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The Pacific large low-shear-velocity province (LLSVP), as revealed by cluster analysis of global tomographic models, hosts multiple internal anomalies, including a notable gap (~20° wide) between the central and eastern Pacific. The cause of the structural gap remains unconstrained. Directly above this structural gap, we identify an anomalously thick mantle transition zone east of the East Pacific Rise, the fastest-spreading ocean ridge in the world, using a dense set of SS precursors. The area of the thickened transition zone exhibits faster-than-average velocities according to recent tomographic images, suggesting perturbed postolivine phase boundaries shifting in response to lowered temperatures. We attribute this observation to episodes of Mesozoic-aged (250 to 120 million years ago) intraoceanic subduction beneath the present-day Nazca Plate. The eastern portion of the Pacific LLSVP was separated by downwelling because of this ancient oceanic slab. Our discovery provides a unique perspective on linking deep Earth structures with surface subduction. [ABSTRACT FROM AUTHOR]

Gravity inversions have contributed greatly to our knowledge of the interior of planetary bodies and the processes that shaped them. However, previous global gravity inversion methods neglect the inference of mantle density anomalies when using techniques to decrease the non‐uniqueness of the inversion. In this work, we present a novel global gravity inversion algorithm, named THeBOOGIe, suited to inferring global‐scale density anomalies within the crust and mantle of planetary bodies. The algorithm embraces the nonuniqueness inherent in gravity inversions by not prescribing at the outset a density interface or depth range of interest. Instead, the method combines a Bayesian approach with a flexible incorporation of prior geological or geophysical information to infer density anomalies at any depth. A validation test using synthetic lunar‐like gravity data shows that THeBOOGIe can constrain the lateral location of crustal density anomalies but tends to overestimate their thicknesses. Importantly, THeBOOGIe can detect deep mantle density anomalies and quantify the level of confidence in the inferred density models. Our results show that THeBOOGIe can provide complementary information to one‐dimensional seismic models of the interior of the terrestrial planets and the Moon by constraining density anomalies that are not spherically symmetric. Additionally, THeBOOGIe is specially suited to constraining the interior of partially differentiated bodies where these large‐scale density anomalies are more likely to exist. Finally, thanks to the flexible use of priors, THeBOOGIe is an essential tool to understand the interior of planetary bodies lacking additional constraints. Plain Language Summary: Gravity data is often the main constraint on the structure of planetary interiors. However, inferring the density structure of a body from gravity data is inherently non‐unique, due to the tradeoff between density anomaly and distance in the definition of gravity anomalies. Traditional methods circumvent this problem by specifying a priori the source of gravity anomalies to crustal thickness variations while neglecting deeper anomalies. We present here an alternative approach where density anomalies are defined over finite‐size objects and the inversion is conducted within a Bayesian framework. Our algorithm, named THeBOOGIe, is able to recover the location of density anomalies in the mantle of the Moon, although the inferred shallow crustal anomalies are spread over a larger depth range than the actual object producing the gravity anomaly. Our approach provides important complementary information about the interior of planetary bodies than the one provided by traditional inversion methods. Key Points: A novel Bayesian gravity inversion method constrains 3‐D global‐scale density anomalies and provides uncertainty on the resulting modelOur algorithm recovers deep mantle anomalies adequately but dilutes shallow crustal density anomalies over a larger depth rangeOur Bayesian framework is especially suited for inverting the gravity field of planetary bodies that lack additional geophysical constraints [ABSTRACT FROM AUTHOR]

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]

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars’ deep mantle is demarcated by a seismic discontinuity arising from the pressureinduced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA’s InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 ± 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 ± 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m². [ABSTRACT FROM AUTHOR]

Seismology provides important constraints on the structure and dynamics of the deep mantle. Computational and methodological advances in the past two decades improved tomographic imaging of the mantle and revealed the fine-scale structure of plumes ascending from the core-mantle boundary region and slabs of oceanic lithosphere sinking into the lower mantle. We discuss the modeling aspects of global tomography including theoretical approximations, data selection, and model fidelity and resolution. Using spectral, principal component, and cluster analyses, we highlight the robust patterns of seismic heterogeneity, which inform us of flow in the mantle, the history of plate motions, and potential compositionally distinct reservoirs. In closing, we emphasize that data mining of vast collections of seismic waveforms and new data from distributed acoustic sensing, autonomous hydrophones, ocean-bottom seismometers, and correlation-based techniques will boost the development of the next generation of global models of density, seismic velocity, and attenuation. ▪ Seismic tomography reveals the 100-km to 1,000-km scale variation of seismic velocity heterogeneity in the mantle. ▪ Tomographic images are the most important geophysical constraints on mantle circulation and evolution. [ABSTRACT FROM AUTHOR]

Inversions of planetary gravity are aimed at constraining the mass distribution within a planet or moon. In many cases, constraints on the interior structure of the planet, such as the depth of density anomalies, must be assumed a priori , to reduce the non-uniqueness inherent in gravity inversions. Here, we propose an alternative approach that embraces the non-uniqueness of gravity inversions and provides a more complete view of related uncertainties. We developed a Transdimensional Hierarchical Bayesian (THB) inversion algorithm that provides an ensemble of mass distribution models compatible with the gravitational field of the body. Using this ensemble of models instead of only one, it is possible to quantify the range of interior parameters that produce a good fit to the gravity acceleration data. To represent the interior structure of the planet or moon, we parametrize mass excess or deficits with point masses. We test this method with synthetic data and, in each test, the algorithm is able to find models that fit the gravity data of the body very well. Three of the target or test models used contain only point mass anomalies. When all the point mass anomalies in the target model produce gravity anomalies of similar magnitudes and the signals from each anomaly are well separated, the algorithm recovers the correct location, number and magnitude of the point mass anomalies. When the gravity acceleration data of a model is produced mostly by a subset of the point mass anomalies in the target model, the algorithm only recovers the dominant anomalies. The fourth target model is composed of spherical caps representing lunar mass concentration (mascons) under major impact basins. The algorithm finds the correct location of the centre of the mascons but fails to find their correct outline or shape. Although the inversion results appear less sharp than the ones obtained by classical inversion methods, our THB algorithm provides an objective way to analyse the interior of planetary bodies that includes epistemic uncertainty. [ABSTRACT FROM AUTHOR]

Low-velocity layers within the crust can indicate the presence of melt and lithologic differences with implications for crustal composition and formation. Seismic wave conversions and reverberations across the base of the crust or intracrustal discontinuities, analysed using the receiver function method, can be used to constrain crustal layering. This is commonly accomplished by inverting receiver functions jointly with surface wave dispersion. Recently, the proliferation of model-space search approaches has made this technique a workhorse of crustal seismology. We show that reverberations from shallow layers such as sedimentary basins produce spurious low-velocity zones when inverted for crustal structure with surface wave data of insufficiently high frequency. Therefore, reports of such layers in the literature based on inversions using receiver function data should be re-evaluated. We demonstrate that a simple resonance-removal filter can suppress these effects and yield reliable estimates of crustal structure, and advocate for its use in receiver-function based inversions. [ABSTRACT FROM AUTHOR]

Inversion of surface wave data for crustal and upper-mantle structure is a staple of passive seismology, particularly since the advent of techniques enabling surface wave dispersion (SWD) and Rayleigh wave ellipticity measurements from ambient noise. Recent development and application of transdimensional Bayesian (TB) seismic inversion offers an approach to quantify model parameter uncertainties and trade-offs with fewer assumptions than traditional methods. Using synthetic tests, we investigate choices in the implementation of TB for the inversion of SWD and Rayleigh wave ellipticity to constrain the structure of Earth's continental lithosphere. We focus on three aspects of the inversion: limitation of data sensitivity, assumed scaling among parameters (compressional wave speed, Vp, shear wave speed, Vs, density and radial anisotropy) and parametrization choices. We show that while surface wave data provide relatively strong constraints on the posterior distribution of Vs and, to a lesser extent, Vp, common parametrization choices can potentially bias structure estimates. This is particularly the case for radial anisotropy (ξ), due to the inability to distinguish variations of Vp and density from those of ξ. Inferred results therefore depend substantially on the parametrization and scaling choices. We illustrate how layered parametrizations can, in the TB framework, recover smoothly varying profiles, and quantify the number of layers recoverable at different levels of measurement uncertainty. Finally, we propose two types of model parametrization for TB inversion involving multiple types of parameters. We demonstrate that by implementing an independent parametrization for different physical quantities, we can avoid imposing identical model geometry across multiple types of model parameters, and obtain better model estimates with reduced trade-offs. We advocate for such a parametrization in TB inversion of radial anisotropy using surface wave data, and when targeting disparate Vp and Vs structures such as those associated with |$\alpha $| - |$\beta $| quartz transtion. [ABSTRACT FROM AUTHOR]

The InSight lander will deliver geophysical instruments to Mars in 2018, including seismometers installed directly on the surface (Seismic Experiment for Interior Structure, SEIS). Routine operations will be split into two services, the Mars Structure Service (MSS) and Marsquake Service (MQS), which will be responsible, respectively, for defining the structure models and seismicity catalogs from the mission. The MSS will deliver a series of products before the landing, during the operations, and finally to the Planetary Data System (PDS) archive. Prior to the mission, we assembled a suite of a priori models of Mars, based on estimates of bulk composition and thermal profiles. Initial models during the mission will rely on modeling surface waves and impact-generated body waves independent of prior knowledge of structure. Later modeling will include simultaneous inversion of seismic observations for source and structural parameters. We use Bayesian inversion techniques to obtain robust probability distribution functions of interior structure parameters. Shallow structure will be characterized using the hammering of the heatflow probe mole, as well as measurements of surface wave ellipticity. Crustal scale structure will be constrained by measurements of receiver function and broadband Rayleigh wave ellipticity measurements. Core interacting body wave phases should be observable above modeled martian noise levels, allowing us to constrain deep structure. Normal modes of Mars should also be observable and can be used to estimate the globally averaged 1D structure, while combination with results from the InSight radio science mission and orbital observations will allow for constraint of deeper structure. [ABSTRACT FROM AUTHOR]

Conversions of shearwaves (S) to compressional waves (P), often analysed as Sp receiver functions, are useful for studying upper-mantle layering, and have been used to map upper-mantle seismic impedance interfaces in various tectonic settings. Recently, common conversion point (CCP) stacking of Sp receiver functions has revealed variations of lithospheric thickness across short horizontal distances. However, compared to Ps receiver functions and reflection, only limited work has been done towards quantifying the interpretability of Sp receiver functions, especially in settings where large lateral structure variations are present. Using the spectral element method, we model wave propagation and S-to-P conversion through simple synthetic models with varying velocity interface topography. We systematically explore the effects of wave frequency content, seismometer spacing and illumination geometry on CCP stacked Sp receiver functions in settings where velocity interface depth varies laterally. We observe that the resolving power of Sp receiver functions decreases with decreasing frequency content, and that upward deflections of velocity interfaces are more difficult to observe than are downward deflections, an asymmetry that primarily arises due to corner diffractions. Furthermore, we document how the relationship between the angle of illumination and the orientation of the topography of the velocity interfaces largely determines the apparent interface slope and strongly affects the amplitude of Sp phases in the CCP stacks. Indeed, under certain illumination geometries, strong velocity contrasts across a dipping lithosphere-asthenosphere boundary may not produce detectable Sp phases at the surface. Furthermore, diffractions arising from corners of interface topography can produce artefacts in CCP stacks that masquerade as mid-lithospheric impedance jumps or drops, as well as gently sloped sublithospheric impedance drops. We find that estimates based on Fresnel zone arguments might, in some cases, underestimate the true resolution, and that they are likely to be only appropriate for situations in which abrupt lateral variations in structure do not produce waveform complexities. These results imply that the interpretation of Sp receiver functions and CCP stacks is not straightforward and that care must be exercised when inferring the presence or absence of lithospheric velocity interfaces. [ABSTRACT FROM AUTHOR]

The article cites a research study that examines elastic parameters for Earth's outer core. It determines the equation-of-state (EoS) parameters of liquid iron alloys at high pressures and temperatures. Earth's magnetic field is generated by convection of the liquid iron alloy outer core and the exact composition of the iron alloy can be constrained by its seismic properties.

Seismic body wave and normal mode analyses have revealed that the inner core is solid, strongly anisotropic, and characterized by dramatic quasi-hemispherical differences in elastic structure and attenuation. Yet, despite these discoveries, the highly heterogeneous and incomplete data coverage of the inner core has impeded the development of tomographic models even at the longest wavelengths. Here, we use a probabilistic and transdimensional tomographic approach (TBI) on a newly expanded dataset of P-wave travel-times sensitive to the upper 120 km of the inner core. The TBI approach yields a ensemble of parsimonious models that simultaneously capture both the dominant hemispheric dichotomy and laterally abrupt velocity variations. Analysis of the model ensemble allows us to determine the locations of the hemisphere boundaries and rule out the presence of hemispheric dichotomy in anisotropy. Instead, we robustly map regional variations in anisotropy beneath Africa and the eastern Pacific, and detect variations at high latitudes suggesting that cylindrical anisotropy may not be adequate for describing the uppermost inner core. • Novel tomographic model of hemispheric and regional structure in inner core. • Uncertainty in hemisphere boundaries constrained with Bayesian method. • Regional variation but no hemispheric dichotomy in cylindrical anisotropy is detected. • Effect of mantle heterogeneity on inner core models determined to be minimal. [ABSTRACT FROM AUTHOR]

The Pacific large low-shear-velocity province (LLSVP), as revealed by cluster analysis of global tomographic models, hosts multiple internal anomalies, including a notable gap (~20° wide) between the central and eastern Pacific. The cause of the structural gap remains unconstrained. Directly above this structural gap, we identify an anomalously thick mantle transition zone east of the East Pacific Rise, the fastest-spreading ocean ridge in the world, using a dense set of SS precursors. The area of the thickened transition zone exhibits faster-than-average velocities according to recent tomographic images, suggesting perturbed postolivine phase boundaries shifting in response to lowered temperatures. We attribute this observation to episodes of Mesozoic-aged (250 to 120 million years ago) intraoceanic subduction beneath the present-day Nazca Plate. The eastern portion of the Pacific LLSVP was separated by downwelling because of this ancient oceanic slab. Our discovery provides a unique perspective on linking deep Earth structures with surface subduction.

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars' deep mantle is demarcated by a seismic discontinuity arising from the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA's InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 [Formula: see text] 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 [Formula: see text] 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m 2 .

A planet's crust bears witness to the history of planetary formation and evolution, but for Mars, no absolute measurement of crustal thickness has been available. Here, we determine the structure of the crust beneath the InSight landing site on Mars using both marsquake recordings and the ambient wavefield. By analyzing seismic phases that are reflected and converted at subsurface interfaces, we find that the observations are consistent with models with at least two and possibly three interfaces. If the second interface is the boundary of the crust, the thickness is 20 ± 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 ± 8 kilometers. Global maps of gravity and topography allow extrapolation of this point measurement to the whole planet, showing that the average thickness of the martian crust lies between 24 and 72 kilometers. Independent bulk composition and geodynamic constraints show that the thicker model is consistent with the abundances of crustal heat-producing elements observed for the shallow surface, whereas the thinner model requires greater concentration at depth., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)