Seismology on Mars: Analysis of Body Waves with Implications for Interior Structure

Abstract

Mars has been a subject of significant scientific interest for centuries but it remains largely unknown compared to Earth. Despite notable differences in characteristics such as size, the absence of a global magnetic field, tectonic plate activity, and active volcanoes, both planets share a similar interior structure, predominantly composed of a solid silicate rock crust and a mantle encasing an iron core. Yet, without comprehensive seismological data Mars's interior is largely unconstrained. The InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) mission, which landed on Mars in late 2018, aimed to understand key aspects of the planet's formation and evolution, including its present level of tectonic activity and impact flux. Equipped with the Seismic Experiments for Interior Structure (SEIS), InSight marked a historic milestone as the first successful mission to collect high-quality seismic data directly from the surface of Mars. SEIS recorded almost continuous data for over more than four Earth years, capturing over 1300 seismic events attributed to both tectonic sources and meteorite impacts. However, SEIS's deployment in Mars's harsh wind and temperature conditions led to inevitable data degradation, with seismic background noise fluctuating throughout the Martian day and various transient signals contaminating the data, challenging the interpretation of seismic data for understanding Mars's interior structure. In line with the primary science goal of the InSight mission, which aims to understand the formation and evolution of terrestrial planets by investigating the interior structure and processes of Mars, the main objective of this thesis is to determine Mars's interior structure through the analysis of seismic data recorded by SEIS. Through data processing, modeling, and inversion techniques, this study aims to achieve the baseline objectives established prior to the mission, including constraining the thickness and structure of the crust, characterizing the composition and structure of the mantle, elucidating the size, composition, and physical state of the core, and investigating the thermal conditions within the planet's interior. Hence, we apply advanced single-station analysis methodologies on SEIS data to identify body waves that propagate through the interior of Mars. These waves offer crucial insights into the planet's internal structure and the characteristics of the layers they encounter. Through iterative application of complementary approaches, we augment the number of detected body-wave phases compared to prior analyses, leading to a substantial gain in information that significantly advances our understanding of Mars's seismic interior structure, from the surface layers to its core. Our findings provide the first constraints on seismic velocities in the deep Martian mantle, revealing deviations from predictions based on thermochemically homogeneous models. Additionally, we detect core-transiting seismic waves from distant events, enabling the construction of the first seismically constrained models for the elastic properties of the Martian core. Furthermore, we find evidence that supports a fully molten silicate layer overlying a smaller, denser liquid core than previously estimated. Moreover, our single-station event-location and structure-inversion methodology proves robust and accurate, as demonstrated by its agreement with the imaged location of an impact event. Lastly, we conduct a detailed analysis of continuous SEIS data to detect Mars's background free oscillations, and evaluate their implications for the planet's internal structure in comparison to results obtained from body waves. Together, this work offers a comprehensive analysis approach to identify body-wave arrivals and background free oscillations in the data recorded by SEIS, contributing to a notable enhancement in information acquisition concerning the interior of Mars. These findings have significantly contributed to a better understanding of Mars's internal structure and composition and prove promising for future planetary missions.