Geophysical evidence for an enriched molten silicate layer above Mars's core

Understanding the internal structure and composition of Mars is crucial for deciphering its formation, evolution, and potential for past habitability. Previous studies, primarily relying on data from the NASA InSight mission, suggested a Martian core radius of approximately 1,830 km, implying a relatively low core density requiring significant light element content (e.g., sulfur, oxygen, hydrogen, carbon). However, these estimates are challenged by several factors. First, the assumption of a compositionally homogeneous mantle is inconsistent with observations of anomalously slow P-wave propagation along the core-mantle boundary (CMB) , indicating mantle heterogeneity. Second, the high abundances of hydrogen and carbon required to achieve the estimated low core density are incompatible with experimental constraints. This study addresses these inconsistencies by proposing a new model for the Martian interior that accounts for the possibility of significant mantle heterogeneity. The core size and density are reassessed using a model incorporating a basal mantle layer (BML) enriched in iron and heat-producing elements. This model is tested against various geophysical data including seismic wave propagation, shear wave attenuation, and tidal dissipation. The aim is to provide a more accurate representation of Mars's internal structure and its implications for planetary evolution, particularly the origin and duration of its ancient magnetic field.

Numerous studies have attempted to determine Mars's internal structure based on data from InSight, focusing on seismic wave arrivals and analysis of core size and composition. Initial estimates of the core size were based on the detection of S waves reflected at the CMB, assuming a homogeneous mantle. However, the high concentrations of light elements necessary to explain the inferred low core density are inconsistent with cosmochemical and experimental constraints. The detection of P waves diffracted along the CMB, along with observations indicating unexpectedly slow wave speeds in the deep mantle, further challenged the homogenous mantle assumption. These inconsistencies prompted the investigation of alternative models, including the possibility of a heterogeneous mantle with a basal layer enriched in iron and heat-producing elements, a concept supported by some isotopic anomalies found in Martian meteorites. The influence of such a layered structure on thermal evolution and dynamo action was also explored in prior studies. This current study builds upon this earlier research to provide a more comprehensive model that integrates seismic data, geodetic observations, and experimental constraints.

To investigate the Martian interior structure, the authors employed a probabilistic inversion method applied to seismic data recorded by the InSight lander. Two inversion sets were considered: one with a compositionally homogeneous mantle (as commonly assumed in previous studies) and another incorporating a basal mantle layer (BML) above the core, enriched in iron and heat-producing elements. The inversion was parameterized in terms of quantities that influence the planet's thermochemical evolution: the viscosity of the mantle (expressed as a function of temperature and pressure using a modified Arrhenius equation), the effective activation energy and volume, and initial temperature profiles. The model considers an evolving lithosphere and crust, a liquid iron core, and a silicate mantle (with or without the BML). The inversion process aimed to find the best-fitting planetary structure parameters that matched observational data, including the arrival times of seismic waves (both direct and reflected/diffracted phases), the amplitude ratios of reflected and direct S waves, and the global quality factor (Q) for seismic waves at various frequencies (including those relevant to Phobos's tides). The results of the inversion were analyzed to compare the models with homogeneous and heterogeneous mantles to determine the better fit to available observational constraints. Additional analysis was conducted to assess the core size, density, and composition within the context of cosmochemical and experimental limits. Analysis also considered the implications of the modeled structure for the Martian dynamo and its long-term evolution.

The study's key findings strongly favor a heterogeneous mantle model with a BML above the core. The model with the BML provides a significantly better fit to the available seismic data, particularly concerning the arrival times of P waves diffracted along the CMB and the observed slow P-wave speeds in the deep mantle. This BML model implies a smaller core radius (1,650 ± 20 km) and a higher core density (6.5 g cm⁻³) compared to previous estimates. The smaller core size and higher density reduce the need for exceptionally high abundances of light elements in the core, bringing the predicted core composition into better agreement with cosmochemical constraints. The BML's presence explains the weak seismic attenuation at high frequencies and the stronger attenuation at longer tidal periods (related to Phobos's tides), suggesting that the partially molten BML acts as a damping mechanism for tidal forces. The study found that the BML model allows the core composition to be consistent with constraints from Mars's nutation. Furthermore, the BML significantly affects Mars's thermal evolution, causing core heating rather than cooling, posing implications for a thermally driven dynamo. Alternative mechanisms for magnetic field generation are suggested. The analysis of shear attenuation using the relative attenuation between direct and reflected S waves supports the BML model.

The findings of this study challenge the widely accepted assumption of a homogeneous Martian mantle and provide compelling evidence for a structurally complex interior with a molten enriched BML above the core. The smaller core size and higher density compared to previous estimations resolves the conflict between earlier seismic models and cosmochemical and experimental constraints. The presence of the BML provides a mechanism to reconcile the observed differences in seismic wave attenuation at various frequencies. The heat buffering effect of the BML presents intriguing challenges to conventional dynamo models, requiring consideration of alternative mechanisms for the generation of Mars's early magnetic field, potentially involving processes like core super-heating or giant impacts. The superior fit to seismic and geodetic data obtained by the BML model demonstrates its importance in understanding Mars's thermal and dynamic evolution.

This study provides strong geophysical evidence for a heterogeneous Martian mantle with a substantial enriched molten silicate layer above the core. This model resolves inconsistencies between previous seismic estimates and cosmochemical constraints on core composition, explains discrepancies in seismic wave attenuation, and has significant implications for our understanding of Mars's thermal evolution and dynamo history. Further research could involve detailed modeling of the BML formation and evolution, exploration of alternative dynamo mechanisms, and refining the analysis of seismic data to reveal additional details of the deep Martian interior.

The study relies heavily on seismic data from the InSight mission, which has limitations in terms of the number and distribution of recorded events. The model relies on several assumptions, including the composition of the BML and the specific rheological parameters used for the mantle viscosity. Further data and advanced modeling techniques could refine the model's parameters and improve its accuracy. The proposed external mechanisms for Mars's early dynamo remain hypothetical and require further investigation.