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.     

Saltation impact as a means for raising dust on Mars

Experiments were conducted under atmospheric pressures appropriate for Earth and Mars to determine the efficiency of sand in saltation as a means for raising dust into the atmosphere under wind speeds which would otherwise be too low for dust entrainment. Experiments involving intimate mixtures of sand and dust (1:1 ratio by mass) showed that after an initial flurry of activity of a few seconds duration, the bed stabilized with little movement of either sand or dust. In contrast, sands set into saltation upwind from dust beds were efficient in injecting the dust into suspension, with low-pressure Martian conditions being some five times more efficient than terrestrial conditions. This result is attributed to the higher kinetic energies of the saltating grains on Mars, which is a consequence of the higher velocities of the grains. These results suggest that sands saltating across dust beds on Mars are an effective means for setting dust into suspension.     

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.     

Device for measuring surface accumulation of dust: applications for future magnetic properties experiments on Mars

A new optical instrument has been developed to precisely measure the local accumulation of dust particles on a surface. This device can be used in combination with applied magnetic or electric fields in order to investigate physical properties of the dust and its interactions with the surface. In this prototype instrument, permanent magnets were used to capture suspended magnetic dust in a Mars simulation wind tunnel. The scientific objectives that may be addressed with such a device are discussed.     

Water-ice clouds and dust in the north polar region of Mars using MGS TES data

Water-ice and dust optical depths in Mars’ north polar region are mapped as function of season, latitude and longitude, and their characteristics and variability on a geographic, seasonal, and interannual basis are discussed. We use water-ice and dust optical depth data provided by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), covering nearly three northern spring and summer periods. We find that interannual variability exists in both the water ice and dust behavior, although there are trends that repeat year to year as well. The optical thickness of the north polar hood (NPH) exhibits interannually varying longitudinal structure, both during springtime recession and late-summer onset. We define the characteristics associated with the transition to and from the NPH and find that the disappearance occurs near L_s=75° and the reappearance near L_s=160-165°. We find that the late spring to early summer time frame is characterized by very low water-ice optical depths and enhanced dust activity, with a preference for lower water-ice and higher dust optical depths in the 0-90°W quadrant. We see possible evidence for stationary wavenumber 2 systems in a few of the maps examined.     

Effect of Mars analogue dust deposition on the automated detection of calcite in visible/near-infrared spectra

We have developed an artificial neural net detector for use on board Mars rovers that correctly identifies calcite under Mars analogue dust (JSC Mars-1 regolith simulant) layers up to ∼100 μm thickness and 80% aerial coverage. Both the detector output and the band depth of the ∼2300 nm CO⁼₃ absorption are linearly related to the surface area of exposed calcite. This detector provides a means for rapid and robust automated recognition of calcite on Mars in areas of active aeolian erosion.     

MOC observations of the 2001 Mars planet-encircling dust storm

From 15 September 1997 through 21 January 2006, only a single planet-encircling martian dust storm was observed by MGS-MOC. The onset of the storm occurred on 26 June 2001 (Ls=184.7°), earliest recorded to date. It was initiated in the southern mid-to-low latitudes by a series of local dust storm pulses that developed along the seasonal cap edge in Malea and in Hellas basin (Ls=176.2°-184.4°). The initial expansion of the storm, though asymmetric, was very rapid in all directions (3-32 m s⁻¹). The main direction of propagation, however, was to the east, with the storm becoming planet encircling in the southern hemisphere on Ls=192.3°. Several distinct centers of active dust lifting were associated with the storm, with the longest persisting for 86 sols (Syria-Claritas). These regional storms helped generate and sustain a dust cloud (“haze”), which reached an altitude of about 60 km and a peak opacity of τdust∼5.0. By Ls=197.0°, the cloud had encircled the entire planet between 59.0° S and 60.0° N, obscuring all but the largest volcanoes. The decay phase began around Ls∼200.4° with atmospheric dust concentrations returning to nominal seasonal low-levels at Ls∼304.0°. Exponential decay time constants ranged from 30-117 sols. The storm caused substantial regional albedo changes (darkening and brightening) as a result of the redistribution (removal and deposition) of a thin veneer of surface dust at least 0.1-11.1 μm thick. It also caused changes in meteorological phenomena (i.e., dust storms, dust devils, clouds, recession of the polar caps, and possibly surface temperatures) that persisted for just a few weeks to more than a single Mars year. The redistribution of dust by large annual regional storms might help explain the long period (∼30 years) between the largest planet-encircling dust storms events.     

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.     

Preliminary analysis of the MER magnetic properties experiment using a computational fluid dynamics model

Motivated by questions raised by the magnetic properties experiments on the NASA Mars Pathfinder and Mars Exploration Rover (MER) missions, we have studied in detail the capture of airborne magnetic dust by permanent magnets using a computational fluid dynamics (CFD) model supported by laboratory simulations. The magnets studied are identical to the capture magnet and filter magnet on MER, though results are more generally applicable.The dust capture process is found to be dependent upon wind speed, dust magnetization, dust grain size and dust grain mass density. Here we develop an understanding of how these parameters affect dust capture rates and patterns on the magnets and set bounds for these parameters based on MER data and results from the numerical model. This results in a consistent picture of the dust as containing varying amounts of at least two separate components with different physical properties.     

A wind tunnel for the calibration of Mars wind sensors

A major limitation in the development of wind sensors for use on Mars is the lack of suitable testing and calibration facilities. A low-density wind tunnel has been developed at Oxford University for calibration of wind sensors for Mars landers, capable of providing stable or dynamically varying winds, of air or carbon dioxide, at Martian pressures (5-10 mbar) and speeds (0.5-30 m/s), and temperatures of 200-300 K. The flow field in the test section was calculated using analytical and computational modelling techniques, and validated experimentally using a pitot probe. This facility's stability and accuracy offer significant advantages with respect to previous calibration facilities.     

The geology of Australian Mars analogue sites

Australia has numerous landforms and features, some unique, that provide a useful reference for interpreting the results of spacecraft orbiting Mars and exploring the martian surface. Examples of desert landforms, impact structures, relief inversion, long-term landscape evolution and hydrothermal systems that are relevant to Mars are outlined and the relevant literature reviewed. The Mars analogue value of Australia's acid lakes, hypersaline embayments and mound spring complexes is highlighted along with the Pilbara region, where the oldest convincing evidence of life guides exploration for early life on Mars. The distinctive characteristics of the Arkaroola Mars Analogue Region are also assessed and opportunities for future work in Australia are outlined.     

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.     

Effect of charging of aerosols in the lower atmosphere of Mars during the dust storm of 2001

Aerosols are very important in the Martian climate system. Aerosols get charged by the attachment of ions in the atmosphere. Charging of aerosols reduces the conductivity of the atmosphere as the very mobile ions are lost during the ion-aerosol attachment. During a dust storm the dust opacity increases and more ion-aerosol attachment process occurs and consequently conductivity reduces further. It was found that with the background aerosols (dust opacity ∼0.2), the conductivity close to the surface of Mars was reduced by a factor of 5, but during the dust storm (opacity ∼5) of 2001 the conductivity decreased by about 2 orders of magnitude.     

Spectral reflectance properties of carbonates from terrestrial analogue environments: Implications for Mars

Spectroscopic analysis of carbonate-bearing samples from a variety of terrestrial environments provides important insights into spectroscopy-based investigations of Mars designed to detect the presence of carbonate minerals. In order to better address the spectral detectability of carbonates on Mars, we examined the spectral reflectance properties of carbonates and carbonate-bearing lithologies from a variety of terrestrial environments, including impact structures (Haughton, St. Martin, Eagle Butte), landslides (Frank), quarrying operations (Hecla), carbonates affected by weathering (Haughton, East German Creek), and sulfide-sulfate-carbonate assemblages (Central Manitoba). The goal is to identify processes and environments that can affect spectroscopy-based carbonate detection, for more detailed follow-on studies. Common carbonates appear to be stable, from a spectroscopic perspective, to various tectonic processes. Iron oxides/hydroxides do not appear to significantly affect spectral detectability of carbonates, as the spectrum-altering effects of these phases are largely restricted to the region below ∼1 μm, while useful carbonate absorption bands occur longward of ∼1.8 μm. Carbonate detection and characterization in the 0.35-2.5-μm region is largely restricted to a single absorption feature in the 2.3-μm region, which can be problematic for robust carbonate identification. While tectonic processes and iron oxide/hydroxide staining do not appear to significantly impair carbonate detection based on the 2.3-μm region absorption band, a number of other factors can affect carbonate detection. These include the fact that this absorption band is weak compared to many other minerals, a number of other minerals also exhibit absorption bands in this wavelength region (leading to possible misidentifications), and that even small abundances of minerals that absorb strongly in this region will reduce the strength of the carbonate absorption band. Identifying the nature of accessory minerals associated with carbonates can be used to constrain possible formation environments. Ongoing research at carbonate-bearing terrestrial analogue sites will continue to provide new insights into the occurrence and detection of carbonates on Mars.     

A distinct class of avalanche scars on Mars

We report observations of a set of surface features on Mars that form a distinct class of avalanche scars. These features have a horizontal scale of hundreds of meters, but a depth scale of meters distinguishes them from the shallower features known as slope streaks. The meters-thick avalanche scars have escaped previous attention because of weak contrast between the interiors of the scarred regions and their surroundings. Often the most visible feature is a shadow cast by the trough wall, a band 1-3 pixels wide in Mars Orbiter Camera narrow angle images, indicating maximum depths of 4-10 m. We investigate the morphology of more than 500 such features. Slopes upon which the avalanches occur average about 27°. Impact craters are seen at the heads of some avalanche scars; this subset exhibits statistically wider opening angles. The scars span an estimated several Ma in age. Those found so far occurred mainly in the Olympus Mons lower aureole. We compare shapes of slope streaks to shapes of meters-thick avalanches, and the results support the notion that the two classes are distinct. The newly-discovered avalanches resemble some terrestrial flows of loose, dry material such as dry snow and glass beads. On the basis of these analogs, we suggest a physical model.     

Albedo models for the residual south polar cap on Mars: Implications for the stability of the cap under near-perihelion global dust storm conditions

It is uncertain whether the residual (perennial) south polar cap on Mars is a transitory or a permanent feature in the current Martian climate. While there is no firm evidence for complete disappearance of the cap in the past, clearly observable changes have been documented. Observations suggest that the perennial cap lost more CO₂ material in the spring/summer season prior to the Mariner 9 mission than in those same seasons monitored by Viking and Mars Global Surveyor. In this paper we examine one process that may contribute to these changes—the radiative effects of a planet encircling dust storm that starts during late Martian southern spring on the stability of the perennial south polar cap. To approach this, we model the radiative transfer through a dusty planetary atmosphere bounded by a sublimating CO₂ surface.A critical parameter for this modeling is the surface albedo spectrum from the near-UV to the thermal-IR, which was determined from both space-craft and Earth-based observations covering multiple wavelength regimes. Such a multi-wavelength approach is highly desirable since one spectral band by itself cannot tightly constrain the three-parameter space for polar surface albedo models, namely photon “scattering length” in the CO₂ ice and the amounts of intermixed water and dust.Our results suggest that a planet-encircling dust storm with onset near solstice can affect the perennial cap's stability, leading to advanced sublimation in a “dusty” year. Since the total amount of solid CO₂ removed by a single storm may be less than the total CO₂ thickness, a series of dust storms would be required to remove the entire residual CO₂ ice layer from the south perennial cap.     

Surficial properties in Gale Crater, Mars, from Mars Odyssey THEMIS data

We examine the nature of the surface layer in Gale Crater as determined from high-resolution thermal and visible Mars Odyssey Thermal Emission Imaging System (THEMIS) data as well as how our conclusions compare to past analyses. At THEMIS resolution, the thermal surface structure is dominated by local control, thus providing us with detailed images that contain thermophysical information as well. Using these data sets we have created a map of the area, defining units based primarily on their geomorphology as determined from the daytime thermal and visible images and then using the nighttime thermal data to interpret the nature of the surface layer within each unit. Seven units have been defined: (i) partially blanketed knobby plateaus, (ii) crater walls with terrain similar to that on the plateaus on the upper half and exposed, rocky surfaces on the lower half, (iii)-(v) three floor units with varying combinations of bedrock and indurated and/or particulate deposits, (vi) sand sheets, and (vii) a central mound, consisting of indurated and/or rocky material forming layers, terraces, and slides, covered by particulate material that tapers in thickness downslope. Additionally, dozens of channels have been observed on the crater walls and central mound. The results indicate that aeolian processes have played a major role in shaping much of the present surface layer within Gale and may still be active today. Because of the dramatic size and structure of Gale, the winds are most likely controlled by the local topography. Additionally, the presence and frequency of channels within Gale bolster hypotheses involving aqueous episodes in the history of the crater.     

Surficial properties in Melas Chasma, Mars, from Mars Odyssey THEMIS data

We examine the nature of the surface layer in a small area of the Melas Chasma region as determined from high-resolution thermal and visible Mars Odyssey Thermal Emission Imaging System (THEMIS) data as well as how our conclusions compare to past analyses. At THEMIS resolution, the thermal structure is dominated by local control and all significant thermal variations can be linked to morphology. Thus, THEMIS provides us with detailed images that contain thermophysical information as well, allowing us to create a surficial geologic map intended to reflect the surface structure of the region. Eight units have been defined: (i) blanketed plateaus with thermally distinct craters and fractures, (ii) blanketed canyon walls with rocky edges, (iii) indurated and/or rocky canyon wall slide material partially covered by aeolian material, (iv) an anomalous wall region with fluvial-like depressions partially filled with particulate material, (v) indurated and/or rocky ridged and non-ridged canyon floor landslide material mingled with aeolian material, (vi) sand sheets, (vii) indurated and/or rocky rounded blocks intermingled with small areas of aeolian material, and (viii) transverse dunes. The THEMIS thermal data support conclusions from previous studies but also reveal much more structure than was seen in the past. We have found that all significant thermal variations in this region can be linked to morphology but all morphological variations cannot be linked to significant thermal variations. THEMIS visible images provide an intermediate resolution that bridges the gap between MOC and Viking and allow for a more meaningful interpretation of the geologic context of a region. Surfaces indicate that landslides were an important geologic process long ago, shaping the canyon walls and floor, while aeolian processes have subsequently altered the surface layer in many locations and may still be active.     

Extension of atmospheric dust loading to high altitudes during the 2001 Mars dust storm: MGS TES limb observations

Mars Global Surveyor (MGS) visible (solarband bolometer) and thermal infrared (IR) spectral limb observations from the Thermal Emission Spectrometer (TES) support quantitative profile retrievals for dust opacity and particle sizes during the 2001 global dust event on Mars. The current analysis considers the behavior of dust lifted to altitudes above 30 km during the course of this storm; in terms of dust vertical mixing, particle sizes, and global distribution. TES global maps of visible (solarband) limb brightness at 60 km altitude indicate a global-scale, seasonally evolving (over 190-240° solar longitudes, L_S) longitudinal corridor of vertically extended dust loading (which may be associated with a retrograde propagating, wavenumber 1 Rossby wave). Spherical radiative transfer analysis of selected limb profiles for TES visible and thermal IR radiances provide quantitative vertical profiles of dust opacity, indicating regional conditions of altitude-increasing dust mixing ratios. Observed infrared spectral dependences and visible-to-infrared opacity ratios of dust scattering over 30-60 km altitudes indicate particle sizes characteristic of lower altitudes (cross-section weighted effective radius, ), during conditions of significant dust transport to these altitudes. Conditions of reduced dust loading at 30-60 km altitudes present smaller dust particle sizes . These observations suggest rapid meridional transport at 30-80 km altitudes, with substantial longitudinal variation, of dust lifted to these altitudes over southern hemisphere atmospheric regions characterized by extraordinary (m/s) vertical advection velocities. By L_S=230° dust loading above 50 km altitudes decreased markedly at southern latitudes, with a high altitude (60-80 km) haze of fine (likely) water ice particles appearing over 10°S-40°N latitudes.