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The shallowest intracrustal layer (extending to 8 ± 2 km depth) beneath the Mars InSight Lander site exhibits low seismic wave velocity, which is likely related to a combination of high porosity and other lithological factors. The SsPp phase, an SV‐ to P‐wave reflection on the receiver side, is naturally suited for constraining the seismic structure of this top crustal layer since its prominent signal makes it observable with a single station without the need for stacking. We have analyzed six broadband and low‐frequency seismic events recorded on Mars and made the first coherent detection of the SsPp phase on the red planet. The timing and amplitude of SsPp confirm the existence of the ∼8 km interface in the crust and the large wave speed (or impedance) contrast across it. With our new constraints from the SsPp phase, we determined that the average P‐wave speed in the top crustal layer is between 2.5 and 3.2 km/s, which is a more precise and robust estimate than the previous range of 2.0–3.5 km/s obtained by receiver function analysis. The low velocity of Layer 1 likely results from the presence of relatively low‐density lithified sedimentary rocks and/or aqueously altered igneous rocks that also have a significant amount of porosity, possibly as much as 22%–30% by volume (assuming an aspect ratio of 0.1 for the pore space). These porosities and average P‐wave speeds are compatible with our current understanding of the upper crustal stratigraphy beneath the InSight Lander site. Plain Language Summary: The NASA InSight mission sent a seismometer to Mars in 2018. One of the science goals of the mission is to better understand how rocky planets form and evolve by investigating the interior structure of Mars. Previous seismological studies with InSight data have revealed a shallow crustal layer (i.e., Layer 1, extending to 8 ± 2 km depth) with low seismic wave speed under the instrument. In this study, we have identified a new seismic signal on the seismograms recorded on Mars. The existence of this seismic phase confirmed the low speed of compressional (P) waves in Layer 1 and provided additional constraints on the average P‐wave speed, that is, between 2.5 and 3.2 km/s. Based on these low speeds, we found that the seismic properties of Layer 1 likely result primarily from the presence of sedimentary rocks and/or aqueously altered igneous rocks that also have a significant amount of porosity, possibly as much as ∼30% by volume. These porosities and average P‐wave speeds are compatible with our current understanding of the upper crustal stratigraphy beneath the InSight Lander site. Key Points: We analyzed marsquakes and made the first coherent detection of the SsPp phase (an SV‐ to P‐wave reflection on the receiver side)We determined that the average P‐wave speed in the top crustal layer (Layer 1, above 8 km) is between 2.5 and 3.2 km/sThe average P‐wave speed in Layer 1 is consistent with the current understanding of the upper crustal stratigraphy beneath InSight [ABSTRACT FROM AUTHOR]

Mars is the first extraterrestrial planet with seismometers (Seismic Experiment for Interior Structure, SEIS) deployed directly on its surface in the framework of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission. The lack of strong Marsquakes, however, strengthens the need of seismic noise studies to additionally constrain the Martian structure. Seismic noise autocorrelations of single‐station recordings permit the determination of the zero‐offset reflection response underneath SEIS. We present a new autocorrelation study which employs state‐of‐the‐art approaches to determine a robust reflection response by avoiding bias from aseismic signals which are recorded together with seismic waves due to unfavorable deployment and environmental conditions. Data selection and segmentation is performed in a data‐adaptive manner which takes the data root‐mean‐square amplitude variability into account. We further use the amplitude‐unbiased phase cross‐correlation and work in the 1.2–8.9 Hz frequency band. The main target are crustal scale reflections, their robustness and convergence. The strongest signal appears at 10.6 s, and, if interpreted as a P‐wave reflection, would correspond to a discontinuity at about 21 km depth. This signal is a likely candidate for a reflection from the base of the Martian crust due to its strength, polarity, and stability. Additionally we identify, among the stable signals, a signal at about 6.15 s that can be interpreted as the P‐wave reflection from the mid‐crust at about 9.5 km depth. Key Points: Estimation of high‐frequency reflection response using phase autocorrelations of seismic noise recorded on Mars at the InSight landing sitePresentation of a new data processing method that avoids aseismic signal bias and stability analysis of the reflection responseA signal at 10.6 s lag time is a possible candidate for a reflection from the base of the crust due to its strength, polarity, and stability [ABSTRACT FROM AUTHOR]

The InSight mission landed on Mars on November 26th, 2018 with several geophysical instruments including a short-period seismometer and a broadband seismometer (SEIS, Seismic Experiment for Interior Structure). Both seismometers are now installed directly on Mars surface and enable to analyze the continuous seismic signal. The purpose of this study is to analyze, quantify and characterize the frequency-dependent polarization of Mars continuous seismic signal. On Earth, the secondary microseisms are the dominant signals in the absence of earthquakes. Secondary microseisms are dominantly Rayleigh waves, with elliptical polarization in the vertical plane. We do not expect to record microseisms on Mars because of the absence of oceans. We adapted the method developed for Earth data by Schimmel et al. (2011) to analyze the frequency-dependent polarization of continuous Mars data. The method enables to detect elliptically polarized signal such as Rayleigh waves and also linearly polarized signals that could be either Love waves or body waves. The method was tested on the two blindtest datasets provided by the Marsquake Service (MQS) and by the Mars Structure Service (MSS). It is then applied to analyze the first data recorded on Mars by the broadband and the short period seismometers. [ABSTRACT FROM AUTHOR]

The InSight lander successfully delivered geophysical instrument package on the Martian surface on November 26th, 2018, including a broadband and a short-period seismometer (Seismic Experiment for Interior Structure, SEIS). The seismic instrument package is specifically designed to record marsquakes and meteoritic impacts in Martian conditions. Routine operations are split into two services: the Mars Structure Service (MSS) and the Marsquake Service (MQS), which are responsible for defining structure models and seismicity catalogs, respectively. The first "deliverable" of the MSS will be a model based on the events detected during the first 3 months of seismic monitoring of the mission, for which only a few quakes might be expected based on current estimates of Mars seismic activity. To test our approach of determining the interior model of Mars and to prepare the InSight science team for data return, we made use of a "blind test" time series for which the Marsquake parameters (location, depth, origin time, and moment tensor) and interior model were unknown to the group at large.In preparation for the mission, the goal was to develop mature algorithms to handle the data as efficiency as possible. Synthetic seismic waveforms were computed in a 1D mantle model with a 3D crust on top using AxiSEM and Salvus. The time series were created by adding seismic noise that relies on pre-landing estimates of noise generated by the sensors, electronic system, environment, and nearby lander. To characterize what we could learn about Mars interior structure with only one station and with the first seismic event, we performed inversions of a synthetic data following a blind test process, where the interior model was unknown to all team members carrying out data analysis and inversion. We detail and compare the results of this "blind test" using different methods including inversion of surface wave dispersion data, body waves travel times, and the waveforms themselves. We have used mainly Bayesian techniques to obtain robust probability density functions of interior structure parameters. The effects on the retrieved model distributions of fixing mars quake location and origin time are investigated, as is the effect of using fixed Vs flexible parameterizations. To allow for tighter constraints, we also test the use of priors based on thermodynamically-constrained models together with seismic observations, as well as seismic confirmation/rejection of models purely based on thermodynamical modelling. These techniques considered here form a large part of the planned modeling of the MSS that will be ultimately employed with the first recording of a seismic event by InSight. [ABSTRACT FROM AUTHOR]