by simplified physics based on Newtonian cooling and Rayleigh friction. The atmosphere and surface are coupled: atmospheric heating depends on the dustiness of the atmosphere, and the surface dust source depends on the strength of the atmospheric winds. Parameters of the model are tuned to fit the output of the NASA AMES GCM and the fit is generally very good.Interannual variability of GDSs is possible in the LOM, but only when stochastic forcing is added to the model. The stochastic forcing could be provided by transient weather systems or some surface process such as redistribution of the sand particles in storm generating zones on the surface. The results are sensitive to the value of the saltation threshold, which hints at a possible feedback between saltation threshold and dust storm activity. According to this hypothesis, erodable material builds up as a result of a local process, whose effect is to lower the saltation threshold until a GDS occurs. The saltation threshold adjusts its value so that dust storms are barely able to occur.     

Further observations of regional dust storms and baroclinic eddies in the northern hemisphere of Mars

We have investigated the near-surface meteorology in the northern hemisphere of Mars through detailed analysis of data obtained with Mars Global Surveyor in January-August 2005. The season in the northern hemisphere ranged from midsummer through winter solstice of Mars Year (MY) 27. We examined composite, wide-angle images from the Mars Orbiter Camera and compiled a catalog of the dust storms that occurred in this interval. As in previous martian years, activity in the northern hemisphere was dominated by regional “flushing” dust storms that sweep southward through the major topographic basins, most frequently in Acidalia Planitia. We also used atmospheric profiles retrieved from radio occultation experiments to characterize eddy activity near the surface at high northern latitudes. There are strong correlations between the two sets of observations, which allowed us to identify three factors that influence the timing and location of the regional dust storms: (1) transitions among baroclinic wave modes, which strongly modulate the intensity of meridional winds near the surface, (2) storms zones, which impose strong zonal variations on the amplitude of some baroclinic eddies, and (3) stationary waves, which further modulate the wind field near the surface. The flushing dust storms ceased abruptly in midautumn, possibly in response to source depletion, CO₂ condensation, a shift in the period of the baroclinic eddies, and changes in the tidal wind field near the surface. Our results extend the meteorological record of the northern hemisphere, substantiate the findings of previous investigations, and further illuminate the climatic impact of baroclinic eddies.     

Investigations of the variability of dust particle sizes in the martian atmosphere using the NASA Ames General Circulation Model

We present a Mars General Circulation Model (GCM) numerical investigation of the physical processes (i.e., wind stress and dust devil dust lifting and atmospheric transport) responsible for temporal and spatial variability of suspended dust particle sizes. Measurements of spatial and temporal variations in airborne dust particles sizes in the martian atmosphere have been derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectral and emission phase function data [Wolff, M.J., Clancy, R.T., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002057. 1-1; Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002058. 2-1]. The range of dust particle sizes simulated by the NASA Ames GCM is qualitatively consistent with TES-derived observations of effective dust particle size variability. Model results suggest that the wind stress dust lifting scheme (which produces regionally confined dust lifting) is the process responsible for the majority of the dust particle size variability in the martian atmosphere. Additionally, model results suggest that atmospheric transport processes play an important role in the evolution of atmospheric dust particles sizes during substantial dust storms on Mars. Finally, we show that including the radiative effects of a spatially variable particle size distribution significantly influences thermal and dynamical fields during the dissipation phase of the simulated global dust storm.     

Bright dust devil tracks on Earth: Implications for their formation on Mars

We report on the first observations of bright dust devil tracks (BDDTs) on Earth, observed in the Turpan depression desert in northwestern China, where raindrop impacts on sand surfaces form aggregates of sand, silt and clay resulting in rough surface textures, which are destroyed by passages of dust devils leading to smooth surface textures within the tracks. The differences in photometric properties between the track and outside the tracks cause the albedo differences leading to the formation of BDDTs and similar processes might lead to BDDTs on Mars in areas with thick dust covers.     

Mesoscale linear streaks on Mars: environments of dust entrainment

Variable surface albedo features on Mars are likely caused by the entrainment and deposition of dust by the wind. Most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface. Some of the largest variable features, here termed mesoscale linear streaks, are up to 400 km in length and repeatedly occur in one of the smoothest regions of Mars: Amazonis Planitia. Their orientations and apparent season of variability as observed by Viking and Mars Orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms. They provide an example of the ability of large-scale winds, without significant local enhancement, to initiate dust motion on Mars. The sizes and spacing of the linear streaks may be controlled by boundary layer rolls. The repetitive formation of these streaks, over a span of more than 11 Mars years, gives one measure of the stability of Mars’ eolian processes.     

Red/violet contrast reversal on Mars: Significance for eolian sediments

Albedo markings on Mars can exhibit reversed contrast with their surroudings when imaged in “red” and “violet” light. A complete search of Viking Orbiter images shows this phenomenon (on scales less than 300 km) is restricted to specific eolian features: intracrater deposits and wind streaks originating from the deposits. Contrast reversal is not found between features (such as lava flows of different ages) that might expose different materials without a largely eolian influence. Laboratory simulations suggest that iron oxides are the most likely materials involved in contrast reversal on the Martian surface. Red/violet contrast reversal is achieved easily (but not exclusively) between samples from which very fine particles (<5 μm) in diameter) have been removed, and corresponding samples in which larger grains are coated by such fine particles. Substantial particle-size-dependent albedo and color viriations exist for material which can be carried in suspension on Mars (<100 μm). Thus, all fine-grained eolian deposits on Mars need not be the same as the brighter parts of Arabia, which have colors and albedos similar to the fine (<10 μm) component of Martian dust storms. The observed contrast reversal characteristics and colors of the intracrater dunes and related sediments can be explained readily if they are essentially free of adhering dust, as would be the case if such eolian features were subject toactive saltation.     

Dust devil sediment flux on Earth and Mars: Laboratory simulations

Laboratory simulations using the Arizona State University Vortex Generator (ASUVG) were run to simulate sediment flux in dust devils in terrestrial ambient and Mars-analog conditions. The objective of this study was to measure vortex sediment flux in the laboratory to yield estimations of natural dust devils on Earth and Mars, where all parameters may not be measured. These tests used particles ranging from 2 to 2000 μm in diameter and 1300 to 4800 kg m⁻³ in density, and the results were compared with data from natural dust devils on Earth and Mars. Typically, the cores of dust devils (regardless of planetary environment) have a pressure decrease of ∼0.1-1.5% of ambient atmospheric pressure, which enhances the lifting of particles from the surface. Core pressure decreases in our experiments ranged from ∼0.01% to 5.00% of ambient pressure (10 mbar Mars cases and 1000 mbar for Earth cases) corresponding to a few tenths of a millibar for Mars cases and a few millibars for Earth cases. Sediment flux experiments were run at vortex tangential wind velocities of 1-45 m s⁻¹, which typically correspond to ∼30-70% above vortex threshold values for the test particle sizes and densities. Sediment flux was determined by time-averaged measurements of mass loss for a given vortex size. Sediment fluxes of ∼10⁻⁶-10⁰ kg m⁻² s⁻¹ were obtained, similar to estimates and measurements for fluxes in dust devils on Earth and Mars. Sediment flux is closely related to the vortex intensity, which depends on the strength of the pressure decrease in the core (ΔP). This study found vortex size is less important for lifting materials because many different diameters can have the same ΔP. This finding is critical in scaling the laboratory results to natural dust devils that can be several orders of magnitude larger than the laboratory counterparts.