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Seismic noise recorded at the surface of Mars has been monitored since February 2019, using the seismometers of the InSight lander. The noise on Mars is 500 times lower than on Earth at ni...

International audience; Key Points: 28 • Ground compliance measurements by InSight depend on the elastic properties in 29 the near surface of Mars 30 • Observed compliance from convective vortex encounters and other pressure fluc-31 tuations opens the way to the exploration of the near surface 32 • Markov chain Monte Carlo inversion for the Young modulus profile suggests strat-33 ification within and below the regolith 34 Corresponding author: Balthasar Kenda, kenda@ipgp.fr-1-Abstract 35 Measurements of ground compliance at the InSight landing site-describing the surface 36 response to pressure loading-are obtained from seismic and meteorological data. Com-37 pliance observations show an increase with frequency indicating the presence of a stiffer 38 rock layer beneath the exposed regolith. We performed a Markov chain Monte Carlo in-39 version to investigate the vertical profile of the elastic parameters down to 20 m below 40 InSight. Compliance was inverted both freely and assuming prior knowledge of compaction 41 in the regolith, and the limitations and strengths of the methods were assessed on the 42 basis of theoretical considerations and synthetic tests. The inverted Young modulus ex-43 hibits an increase by a factor of 10-100 over the first 10-15 meters, compatible with a struc-44 tural discontinuity between 0.7 and 7 m. The proposed scheme can be used for joint in-45 version of other seismic, geological or mechanical constraints to refine the resulting ver-46 tical section. 47 Plain Language Summary 48 Pressure fluctuations of the Mars' atmosphere induce tiny deformations of the ground 49 that can be measured by the very sensitive seismometer of the InSight mission. The amount 50 of deformation depends on the elastic properties of the sandy regolith (the surface layer 51 exposed and highly fractured by impacts) and of the underlying rocks, and can thus be 52 used to explore beneath the surface. In this work, we review the theory describing the 53 ground motion caused by moving pressure perturbations, and we analyze the effect of 54 various parameters (wind speed, layering in the subsurface). We then develop a method 55 to retrieve a vertical profile of the elastic parameters beneath the lander from the mea-56 surements. After testing the method on ideal cases, we apply it to data from Mars: the 57 results show that the regolith becomes stiffer with depth, and that a layer of harder rock 58 may be present below, with the interface possibly located between 0.7 and 7 depth. De-59 termining the structure of the near surface provides constraints on the geologic history 60 of the landing site and contributes to the explanation of measured seismic signals.

Mars atmospheric pressure variations induce ground displacements through elastic deformations. The various sensors of INSIGHT mission were designed in order to be able to understand and correct these ground deformations induced by atmospheric effects. Particular efforts were done on one side to avoid direct pressure and wind effects on the seismometer, and on the other side to have a high performance pressure sensor operating in the same frequency range than the seismometer.As a consequence of the high performances of both instruments, their very efficient protection systems against direct atmospheric disturbances, and the low Mars background seismic noise, INSIGHT mission is opening a new science domain for which the ground displacements can be used to perform atmospheric science.This study presents an analysis of pressure and seismic signals and their relations. After a short description of the pressure and seismic sensors deployed by INSIGHT, we present an analysis of these signals as a function of local time at INSIGHT location.Then, the background and event like coherent signals between Pressure and seismometer sensors are interpreted in terms of various atmospheric excitations and induced ground deformation processes. Different methods to remove the pressure effects recorded by SEIS sensors are presented, and their efficiency is estimated. Finally, we demonstrate that the pressure and ground deformations measurements can be used to decipher between various atmospheric excitation types (meteorological pressure variations, acoustic and gravity waves) and characterize these. Effects of the local sub-surface structure are also suggested by the data analysis.

SEIS, the seismometer of the InSight mission, which landed on Mars on 26 November 2018, is monitoring the seismic activity of the planet. The goal of the Mars Structure Service (MSS) is to provide, as a mission product, the first average 1-D velocity model of Mars from the recorded InSight data. Prior to the mission, methodologies have been developed and tested to allow the location of the seismic events and estimation of the radial structure, using surface waves and body waves arrival times, and receiver functions. The paper describes these validation tests and compares the performance of the different algorithms to constrain the velocity model below the InSight station and estimate the 1-D average model over the great circle path between source and receiver. These tests were performed in the frame of a blind test, during which synthetic data were inverted. In order to propagate the data uncertainties on the output model distribution, Bayesian inversion techniques are mainly used. The limitations and strengths of the methods are assessed. The results show the potential of the MSS approach to retrieve the structure of the crust and underlying mantle. However, at this time, large quakes with clear surface waves have not yet been recorded by SEIS, which makes the estimation of the 1-D average seismic velocity model challenging. Additional locatable events, especially at large epicentral distances, and development of new techniques to fully investigate the data, will ultimately provide more constraints on the crust and mantle of Mars. ©2020 The Authors., Xu and Beghein were funded by NASA InSight PSP Grant 80NSSC18K1679. M. P. P., W. B. B., S. E. S., and S. T. were supported the NASA InSight mission and funds from the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The French authors acknowledge the French Space Agency CNES as well as CNRS and the French team universities for personal and infrastructure supports. The development of the MSS and associated software was provided by ANR (ANR‐14‐CE36‐0012‐02 and ANR‐19‐CE31‐0008‐08) and, for IPGP team, by UnivEarthS Labex program (ANR‐10‐LABX‐0023), IDEX Sorbonne Paris Cit (ANR‐11‐IDEX‐0005‐0). M1a and M2b models computation used HPC resources of CINES under the allocation A0050407341 and A0070407341 attributed by GENCI (Grand Equipement National de Calcul Intensif). M1b model is computed at the Centre de calcul des pays de la Loire (ccipl). This is InSight Contribution Number 141 and IPGP Contribution 4104. M. v. D. and S. C. S. were supported by a grant from the Swiss National Science Foundation (SNF‐ANR Project 157133 Seismology on Mars) and the Swiss National Supercomputing Center (CSCS) under Project ID sm682. A. K. and D. G. would like to acknowledge support from the Swiss National Science Foundation (SNF‐ANR project 172508 Mapping the internal structure of Mars).

In December 2018, the National Aeronautics and Space Administration (NASA) Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission deployed a seismometer on the surface of Mars. In preparation for the data analysis, in July 2017, the marsquake service initiated a blind test in which participants were asked to detect and characterize seismicity embedded in a one Earth year long synthetic data set of continuous waveforms. Synthetic data were computed for a single station, mimicking the streams that will be available from InSight as well as the expected tectonic and impact seismicity, and noise conditions on Mars (Clinton et al., 2017). In total, 84 teams from 20 countries registered for the blind test and 11 of them submitted their results in early 2018. The collection of documentations, methods, ideas, and codes submitted by the participants exceeds 100 pages. The teams proposed well established as well as novel methods to tackle the challenging target of building a global seismicity catalog using a single station. This article summarizes the performance of the teams and highlights the most successful contributions., This work was jointly funded by Swiss National Science Foundation and French Agence Nationale de la Recherche (SNF-ANR project 157133 "Seismology on Mars"); Swiss State Secretariat for Education, Research and Innovation (project "MarsQuake Service-Preparatory Phase"); and ETH Zurich (project "Preparatory phase for Mars InSight Ground Segment Support"). Additional support came from the Swiss National Supercomputing Centre (CSCS) under Project ID s682. Some of the research described in this article was supported by the Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) project, Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). The Houston team was partially funded by EAR-1621878. A. Spiga and L. Rolland acknowledge funding by Centre National d'Etudes Spatiales (CNES). This article constitutes InSight Contribution Number 93.

Impact investigations will be an important aspect of the InSight mission. One of the scientific goals of the mission is a measurement of the current impact rate at Mars. Impacts will additionally inform the major goal of investigating the interior structure of Mars. In this paper, we review the current state of knowledge about seismic signals from impacts on the Earth, Moon, and laboratory experiments. We describe the generalized physical models that can be used to explain these signals. A discussion of the appropriate source time function for impacts is presented, along with spectral characteristics including the cutoff frequency and its dependence on impact momentum. Estimates of the seismic efficiency (ratio between seismic and impact energies) vary widely. Our preferred value for the seismic efficiency at Mars is $5 \times 10^{- 4}$ , which we recommend using until we can measure it during the InSight mission, when seismic moments are not used directly. Effects of the material properties at the impact point and at the seismometer location are considered. We also discuss the processes by which airbursts and acoustic waves emanate from bolides, and the feasibility of detecting such signals. We then consider the case of impacts on Mars. A review is given of the current knowledge of present-day cratering on Mars: the current impact rate, characteristics of those impactors such as velocity and directions, and the morphologies of the craters those impactors create. Several methods of scaling crater size to impact energy are presented. The Martian atmosphere, although thin, will cause fragmentation of impactors, with implications for the resulting seismic signals. We also benchmark several different seismic modeling codes to be used in analysis of impact detections, and those codes are used to explore the seismic amplitude of impact-induced signals as a function of distance from the impact site. We predict a measurement of the current impact flux will be possible within the timeframe of the prime mission (one Mars year) with the detection of ∼ a few to several tens of impacts. However, the error bars on these predictions are large. Specific to the InSight mission, we list discriminators of seismic signals from impacts that will be used to distinguish them from marsquakes. We describe the role of the InSight Impacts Science Theme Group during mission operations, including a plan for possible night-time meteor imaging. The impacts detected by these methods during the InSight mission will be used to improve interior structure models, measure the seismic efficiency, and calculate the size frequency distribution of current impacts.

International audience; Based on an updated model of the regolith’s elastic properties, we simulate the ambient vibrations background wavefield recorded by InSight’s Seismic Experiment for Interior Structure (SEIS) on Mars to characterise the influence of the regolith and invert SEIS data for shallow subsurface structure. By approximately scaling the synthetics based on seismic signals of a terrestrial dust devil, we find that the high-frequency atmospheric background wavefield should be above the self-noise of SEIS’s SP sensors, even if the signals are not produced within 100–200 m of the station. We compare horizontal-to-vertical spectral ratios and Rayleigh wave ellipticity curves for a surface-wave based simulation on the one hand with synthetics explicitly considering body waves on the other hand and do not find any striking differences. Inverting the data, we find that the results are insensitive to assumptions on density. By contrast, assumptions on the velocity range in the upper-most layer have a strong influence on the results also at larger depth. Wrong assumptions can lead to results far from the true model in this case. Additional information on the general shape of the curve, i.e. single or dual peak, could help to mitigate this effect, even if it cannot directly be included into the inversion. We find that the ellipticity curves can provide stronger constraints on the minimum thickness and velocity of the second layer of the model than on the maximum values. We also consider the effect of instrumentation resonances caused by the lander flexible modes, solar panels, and the SEIS levelling system. Both the levelling system resonances and the lander flexible modes occur at significantly higher frequencies than the expected structural response, i.e. above 35 Hz and 20 Hz, respectively. While the lander and solar panel resonances might be too weak in amplitude to be recorded by SEIS, the levelling system resonances will show up clearly in horizontal spectra, the H/V and ellipticity curves. They are not removed by trying to extract only Rayleigh-wave dominated parts of the data. However, they can be distinguished from any subsurface response by their exceptionally low damping ratios of 1% or less as determined by random decrement analysis. The same applies to lander-generated signals observed in actual data from a Moon analogue experiment, so we expect this analysis will be useful in identifying instrumentation resonances in SEIS data.

International audience; The atmospheric pressure fluctuations on Mars induce an elastic response in the ground that creates a ground tilt, detectable as a seismic signal on the InSight seismometer SEIS. The seismic pressure noise is modeled using Large Eddy Simulations (LES) of the wind and surface pressure at the InSight landing site and a Green’s function ground deformation approach that is subsequently validated via a detailed comparison with two other methods: a spectral approach, and an approach based on Sorrells’ theory (Sorrells,Geophys. J. Int. 26:71–82, 1971; Sorrells et al., Nat. Phys. Sci. 229:14–16, 1971). The horizontal accelerations as a result of the ground tilt due to the LES turbulence-induced pressure fluctuations are found to be typically ∼ 2–40 nm/s2 in amplitude, whereas the direct horizontal acceleration is two orders of magnitude smaller and is thus negligible in comparison. The vertical accelerations are found to be ∼ 0.1–6 nm/s2 in amplitude. These are expected to be worst-case estimates for the seismic noise as we use a half-space approximation; the presence at some (shallow) depth of a harder layer would significantly reduce quasi-static displacement and tilt effects. We show that under calm conditions, a single-pressure measurement is representative of the large-scale pressure field (to a distance of several kilometers), particularly in the prevailing wind direction. However, during windy conditions, small-scale turbulence results in a reduced correlation between the pressure signals, and the single-pressure measurement becomes less representative of the pressure field. The correlation between the seismic signal and the pressure signal is found to be higher for the windiest period because the seismic pressure noise reflects the atmospheric structure close to the seismometer. In the same way that we reduce the atmospheric seismic signal by making use of a pressure sensor that is part of the InSight Auxiliary Payload Sensor Suite, we also the use the synthetic noise data obtained from the LES pressure field to demonstrate a decorrelation strategy. We show that our decorrelation approach is efficient, resulting in a reduction by a factor of ∼ 5 in the observed horizontal tilt noise (in the wind direction) and the vertical noise. This technique can, therefore, be used to remove the pressure signal from the seismic data obtained on Mars during the InSight mission.