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We present the first observations of seismic waves propagating through the core of Mars. These observations, made using seismic data collected by the InSight geophysical mission, have allowed us to construct the first seismically constrained models for the elastic properties of Mars' core. We observe core-transiting seismic phase SKS from two farside seismic events detected on Mars and measure the travel times of SKS relative to mantle traversing body waves. SKS travels through the core as a compressional wave, providing information about bulk modulus and density. We perform probabilistic inversions using the core-sensitive relative travel times together with gross geophysical data and travel times from other, more proximal, seismic events to seek the equation of state parameters that best describe the liquid iron-alloy core. Our inversions provide constraints on the velocities in Mars' core and are used to develop the first seismically based estimates of its composition. We show that models informed by our SKS data favor a somewhat smaller (median core radius = 1,780 to 1,810 km) and denser (core density = 6.2 to 6.3 g/cm3) core compared to previous estimates, with a P-wave velocity of 4.9 to 5.0 km/s at the core-mantle boundary, with the composition and structure of the mantle as a dominant source of uncertainty. We infer from our models that Mars' core contains a median of 20 to 22 wt% light alloying elements when we consider sulfur, oxygen, carbon, and hydrogen. These data can be used to inform models of planetary accretion, composition, and evolution. [ABSTRACT FROM AUTHOR]

With the recent successful landing and the ongoing deployment of its instruments, the InSight mission will soon provide the first series of Martian seismic recordings. Such data will allow the present-day deep structure of the planet to be revealed. While inverse methods are powerful tools to achieve this goal, they are prone to two main limitations: the large size of the parameter space to be explored, and the non-uniqueness of the solution. This is particularly true for spatially uneven and relatively small amounts of seismic recordings, as in the case of InSight, which consists of a single station, and a precious, but limited amount of expected Marsquakes.To better constrain the interior of Mars using the upcoming InSight recordings, we have developed a Monte-Carlo Markov-Chain inversion of seismic data, where the modeling of Mars' thermochemical history is part of the forward problem. Such a modeling accounts for the main Martian envelopes: an essentially metallic core surrounded by a convective silicate envelope, overlaid by a stagnant lithospheric lid. The latter includes a crust enriched in time-decaying, heat-producing radioactive elements. This thermo-chemical frame is integrated via a parametrized approach, allowing the long-term planetary evolution to be accurately modeled at a reasonable computational cost.Consequently, instead of varying independently seismological parameters along the inversion process (crustal thickness, core size, seismic velocities, etc.), our built-in geodynamic frame significantly reduces the parameter space, by self-consistently accounting for the interdependencies between various quantities: temperature, composition, rheology, and their influences on crustal thickness. Further constraints on these different physical parameters will come from recent measurements directly performed under Mars' interior P-T conditions. The resulting output parameters from the thermo-chemical modeling yield the seismic structure required for the inversion process. Our approach can also constrain the value of physical quantities inaccessible to direct measurements, such as the rheology of the Martian mantle. Finally, our method naturally yields the entire thermo-chemical history of Mars associated with each model.Our implementation has been benchmarked against analytical and numerical solutions in spherical geometry, and we successfully tested our inversion approach against synthetic data.We will present the details of our inversion method, together with its performances using synthetic data. Finally, we will apply our approach to the data that InSight instruments are expected to collect, in order to constrain the current state of Mars' interior, and to infer its long-term evolution. [ABSTRACT FROM AUTHOR]

Clues to a planet’s geologic history are contained in its interior structure, particularly its core. We detected reflections of seismic waves from the core-mantle boundary of Mars using InSight seismic data and inverted these together with geodetic data to constrain the radius of the liquid metal core to 1830 ± 40 kilometers. The large core implies a martian mantle mineralogically similar to the terrestrial upper mantle and transition zone but differing from Earth by not having a bridgmanite-dominated lower mantle. We inferred a mean core density of 5.7 to 6.3 grams per cubic centimeter, which requires a substantial complement of light elements dissolved in the iron-nickel core. The seismic core shadow as seen from InSight’s location covers half the surface of Mars, including the majority of potentially active regions—e.g., Tharsis—possibly limiting the number of detectable marsquakes. [ABSTRACT FROM AUTHOR]

Authors:T. Kawamura(1), N. C. Schmerr(2), L. Margerin(3), M. van Driel(4), R. Garcia(5), F. Karakostas(1,2), Benoit Tauzin (6), M. Drilleau(1), P. Lognonné(2),Affiliations:1 Institut de Physique du Globe de Paris2 University of Maryland3 CNRS / Université de Toulouse 4 ETH Zurich5 ISAE-SUPAERO6 Université de LyonTitle:Measuring the Scattering and Atteunation of Seismic Waves in Mars with the InSight Seismometers. Abstract: The amplitudes of seismic waves decrease over distance due frequency dependent energy loss via scattering and attenuation. Scattering, or extrinsic attenuation, is dependent upon the size, strength, and abundance of elastic heterogeneities within the medium. Intrinsic attenuation results from dissipation due to anelastic processes and internal friction between molecules and defects within crystal lattices. Both forms of attenuation are described using the seismic quality factor, Q. Scattering can arise from compositional heterogeneities, mechanical boundaries (such as fractures, cracks, veins, cavities), and other major non-radial changes in material properties of an object. Intrinsic attenuation is tied to grain boundary processes, crystal defects, fluid-filled cracks, and other dissipative viscoelastic processes. Attenuation is vital for understanding the detectability, travel times, and propagation properties of seismic waves, as well as the properties of the interior of a world. The Interior Exploration using Seismic Investigations Geodesy and Heat Transport (InSight) Seismic Experiment for Interior Structure (SEIS) has placed a very broadband (VBB) and short period (SP) seismometers on Mars that allows us to investigate the fundamental geophysical process of seismic attenuation at Mars, which is expected to have properties falling between the Earth and the Moon. In the Earth, attenuation by seismic scattering is found to be relatively weak, and is typically confined to small-scale compositional heterogeneities in the crust, lithosphere, and mantle. Intrinsic attenuation varies throughout the Earth, and is especially high where fluids, elevated temperatures, and changes in grain size are present. On the Moon, seismic scattering is pervasive in the highly fractured and dry crust and uppermost mantle, and intrinsic attenuation is large in the deep interior. At present, the total Q, layering in Q, and relative proportion of seismic scattering to intrinsic attenuation within Mars remains unconstrained. [ABSTRACT FROM AUTHOR]

Mars atmospheric pressure variations induce ground deformations that are the main source of environmental noise on the InSight SEIS instrument. Phenomena generating pressure variations, such as meteorological wavefronts, gravity waves, convective vortices and small-scale turbulence are covering the whole frequency range of SEIS broadband seismometers. These signals impact the capability of SEIS sensors to detect seismic waves propagating in Mars interior. However, the relation between pressure variations measured by the InSight pressure sensor and ground movements measured by SEIS being essentially linear, it is possible to invert both signals to find the transfer function between pressure and ground motion. Then, pressure generated ground movements can be substracted from SEIS records in order to improve their sensitivity to seismic waves. The transfer function, usually called compliance, depends on the ground properties. Thus, the decorrelation of SEIS records from pressure effects will also allow us to constrain the subsurface properties at the InSight landing site.Our study compares two different pressure decorrelation methods applied to Mars synthetic data, and records by both short period and very broadband SEIS sensors. The pressure-decorrelation methods are presented and their differences in terms of underlying assumptions and implementations are explained. Their performances and drawbacks, analyzed on realistic synthetic datasets, are discussed. We plan to apply these methods to real Mars data and use them to retrieve the ground compliance at the InSight landing site over a broad frequency range. Accordingly, we expect to extract elastic properties of the subsurface through the compliance measurements. Wind measurements by the TWINS wind sensor will be integrated in order to separate the effects of various atmospheric phenomena. This sensor is almost identical to the Rover Environmental Monitoring Station sent to Mars as part of the Mars Science Laboratory (MSL), which operated even after the damage suffered during the MSL landing for more than two Martian years. Thus, wind measured by TWINS will provide valuable inputs to the decorrelation on the InSight SEIS instrument. Finally, the variations of decorrelation performances and compliance measurements with the diurnal Mars cycle will be analyzed. [ABSTRACT FROM AUTHOR]