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Analyses of seismic data from the InSight mission have provided the first in situ constraints on the thickness of the crust of Mars. These crustal thickness constraints are currently limited to beneath the lander that is located in the northern lowlands, and we use gravity and topography data to construct global crustal thickness models that satisfy the seismic data. These models consider a range of possible mantle and core density profiles, a range of crustal densities, a low-density surface layer, and the possibility that the crustal density of the northern lowlands is greater than that of the southern highlands. Using the preferred InSight three-layer seismic model of the crust, the average crustal thickness of the planet is found to lie between 30 and 72 km. Depending on the choice of the upper mantle density, the maximum permissible density of the northern lowlands and southern highlands crust is constrained to be between 2,850 and 3,100 kg m−3. These crustal densities are lower than typical Martian basaltic materials and are consistent with a crust that is on average more felsic than the materials found at the surface. We argue that a substantial portion of the crust of Mars is a primary crust that formed during the initial differentiation of the planet. Various hypotheses for the origin of the observed intracrustal seisimic layers are assessed, with our preferred interpretation including thick volcanic deposits, ejecta from the Utopia basin, porosity closure, and differentiation products of a Borealis impact melt sheet.

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.

The mantle transition zone is believed to play an important role in the thermochemical evolution of our planet and in its deep water cycle. Constraining mantle flow at these depths can help elucidate its nature and better understand mantle dynamics and the history of plate tectonics. Seismic anisotropy, i.e., the directional dependence of seismic wave velocity, provides us with the most direct constraints on mantle deformation. Its detection below ~250 km depth is challenging, and it is often assumed that the deep upper mantle is seismically isotropic due to a change in mantle deformation mechanism. Here, we present a global model of azimuthal anisotropy for the top 1000. km of the mantle. We used a dataset composed of fundamental and higher mode Rayleigh wave phase velocity maps, which provides resolution of azimuthal anisotropy to much greater depths than in previous studies. Our model unravels the presence of significant anisotropy in the transition zone, challenging common views of mantle deformation mechanisms, and reveals a striking correlation between changes in anisotropy amplitudes and in the fast direction of wave propagation where upper mantle phase transitions occur. The newly found relation between anisotropy changes and phase transformations gives new insight on the nature of the MTZ and suggests that the anisotropy originates from the lattice preferred orientation of anisotropic material. While the interpretation of our results in terms of mantle deformation is not straightforward due to many possible scenarios, possible explanations for our findings include recrystallization during phase transformations, or changes in the slip system across the MTZ boundaries, which in turn could be explained by changes in the water content of mantle material, consistent with the idea that the transition zone acts as a water filter. © 2013.

The mantle transition zone (MTZ) of Earth is demarcated by solid‐to‐solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660‐km depths. Mineral physics experiments predict that wadsleyite can have strong single‐crystal anisotropy at the pressure and temperature conditions of the MTZ. Thus, significant seismic anisotropy is possible in the upper MTZ where lattice‐preferred orientation of wadsleyite is produced by mantle flow. Here, we use a body wave method, SS precursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. We stack the data to explore the azimuthal dependence of travel‐times and amplitudes of SS precursors and constrain the azimuthal anisotropy in the MTZ. Beneath the central Pacific, we find evidence for ~4% anisotropy with a SE fast direction in the upper mantle and no significant anisotropy in the MTZ. In subduction zones, we observe ~4% anisotropy with a trench‐parallel fast direction in the upper mantle and ~3% anisotropy with a trench‐perpendicular fast direction in the MTZ. The transition of fast directions indicates that the lattice‐preferred orientation of wadsleyite induced by MTZ flow is organized separately from the flow in the upper mantle. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the upper mantle but negligible anisotropy (<1%) in the MTZ. Finally, we correct for the upper mantle and MTZ anisotropy structures to obtain a new MTZ topography model. The anisotropy correction produces ±3 km difference and therefore has minor overall effects on global MTZ topography. Key Points: The MTZ anisotropy is investigated from the SS precursors at both regional and global scalesWe observe ~3% anisotropy in the MTZ beneath subduction zones but negligible MTZ anisotropy (<1%) at a global scaleWe propose a new MTZ topography model corrected for the upper mantle and MTZ anisotropy structures [ABSTRACT FROM AUTHOR]

We applied Sambridge's Neighbourhood Algorithm (NA) to degree-8 fundamental mode Love and Rayleigh wave phase velocity maps between 40 and 150 s to find models of radial anisotropy in the upper 220 km of the mantle. The NA is a powerful tool to explore a multidimensional model space and retrieve an ensemble of models from which statistical inferences (posterior probability density functions (PPDFs) and trade-offs) can be made. We sought solutions for density anomalies and perturbations in the five elastic coefficients that describe transverse isotropy and obtained independent probability density functions for S-wave anisotropy, P-wave anisotropy, intermediate parameter η, Vp, Vs and density anomalies. We find robust departures from PREM in S-wave anisotropy (ξ) under cratons and oceans alike, with a clear change of sign in the anomalies with respect to the reference model at approximately 100 km depth. No significant difference is observed between cratons and oceans, both in the amplitude and depth variation of ξ. The signal within continents is clearly age related, with platforms and tectonically active regions characterized by a rapid decrease in ξ with depth, while cratons display a more constant signal. A similar age dependence in S-wave anisotropy is also observed beneath oceans: a strong and rapidly decreasing anisotropy for young oceans and a more constant anisotropy for older oceans. Perturbations in P-wave anisotropy (φ) are small and limited to the shallowest part of the continents. A small age-dependent signal for φ is observed beneath oceans. Anomalies in intermediate parameter η are similar to those in φ (but not globally correlated), but the deviation from PREM is stronger for η than for φ. Cratons appear to be devoid of any η anisotropy in the top 100 km. There is no obvious global correlation between deviations in φ and deviations in ξ, and the ratio between dη and dξ is clearly regionally variable, which cautions against the use of commonly used proportionality factors between these variables in inversions. In all regions, we found a good correlation between the equivalent isotropic P- and S-wave velocity anomalies, with a ratio d  ln  Vs/ d  ln  Vp close to 1. Density anomalies were not sufficiently well resolved with fundamental mode data alone, but do not influence the results for anisotropy. [ABSTRACT FROM AUTHOR]