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Located on top of the mast of the Mars 2020 Perseverance rover, the SuperCam instrument suite includes a microphone to record audible sounds from 100 Hz to 10 kHz on the surface of Mars. It will support SuperCam’s Laser-Induced Breakdown Spectroscopy investigation by recording laser-induced shock-waves but it will also record aeroacoustic noise generated by wind flowing past the microphone. This experimental study was conducted in the Aarhus planetary wind-tunnel under low CO 2 pressure with wind generated at several velocities. It focused on understanding the wind-induced acoustic signal measured by microphones instrumented in a real scale model of the rover mast as a function of the wind speed and wind orientation. Acoustic spectra recorded under a wind flow show that the low-frequency range of the microphone signal is mainly influenced by the wind velocity. In contrast, the higher frequency range is seen to depend on the wind direction relative to the microphone. On the one hand, for the wind conditions tested inside the tunnel, it is shown that the Root Mean Square of the pressure, computed over the 100 Hz to 500 Hz frequency range, is proportional to the dynamic pressure. Therefore, the SuperCam microphone will be able to estimate the wind speed, considering an in situ cross-calibration with the Mars Environmental Dynamic Analyzer. On the other hand, for a given wind speed, it is observed that the root mean square of the pressure, computed over the 500 Hz to 2000 Hz frequency range, is at its minimum when the microphone is facing the wind whereas it is at its maximum when the microphone is pointing downwind. Hence, a full 360 ∘ rotation of the mast in azimuth in parallel with sound recording can be used to retrieve the wind direction. We demonstrate that the SuperCam Microphone has a priori the potential to determine both the speed and the direction of the wind on Mars, thus contributing to atmospheric science investigations.

Aeolian sediment transport is observed to occur on Mars as well as other extraterrestrial environments, generating ripples and dunes as on Earth. The search for terrestrial analogues of planetary bedforms, as well as environmental simulation experiments able to reproduce their formation in planetary conditions, are powerful ways to question our understanding of geomorphological processes towards unusual environmental conditions. Here, we perform sediment transport laboratory experiments in a closed-circuit wind tunnel placed in a vacuum chamber and operated at extremely low pressures to show that Martian conditions belong to a previously unexplored saltation regime. The threshold wind speed required to initiate saltation is only quantitatively predicted by state-of-the art models up to a density ratio between grain and air of $4 \times 10^5$, but unexpectedly falls to much lower values for higher density ratios. In contrast, impact ripples, whose emergence is continuously observed on the granular bed over the whole pressure range investigated, display a characteristic wavelength and propagation velocity essentially independent of pressure. A comparison of these findings with existing models suggests that sediment transport at low Reynolds number but high grain-to-fluid density ratio may be dominated by collective effects associated with grain inertia in the granular collisional layer., Comment: 38 pages, 21 figures

The resuspension of volcanic ash by wind is a significant source of hazard during and after volcanic eruptions. Parameterizing and modeling ash resuspension requires direct measurement of the minimum wind shear stress required to move particles, usually expressed as the threshold friction velocity U ⁎ th, a parameter that, for volcanic ash, has been measured only scarcely and always in the laboratory. Here, we report the first field measurements of U ⁎ th for volcanic ash, with a portable wind tunnel specifically developed, calibrated, and tested. Field measurements, performed on natural reworked ash deposits from Sakurajima (Japan) and Cordón Caulle (Chile) volcanoes, agree well with our laboratory determinations on ash from the same deposits, with values of U ⁎ th ranging from 0.13 to 0.38 m/s. Our results show that the median grain size of the deposit and particle shape have a stronger control on U ⁎ th than the local substratum nature and deposit texture.

International audience; The SuperCam instrument suite onboard the Mars 2020 rover will include the Mars Microphone, an experiment designed to record the sounds of the SuperCam laser strikes on rocks and also aeolian noise. In order to record shock waves produced by the laser blasts, the Mars Microphone must be able to record audio signals from 100 Hz to 10 kHz on the surface of Mars, with a sensitivity sufficient to monitor a laser impact at distances up to 4 m. The Aarhus planetary simulator facility has been used to test the Mars 2020 rover microphone in a controlled Martian environment. The end-to-end tests performed in a 6 mbar CO2 atmosphere, with wind, and also with the microphone at −80° C have demonstrated that the SuperCam/Mars Microphone requirements are satisfied. Tests were also performed on Martian soil simulant targets showing that the variation of the acoustic energy of the shock wave depends on the level of compaction of the target.

We have experimentally investigated CO2 ice condensation under conditions similar to those expected in the Martian polar areas using the Environmental Wind Tunnel of Aarhus University in the framework of Trans-National Access opportunities within the EuroPlanet Research Infrastructure. Our goal was to condense CO2 directly from the chamber's atmosphere onto a specially designed cooling plate. We investigated ranges of temperatures and pressures similar to those in Martian polar areas, observed the texture of the created CO2 ice layer, and measured its optical properties. Most importantly, we find that under conditions usual for Martian polar areas in fall and winter, CO2 ice always deposits as a translucent slab. The maximum thickness of the ice that we have achieved approached 2 cm and this CO2 slab ice layer was either transparent or highly translucent. Under significantly lower temperatures or in states away from equilibrium (for example, when CO2 gas input into the chamber was fast and created over-pressure) CO2 deposited as different crystalline structures. In this case, when a thick layer was created, it was not visually transparent. However, even in those cases it still retains some level of translucency. Our results will advance the analysis of remote-sensing data related to CO2 ice, Mars’ seasonal cycles, and various models considering CO2 ice condensation-sublimation related processes.

As a method to experimentally study aerodynamic drag the terminal settling velocities of aerosolized silica and glass microspheres (with sizes of 1–44 μm) have been measured at various gas pressures in the range 0.6–10 mbar and using various gas compositions including He, Ar, Xe, Air, CO 2 , and water vapour. Within the molecular scattering regime, i.e. where the Knudsen number Kn > 10 and up to 500, reasonable agreement has been found with the model of Epstein (1924). Values of the scattering parameter δ were observed to be within the expected range of 1–1.44 for respectively specular/evaporative - diffuse molecular scattering processes. However, δ was seen to depend upon gas composition, specifically for H 2 O; δ = 0.96 ± 0.07, CO 2 ; δ = 1.16 ± 0.07, noble gasses; δ = 1.25–1.44 and in the case of Air δ = 1.18 ± 0.07 which is not in agreement with that conventionally used of around 1.3–1.4. These observations imply disagreement with the concept of a general or universal model of drag for all particle surfaces and atmospheres. Similarly, for Kn < 10 these results were not well described by conventional models such as the semi-empirical expression of Knudsen-Weber (1911) or the general law of fall proposed by Millikan (1923b).

The increasing number of missions to Mars also increases the risk of forward contamination. Consequently there is a need for effective protocols to ensure efficient protection of the Martian environment against terrestrial microbiota. Despite the fact of constructing sophisticated clean rooms for spacecraft assembly a 100 % avoidance of contamination appears to be impossible. Recent surveys of these facilities have identified a significant number of microbes belonging to a variety of taxonomic groups that survive the harsh conditions of clean rooms. These microbes may have a strong contamination potential, which needs to be investigate to apply efficient decontamination treatments. In this study we propose a series of tests to evaluate the potential of clean room contaminants to survive the different steps involved in forward contamination. We used Staphylococcus xylosus as model organism to illustrate the different types of stress that potential contaminants will be subjected to on their way from the spacecraft onto the surface of Mars. Staphylococcus xylosus is associated with human skin and commonly found in clean rooms and could therefore contaminate the spacecraft as a result of human activity during the assembling process. The path the cell will take from the surface of the spacecraft onto the surface of Mars was split into steps representing different stresses that include desiccation, freezing, aeolian transport in a Martian-like atmosphere at Martian atmospheric pressure, and UV radiation climate. We assessed the surviving fraction of the cellular population after each step by determining the integrated metabolic activity of the survivor population by measuring their oxygen consumption rate. The largest fraction of the starting culture (around 70 %) was killed during desiccation, while freezing, Martian vacuum and short-term UV radiation only had a minor additional effect on the survivability of Staphylococcus xylosus. The study also included a simulation of atmospheric transport on Martian dust, which did not significantly alter the metabolic potential of the cells. The high survival potential of skin microbes, which are not among the most robust isolates, clearly underlines the necessity for efficient decontamination protocols and of adequate planetary protection measures. Thus we propose a series of tests to be included into the description of isolates from spacecraft assembly clean rooms in order to assess the forward contamination potential of the specific isolate and to categorize the risk level according to the organisms survival potential. We are aware that the tests that we propose do not exhaust the types of challenges that the microbes would meet on their way and therefore the series of tests is open to being extended.

The results of the Labeled Release and the Gas Exchange experiments conducted on Mars by the Viking Landers show that compounds in the Martian soil can cause oxidation of organics and a release of oxygen in the presence of water. Several sources have been proposed for the oxidizing compounds, but none has been validated in situ and the cause of the observed oxidation has not been resolved. In this study, laboratory simulations of saltation were conducted to examine if and under which conditions wind abrasion of silicates, a process that is common on the Martian surface, can give rise to oxidants in the form of hydrogen peroxide ( H 2 O 2 ) and hydroxyl radicals (⋅OH). We found that silicate samples abraded in simulated Martian atmospheres gave rise to a significant production of H 2 O 2 and ⋅OH upon contact with water. Our experiments demonstrated that abraded silicates could lead to a production of H 2 O 2 facilitated by atmospheric O 2 and inhibited by carbon dioxide. Furthermore, during simulated saltation the silicate particles became triboelectrically charged and at pressures similar to the Martian surface pressure we observed glow discharges. Electrical discharges can cause dissociation of CO 2 and through subsequent reactions lead to a production of H 2 O 2 . These results indicate that the reactions linked to electrical discharges are the dominant source of H 2 O 2 during saltation of silicates in a simulated Martian atmosphere, given the low pressure and the relatively high concentration of CO 2 . Our experiments provide evidence that wind driven abrasion could enhance the reactivity of the Martian soil and thereby could have contributed to the oxidation of organic compounds and the O 2 release observed in the Labeled Release and the Gas Exchange experiments. Furthermore, the release of H 2 O 2 and ⋅OH from abraded silicates could have a negative effect on the persistence of organic compounds in the Martian soil and the habitability of the Martian surface.

Numerical modeling of ash plume dispersal is an important tool for forecasting and mitigating potential hazards from volcanic ash erupted during explosive volcanism. Recent tephra dispersal models have been expanded to account for dynamic ash aggregation processes. However, there are very few studies on rates of disaggregation during transport. It follows that current models regard ash aggregation as irrevocable and may therefore overestimate aggregation-enhanced sedimentation. In this experimental study, we use industrial granulation techniques to artificially produce aggregates. We subject these to impact tests and evaluate their resistance to break-up processes. We find a dependence of aggregate stability on primary particle size distribution and solid particle binder concentration. We posit that our findings could be combined with eruption source parameters and implemented in future tephra dispersal models.

The contact electrification of aerosolized micro particles has been studied using a novel technique involving laser velocimetry. This has allowed the simultaneous determination of size and electrical charge of individual silica microspheres (in the range 1 – 8 µm). Interestingly the particles interacting with the injector tube have been seen to become electrified with a relatively narrow range of surface charge concentration of around Q/4πr 2 ~ −100 e - /µm 2 (~ −0.02 mC/m 2 ) for all particle sizes. Several combinations of aerosol particle and injector tube composition were also investigated, some of which led to positive particle electrification and all of which resulted in similar values of measured surface charge concentration. The electrification was not seen to be strongly affected by gas composition and is in reasonable agreement with the expected maximum surface charge observed in previous experiments ( 2 ). Possible explanations for this effect are discussed, including the possibility of field emission at the contact site. In the future this technique is intended also to be applied to particle-particle induced contact electrification and its material dependence.

International audience; Atmospheric transport and suspension of dust frequently brings electrification, which may be substantial. Electric fields of 10 kV m−1 to 100 kV m−1 have been observed at the surface beneath suspended dust in the terrestrial atmosphere, and some electrification has been observed to persist in dust at levels to 5 km, as well as in volcanic plumes. The interaction between individual particles which causes the electrification is incompletely understood, and multiple processes are thought to be acting. A variation in particle charge with particle size, and the effect of gravitational separation explains to, some extent, the charge structures observed in terrestrial dust storms. More extensive flow-based modelling demonstrates that bulk electric fields in excess of 10 kV m−1 can be obtained rapidly (in less than 10 s) from rotating dust systems (dust devils) and that terrestrial breakdown fields can be obtained. Modelled profiles of electrical conductivity in the Martian atmosphere suggest the possibility of dust electrification, and dust devils have been suggested as a mechanism of charge separation able to maintain current flow between one region of the atmosphere and another, through a global circuit. Fundamental new understanding of Martian atmospheric electricity will result from the ExoMars mission, which carries the DREAMS (Dust characterization, Risk Assessment, and Environment Analyser on the Martian Surface)—MicroARES (Atmospheric Radiation and Electricity Sensor) instrumentation to Mars in 2016 for the first in situ electrical measurements.

International audience; We review selected experimental saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed.We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4m long, and focuses on the transitional behavior near the upwind roughness discontinuity where saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearlywith the impact velocity of the saltating particles. Studies of saturated saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reachmuch higher than Bagnold's focal point is virtually independent of Shields parameter—at least for lowor intermediate u⁎-values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40mmindicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreementwith a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow.Quantitative understanding of saltation characteristics onMars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter N 0.6mm) and that saltation occurs at high wind speeds (N26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be b20 m/s, with even turbulent gusts not exceeding 25 m/s. Electrification is seen as a dominant factor in the transport dynamics of dust onMars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments.

Wind tunnels have been used extensively in industry and research applications over the past 50 years. They vary greatly in scale and geometry, with some large enough to house and test small aircraft (see for example NASA, ATP facilities) and others are miniaturized flow generators used in the calibration of small sensors. However they invariably utilize the same basic technology and design elements. Similarly environmental simulators are also used widely in research, for example in climate and planetary studies. Here again they superficially vary greatly in size and configuration, but basically consist of a hermetic chamber with some form of temperature control [Jensen et al. 2008]. There is therefore a broad array of standard and often commercial technologies and construction techniques which have been successfully applied within the fields of wind tunnel and environmental simulator design. Some of these technologies and techniques will be outlined in this chapter to aid researchers or technology developers in their efforts to design or use environmental wind tunnels and also serve as an informative guide to those new to these fields of investigation. The fusion of an environmental simulator and a wind tunnel is a natural evolution of laboratory based technology to fulfill the need to reproduce specific physical conditions found in nature. Although facilities of this kind are only now being fully developed, they have the potential to expand into a new research field that could substantially contribute to our understanding of climate and mediate growth in advanced sensor technologies. In this chapter many of the challenges in designing and constructing environmental wind tunnels will be introduced and possible solutions presented, with some emphasis placed on extreme terrestrial and Martian planetary conditions. In addition some of the many and varied scientific and industrial applications will be discussed. Generally environmental wind tunnels are already in current use as a method of testing and calibrating meteorology sensors of various kinds especially wind flow sensors (anemometers). Application of wind tunnels in civil engineering and town planning is becoming common place. Here through wind tunnel simulation and modeling the flow of air around buildings and through built-up areas may be useful to avoid the generation of high wind shear and hazardous vortices at periods of high wind or storms. Such simulations can also aid in the design and placement of wind generation systems such as wind turbines. The formalized scaling laws developed by Reynolds (Reynolds equations) allows measurements, for example in smaller scale laboratory wind tunnels, which generate the same (or extremely similar) flow to that generated in the natural setting [Monin and Yaglom

Wind tunnels for investigating the effect of wind on sand and soil movement have been constructed by many groups since Brigadier R.A. Bagnold build one of the first wind tunnels for investigating the threshold of motion of sand (Bagnold 1941). Initiated by the serious soil erosion in the American Midwest during the 1930’s another wind tunnel facility was set up later in the decade at Kansas State University at the USDA-ARS Wind Erosion Research Unit for soil erosion research. A wind tunnel for studying particle motion under fluid densities as on the planet Mars was as the first of its kind build at the NASA Ames Research Centre, California in the mid 1970’s under guidance of Dr. Ronald Greeley (Greeley et al. 1981). The wind tunnel laboratory at Aarhus University was started in the 1970’s and have to day two sand transport wind tunnels and two low pressure wind tunnels for studying sand and dust transport under Martian conditions. This chapter will deal with descriptions of the materials used in wind tunnel experiments and research lay-outs for the wind tunnels at the Aarhus University wind tunnel laboratory.

A long-standing unresolved puzzle related to the Danish temperate humid climate is the presence of extended areas with large Fe contents, where goethite and ferrihydrite are present in the topsoil along with hematite and maghemite. Hematite and, particularly, maghemite would normally be interpreted as the result of high temperature as found after forest fires. However, a body of evidence argues against these sites having been exposed to fire. In an attempt to get closer to an explanation of this Fe mineralogy, an experimental forest fire was produced. The results showed a clear mineralogical zonation down to 10 cm depth. This was not observed at the natural sites, which contained a mixture of goethite/ferrihydrite, hematite and maghemite down to 20 cm depth. The experimental forest fire left charcoal and ashes at the topsoil, produced high pH and decreased organic matter content, all of which is in contrast to the natural sites. The conclusion from this work is that the mineralogy of these sites is not consistent with exposure to forest fire, but may instead result from long-term transformation in a reducing environment, possibly involving microbiology.

Particle lifting in dust devils on both Earth and Mars has been studied from many different perspectives, including how dust devils could influence the dust cycles of both planets. Here we review our current understanding of particle entrainment by dust devils by examining results from field observations on Earth and Mars, laboratory experiments (at terrestrial ambient and Mars-analog conditions), and analytical modeling. By combining insights obtained from these three methodologies, we provide a detailed overview on interactions between particle lifting processes due to mechanical, thermal, electrodynamical and pressure effects, and how these processes apply to dust devils on Earth and Mars. Experiments and observations have shown dust devils to be effective lifters of dust given the proper conditions on Earth and Mars. However, dust devil studies have yet to determine the individual roles of each of the component processes acting at any given time in dust devils.

Ash deposited during volcanic eruptions can be resuspended by wind and become hazardous for health and infrastructure hours to decades after an eruption. Accurate resuspension forecasting requires accurate modelling of the threshold friction velocity of the volcanic particles (Uth*), which is the key parameter controlling volcanic ash detachment by wind. Using an environmental wind tunnel facility this study provides much needed experimental data on volcanic particle resuspension, with the first systematic parameterization of Uth* for ash from the regions Campi Flegrei in Italy and also Eyjafjallajökull in Iceland. In this study atmospheric relative humidity (and related ash moisture content) was systematically varied, from 90%, which in the case of the Eyjafjallajökull fine ash (Uth*. Using the Campi Flegrei fine ash (Uth* of only around a factor of 1.5 was observed. Reasonable agreement with force balance resuspension models was seen, which implied an increase in interparticle adhesion force of up to a factor of six due to high humidity. Our results imply that, contrary to dry conditions, one single modelling scheme may not satisfy the resuspension of volcanic ash from different eruptions under wet conditions.

In-Situ Resource Utilisation (ISRU) can reduce the mass and cost of planetary missions. The Mars Oxygen ISRU Experiment (MOXIE) on the Mars 2020 rover Perseverance will demonstrate ISRU on Mars for the first time by producing oxygen from atmospheric carbon dioxide via solid oxide electrolysis. To protect the solid oxide electrolysis subsystem from contamination by dust, a High Efficiency Particulate Air (HEPA) filter is used. However, the performance of HEPA filters in Martian atmospheric conditions is not well understood. The theory of filtration was reviewed in the context of filtration of Mars’ atmosphere, and an experimental investigation was carried out to determine the dust loading rate and pressure drop as a function of dust loading and filtration velocity for a flight-representative pleated and baffled MOXIE HEPA filter using wind tunnels and Martian dust simulant. In simulated atmospheric conditions of 10.3 ​mbar carbon dioxide at room temperature with a horizontal wind speed of 3 ​m ​s−1 and filter inlet face velocity of 7.1 ​cm ​s−1, the dust loading rate was (0.19 ± 0.02) mg ​m−2 h−1. This is likely a lower bound: analytical approaches estimate dust loading rates of up to approximately 20 ​mg ​m−2 h−1. The pressure drop ΔP (mbar) as a function of dust loading m (g ​m−2) and filtration velocity UF (cm ​s−1) was ΔP=am+bUF, where a = 0.0012(1)mbar (g m-2)-1 (cm s-1)-1 and b = 0.063(1) mbar (cm s-1)-1. Due to operation outside the continuum flow regime, pressure drop increased with atmospheric pressure, unlike HEPA filters on Earth where pressure drop is independent of atmospheric pressure. Dust is unlikely to produce a problematic pressure drop for MOXIE, but needs to be considered for large-scale filtration if the benefits of atmospheric ISRU on Mars are to be fully realised.