Martian Boundary Layer Research Articles

Inferring the Speed of Sound and Wind in the Nighttime Martian Boundary Layer From Impact‐Generated Infrasound

AbstractThe properties of the first kilometers of the Martian atmospheric Planetary Boundary Layer have until now been measured by only a few instruments and probes. InSight offers an opportunity to investigate this region through seismoacoustics. On six occasions, its seismometers recorded short low‐frequency waveforms, with clear dispersion between 0.4 and 4 Hz. These signals are the air‐to‐ground coupling of impact‐generated infrasound, which propagated in an low‐altitude atmospheric waveguide. Their group velocity depends on the structure of effective sound speed in the boundary layer. Here, we conduct a Bayesian inversion of effective sound speed up to 2,000 m altitude using the group velocity measured for events S0981c, S0986c and S1034a. The inverted effective sound speed profiles are in good agreement with estimates provided by the Mars Climate Database. Differences between inverted and modeled profiles can be attributed to a local wind variation in the impact→station direction, of amplitude smaller than 2 m/s.

Investigating Diurnal and Seasonal Turbulence Variations of the Martian Atmosphere Using a Spectral Approach

We use a spectral approach to analyze the pressure and wind data from the InSight mission and investigate the diurnal and seasonal trends. Our analyses show that the daytime pressure and wind spectra have slopes of approximately −1.7 and −1.3 and, therefore, do not follow the Kolmogorov scaling (as was also previously reported for a reduced data set in Banfield et al.). We find that the nighttime pressure spectral slope is close to −1 (as reported in Temel et al.), and that the wind speed spectral slope is close to −0.5, flatter than the theoretical slope expected for the shear-dominated regime. We observe strong nocturnal (likely shear-generated) turbulent behavior starting around L s = 150° (InSight sol 440) that shifts to progressively earlier local times before reaching the “5th season” (InSight sols 530–710) identified by Chatain et al.. The diurnal spectral slope analyses indicate an asymmetry in the diurnal behavior of the Martian boundary layer, with a slow growth and fast collapse mechanism. Finally, the low-frequency (5–30 mHz) pressure data exhibit large spectral slope oscillations. These occur particularly during the periods with a highly stable atmosphere and, therefore, may be linked to gravity wave activity.

Design and construction of a ground-based plasma device for Martian boundary layer investigation

Laboratory experiments have contributed significantly to the exploration of the fundamental physics of Earth’s and planetary space plasma. In this work, a ground-based plasma device was designed and constructed to study the plasma physics in the Martian boundary layer, where the solar wind plasma directly interacts with the Martian ionospheric plasma, and the underlying physical mechanisms and processes are not fully understood. The solar wind plasma and the Martian ionospheric plasma were simulated using two independent plasma sources. A Kaufman source was used to simulate the low-density, high-velocity solar wind plasma, and a lanthanum hexaboride source was utilized to simulate the high-density, low-velocity Martian ionospheric plasma. Additionally, the crustal magnetic field of Mars was modeled using a rotatable electromagnet. The initial results suggest that the interaction in the Martian boundary layer can be simulated based on the device, and the boundary layer’s structure, location, and physical processes can be experimentally studied in the future.

Spectral Analysis of the Martian Atmospheric Turbulence: InSight Observations

AbstractIn this study, we perform spectral analysis of Martian near‐surface turbulence using the in‐situ observations by NASA's InSight lander on Mars. A recent study on the daytime Martian boundary layer using the InSight observations showed that the conventional Kolmogorov energy cascade of inertial subrange fails to predict the spectral density of pressure (Banfield et al., 2020, https://doi.org/10.1038/s41561-020-0544-y). Here we extend this by investigating diurnal and seasonal variations in the spectral density of pressure, as the indicator of the Martian atmospheric turbulence. We show distinct spectral behaviors for the daytime and nighttime conditions. Moreover, we report the important effects of regional dust storms, gravity waves (GWs), bore, and solitary waves on the turbulent energy cascade. Our results show that the presence of a dust storm and GW activity can enhance the turbulence of the nighttime boundary layer of Mars despite the extreme stably stratified conditions compared to the terrestrial case.

Estimating the Heights of Martian Vortices from Mars 2020 MEDA Data

Small convective vortices occur ubiquitously on Mars, frequently as dust devils, and they produce detectable signals in meteorological data—in pressure, temperature, and wind speed and direction. In addition to being important contributors to the Martian dust budget, convective vortices may serve as probes of the boundary layer, providing clues to convective instability, boundary layer diurnal evolution, and surface–atmosphere interactions. Using vortices as boundary layer probes requires a detailed understanding of the link between their properties and occurrence rates and the conditions that produce them. Fortunately, the growing cache of data from the Mars Environmental Dynamics Analyzer instrument suite on board the Mars 2020 Perseverance rover promises to elucidate these relationships. In this study, we present a catalog of vortex detections from mission sols 90 through 179 to bolster our previous catalog based on sols 15 through 89. Consistent with predictions, we find more vortex encounters during this second half of the mission than from the first half. In addition to analyzing the pressure signals from these vortex encounters, we also use a Gaussian process analysis to recover contemporaneous temperature signals. By combining these signals with a long-established thermodynamics model, we estimate heights of the vortices and find some agreement with previous work and evidence for the diurnal growth and decay of the Martian boundary layer. We also discuss prospects for additional boundary layer studies using Martian vortex encounters.

Interannual, Seasonal and Regional Variations in the Martian Convective Boundary Layer Derived From GCM Simulations With a Semi‐Interactive Dust Transport Model

AbstractWe present interannual, seasonal, and regional variations in the daytime Martian convective boundary layer (CBL). Martian CBL meteorology is driven both by the effect of diurnal and seasonal cycles as well as complex Martian topography. One of the most important components of the Martian atmosphere is its dust cycle. Here, we develop a novel semi‐interactive dust transport model within the MarsWRF framework, in which the dust is lifted, advected by model winds, mixed, and allowed to sediment, but is then scaled to match two‐dimensional maps of the observed daily column‐integrated dust opacity. This allows the vertical dust distribution and associated dust radiative heating to be controlled by model processes, while the horizontal dust distribution is constrained to follow observations. We report the impact of the dust cycle on Martian boundary layer meteorology. Enhanced dust transport lowers the global net surface heating rates, decreasing the turbulent mixing in CBL to virtually zero (within the dust storm season) and, enhances the wind shear on average by almost 50%. As a superposition of both impacts, during global dust storms (GDS) in Mars Year (MY) 25 and 34, we find that long‐lasting extremely shallow daytime boundary layers can globally form as shallow as 0.5 km (but not for the less intense GDS in MY 28), unlike the 9 km deep and highly turbulent CBL formation at GDS onset and decay. Based on our GCM results, strong CBL suppression lasts as long as approximately 67 and 57 sols during GDS events in MY 25 and 34.

Martian boundary layer wind profiles during the landings of Viking and InSight

Interaction between mesoscale, synoptic and planetary scale circulations on Mars can combine to produce locally complex winds that make interpretation of lander wind data a challenge. We investigate the structure of the boundary layer winds by obtaining new wind speed and directions from trajectory analysis of the Viking 1, Viking 2 and InSight landings. We use the new wind data for InSight together with a column model to investigate the near-surface winds at the InSight landing site.We use our Trajectory Model and Analysis Tool (TMAT) to verify wind data obtained by Viking lander 1 (VL-1) as it passed through a region of strong winds. This is important as the winds may be evidence of Western Boundary Currents. We constrain the vertical extent of this wind layer. We also verify the wind data for Viking lander 2 (VL-2). For VL-2 the winds remain constant from parachute deployment to lander release below which there is a large change in wind direction. For InSight our analysis indicates the wind speeds remain approximately constant with altitude. We found the wind blows from the west in the upper part of InSight's descent and from the north northwest closer to the surface.We use our new TMAT inferred wind data for InSight to initialise the winds of a column model (University of Helsinki/Finnish Meteorological Institute). We investigate the influence of slope winds on the local winds at the InSight landing site. Our results demonstrate that Martian slopes and large-scale circulations can interact in a complex way to produce the near-surface wind patterns as observed by InSight.We have been able to obtain new wind velocity data at several altitudes and locations on Mars by utilizing atmospheric modelling and the known locations of landed hardware from Mars surface missions. We have applied these new velocity data to investigate the interactions of large-scale winds and the Martian topography.

Turbulent Shear Flow Over Large Martian Ripples

AbstractOn Mars, large aeolian ripples with wavelengths typically 1–3 m but lacking very coarse sand at crests have been encountered by rovers and observed from orbit. These bedforms have no terrestrial counterpart and several hypotheses for origins have been proposed. This work reports results of Computational Fluid Dynamics (CFD) experiments with ANSYS Fluent under terrestrial and Martian boundary layer conditions, using the k − ω SST turbulence model to evaluate shear stress along a topographic profile of large Martian ripples at different boundary layer wind speeds. Results indicate that, compared with Earth conditions: (1) boundary‐layer flow along large ripples under Martian conditions is less turbulent due to higher kinematic viscosity; (2) reverse‐flow vortex regions from crests at ripple lee flanks are larger; and (3) shear stresses at crests of large ripples are relatively low, so ripple flattening is less likely at high wind speeds. These results indicate Martian ripples formed by the saltation impact splash mechanism should be less constrained by shear stress effects limiting growth of exposed ripple crests, because the low‐density Martian atmosphere applies relatively low wind‐related shear stress to ripple surfaces. Other origins for the large Martian ripples are not excluded, however. On Earth, very large ripples with crests unprotected by very coarse grains do not develop due to higher wind‐related shear stresses.

Measurement of Martian boundary layer winds by the displacement of jettisoned lander hardware

Martian boundary layer wind speed and direction measurements, from a variety of locations, seasons and times, are provided. For each lander sent to Mars over the last four decades a unique record of the winds blowing during their descent is preserved at each landing site. By comparing images acquired from orbiting spacecraft of the impact points of jettisoned hardware, such as heat shields and parachutes, to a trajectory model the winds can be measured. We start our investigations with the Viking lander 1 mission and end with Schiaparelli. In-between we extract wind measurements based on observations of the Beagle 2, Spirit, Opportunity, Phoenix and Curiosity landing sites.With one exception the wind at each site during the lander’s descent were found to be < 8 m s−1. High speed winds were required to explain the displacement of jettisoned hardware at the Phoenix landing site. We found a tail wind ( > 20 m s−1), blowing from the north-west was required at a high altitude ( > 2 km) together with a gust close to the surface ( < 500 m altitude) originating from the north. All in all our investigations yielded a total of ten unique wind measurements in the PBL. One each from the Viking landers and one each from Beagle 2, Spirit, Opportunity and Schiaparelli. Two wind measurements, one above about 1 km altitude and one below, were possible from observations of the Curiosity and Phoenix landing site.Our findings are consistent with a turbulent PBL in the afternoon and calm PBL in the morning. When comparing our results to a GCM we found a good match in wind direction but not for wind speed. The information provided here makes available wind measurements previously unavailable to Mars atmosphere modellers and investigators.

Field measurements of horizontal forward motion velocities of terrestrial dust devils: Towards a proxy for ambient winds on Mars and Earth

Dust devils – convective vortices made visible by the dust and debris they entrain – are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites.We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a 100 active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements.Daily (10:00–16:00 local time) and 2-h averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-min smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10–20% faster than ambient wind speed measured at 10m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction.The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain.

Accommodation Study for an Anemometer on a Martian Lander

Abstract Measuring the wind velocity and its turbulent fluctuations near the surface of Mars is an important component of the future exploration of Mars, not only to minimize risk in landing, but also to understand some of the most important fundamental processes that dominate Mars’ behavior today. Previous missions have included instrumentation to measure 2D mean winds, but a more sophisticated instrument has been designed that allows for fast, precise 3D measurements of the wind and its turbulent properties. These richer observations raise the question of how best to place such an instrument on a future Martian lander to minimize the flow distortions imposed by the lander, and how to correct for the perturbations that cannot be avoided. To carry out this research, computational fluid dynamic simulations in three dimensions were performed using Fluent, a commercially available software. The first step was to model the conditions at the surface of Mars and, more particularly, the quantities describing the flow in the boundary layer. Using these models, simulations were conducted with two simple shapes for the lander and with eight turbulence conditions representing neutral stability flows in the Martian boundary layer. The results are believed to be generally robust for neutral stability cases because the simulations exhibited little variability as a function of the turbulence conditions. This allowed criteria to be established that would optimize the placement of an anemometer in close proximity to a Martian lander under these conditions. The optimal position that has been derived is an elevation of 55° with a minimum distance from the lander of 0.8 times the characteristic size of the lander.

Large-Eddy Simulations of Vertical Vortex Formation in the Terrestrial and Martian Convective Boundary Layers

Large-eddy simulations (LES) of the Martian and terrestrial convective boundary layers (CBL) are performed to compare the physical characteristics of simulated convective vertical vortices to those of observed dust devils and vortices. Martian and terrestrial CBLs are outwardly found to have similar structures and turbulence statistics based on primary mechanisms for the transfer of energy from the insolated surface. Applying the heating effect of radiative flux divergence in the Martian atmosphere caused differences in atmospheric vertical profiles in the surface and mixed layers of Mars. In general, the Martian boundary layer is found to be roughly four times deeper than Earth’s, indicating that convection on Mars is more intense than that on Earth due to a lower atmospheric density. Performing fine-resolution simulations in quiescent atmospheres of the two planets, it is found that the general vorticity development in all cases is similar and that the Martian vorticity columns extend six times higher and are 10 times wider than those on Earth. The accuracy of the simulated vortices as compared with observed physical characteristics is discussed. This study is a necessary part of a larger effort for the Phoenix Mars mission and examines the possible formation and maintenance mechanisms for vertical vortices in the Martian convective boundary layer at the Phoenix lander site.

Elements of comparison between Martian and terrestrial mesoscale meteorological phenomena: Katabatic winds and boundary layer convection

Terrestrial and Martian atmospheres are both characterised by a large variety of mesoscale meteorological events, occurring at horizontal scales of hundreds of kilometres and below. Available measurements from space exploration and recently developed high-resolution numerical tools have given insights into Martian mesoscale phenomena, as well as similarities and differences with their terrestrial counterparts. The remarkable intensity of Martian mesoscale events compared to terrestrial phenomena mainly results from low density and strong radiative control. This is exemplified in the present paper by discussing two mesoscale phenomena encountered in the lowest atmospheric levels of both planets with notable differences: nighttime katabatic winds (drainage flow down sloping terrains) and daytime boundary layer convection (vertical growth of mixed layer over heated surfaces). While observations of katabatic events are difficult on Earth, except over vast ice sheets, intense clear-cut downslope circulations are expected to be widespread on Mars. Convective motions in the daytime Martian boundary layer are primarily driven by radiative contributions, usually negligible on Earth where sensible heat flux dominates, and exhibit turbulent variances one order of magnitude larger. Martian maximum heat fluxes are not attained close to the surface as on Earth but a few hundreds of metres above, which implies generalised definitions for mixing layer scales such as vertical velocity w ⁎. Measurements on Mars of winds in uneven topographical areas and of heat fluxes over flat terrains could be useful to assess general principles of mesoscale meteorology applicable to both terrestrial and Martian environments.

Thermal structure of the atmospheric boundary layer on Mars based on Mini‐TES observations

AbstractThis paper presents and discusses the vertical profiles of the potential temperature in the Martian boundary layer. The profiles were remotely sensed from on‐board the Spirit Mars Exploration Rover during the first 702 sols of its operation in 2004–2005. The rover carried the Miniature Thermal Emission Spectrometer (Mini‐TES), which allowed it to retrieve temperature profiles up to a level 2 km above the ground surface. The data analysed here permit a reconstruction of the diurnal and seasonal structure of the lower part of the Martian convective boundary layer, an evaluation of the boundary layer depth, and a computation of the temperature heat flux. Copyright © 2009 Royal Meteorological Society

A model of dust in the Martian lower atmosphere

A coupled boundary layer–Aeolian dust model of the Martian atmosphere is presented. This model was developed to determine how radiation (scattering, absorption, and emission) by dust affects the boundary layer and, in turn, how this affects the dust distribution in the atmosphere. This was achieved by coupling a planetary boundary layer (PBL) model with the 2007 dust model of P. A. Taylor et al. The principle motivation here is to determine whether it will be possible to use the lidar on board the Phoenix lander to detect the depth of the Martian boundary layer from the dust distribution. Runs were conducted for different dust profiles, roughness lengths, and geostrophic winds. These indicate that there should be a distinct horizon in dust concentration which will be detectable by the Phoenix lidar. The model is also applied to the 1977B dust storm optical data of Viking Lander 1, and our analysis indicates a significant improvement over previous 1‐D studies of dust storm decay.

Simulating Martian boundary layer water ice clouds and the lidar measurements for the Phoenix mission

Diurnal variation of ground fog and water ice cloud formation at the NASA Phoenix lander site is investigated using a one‐dimensional Mars Microphysical Model (MMM) coupled with the results from the one‐dimensional University of Helsinki atmospheric boundary layer (ABL) model. Phoenix is scheduled to reach Mars in May 2008 and land in the northern plains (65°–72°N). Observations from Mars Global Surveyor Thermal Emission Spectrometer for the proposed landing site and season Ls = 76°–125° have been used for the model initialization, both in the ABL and MMM. The diurnal variations of temperature and eddy diffusion coefficients produced by the uncoupled ABL are then applied to the MMM. Extinction and backscattering coefficients and lidar ratios are presented for the simulated dust and water ice clouds at the Phoenix location. Results of the dust and ice clouds are then used to simulate the Phoenix lidar measurements at two wavelengths, 532 and 1064 nm.

Modelling dust distributions in the atmospheric boundary layer on Mars

A time and height dependent eddy diffusion model is used to investigate possible scenarios for the size distribution of dust in the lower atmosphere of Mars. The dust is assumed to either have been advected from a distant source or to have originated locally. In the former case, the atmosphere is assumed to initially contain dust particles with sizes following a modified gamma distribution. Larger particles are deposited relatively rapidly while small particles are well mixed up to the maximum height of the afternoon boundary layer and are deposited more slowly. In other cases, a parameterization of the dust source at the surface is proposed. Model results show that smaller particles are rapidly mixed within the Martian boundary layer, while larger particles (r > 10 μm) are concentrated near the ground with a stronger diurnal cycle. In all simulations we assume that the initial concentration or surface source depend on a modified gamma function distribution. For small particles (cross-sectional area weighted mean radius, reff = 1.6 μm) distributions retain essentially the same form, though with variations in the mean and variance of the area-weighted radius, and the gamma function can be used to represent the particle size distribution reasonably well at most heights within the boundary layer. In the case of a surface source of larger particles (mean radius 50 μm) the modified gamma function does not fit the resulting particle size distribution. All results are normalised by a scaling factor that can be adjusted to correspond to an optical depth for assumed particle optical scattering properties.

Statistics of shallow convection on Mars based on large-eddy simulations. Part 2: effects of wind shear

Effects of wind on quasi-steady, shallow convection in the Martian boundary layer are studied using a large-eddy simulation model. Convection in the model is generated by the radiative flux divergence and the strength of the surface heat flux, which do not vary in time. The resulting convective boundary layer exhibits transient, irregular, horizontal cellular structures, transported by wind, and a lack of well-pronounced regular horizontal rolls, observed for analogous conditions on Earth. The dimensionless statistics of turbulence are generally similar to those generated in the windless conditions, and depend on the ratio F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer. The simulations show that variations of the radiative heating influence the temperature statistics, while their effects on the wind velocity are relatively small. The horizontal velocity variances do not show a strong dependence on parameter F, in contrast with the vertical velocity variances, which are strongly dependent on F.

Statistics of shallow convection on Mars based on large-eddy simulations. Part 1: shearless conditions

Results of large-eddy simulations of shallow, quasi-steady, shear-less convection in the Martian boundary layer are presented and discussed. In the considered three cases, turbulence is forced by the radiative flux divergence, prescribed as given functions of height, and the strength of the surface heat flux. It is constrained by the temperature inversion at the boundary-layer top. The resulting convective boundary layer exhibits horizontal cellular structures. The presence of radiative heating causes dimensionless statistics of turbulence to depend on the parameter F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer.

First Atmospheric Science Results from the Mars Exploration Rovers Mini-TES

Thermal infrared spectra of the martian atmosphere taken by the Miniature Thermal Emission Spectrometer (Mini-TES) were used to determine the atmospheric temperatures in the planetary boundary layer and the column-integrated optical depth of aerosols. Mini-TES observations show the diurnal variation of the martian boundary layer thermal structure, including a near-surface superadiabatic layer during the afternoon and an inversion layer at night. Upward-looking Mini-TES observations show warm and cool parcels of air moving through the Mini-TES field of view on a time scale of 30 seconds. The retrieved dust optical depth shows a downward trend at both sites.