Martian Pressure Research Articles

Generation and migration of CO in CO2 DBD glow plasma under Martian pressure

AbstractDielectric barrier discharge (DBD) plasma is a potential tool in the field of in situ CO2 conversion with the low‐pressure environment of Mars. CO is an important intermediate product in the conversion process of CO2. Understanding the pathways and dynamics that govern the generation of CO in CO2 plasmas establishes the foundation for effective regulation. In this work, parallel‐plate DBD structure was employed in our experiment and one‐dimensional fluid simulation model. The findings indicate that CO primarily originates at the boundary of the cathode potential fall region, and it subsequently migrates toward the surface of instantaneous cathode where it accumulates. The thickness of CO‐enriched region is approximately 0.8 mm. During this process, CO migration speed reaches about 2000 m/s. It is worth noting that surface reactions at the instantaneous cathode and anode surfaces contribute only 0.24% to CO generation, in contrast to the predominant influence of impact dissociation reaction between CO2 and electrons (e + CO2 → 2e + CO + O+) at 53.21%, and two‐body decomposition reaction between O+ and CO2 (O+ + CO2 → O +2 + CO) at 35.88%. Finally, the primary factors influencing the migration of CO from production sites to enrichment regions are determined to be particle collisions and momentum exchange between ions and CO, followed by electro‐hydro dynamics force, while dielectrophoresis forces have minimal effect.

Real-Time Calculation of CO2 Conversion in Radio-Frequency Discharges under Martian Pressure by Introducing Deep Neural Network

In recent years, the in situ resource utilization of CO2 in the Martian atmosphere by low-temperature plasma technology has garnered significant attention. However, numerical simulation is extremely time-consuming for modeling the complex CO2 plasma, involving tens of species and hundreds of reactions, especially under Martian pressure. In this study, a deep neural network (DNN) with multiple hidden layers is introduced to investigate the CO2 conversion in radio-frequency (RF) discharges at a given power density under Martian pressure in almost real time. After training on the dataset obtained from the fluid model or experimental measurements, the DNN shows the ability to accurately and efficiently predict the various discharge characteristics and plasma chemistry of RF CO2 discharge even in seconds. Compared with conventional fluid models, the computational efficiency of the DNN is improved by nearly 106 times; thus, a real-time calculation of RF CO2 discharge can almost be achieved. The DNN can provide an enormous amount of data to enhance the simulation results due to the very high computational efficiency. The numerical data also suggest that the CO2 conversion increases with driving frequency at a fixed power density. This study shows the ability of the DNN-based approach to investigate CO2 conversion in RF discharges for various applications, providing a promising tool for the modeling of complex non-thermal plasmas.

The Dynamics of CO2‐Driven Granular Flows in Gullies on Mars

AbstractMartian gullies are landforms consisting of an erosional alcove, a channel, and a depositional apron. A significant proportion of Martian gullies at the mid‐latitudes is active today. The seasonal sublimation of CO2 ice has been suggested as a driver behind present‐day gully activity. However, due to a lack of in situ observations, the actual processes causing the observed changes remain unresolved. Here, we present results from flume experiments in environmental chambers in which we created CO2‐driven granular flows under Martian atmospheric conditions. Our experiments show that under Martian atmospheric pressure, large amounts of granular material can be fluidized by the sublimation of small quantities of CO2 ice in the granular mixture (only 0.5% of the volume fraction of the flow) under slope angles as low as 10°. Dimensionless scaling of the CO2‐driven granular flows shows that they are dynamically similar to terrestrial two‐phase granular flows, that is, debris flows and pyroclastic flows. The similarity in flow dynamics explains the similarity in deposit morphology with levees and lobes, supporting the hypothesis that CO2‐driven granular flows on Mars are not merely modifying older landforms, but they are actively forming them. This has far‐reaching implications for the processes thought to have formed these gullies over time. For other planetary bodies in our solar system, our experimental results suggest that the existence of gully like landforms is not necessarily evidence for flowing liquids but that they could also be formed or modified by sublimation‐driven flow processes.

How, when and where current mass flows in Martian gullies are driven by CO2 sublimation

Martian gullies resemble water-carved gullies on Earth, yet their present-day activity cannot be explained by water-driven processes. The sublimation of CO2 has been proposed as an alternative driver for sediment transport, but how this mechanism works remains unknown. Here we combine laboratory experiments of CO2-driven granular flows under Martian atmospheric pressure with 1D climate simulation modelling to unravel how, where, and when CO2 can drive present-day gully activity. Our work shows that sublimation of CO2 ice, under Martian atmospheric conditions can fluidize sediment and creates morphologies similar to those observed on Mars. Furthermore, the modelled climatic and topographic boundary conditions for this process, align with present-day gully activity. These results have implications for the influence of water versus CO2-driven processes in gully formation and for the interpretation of gully landforms on other planets, as their existence is no longer definitive proof for flowing liquids.

Formation and Stability of Salty Soil Seals in Mars‐Like Conditions. Implications for Methane Variability on Mars

AbstractMethane spikes observed in the Martian atmosphere require the abrupt release of large amounts of methane from the Martian subsurface. The mechanism for such release has not been identified. We tested whether gas traps can form under Mars‐like conditions in the shallow Martian regolith due to salt migration in the icy soil. Experiments were performed on various soil samples in a Mars Simulation Chamber comprising different perchlorate salts and water concentrations mixed with JSC Mars‐1A. Inside the chamber, the samples were exposed to a range of temperatures from −20°C to 10°C and maintained CO2 gaseous pressure between 8 and 10 mbars. As a methane analog, Neon was injected periodically underneath the soil sample. It was found that over a wide range of Mars‐like soil parameters, a gas impermeable soil seal can form over a relatively short period (3–13 days) but requires 5%–10% of perchlorate salt content in the soil. It was determined that such a seal could sustain several mbars of neon above the Martian atmospheric pressure in the soil. Based on our experiments, substantial amounts of gaseous methane may accumulate under the soil seal and get released abruptly into the atmosphere upon seal cracking. An abrupt release of methane from the shallow subsurface may help explain methane variability at the Martian surface, as the Mars Science Laboratory detected.

Perseverance MEDA Atmospheric Pressure Observations—Initial Results

AbstractThe Mars2020 Perseverance Rover landed successfully on the Martian surface on the Jezero Crater floor (18.44°N, 77.45°E) at Martian solar longitude, Ls, ∼5° in February 2021. Since then, it has produced highly valuable environmental measurements with a versatile scientific payload including the MEDA (Mars Environmental Dynamics Analyzer) suite of environmental sensors. One of the MEDA systems is the PS pressure sensor system, which weighs 40 g and has an estimated absolute accuracy of better than 3.5 Pa and a resolution of 0.13 Pa. We present initial results from the first 414 sols of Martian atmospheric surface pressure observations by the PS, whose performance was found to meet its specifications. Observed sol‐averaged atmospheric pressures follow an anticipated pattern of pressure variation in the course of the advancing season and are consistent with data from other landing missions. The observed daily pressure amplitude varies by ∼2%–5 % of the sol‐averaged pressure, with absolute amplitude 10–35 Pa in an approximately direct relationship with airborne dust. During a regional dust storm, which began at Ls ∼ 135°, the daily pressure amplitude roughly doubled. The daily pressure variations were found to be remarkably sensitive to the seasonal evolution of the atmosphere. In particular, analysis of the daily pressure signature revealed diagnostic information likely related to the regional scale structure of the atmosphere. Comparison of Perseverance pressure observations with data from other landers reveals the global scale seasonal behavior of Mars' atmosphere.

Survival of Environment-Derived Opportunistic Bacterial Pathogens to Martian Conditions: Is There a Concern for Human Missions to Mars?

The health of astronauts during space travel to new celestial bodies in the Solar System is a critical factor in the planning of a mission. Despite cleaning and decontamination protocols, microorganisms from the Earth have been and will be identified on spacecraft. This raises concerns for human safety and planetary protection, especially if these microorganisms can evolve and adapt to the new environment. In this study, we examined the tolerance of clinically relevant nonfastidious bacterial species that originate from environmental sources (Burkholderia cepacia, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Serratia marcescens) to simulated martian conditions. Our research showed changes in growth and survival of these species in the presence of perchlorates, under desiccating conditions, exposure to ultraviolet radiation, and exposure to martian atmospheric composition and pressure. In addition, our results demonstrate that growth was enhanced by the addition of a martian regolith simulant to the growth media. Additional future research is warranted to examine potential changes in the infectivity, pathogenicity, and virulence of these species with exposure to martian conditions.

Volumetric Changes of Mud on Mars: Evidence From Laboratory Simulations

AbstractSubtle mounds have been discovered in the source areas of Martian kilometer‐sized flows and on top of summit areas of domes. These features have been suggested to be related to subsurface sediment mobilization, opening questions regarding their formation mechanisms. Previous studies hypothesized that they mark the position of feeder vents through which mud was brought to the surface. Two theories have been proposed: (a) ascent of more viscous mud during the late stage of eruption and (b) expansion of mud within the conduit due to the instability of water under Martian conditions. Here, we present experiments performed inside a low‐pressure chamber designed to investigate whether the volume of mud changes when exposed to a Martian atmospheric pressure. Depending on the mud viscosity, we observe a volumetric increase of up to 30% at the Martian average pressure of ∼6 mbar. The reason is that the low pressure causes instability of the water within the mud, leading to the bubble formation that increases the volume of the mixture. This mechanism bears resemblance to the volumetric changes associated with the degassing of terrestrial lava or mud volcano eruptions caused by a rapid pressure drop. We conclude that the mounds associated with putative Martian sedimentary volcanoes might indeed be explained by volumetric changes in the mud. We also show that mud flows on Mars and elsewhere in the Solar System could behave differently to those found on Earth because mud dynamics are affected by the formation of bubbles in response to the different atmospheric pressures.

Experimental study of sediment transport processes by liquid water and brine under Martian pressure

We present here an experimental study to compare the behaviour of water and brine releases over loose sediments under present-day Martian pressure. Water has been invoked to explain current or past Martian surface features for decades. Recent studies have indicated that current surface conditions are, in certain times and places, compatible with the transient existence of liquid water or brine. However, the behaviour of water or brine releases over loose sediments under low Martian atmospheric pressure has been poorly studied.We performed 33 experiments of water and brine (MgSO4 at 19 wt%) releases over sandy slopes of various temperatures (0 °C to 20 °C), in a chamber allowing the reproduction of Martian pressure 6–7 millibars (mbar). We observe sediment transport mechanisms, that do not occur on Earth, caused by the boiling of water or brine at Martian pressures: grain ejection and “levitation” of sand pellets on cushions of vapour. The main parameter controlling the behaviour of the flow is the temperature of the substrate. Water and brine flows transport similar volumes of sediment under Martian pressure. We show that the grain ejection is the most efficient transport mechanism, dominating the volumes of sediment transported. Pellet “levitation” should lead to longer features formed with brine than with pure water on Mars. Boiling induced sediment transport requires much less water than sediment transport by overland flow to form morphologies similar in size or volume. Moreover, our one-dimensional climate model runs reveal that the temperatures at which we observe those types of transport are predicted to occur at the Martian surface today and in the past. When scaled to Mars, the morphologies we observe with water and brine experiments should be resolvable using the High-Resolution Science Experiment (HiRISE) camera at ∼25 cm/pix. Overall, our results show that boiling must be taken into account when considering sediment-rich flows under recent or current Martian conditions.

Study on the conversion mechanism of CO2 to O2 in pulse voltage dielectric barrier discharge at Martian pressure

CO2 constitutes around 95 % of the atmosphere on Mars, and its in-situ utilization is of great value. O2 is a significant target product transformed from CO2 that can support both human respiration and fuel combustion, its conversion mechanism remains unclear. In this study, using a dielectric barrier discharge (DBD) with parallel plate electrodes, simulation and experimental results are combined to discuss the discharge characteristics and CO2 conversion process, under Mars conditions. The DBD was driven by a repetitive frequency microsecond pulse voltage. According to the discharge current, the discharge process can be split into four stages, namely the positive (negative) discharge stage and the positive (negative) discharge extinguishing stage. With a contribution up to 98 % and primarily taking place in the positive discharge extinguishing stage, the recombination decomposition reaction of electron and CO+2 is the primary pathway of O2 synthesis. At the position near the edge of the cathode potential drop zone, electron impact ionization of CO2 ground state molecules dominates the formation of CO+2. Under the range of pulse parameters considered in this paper, increasing the pulse width can lengthen the time of recombination decomposition reaction of electron and CO+2 in positive discharge extinguishing stage, which is favorable for the formation of O2. Likewise, reducing the pulse's rising edge can increase the discharge intensity and CO+2 density, which is also favorable to O2 production.

Modeling study on temporal nonlinear behaviors in CO2 dielectric barrier discharges under Martian pressure

In recent years, dielectric barrier discharges (DBDs) have become one of the most potential candidates for in situ resource utilization of CO 2 ${\text{CO}}_{2}$ in the Martian atmosphere. The nonlinear behaviors in DBDs are directly associated with the stability of discharges, and understanding the formation mechanism of the nonlinear behaviors can effectively enhance the ability to control the plasmas. In this paper, to further explore the underpinning physics of temporal nonlinear behaviors in CO 2 ${\text{CO}}_{2}$ discharges, the tailored sinusoidal voltages with power-on and power-off phases are applied to ignite the plasmas under Martian pressure. The simulation results from a fluid model with hundreds of reactions show that the discharge can evolve from period-one state into period-four state via period-two state by reducing the power-off duration; the electron density drops rapidly and the heavy ions of CO 2 + ${\text{CO}}_{2}^{+}$ and CO 3 − ${\text{CO}}_{3}^{-}$ become the dominant charged particles after the discharge is extinguished. With the power-off duration further reduced, the heavy ions produced by the previous discharge cannot be completely dissipated and remain in the sheath region to enhance the electric field before the next discharge event; thus, the next ignition takes place more easily with a reduced breakdown voltage and a lower discharge current, indicating the discharge transits from a period-one state to a period-two state. In the transition from period-two discharge to period-four discharge, the heavy ions produced by the second discharge event cannot be completely transported to the surface of barriers, affecting the ignition in the third voltage period. When the power-off duration is less than 10 μs $\mathrm{\mu s}$ in the simulation, electrons become the dominant residual negative charges in the discharge region, and the rapid increase in memory voltage results in a smaller positive discharge current and a stronger negative discharge current. As the power-off duration continues to decrease, the discharge undergoes an inverse period-doubling bifurcation from reverse period-four discharge to reverse period-two discharge. This study provides a deeper insight into the underlying physics of nonlinear behaviors in CO 2 ${\text{CO}}_{2}$ DBDs under Martian pressure.

Frequency Effects on the Vibrational States and Conversion of CO2 in Radio Frequency Discharges Under Martian Pressure

In recent years, with the progress of Mars exploration, the in situ utilization of CO2 resources in the Martian atmosphere has become a widely discussed subject. Nonequilibrium plasma technology as an efficient molecular activation approach has great potential for CO2 conversion. In this study, a fluid model is established to discuss driving frequency effects on the vibrational states and conversion of CO2 in radio frequency (RF) discharges at a given power density under Martian pressure. The calculated CO2 conversion and energy efficiency depending on the specific energy input are in good agreement with the experimental measurements. The simulation results found that a considerable fraction of electron energy is transferred to the asymmetric stretching modes of CO2, and the characteristic times for electron impact vibrational excitation and vibration–vibration (VV) relaxation are small, which results in a larger density of higher vibrational levels. The density of vibrational levels of CO2 increases with the driving frequency at a constant power density, which can be explained by the enhancement of vibrational excitation and VV relaxation. The computational data also show that more CO, O, and O2 are produced as the driving frequency increases at a constant power density, and the vibration-induced dissociation with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textrm {CO}_{2}\textrm {v}_{8}$ </tex-math></inline-formula> gradually replaces the direct dissociation to be the main reaction for the production of CO and O. In addition, the increase in driving frequency causes more electron energy to be transferred to the asymmetrical stretch modes, and the vibrational energy of these vibrational states of CO2 can reduce the activation barrier of the reaction, improving the CO2 conversion and energy efficiency. This study contributes to a deeper insight into the frequency effects on plasma chemistry in RF CO2 discharges under Martian pressure and shows the optimized ways for the decomposition of CO2 on Mars.

Comprehensive study on plasma chemistry and products in [formula omitted] pulsed discharges under Martian pressure

With the development of Mars exploration, the In-Situ resource utilization on Mars has been drawn significant attention in recent years. The abundant CO2 in the Martian atmosphere makes it possible to decompose CO2 to produce high-value platform chemicals by plasma technology. In this comprehensive study, the discharge characteristics, products distribution, and reaction pathways in CO2 pulsed discharges under Martian pressure are investigated, and the effects of pulsed waveforms on discharge characteristics and produced species are also discussed. The simulation results show that electrons and CO2+ ions are the main components of charged particles in the discharge region; CO and O are mainly produced by the electron impact dissociation and electron impact attachment reactions with ground state and vibrationally excited state of CO2 during the power-on phase and consumed by the ion–neutral reactions. The recombination reaction of electrons and CO2+ ions is very important for the formation of O2. The simulation data also give the main generation and consumption pathways of vibrationally excited CO2 molecules, and indicate that the vibrational energy of excited CO2 molecules can lower the energy barrier of the reaction and facilitate the decomposition of CO2. In addition, the experimental and computational data also suggest that the pulsed waveforms could affect the discharge characteristics and plasma chemistry in CO2 pulsed discharges under Martian pressure, the densities of product species in CO2 plasma increase with the pulse rise rate and pulse rise duration of pulsed voltage. This study provides an enhanced understanding of discharge characteristics and plasma chemistry in the CO2 pulsed discharges under Martian pressure, and suggests optimized ways to utilize the CO2 on Mars.

Collisional Charging in the Low Pressure Range of Protoplanetary Disks

Abstract In recent years, collisional charging has been proposed to promote the growth of pebbles in early phases of planet formation. Ambient pressure in protoplanetary disks spans a wide range from below 10−9 mbar up to way beyond mbar. Yet, experiments on collisional charging of same material surfaces have only been conducted under Earth atmospheric pressure, Martian pressure and more generally down to 10−2 mbar thus far. This work presents first pressure dependent charge measurements of same material collisions between 10−8 and 103 mbar. Strong charging occurs down to the lowest pressure. In detail, our observations show a strong similarity to the pressure dependence of the breakdown voltage between two electrodes and we suggest that breakdown also determines the maximum charge on colliding grains in protoplanetary disks. We conclude that collisional charging can occur in all parts of protoplanetary disks relevant for planet formation.

Use of NanoSIMS to Identify the Lower Limits of Metabolic Activity and Growth by Serratia liquefaciens Exposed to Sub-Zero Temperatures.

Serratia liquefaciens is a cold-adapted facultative anaerobic astrobiology model organism with the ability to grow at a Martian atmospheric pressure of 7 hPa. Currently there is a lack of data on its limits of growth and metabolic activity at sub-zero temperatures found in potential habitable regions on Mars. Growth curves and nano-scale secondary ion mass spectrometry (NanoSIMS) were used to characterize the growth and metabolic threshold for S. liquefaciens ATCC 27,592 grown at and below 0 °C. Cells were incubated in Spizizen medium containing three stable isotopes substituting their unlabeled counterparts; i.e., 13C-glucose, (15NH4)2SO4, and H218O; at 0, −1.5, −3, −5, −10, or −15 °C. The isotopic ratios of 13C/12C, 15N/14N, and 18O/16O and their corresponding fractions were determined for 240 cells. NanoSIMS results revealed that with decreasing temperature the cellular amounts of labeled ions decreased indicating slower metabolic rates for isotope uptake and incorporation. Metabolism was significantly reduced at −1.5 and −3 °C, almost halted at −5 °C, and shut-down completely at or below −10 °C. While growth was observed at 0 °C after 5 days, samples incubated at −1.5 and −3 °C exhibited significantly slower growth rates until growth was detected at 70 days. In contrast, cell densities decreased by at least half an order of magnitude over 70 days in cultures incubated at ≤ −5 °C. Results suggest that S. liquefaciens, if transported to Mars, might be able to metabolize and grow in shallow sub-surface niches at temperatures above −5 °C and might survive—but not grow—at temperatures below −5 °C.

Investigation on the products distribution, reaction pathway, and discharge mechanism of low‐pressure CO2 discharge by employing a 1D simulation model

AbstractMars has a special carbon dioxide environment. The surface and atmosphere of Mars contain a large amount of solid and gaseous carbon dioxide, which makes the in‐situ resource utilization of Martian carbon dioxide attract widespread attention. The conversion of carbon dioxide into fuel or high value‐added products through electrical discharge has become a research focus, in which numerical simulation is one of the important means to explain the experimental phenomenon and the mechanism. Therefore, a one‐dimensional fluid model was established in this paper to simulate the discharge and conversion of carbon dioxide under simulated Martian atmospheric pressure. The discharge mode, particle distribution, and discharge mechanism were studied by considering CO2 vibrational states and vibration relaxation reaction in the model. The results show that the discharge mode is glow discharge with an obvious cathode fall region, negative glow space, and positive column. The density of the four discharge products, C, O2, CO, and O, shows a step‐up trend, showing a significant cumulative effect, and the peak values appear near both electrodes. The density of vibrational states of CO2 molecule, which is highest in the neutral products, increases during the discharge stage and decreases after the discharge is extinguished. The temporal and spatial distribution of reaction rates shows that vibrational excitation, electronic excitation, and vibrational relaxation reactions dominate the loss and formation of CO2. The density of ground‐state CO2 decreases obviously in the discharge stage, and the minimum density appears near the instantaneous cathode. After the discharge is extinguished, the CO2 near the instantaneous cathode increases gradually.

The formation of araneiforms by carbon dioxide venting and vigorous sublimation dynamics under martian atmospheric pressure

The local redistribution of granular material by sublimation of the southern seasonal {hbox {CO}}_2 ice deposit is one of the most active surface shaping processes on Mars today. This unique geomorphic mechanism is hypothesised to be the cause of the dendritic, branching, spider-like araneiform terrain and associated fans and spots—features which are native to Mars and have no Earth analogues. However, there is a paucity of empirical data to test the validity of this hypothesis. Additionally, it is unclear whether some araneiform patterns began as radial and then grew outward, or whether troughs connected at mutual centres over time. Here we present the results of a suite of laboratory experiments undertaken to investigate if the interaction between a sublimating {hbox {CO}}_2 ice overburden containing central vents and a porous, mobile regolith will mobilise grains from beneath the ice in the form of a plume to generate araneiform patterns. We quantify the branching and area of the dendritic features that form. We provide the first observations of plume activity via {hbox {CO}}_2 sublimation and consequent erosion to form araneiform features. We show that {hbox {CO}}_2 sublimation can be a highly efficient agent of sediment transport under present day Martian atmospheric pressure and that morphometry is governed by the Shields parameter.

Will the Mars Helicopter Induce Local Martian Atmospheric Breakdown?

Abstract Any rotorcraft on Mars will fly in a low-pressure and dusty environment. It is well known that helicopters on Earth become highly charged due, in part, to triboelectric effects when flying in sandy conditions. We consider the possibility that the Mars Helicopter Scout (MHS), called Ingenuity, flying at Mars as part of the Mars 2020 Perseverance mission, will also become charged due to grain-rotor triboelectric interactions. Given the low Martian atmospheric pressure of ∼5 Torr, the tribocharge on the blade could become intense enough to stimulate gas breakdown near the surface of the rotorcraft. We modeled the grain–blade interaction as a line of current that forms along the blade edge in the region where grain–blade contacts are the greatest. This current then spreads throughout the connected quasi-conductive regions of the rotorcraft. Charge builds up on the craft, and the dissipative pathway to remove charge is back into the atmosphere. We find that for blade tribocharging currents that form in an ambient atmospheric dust load, system current balance and charge dissipation can be accomplished via the nominal atmospheric conductive currents. However, at takeoff and landing, the rotorcraft could be in a rotor-created particulate cloud, leading to local atmospheric electrical breakdown near the rotorcraft. We especially note that the atmospheric currents in the breakdown are not large enough to create any hazard to Ingenuity itself, but Ingenuity operations can be considered a unique experiment that provides a test of the electrical properties of the Martian near-surface atmosphere.

A lower-than-expected saltation threshold at Martian pressure and below

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 analogs 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 toward 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 [Formula: see text] 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.

Ne-Ar separation using a permeable membrane to measure Ne isotopes for future planetary explorations

We propose a method using a permeable membrane to separate Ne from Ar for Ne isotope analyses in a future planetary exploration, in particular, for the Martian surface. The results of simulation based on permeation data and theoretical formulas show that significant amounts of 20Ne without 40Ar permeate through Viton (fluoropolymer elastomer) and Kapton (polyimide) with appropriate thicknesses. Considering the simulation results, the amounts of 20Ne and 40Ar that permeate through 1 ​mm-thick Viton and 125 ​μm-thick Kapton were determined experimentally. The amounts of permeated 20Ne agree with those of 20Ne obtained by the theoretical calculation within experimental uncertainties. The amounts of permeated 40Ar are as small as those of the background, which is also consistent with the fact that the calculated amounts of 40Ar through those membranes are several orders of magnitude lower than the background 40Ar. In the present experiments, 20Ne/40Ar ratios increased up to 1 for Viton and 200 for Kapton from the terrestrial atmospheric ratio of 1.8 ​× ​10−3 after permeation for 30 and 60 ​min, respectively. A Kapton sheet efficiently enhances the 20Ne/40Ar ratio and enables measurement of the Ne isotope composition without the significant 40Ar++ interference in planetary exploration. Assuming the Martian atmospheric pressure (7 ​hPa) and composition (2.3 ​ppm for 20Ne and 1.9% for 40Ar), the amounts of 20Ne and 40Ar that permeate through Kapton were calculated. The 20Ne/40Ar ratio can be increased up to ca. 1 using 75 ​μm-thick Kapton with a duration time of 30–60 ​min, where the contribution of 40Ar++ is approximately 10% of 20Ne+ and the uncertainty after the 40Ar++ correction is estimated to be a few percent. An effective area of 50 ​cm2 is required if the sensitivity of the QMS used in this study is assumed.