Constraints for the Martian Crustal Structure From Rayleigh Waves Ellipticity of Large Seismic Events

For the first time, we measured the ellipticity of direct Rayleigh waves at intermediate periods (15-35 s) on Mars using the recordings of three large seismic Martian events, including S1222a, the largest event recorded by the InSight mission. These measurements, together with P-to-s receiver functions and P-wave reflection times, were utilized for performing a joint inversion of the local crustal structure at the InSight landing site. Our inversion results are compatible with previously reported intra-crustal discontinuities around 10 and 20 km depths, whereas the preferred models show a strong discontinuity at ~37 km, which is interpreted as the crust-mantle interface. Additionally, we support the presence of a shallow low-velocity layer of 2-3 km thickness. Compared to nearby regions, lower seismic wave velocities are derived for the crust, suggesting a higher porosity or alteration of the whole local crust

The Polarization of Ambient Noise on Mars

Seismic noise recorded at the surface of Mars has been monitored since February 2019, using the InSight seismometers. This noise can reach −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

The Far Side of Mars: Two Distant Marsquakes Detected by InSight

For over three Earth years the Marsquake Service has been analyzing the data sent back from the Seismic Experiment for Interior Structure¿the seismometer placed on the surface of Mars by NASA¿s InSight lander. Although by October 2021, the Mars seismic catalog included 951 events, until recently all these events have been assessed as lying within a radius of 100° of InSight. Here we report two distant events that occurred within days of each other, located on the far side of Mars, giving us our first glimpse into Mars¿ core shadow zone. The first event, recorded on 25 August 2021 (InSight sol 976), shows clear polarized arrivals that we interpret to be PP and SS phases at low frequencies and locates to Valles Marineris, 146° ± 7° from InSight. The second event, occurring on 18 September 2021 (sol 1000), has significantly more broadband energy with emergent PP and SS arrivals, and a weak phase arriving before PP that we interpret as Pdiff¿. Considering uncertain pick times and poorly constrained travel times for Pdiff¿, we estimate this event is at a distance between 107° and 147° from InSight. With magnitudes of MMaw 4.2 and 4.1, respectively, these are the largest seismic events recorded so far on Mars.Anna C. Horleston, Jessica C. E. Irving,and Nicholas A. Teanby are funded by the UKSA under Grant Numbers ST/R002096/1, ST/W002523/1, and ST/W002515/1.Nikolaj L. Dahmen, Cecilia Duran, Géraldine Zenhäusern, andSimon C. Stähler would like to acknowledge support from Eidgenössische Technische Hochschule (ETH) through the ETH+ funding scheme (ETH+02 19-1: “Planet Mars”). The French coauthors acknowledge the funding support provided by CNES and the Agence Nationale de la Recherche (ANR-19-CE31-0008-08 MAGIS) for SEIS operation and SEIS Science analysis. Alexander E. Stott acknowledges the French Space Agency CNES and ANR (ANR-19-CE31-0008-08). Caroline Beghein and Jiaqi Li were supported by NASA InSight Participating Scientist Program (PSP) Grant Number 80NSSC18K1679. This article is InSight Contribution Number 236

The interior of Mars as seen by InSight (Invited)

InSight is the first planetary mission dedicated to exploring the whole interior of a planet using geophysical methods, specifically seismology and geodesy. To this end, we observed seismic waves of distant marsquakes and inverted for interior models using differential travel times of phases reflected at the surface (PP, SS...) or the core mantle-boundary (ScS), as well as those converted at crustal interfaces. Compared to previous orbital observations1-3, the seismic data added decisive new insights with consequences for the formation of Mars: The global average crustal thickness of 24-75 km is at the low end of pre-mission estimates5. Together with the the thick lithosphere of 450-600 km5, this requires an enrichment of heat-producing elements in the crust by a factor of 13-20, compared to the primitive mantle. The iron-rich liquid core is 1790-1870 km in radius6, which rules out the existence of an insulating bridgmanite-dominated lower mantle on Mars. The large, and therefore low-density core needs a high amount of light elements. Given the geochemical boundary conditions, Sulfur alone cannot explain the estimated density of ~6 g/cm3 and volatile elements, such as oxygen, carbon or hydrogen are needed in significant amounts. This observation is difficult to reconcile with classical models of late formation from the same material as Earth. We also give an overview of open questions after three years of InSight operation on the surface of Mars, such as the potential existence of an inner core or compositional layers above the CM

Atmospheric Science with InSight

International audienceIn November 2018, for the first time a dedicated geophysical station, the InSight lander, will be deployed on the surface of Mars. Along with the two main geophysical packages, the Seismic Experiment for Interior Structure (SEIS) and the Heat-Flow and Physical Properties Package (HP3), the InSight lander holds a highly sensitive pressure sensor (PS) and the Temperature and Winds for InSight (TWINS) instrument, both of which (along with the InSight FluxGate (IFG) Magnetometer) form the Auxiliary Sensor Payload Suite (APSS). Associated with the RADiometer (RAD) instrument which will measure the surface brightness temperature, and the Instrument Deployment Camera (IDC) which will be used to quantify atmospheric opacity, this will make InSight capable to act as a meteorological station at the surface of Mars. While probing the internal structure of Mars is the primary scientific goal of the mission, atmospheric science remains a key science objective for InSight. InSight has the potential to provide a more continuous and higher-frequency record of pressure, air temperature and winds at the surface of Mars than previous in situ missions. In the paper, key results from multiscale meteorological modeling, from Global Climate Models to Large-Eddy Simulations, are described as a reference for future studies based on the InSight measurements during operations. We summarize the capabilities of InSight for atmospheric observations, from profiling during Entry, Descent and Landing to surface measurements (pressure, temperature, winds, angular momentum), and the plans for how InSight’s sensors will be used during operations, as well as possible synergies with orbital observations. In a dedicated section, we describe the seismic impact of atmospheric phenomena (from the point of view of both “noise” to be decorrelated from the seismic signal and “signal” to provide information on atmospheric processes). We discuss in this framework Planetary Boundary Layer turbulence, with a focus on convective vortices and dust devils, gravity waves (with idealized modeling), and large-scale circulations. Our paper also presents possible new, exploratory, studies with the InSight instrumentation: surface layer scaling and exploration of the Monin-Obukhov model, aeolian surface changes and saltation / lifing studies, and monitoring of secular pressure changes. The InSight mission will be instrumental in broadening the knowledge of the Martian atmosphere, with a unique set of measurements from the surface of Mars

Instabilités de flambage dans les fluides visqueux - Du laboratoire au manteau terrestre

Un mince filet de fluide visqueux qui tombe sur une surface s’enroule en hélice tournante, alors que le même fluide en forme de nappe se replie de manière périodique. Ces instabilités, dites « de flambage », cachent une physique d’une grande quatre régimes qui correspondent aux différents équilibres des forces visqueuse, gravitationnelle et inertielle. La gamme de leurs applications va des industries de l’alimentation et de fabrication des polymères jusqu’à la dynamique du manteau de notre planète

The Effect of Water Column Resonance on the Spectra of Secondary Microseism P Waves

International audienceWe compile and analyze a data set of secondary microseismic P wave spectra that were observed by North American seismic arrays. Two distinct frequency bands, 0.13-0.15 Hz and 0.19-0.21 Hz, with enhanced P wave energy characterize the data set. Cluster analysis allows to classify the spectra and to associate typical spectral shapes with geographical regions: Low-frequency-dominated spectra (0.13-0.15 Hz) are mostly detected in shallower regions of the North Atlantic and the South Pacific, as well as along the Central and South American Pacific coast. High-frequency-dominated spectra (0.19-0.21 Hz) are mostly detected in deeper regions of the northwestern Pacific and the South Pacific. For a selected subset of high-quality sources, we compute synthetic spectra from an ocean wave hindcast. These synthetic spectra are able to reproduce amplitude and shape of the observed spectra, but only if P wave resonance in the water column at the source site is included in the model. Our data sets therefore indicate that the spectral peaks at 0.13-0.15 Hz and 0.19-0.21 Hz correspond to the first and second harmonics of P wave resonance in the water column that occur in shallower ocean depths (<3,000 m) and in the deep ocean (∼5,000 m), respectively. This article demonstrates the important effect of water column resonance on the amplitude and frequency of P waves that are generated by secondary microseisms and that the amplitude of high-quality sources can be predicted from ocean wave hindcasts within a factor of 0.4-6

On the shaping factors of the secondary microseismic wavefield

International audienceSeismic noise in the period band 3–10 s is known as secondary microseism, and it is generated at the ocean surface by the interaction of ocean gravity waves coming from nearly opposite directions. In this paper, we investigate the seismic content of the wavefield generated by a source at the ocean surface and three of the major wavefield shaping factors using the 2-D spectral element method: the ocean-continent boundary, the source site effect, and the thickness of seafloor sediments. The seismic wavefield recorded on the vertical component seismograms below the seafloor is mainly composed of the fundamental mode and the first overtone of Rayleigh waves. A mode conversion from the first overtone to the fundamental mode of Rayleigh waves occurs at the ocean-continent boundary. The presence of a continental shelf at the ocean-continent boundary produces a negligible effect on land-recorded seismograms, whereas the source site effect, i.e., the source location with respect to the local ocean depth and sediment thickness, plays the major role. A source in shallow water mostly enhances the fundamental mode of Rayleigh waves, whereas a source in deep water mainly enhances the first overtone of Rayleigh waves. Land-recorded long-period signals (T > 6 s) are mostly due to deep water sources, whereas land-recorded short-period signals (T 6 s)

From strain to displacement: using deformation to enhance distributed acoustic sensing applications

International audienceOver a period of less than a decade, distributed acoustic sensing (DAS) has become a well-established technology in seismology. For historical and practical reasons, DAS manufacturers usually provide instruments that natively record strain (rate) as the principal measurement. While at first glance strain recordings seem related to ground motion waveforms (displacement, velocity and acceleration), not all the seismological tools developed over the past century (e.g. magnitude estimation, seismic beamforming, etc.) can be readily applied to strain data. Notably, the directional sensitivity of DAS is more limited than conventional particle motion sensors, and DAS experiences an increased sensitivity to slow waves, often highly scattered by the subsurface structure and challenging to analyse. To address these issues, several strategies have been already proposed to convert strain rate measurements to particle motion. In this study, we focus on strategies based on a quantity we refer to as ‘deformation’. Deformation is defined as the change in length of the cable and is closely related to displacement, yet both quantities differ from one another: deformation is a relative displacement measurement along a curvilinear path. We show that if the geometry of the DAS deployment is made of sufficiently long rectilinear sections, deformation can be used to recover the displacement without the need of additional instruments. We validate this theoretical result using full-waveform simulations and by comparing, on a real data set, the seismic velocity recovered from DAS with that recorded by collocated seismometers. The limitations of this approach are discussed, and two applications are shown: enhancing direct P-wave arrivals and simplifying the magnitude estimation of seismic events. Converted displacement provides better sensitivity to high velocity phases, improves broadside response and permits the direct application of conventional seismological tools that are less effective when applied to strain (rate) data