Dust devil speeds, directions of motion and general characteristics observed by the Mars Express High Resolution Stereo Camera

A total of 205 dust devils were detected in 23 High Resolution Stereo Camera (HRSC) images taken between January 2004 and July 2006 with the ESA Mars Express orbiter, in which average dust devil heights were ∼660 m and average diameters were ∼230 m. For the first time, dust devil velocities were directly measured from orbit, and range from 1 to 59 m/s. The observed dust devil directions of motion are consistent with data derived from a General Circulation Model (GCM). In some respects HRSC dust devil properties agree favorably with data from the NASA Mars Exploration Rover Spirit dust devil analyses. The spatial distribution of the active dust devils detected by HRSC supports the conjecture that the ascending branch of the Hadley circulation is responsible for the increase in dust devil activity, especially observed during southern summer between 50° and 60° S latitude. Combining the dust-lifting rate of 19 kg/km²/sol derived from the Spirit observations with the fewer in number but larger in size dust devils from various other locations observed by HRSC, we suggest that dust devils make a significant contribution to the dust entrainment into the atmosphere and to the martian dust cycle.     

Power law distribution of pressure drops in dust devils: Observation techniques and Earth–Mars comparison

Data from the Pathfinder and Phoenix landers on Mars show transient pressure drops (～1–4 per day) attributed to nearby encounters with dust devils or dust-free vortices. The distribution of pressure drop amplitudes is consistent with a truncated power law distribution with a slope of ?2, similar to that suggested previously for the optical diameters of dust devils. Comparable data from terrestrial field observations are very sparse: the only published dataset is half a century old and lists only 19 pressure drops. That dataset is too small to permit a robust comparison with Mars and likely suffers from a low detection efficiency at small dust devil sizes. Observed pressure drops in these fixed-station Mars datasets (30–300 μbar) are 10× lower than those typically observed on Earth (0.3–3 mbar): some higher drops have been reported for large terrestrial devils sampled by pursuing vehicles. The needed terrestrial data for comparison with Mars in-situ data (soon to be augmented, we hope, by the Mars Science Laboratory mission) is noted. Prospects for obtaining such data via field campaigns using new data acquisition technology, and with microbarographs for nuclear test monitoring, are discussed.     

Multitemporal observations of identical active dust devils on Mars with the High Resolution Stereo Camera (HRSC) and Mars Orbiter Camera (MOC)

Active dust devils were observed in Syria Planum in Mars Observer Camera - Wide Angle (MOC-WA) and High Resolution Stereo Camera (HRSC) imagery acquired on the same day with a time delay of ∼26 min. The unique operating technique of the HRSC allowed the measurement of the traverse velocities and directions of motion. Large dust devils observed in the HRSC image could be retraced to their counterparts in the earlier acquired MOC-WA image. Minimum lifetimes of three large (avg. ∼700 m in diameter) dust devils are ∼26 min, as inferred from retracing. For one of these large dust devil (∼820 m in diameter) it was possible to calculate a minimum lifetime of ∼74 min based on the measured horizontal speed and the length of its associated dust devil track. The comparison of our minimum lifetimes with previous published results of minimum and average lifetimes of small (∼19 m in diameter, avg. min. lifetime of ∼2.83 min) and medium (∼185 m in diameter, avg. min. lifetime of ∼13 min) dust devils imply that larger dust devils on Mars are active for much longer periods of time than smaller ones, as it is the case for terrestrial dust devils. Knowledge of martian dust devil lifetimes is an important parameter for the calculation of dust lifting rates. Estimates of the contribution of large dust devils (>300-1000 m in diameter) indicate that they may contribute, at least regionally, to ∼50% of dust entrainment by dust devils into the atmosphere compared to the dust devils <300 m in diameter given that the size-frequency distribution follows a power-law. Although large dust devils occur relatively rarely and the sediment fluxes are probably lower compared to smaller dust devils, their contribution to the background dust opacity by dust devils on Mars could be at least regionally large due to their longer lifetimes and ability of dust lifting into high atmospheric layers.     

Assessing the power law hypothesis for the size-frequency distribution of terrestrial and martian dust devils

Competing hypotheses for the diameter dependence of terrestrial and martian dust devil frequency are assessed using new field observations from two sites in the southwestern United States. We show that at diameters less than 12 m, our observed dust devil size-frequency distributions are better fit by an exponential function than by a power law formulation, and discuss the implications for larger dust devils on Earth and Mars.     

Convective vortices on Mars: a reanalysis of Viking Lander 2 meteorological data, sols 1-60

Dust devil data from Mars is limited by a lack of data relating to diurnal dust devil behaviour. Previous work looking at the Viking Lander meteorological data highlighted seasonal changes in temporal occurrence of dust devils and gave an indication of typical dust devil diameter, size, and internal dynamics. The meteorological data from Viking Lander 2 for sols 1 to 60 have been revisited to provide detailed diurnal dust devil statistics. Results of our analysis show that the Viking Lander 2 experienced a possible 38 convective vortices in the first 60 sols of its mission with a higher occurrence in the morning compared to Earth, possibly as a result of turbulence generated by the Lander body. Dust devil events have been categorised by statistical confidence and intensity. Some initial analysis and discussion of the results is also presented. Assuming a similar dust loading to the vortices seen by Mars Pathfinder, it is estimated that the amount of dust lofted in the locality of the Lander is approximately 800 ± 10 kgsol⁻¹km⁻².     

The meteorological signatures of dust devils on Mars

This paper describes a method for identifying martian dust devils and convective vortices in meteorological data. We have combined analysis of terrestrial dust devil fieldwork, re-analysis of martian meteorological data and laboratory experiments to explore fully the meteorological signature of dust devils. Both martian and terrestrial dust devils have similar characteristics implying a common formation mechanism. Terrestrial fieldwork therefore provides vital data that can be used to aid in the identification of their martian counterparts. Finally, a martian surface instrument package is suggested that will best detect dust devils and convective vortices without the need for visual confirmation.     

Mars mesospheric zonal wind around northern spring equinox from infrared heterodyne observations of CO2

We present direct observations of Mars zonal wind velocities around northern spring equinox (L_S = 336°, L_S = 355°, L_S = 42°) during martian year 27 and 29. Data was acquired by means of infrared heterodyne spectroscopy of CO₂ features at 959.3917 cm^?1 (10.4232 μm) and 957.8005 cm^?1 (10.4405 μm) using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) at the McMath–Pierce telescope of the National Solar Observatory on Kitt Peak in Arizona and the NASA Infrared Telescope Facility on Mauna Kea, Hawaii between 2005 and 2008. Winds were measured on the dayside of Mars with an unprecedented spatial resolution allowing sampling of up to nine independent latitudes over the martian disk. Retrieved wind velocities depend strongly on latitude and season with values ranging from 180 m/s prograde to ?94 m/s retrograde. A comparison of the observational results to predicted values from the Mars Climate Database yield a reasonable agreement between modeling and observation.     

Dust ejection from planetary bodies by temperature gradients: Laboratory experiments

Laboratory experiments show that dusty bodies in a gaseous environment eject dust particles if they are illuminated. We find that even more intense dust eruptions occur when the light source is turned off. We attribute this to a compression of gas by thermal creep in response to the changing temperature gradients in the top dust layers. The effect is studied at a light flux of 13 kW/m² and 1 mbar ambient pressure. The effect is applicable to protoplanetary disks and Mars. In the inner part of protoplanetary disks, planetesimals can be eroded especially at the terminator of a rotating body. This leads to the production of dust which can then be transported towards the disk edge or the outer disk regions. The generated dust might constitute a significant fraction of the warm dust observed in extrasolar protoplanetary disks. We estimate erosion rates of about 1 kg s^?1 for 100 m parent bodies. The dust might also contribute to subsequent planetary growth in different locations or on existing protoplanets which are large enough not to be susceptible to particle loss by light induced ejection. Due to the ejections, planetesimals and smaller bodies will be accelerated or decelerated and drift outward or inward, respectively. The effect might also explain the entrainment of dust in dust devils on Mars, especially at high altitudes where gas drag alone might not be sufficient.     

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.     

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.     

Dynamical evolution of the Hungaria asteroids

The Hungarias are a stable asteroid group orbiting between Mars and the main asteroid belt, with high inclinations (16–30°), low eccentricities (e < 0.18), and a narrow range of semi-major axes (1.78–2.06 AU). In order to explore the significance of thermally-induced Yarkovsky drift on the population, we conducted three orbital simulations of a 1000-particle grid in Hungaria a–e–i space. The three simulations included asteroid radii of 0.2, 1.0, and 5.0 km, respectively, with run times of 200 Myr. The results show that mean motion resonances—martian ones in particular—play a significant role in the destabilization of asteroids in the region. We conclude that either the initial Hungaria population was enormous, or, more likely, Hungarias must be replenished through collisional or dynamical means. To test the latter possibility, we conducted three more simulations of the same radii, this time in nearby Mars-crossing space. We find that certain Mars crossers can be trapped in martian resonances, and by a combination of chaotic diffusion and the Yarkovsky effect, can be stabilized by them. Therefore, some Hungarias (around 5% of non-family members with absolute magnitudes H < 15.5 and 10% for H < 17) may represent previously transient Mars crossers that have been adopted in this manner.     

The atmospheric circulation and dust activity in different orbital epochs on Mars

A general circulation model is used to evaluate changes to the circulation and dust transport in the martian atmosphere for a range of past orbital conditions. A dust transport scheme, including parameterized dust lifting, is incorporated within the model to enable passive or radiatively active dust transport. The focus is on changes which relate to surface features, as these may potentially be verified by observations. Obliquity variations have the largest impact, as they affect the latitudinal distribution of solar heating. At low obliquities permanent CO₂ ice caps form at both poles, lowering mean surface pressures. At higher obliquities, solar insolation peaks at higher summer latitudes near solstice, producing a stronger, broader meridional circulation and a larger seasonal CO₂ ice cap in winter. Near-surface winds associated with the main meridional circulation intensify and extend polewards, with changes in cap edge position also affecting the flow. Hence the model predicts significant changes in surface wind directions as well as magnitudes. Dust lifting by wind stress increases with obliquity as the meridional circulation and associated near-surface winds strengthen. If active dust transport is used, then lifting rates increase further in response to the larger atmospheric dust opacities (hence circulation) produced. Dust lifting by dust devils increases more gradually with obliquity, having a weaker link to the meridional circulation. The primary effect of varying eccentricity is to change the impact of varying the areocentric longitude of perihelion, l, which determines when the solar forcing is strongest. The atmospheric circulation is stronger when l aligns with solstice rather than equinox, and there is also a bias from the martian topography, resulting in the strongest circulations when perihelion is at northern winter solstice. Net dust accumulation depends on both lifting and deposition. Dust which has been well mixed within the atmosphere is deposited preferentially over high topography. For wind stress lifting, the combination produces peak net removal within western boundary currents and southern midlatitude bands, and net accumulation concentrated in Arabia and Tharsis. In active dust transport experiments, dust is also scoured from northern midlatitudes during winter, further confining peak accumulation to equatorial regions. As obliquity increases, polar accumulation rates increase for wind stress lifting and are largest for high eccentricities when perihelion occurs during northern winter. For dust devil lifting, polar accumulation rates increase (though less rapidly) with obliquity above o=25°, but increase with decreasing obliquity below this, thus polar dust accumulation at low obliquities may be increasingly due to dust lifted by dust devils. For all cases discussed, the pole receiving most dust shifts from north to south as obliquity is increased.     

Active Cosmic Dust Collector

The Stardust mission returned two types of unprecedented extraterrestrial samples: the first samples of material from a known solar system body beyond the moon, the comet 81P/Wild2, and the first samples of contemporary interstellar dust. Both sets of samples were captured in aerogel and aluminum foil collectors and returned to Earth in January 2006. While the analysis of particles from comet Wild 2 yielded exciting new results, the search for and analysis of collected interstellar particles is more demanding and is ongoing.Novel dust instrumentation will tremendously improve future dust collection in interplanetary space: an Active Cosmic Dust Collector is a combination of an in-situ dust trajectory sensor (DTS) together with a dust collector consisting of aerogel and/or other collector materials, e.g. such as those used by the Stardust mission. Dust particles’ trajectories are determined by the measurement of induced electrical signals when charged particles fly through a position sensitive electrode system. The recorded waveforms enable the reconstruction of the velocity vector with high precision.The DTS described here was subject to performance tests at the Heidelberg dust accelerator at the same time as the recording of impact signals from potential collector materials. The tests with dust particles in the speed range from 3 to 40 km/s demonstrate that trajectories can be measured with accuracies of ～1° in direction and ～1% in speed. The sensitivity of the DTS electronics is of the order of 10^?16 C and thus the trajectory of cosmic dust particles as small as 0.4 μm size can be measured. The impact position on the collector can be determined with better than 1 mm precision, which will ease immensely the task of locating sub-micron-sized particles on the collector. Statistically significant numbers of trajectories of interplanetary and interstellar dust particles can thus be collected in interplanetary space and their compositions correlated with their trajectories.     

Martian winds and dust clouds

Previous calculations of the surface wind stress required to raise dust on Mars are reconsidered and the threshold friction velocity is found to be about 2.0 m sec^?1 with particles of 200–300 μm being the most easily lifted. With this friction velocity, the planetary resistance law yields a corresponding wind at the top of the Ekman layer of 60 m sec^?1, and the logarithmic wind law yields a corresponding wind at the top of the Prandtl layer of 38 m sec^?1. These speeds are somewhat lower than those used by previous investigators.Various mechanisms for producing such strong winds are examined and it is concluded that the general circulation, thermal effects of topography, mechanical effects of topography and dust devils are all capable of doing so.Dust storms associated with small-scale disturbances are found to be incapable of growth. A scaling analysis of the equations of horizontal motion and of hydrostatic balance shows that a dust cloud at least 10 km thick and several tens of km in radius can, by absorption of sunlight, generate temperature gradients that, in turn, produce winds capable of raising more dust. Thus, a feedback mechanism is suggested in which an initial dust cloud exceeding certain critical dimensions can grown to planetary size. The preference of large dust storms to occur at southern hemisphere summer solstice is attributed to the maximum of insolation at that time. It is suggested that the frequent origin in the Noachis-Hellas region may be due to orographic features of the right scale and to low height in that area.     

Periglacial mass-wasting landforms on Mars suggestive of transient liquid water in the recent past: Insights from solifluction lobes on Svalbard

On Earth, periglacial solifluction is a slow mass-wasting process related to freeze–thaw activity. We compare the morphology of small-scale lobate features on Mars to solifluction lobes in Svalbard to constrain their processes of formation. The analysis is based on high-resolution satellite imagery of Mars (HiRISE, ～25 cm/pxl), aerial images of Svalbard with a similar spatial resolution (HRSC–AX, ～20 cm/pxl) acquired through an air campaign in summer 2008, and ground truth obtained during two summer expeditions in 2009 and 2011 on Svalbard. We present a detailed study of two crater environments on Mars displaying two types of lobate forms, characterized as sorted (clast-banked) and non-sorted lobes. On both Svalbard and Mars such lobes typically occur as clusters of overlapping risers (lobe fronts), pointing to differential velocities in the soil. The martian small-scale lobes have well-defined arcuate risers and lobe treads (surface). Lobe widths range between 14 and 127 m and tread lengths between 13 and 105 m. Riser height is estimated to be approximately 1–5 m. The lobes on Mars share the plan view morphology of solifluction lobes on Svalbard and their morphometry is within the range of values of terrestrial solifluction lobes. The lobes are distinct from permafrost-creep landforms such as rock glaciers. We show the results of a survey of 53 HiRISE images covering latitudes between 59°N and 81°N. Similar to Svalbard, the studied lobate features on Mars occur in close spatial proximity to gullies and thermal contraction polygons. The widespread distribution of the lobate forms in the northern hemisphere and their close association to ground-ice and gullies are best explained by mass-wasting processes related to frost creep, gelifluction and/or plug-like flow. This suggests a protracted process (thousand to several thousands of years) of freeze–thaw activity at the northern high latitudes on Mars. Age constraints on lobe deposits and superposition relationships with gullies and polygons imply a process involving liquid water within the last few million years.     

Free collisions in a microgravity many-particle experiment. I. Dust aggregate sticking at low velocities

Over the past years the processes involved in the growth of planetesimals have extensively been studied in the laboratory. Based on these experiments, a dust-aggregate collision model was developed upon which computer simulations were based to evaluate how big protoplanetary dust aggregates can grow and to analyze which kinds of collisions are relevant in the solar nebula and are worth further studies in the laboratory. The sticking threshold velocity of millimeter-sized dust aggregates is one such critical value that have so far only theoretically been derived, as the relevant velocities could not be reached in the laboratory. We developed a microgravity experiment that allows us for the first time to study free collisions of mm-sized dust aggregates down to velocities of ～0.1 cm s^?1 to assess this part of the protoplanetary dust evolution model. Here, we present the results of 125 free collisions between dust aggregates of 0.5–2 mm diameter. Seven collisions with velocities between 0.2 and 3 cm s^?1 led to sticking, suggesting a transition from perfect sticking to perfect bouncing with a certain sticking probability instead of a sharp velocity threshold. We developed a model to explain the physical processes involved in dust-aggregate sticking, derived dynamical material properties of the dust aggregates from the results of the collisions, and deduced the velocity below which dust aggregates always stick. For millimeter-sized porous dust aggregates this velocity is 8 × 10^?5 m s^?1.     

A one-year seasonal analysis of martian gravity waves using MCS data

Temperature measurements from the Mars Climate Sounder on NASA’s Mars Reconnaissance Orbiter are examined for gravity wave signals using the Stockwell transform, a technique previously applied to terrestrial temperature profiles. An analysis is presented for internal-gravity waves throughout the martian atmosphere for the period July 2007–May 2009, representing a full martian year of data, divided by season. Momentum fluxes observed in the altitude range 200–20 Pa are measured as ～10^?5–10^?3 Pa, significantly higher than at equivalent altitudes on Earth, and are primarily peaked around the tropics. Observed orographic effects are minimal; the primary observed effects are seasonal. Waves are shown to vary significantly in wavelength with latitude, but are generally approximately zonally symmetric. Horizontal wavelengths are consistently much shorter in the northern hemisphere than the southern.     

Accretion of dust by chondrules in a MHD-turbulent solar nebula

Numerical magnetohydrodynamic (MHD) simulations of a turbulent solar nebula are used to study the growth of dust mantles swept up by chondrules. A small neighborhood of the solar nebula is represented by an orbiting patch of gas at a radius of 3 AU, and includes vertical stratification of the gas density. The differential rotation of the nebular gas is replaced by a shear flow. Turbulence is driven by destabilization of the flow as a result of the magnetorotational instability (MRI), whereby magnetic field lines anchored to the gas are continuously stretched by the shearing motion. A passive contaminant mimics small dust grains that are aerodynamically well coupled to the gas, and chondrules are modeled by Lagrangian particles that interact with the gas through drag. Whenever a chondrule enters a region permeated by dust, its radius grows at a rate that depends on the local dust density and the relative velocity between itself and the dust. The local dust abundance decreases accordingly. Compaction and fragmentation of dust aggregates are not included. Different chondrule volume densities ρ_c lead to varying depletion and rimmed-chondrule size growth times. Most of the dust sweep-up occurs within ～1 gas scale-height of the nebula midplane. Chondrules can reach their asymptotic radius in 10–800 years, although short growth times due to very high ρ_c may not be altogether realistic. If the sticking efficiency Q of dust to chondrules depends on their relative speed δv, such that Q < 10^?2 whenever δv > v_stick ≈ 34 cm/s (with v_stick a critical sticking velocity), then longer growth times result due to the prevalence of high MRI-turbulent relative velocities. The vertical variation of nebula turbulent intensity results in a moderate dependence of mean rimmed-chondrule size with nebula height, and in a ～20% dispersion in radius values at every height bin. The technique used here could be combined with Monte Carlo (MC) methods that include the physics of dust compaction, in a self-consistent MHD-MC model of dust rim growth around chondrules in the solar nebula.     

Statistical distribution of atmospheric dust devils

Theoretical predictions with regard to dust devil (apparent) angular size–frequency distribution are made and critically compared with Mars Exploration Rover (MER) Spirit optical observations. For an idealized horizontal viewing geometry one should expect that the number of dust devils having the apparent angular diameter greater than a given angle α is inversely proportional to α squared. The actual dependency for Spirit dust devils is in between the inverse-squared and simple inverse laws, and close to the latter one for small and moderate angles α. It is emphasized that such a comparison can be considered as a benchmark for completeness and adequateness of dust-devil optical observations and correctness of competing analytical formulations for dust devil size–frequency distribution.