Search

This paper presents a theoretical analysis of the data on wave velocity measurements at small stresses presented in a companion paper describing the experimental results on a Martian regolith loose sandy simulant (Fontainebleau sand) of the soil at the InSight landing site on Mars (Elysium Planitia). Experimental data of wave velocities and Poisson's ratio are interpreted in the light of a granular contact mechanics theory and completed accounting for rugosity effects that are suspected to have stronger effects in sands under low stresses. The asperities of a grain of Fontainebleau sand were investigated through Atomic Force Microscopy, but larger asperities had to be adopted so as to better fit the model prediction with experimental data. A good agreement between the experimental data and the model predictions is obtained for stress above 10 kPa. Below 5 kPa, an area in which asperities are suspected to have a stronger influence, the model is not fully satisfactory, showing that further experimental and theoretical investigation is necessary in a stress zone particularly relevant to surface soils in planets, with probably enhanced effects of asperities on the intergrain contact mechanics. Plain Language Summary: To better understand seismic wave propagation at the surface of the InSight landing site on Mars, this paper presents a theoretical interpretation of the seismic wave velocities at low stress measured in a sand sample used as a Martian regolith simulant and presented in a companion paper. The theory is based on both a theoretical elastic model of a pack of smooth spheres of same diameter and approach accounting for the effects of local rugosity at the contact between sand grains (rugosity is supposed to have a stronger influence at the low stresses resulting from the smaller gravity on Mars). The sphere pack model shows that some slippage between grains is necessary to properly account for the radial deformation of a cylindrical sample subjected to an increase in axial stress (as described by an elastic parameter called the Poisson ratio). Accounting for rugosity effects allows a better prediction of the changes in wave velocity with respect to stress. The decrease in velocity under decreased stress is confirmed, but the rugosity parameter measured using an Atomic Force Microscope was under‐estimated, probably due to the over‐simplistic hypothesis of considering spheres. A proper prediction was obtained by fitting this parameter at a twice larger value. Key Points: A novel theory based on contact mechanics was proposed to calculate Vp and Vs of uniform granular materialsThe theory considers the roughness of the particles, the stress, the porosity of the granular assembly, and the properties of the grainsTheoretical results agree with experimental measurements on loose samples of Fontainebleau sand used as a Mars regolith simulant [ABSTRACT FROM AUTHOR]

Wave velocity measurements were performed on Fontainebleau sand samples used as a Martian regolith simulant to investigate the elastic properties of the surface material at the InSight landing site on Mars (Elysium Planitia). Loose samples (density 1.4 Mg/m3, density index 6%) were prepared by using the pluviation method to mimic the low regolith density at the InSight landing site. A novel device derived from triaxial testing was designed to measure wave velocities at low stresses along a horizontal cylindrical specimen. Four tests were made, in which the confining stress was applied by applying a vacuum between 1 and 80 kPa. Wave velocities were measured by using bender elements under stresses as low as 1.75 kPa, a very low value compared to the standard stress ranges generally considered in terrestrial geotechnics (>10 kPa). The changes in compression and shear wave velocities obey a standard power law, with two slightly different exponents for Vp and Vs, indicating a not perfectly elastic behavior. Data showed greater variability below 5 kPa, indicating some limitations of the bender element technique in this range. A slight decrease in Poisson ratio was detected below 5 kPa, which certainly deserves more investigation. This investigation is useful to better analyze the data of the InSight mission, both in terms of wave propagation at the surface and to interpret some in situ elastic tests carried out with the scoop. These data are interpreted in the light of a granular contact mechanics theory in a companion paper. Plain Language Summary: The InSight mission lander is a highly sensitive seismometer placed on Mars to detect seismic waves from Marsquakes and meteorite impacts. To help analyzing the seismic waves, a special laboratory device was developed to measure wave velocities on a loose sandy simulant mimicking the surface regolith, with wave emitting/receiving transducers placed on both sides of a cylindrical sand sample subjected to external stress. Due to reduced gravity, the stresses that control wave velocity (larger stress, faster waves) are very low at the Mars surface. To impose low stresses (down to 1.75 kPa), we used a horizontal sample with a careful stress calculation along the sample axis, along which waves propagate. We determined the changes in wave velocity with stress (governed by a power law) and observed greater variability under 5 kPa, showing the limit of our transducers at low stresses. Based on elasticity theory, derived from wave velocity parameters governing sample deformations under strain changes. We suspect a slight decrease in the Poisson ratio (that characterizes radial elastic strain of a cylindrical sample under axial loading) below 5 kPa. Data is useful to better constrain the response of Mars sandy surface materials at the InSight site and other areas for future Mars missions. Key Points: Bender Element measurements of Vp and Vs carried out on a novel device on loose Fontainebleau sand samples used as a Mars regolith simulantInvestigation carried out within a so far unexplored low stress range, below 10 kPa and as low as 1.75 kPaPoisson ratio observed to remain mostly constant at low stress, with a possible decrease in the zone below 5 kPa [ABSTRACT FROM AUTHOR]

In an attempt to improve the quality of the seismic signals provided by the seismometer of the InSight mission (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) on Mars, part of the tether linking the seismometer to the InSight lander was buried by some regolith using the scoop of the articulated robotic arm. The regolith in a source area was scraped into piles, scooped and dumped by the scoop from a height of ∼50 cm above the surface onto the tether. Part of the regolith was carried away by the wind and dispersed 1–2 m downwind, as evidenced by the comparison between images taken from the lander before and after the regolith pouring. Using both ballistic trajectory and wind dispersion effects as a sorter, the grain size range was determined through numerical fluid mechanics simulations. The trajectory of the poured grains is determined by the Martian atmospheric and gravimetry conditions, the initial conditions of scoop pouring and grain lithology. The spatial grain distribution on the ground shows a downwind decrease in grain size from the pouring point, with a size ranging from 1 mm near the dump point to ∼100 μm at the farthest area observed on the images. We find that the deposit of grains coarser than 500 μm is controlled mainly by gravity. Grains finer than 100 μm are present in the regolith, but they are not quantifiable with this method because they are blown away by the wind. Plain Language Summary: The soil on which the lander of the geophysical InSight mission on Mars has landed is partially known, mainly through visual observations by the lander's cameras and calculations based on their thermal properties (derived from both orbiter and lander measurements). Some soil scraped by the scoop of the lander's robotic arm was dumped on the tether connecting the InSight seismometer, so as to improve the quality of the seismic signals. During these dumpings, it was observed that some particles were blown away by Martian winds that were simultaneously recorded by the lander instruments. Given that they are subjected to the combined effect of gravity and wind, larger particles rapidly fall below the scoop, whereas smaller ones are blown away; the smaller the particle, the larger the distance. The trajectory of the poured particles was numerically calculated using fluid mechanics to account for wind effects, and the resulting downwind deposit was compared to that given by photos. It was shown that particles larger than 500 μm fell close to the pile formed below the scoop, whereas smaller ones (>100 μm) fell at distances of 2–4 m. Smaller particles (<100 μm) including dust (<5–10 μm) were blown further away. Key Points: The grain size distribution of the InSight Martian regolith is better constrained by using wind as a sorter during the burial of a tetherBased on image observations and numerical fluid mechanics simulations, the regolith mainly contains grains between 100 μm and 1 mmGrains smaller than 100 μm are not quantifiable because they have been blown away by the wind [ABSTRACT FROM AUTHOR]

Seismic noise recorded at the surface of Mars has been monitored since February 2019, using the InSight seismometers. This noise can reach â'200 dB. It is 500 times lower than on Earth at night and it increases of 30 dB during the day. We analyze its polarization as a function of time and frequency in the band 0.03-1 Hz. We use the degree of polarization to extract signals with stable polarization independent of their amplitude and type of polarization. We detect polarized signals at all frequencies and all times. Glitches correspond to linear polarized signals which are more abundant during the night. For signals with elliptical polarization, the ellipse is in the horizontal plane below 0.3 Hz. In the 0.3-1Hz high frequency band (HF) and except in the evening, the ellipse is in the vertical plane and the major axis is tilted. While polarization azimuths are different in the two frequency bands, they both vary as a function of local hour and season. They are also correlated with wind direction, particularly during the daytime. We investigate possible aseismic and seismic origins of the polarized signals. Lander or tether noise can be discarded. Pressure fluctuations transported by wind may explain part of the HF polarization but not the tilt of the ellipse. This tilt can be obtained if the source is an acoustic emission coming from high altitude at critical angle. Finally, in the evening when the wind is low, the measured polarized signals may correspond to the seismic wavefield of the Mars background noise. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate the possibility to determine the true North direction on Mars by using a gnomon on the InSight mission. The Sun local coordinates are computed using the analytical solution VSOP87 for the planetary ephemeris. The Sun position in the sky induce specific gnomon shadow orientation and length that can be read on a target. By comparing the images of the shadow with the expected position of the Sun, we can determine the true North direction with an accuracy up to 1∘, well within the seismic requirement of 5∘, even if the lander position or the gnomon verticality are not accurate. To achieve this result, we determine the best periods of observation during the SEIS deployment phase in December 2018. A total period of 5 hours has been identified on mornings and evenings on Mars, during which the impact of errors on position and verticality on the true North direction are minimised. [ABSTRACT FROM AUTHOR]

We investigate the scattering attenuation characteristics of the Martian crust and uppermost mantle to understand the structure of the Martian interior. We examine the energy decay of the spectral envelopes for 21 high-quality Martian seismic events from Sol 128 to Sol 500 of InSight operations. We use the model of Dainty et al. (1974b) to approximate the behavior of energy envelopes resulting from scattered wave propagation through a single diffusive layer over an elastic half-space. Using a grid search, we mapped the layer parameters that fit the observed InSight data envelopes. The single diffusive layer model provided better fits to the observed energy envelopes for High Frequency (HF) and Very High Frequency (VF) than for the Low Frequency (LF) and Broadband (BB) events. This result is consistent with the suggested source depths (Giardini et al., 2020) for these families of events and their expected interaction with a shallow scattering layer. The shapes of the observed data envelopes do not show a consistent pattern with event distance, suggesting that the diffusivity and scattering layer thickness is non-uniform in the vicinity of InSight at Mars. Given the consistency in the envelope shapes between HF and VF events across epicentral distances and the tradeoffs between the parameters that control scattering, the dimensions of the scattering layer remain unconstrained but require that scattering strength decreases with depth and that the rate of decay in scattering strength is fastest near the surface. This is generally consistent with the processes that would form scattering structures in planetary lithospheres., Competing Interests: CONFLICT OF INTEREST The authors acknowledge there are no conflicts of interest recorded.

Measurements from the InSight lander radiometer acquired after landing are used to characterize the thermophysical properties of the Martian soil in Homestead hollow. This data set is unique as it stems from a high measurement cadence fixed platform studying a simple well-characterized surface, and it benefits from the environmental characterization provided by other instruments. We focus on observations acquired before the arrival of a regional dust storm (near Sol 50), on the furthest observed patch of soil (i.e., ∼3.5 m away from the edge of the lander deck) where temperatures are least impacted by the presence of the lander and where the soil has been least disrupted during landing. Diurnal temperature cycles are fit using a homogenous soil configuration with a thermal inertia of 183 ± 25 J m -2  K -1  s -1/2 and an albedo of 0.16, corresponding to very fine to fine sand with the vast majority of particles smaller than 140 μm. A pre-landing assessment leveraging orbital thermal infrared data is consistent with these results, but our analysis of the full diurnal temperature cycle acquired from the ground further indicates that near surface layers with different thermophysical properties must be thin (i.e., typically within the top few mm) and deep layering with different thermophysical properties must be at least below ∼4 cm. The low thermal inertia value indicates limited soil cementation within the upper one or two skin depths (i.e., ∼4-8 cm and more), with cement volumes <<1%, which is challenging to reconcile with visible images of overhangs in pits., (© 2021. Jet Propulsion Laboratory. California Institute of Technology. Government sponsorship acknowledged.)