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We report observations of Rayleigh waves that orbit around Mars up to three times following the S1222a marsquake. Averaging these signals, we find the largest amplitude signals at 30 and 85 s central period, propagating with distinctly different group velocities of 2.9 and 3.8 km/s, respectively. The group velocities constraining the average crustal thickness beneath the great circle path rule out the majority of previous crustal models of Mars that have a >200 kg/m3 density contrast across the equatorial dichotomy between northern lowlands and southern highlands. We find that the thickness of the Martian crust is 42–56 km on average, and thus thicker than the crusts of the Earth and Moon. Considered with the context of thermal evolution models, a thick Martian crust suggests that the crust must contain 50%–70% of the total heat production to explain present‐day local melt zones in the interior of Mars. Plain Language Summary: The NASA InSight mission and its seismometer installed on the surface of Mars is retired after ∼4 years of operation. From the largest marsquake recording during the entire mission, we observe clear seismic signals from surface waves called Rayleigh waves that orbit around Mars up to three times. By measuring the wavespeeds with which these surface waves travel at different frequencies, we obtain the first seismic evidence that constrains the average crustal and uppermost mantle structures beneath the traveling path on a planetary scale. Using the new seismic observations together with gravity data, we confirm that the density of the crust in the northern lowlands and the southern highlands is similar, differing by no more than 200 kg/m3. Furthermore, we find that the global average crustal thickness on Mars is 42–56 km, much thicker than the Earth's and Moon's crusts. By exploring Mars' thermal history, a thick Martian crust requires about 50%–70% of the heat‐producing elements such as thorium, uranium, and potassium to be concentrated in the crust in order to explain local regions in the Martian mantle that can still undergo melting at present day. Key Points: We present the first observation of Rayleigh waves that orbit around Mars up to three timesGroup velocity measurements and 3‐D simulations constrain the average crustal and uppermost mantle velocities along the great‐circle propagation pathThe global average crustal thickness is 42–56 km and requires a large enrichment of heat‐producing elements to explain local melt zones [ABSTRACT FROM AUTHOR]

Recent estimates of the crustal thickness of Mars show a bimodal result of either ∼20 or ∼40 km beneath the InSight lander. We propose an approach based on random matrix theory applied to receiver functions (RFs) to further constrain the subsurface structure. Assuming a spiked covariance model for our data, we first use the phase transition properties of the singular value spectrum of random matrices to detect coherent arrivals in the waveforms. Examples from terrestrial data show how the method works in different scenarios. We identify three previously undetected converted arrivals in the InSight data, including the first multiple from a deeper third interface. We then use this information to jointly invert RFs with the absolute S‐wave velocity information in the polarization of body waves. Results show a crustal thickness of 43 ± 5 km beneath the lander with two mid‐crustal interfaces at depths of 8 ± 1 and 21 ± 3 km. Plain Language Summary: Recent analysis of seismic data from InSight shows that the crustal thickness beneath the InSight lander can be either 20 or 40 km. To resolve this ambiguity, we apply results from random matrix theory to receiver function (RF) analysis. The distribution of singular values of a random matrix shows well‐behaved deterministic properties that can be used to separate them from those of an underlying coherent signal if present. We use examples from terrestrial data to show how the method works. When applied to RFs computed from InSight seismic data, we identify three new energy arrivals, including one that supports the existence of a deeper third layer. Using this information, we simultaneously inverted the RF data along with the measured incidence angle of body waves. Results show a crustal thickness of 43 ± 5 km beneath the lander with two mid‐crustal interfaces at depths of 8 ± 1 and 21 ± 3 km. Key Points: We apply recent results from random matrix theory to identify crustal phases in noisy receiver functions for Mars from InSight dataOnce identified, we jointly invert these phases with frequency‐dependent apparent S‐wave velocity curvesResults show a crustal thickness of 43 km with two inter‐crustal discontinuities at 8 and 21 km beneath the lander [ABSTRACT FROM AUTHOR]

The geodynamical response of the lithosphere under stresses imposed by the geologically young north polar cap is one of the few clues we have to constrain both the polar cap composition and the present‐day thermal state of Mars. Here we combine radar data with a flexural loading model to self‐consistently estimate the density (ρ) and real dielectric constant (ε′) of the polar cap, and the elastic thickness of the lithosphere underneath (Te). Our results show that ρ ranges from 920 to 1,520 kg m −3, ε′ is 2.75 (+0.40, −0.35), and Te is between 330 and 450 km. We determine a polar cap volume that is up to 30% larger than current estimates that all neglect lithospheric flexure. Our inferred compositions suggest that, for dust content larger than about 6 vol%, 10 vol% CO 2 ice is mixed within the polar deposits, which has important implications for the climate evolution of Mars. Plain Language Summary: The north polar cap of Mars is a tremendous reservoir of ices and dust of unknown concentration and composition. It is transparent to radar giving us a unique insight into its structure and composition. Here we use a novel technique that combines radar and elevation data along with a flexure model, to invert for the polar cap composition and the strength of the underlying lithosphere. Similar to previous studies, we find that the lithosphere below the north pole is extremely rigid and does not deform much under the load of the polar cap. This implies that the north polar region is currently colder than the rest of the planet, which has profound implications for our understanding of the structure and evolution of the Martian interior. Our inferred compositions suggest that for reasonable dust contents, about 10 vol% CO 2 ice is mixed within the polar deposits. This is the first time a large quantity of CO 2 ice is constrained to exist in the north polar cap. Like on Earth, where the composition of buried ices gives hints on the climatic evolution, having CO 2 ice at the north pole of Mars will help improve scenarios for the climate evolution of the planet. Key Points: Radar data and a flexural loading model are used to probe the composition of the north polar cap of Mars and the strength of the lithosphereThe elastic thickness beneath the north polar cap is found to be between 330 and 450 km, implying regional heat flows of 11 to 16 mW m −2For reasonable dust contents, large quantities of CO 2 ice must be mixed within the polar cap to be consistent with our inferred compositions [ABSTRACT FROM AUTHOR]