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The shallow crustal structure of Mars records the evolutionary history of the planet, which is crucial for understanding the early Martian geological environment. Until now, seismic constraints on the Martian crust have come primarily from the receiver functions (RFs). However, analysis of the Mars RFs did not focus on the shallow structure (1–5 km) so far due to the limitation of the signal‐to‐noise ratio at high frequencies for most events. Here, we take advantage of the S1222a and six other marsquakes, which exhibit high signal‐to‐noise ratios, to probe the shallow structure of Mars. We observe a converted S‐wave at approximately 1 s after the direct P‐wave in the high‐frequency P‐wave RFs. This suggests a discontinutity at 2‐km depth between highly fractured and more coherent crustal materials. Plain Language Summary The Martian shallow crustal structure is essential for understanding the geological evolution of Mars. The InSight lander successfully deployed a seismic station on Mars in late 2018, aiming to investigate the internal structure of Mars. Since most marsquakes detected previously have a low signal‐to‐noise ratio (SNR) at high frequencies, most seismic analyses do not focus on the shallow structure of Mars (1–5 km). However, when the InSight seismometer was near the end of its observational lifetime, a large marsquake occurred on sol 1222 with significant high‐frequency energy, far more than the noise level, allowing us to study the Martian shallow structure. We calculate the high‐frequency P‐wave receiver function (RF) of S1222a and extract a converted S‐wave at approximately 1 s after the direct P‐wave. To confirm the result, we also compute P‐wave RFs for high SNR events that occurred before. We observe this ∼1‐s signal in the high‐frequency P‐wave RFs of two additional large events as well. Combined with the geological analysis adjacent to the InSight lander, we attribute this 1‐s converted S‐wave to a discontinuity at approximately 2 km depth, probably corresponding to the bottom of highly fractured crustal materials beneath the InSight landing site. Key Points We calculate high‐frequency P‐wave receiver functions (RF) from InSight seismic data of seven marsquakes with high signal‐to‐noise ratios The high‐frequency RFs exhibit a converted S‐wave at approximately 1 s The ∼1‐s converted S‐wave suggests a discontinuity at a depth of approximately 2 km beneath the InSight lander

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 times Group velocity measurements and 3‐D simulations constrain the average crustal and uppermost mantle velocities along the great‐circle propagation path The global average crustal thickness is 42–56 km and requires a large enrichment of heat‐producing elements to explain local melt zones

Martian thermal state and evolution depend principally on the radiogenic heat‐producing element (HPE) distributions in the planet's crust and mantle. The Gamma‐Ray Spectrometer (GRS) on the 2001 Mars Odyssey spacecraft has mapped the surface abundances of HPEs across Mars. From these data, we produce the first models of global and regional surface heat production and crustal heat flow. As previous studies have suggested that the crust is a repository for approximately 50% of the radiogenic elements on Mars, these models provide important, directly measurable constraints on Martian heat generation. Our calculations show considerable geographic and temporal variations in crustal heat flow, and demonstrate the existence of anomalous heat flow provinces. We calculate a present day average surface heat production of 4.9 ± 0.3 × 10−11 W · kg−1. We also calculate the average crustal component of heat flow of 6.4 ± 0.4 mW · m−2. The crustal component of radiogenically produced heat flow ranges from <1 mW · m−2 in the Hellas Basin and Utopia Planitia regions to ∼13 mW · m−2 in the Sirenum Fossae region. These heat production and crustal heat flow values from geochemical measurements support previous heat flow estimates produced by different methodologies. Key Points We present methods and results for new crustal heat flow calculations for Mars Crustal heat flow varies significantly over the surface of Mars GRS Heat flow values agree well with previous geophysical estimates

Mineral deposits on the martian surface can elucidate ancient environmental conditions on the planet. Opaline silica deposits (as much as 91 weight percent SiO₂) have been found in association with volcanic materials by the Mars rover Spirit. The deposits are present both as light-toned soils and as bedrock. We interpret these materials to have formed under hydrothermal conditions and therefore to be strong indicators of a former aqueous environment. This discovery is important for understanding the past habitability of Mars because hydrothermal environments on Earth support thriving microbial ecosystems.

The Mars Exploration Rover Opportunity has spent more than 2 years exploring Meridiani Planum, traveling ~8 kilometers and detecting features that reveal ancient environmental conditions. These include well-developed festoon (trough) cross-lamination formed in flowing liquid water, strata with smaller and more abundant hematite-rich concretions than those seen previously, possible relict \"hopper crystals\" that might reflect the formation of halite, thick weathering rinds on rock surfaces, resistant fracture fills, and networks of polygonal fractures likely caused by dehydration of sulfate salts. Chemical variations with depth show that the siliciclastic fraction of outcrop rock has undergone substantial chemical alteration from a precursor basaltic composition. Observations from microscopic to orbital scales indicate that ancient Meridiani once had abundant acidic groundwater, arid and oxidizing surface conditions, and occasional liquid flow on the surface.

The Mars Exploration Rover Opportunity has investigated the landing site in Eagle crater and the nearby plains within Meridiani Planum. The soils consist of fine-grained basaltic sand and a surface lag of hematite-rich spherules, spherule fragments, and other granules. Wind ripples are common. Underlying the thin soil layer, and exposed within small impact craters and troughs, are flat-lying sedimentary rocks. These rocks are finely laminated, are rich in sulfur, and contain abundant sulfate salts. Small-scale cross-lamination in some locations provides evidence for deposition in flowing liquid water. We interpret the rocks to be a mixture of chemical and siliciclastic sediments formed by episodic inundation by shallow surface water, followed by evaporation, exposure, and desiccation. Hematite-rich spherules are embedded in the rock and eroding from them. We interpret these spherules to be concretions formed by postdepositional diagenesis, again involving liquid water.

Gale crater on Mars was once a lake fed by rivers and groundwater. Hurowitz et al . analyzed 3.5 years of data from the Curiosity rover’s exploration of Gale crater to determine the chemical conditions in the ancient lake. Close to the surface, there were plenty of oxidizing agents and rocks formed from large, dense grains, whereas the deeper layers had more reducing agents and were formed from finer material. This redox stratification led to very different environments in different layers, which provides evidence for Martian climate change. The results will aid our understanding of where and when Mars was once habitable. Science , this issue p. eaah6849 Gale crater on Mars was once a lake that separated into layers with differing chemical conditions. In 2012, NASA’s Curiosity rover landed on Mars to assess its potential as a habitat for past life and investigate the paleoclimate record preserved by sedimentary rocks inside the ~150-kilometer-diameter Gale impact crater. Geological reconstructions from Curiosity rover data have revealed an ancient, habitable lake environment fed by rivers draining into the crater. We synthesize geochemical and mineralogical data from lake-bed mudstones collected during the first 1300 martian solar days of rover operations in Gale. We present evidence for lake redox stratification, established by depth-dependent variations in atmospheric oxidant and dissolved-solute concentrations. Paleoclimate proxy data indicate that a transition from colder to warmer climate conditions is preserved in the stratigraphy. Finally, a late phase of geochemical modification by saline fluids is recognized.

Opportunity has investigated in detail rocks on the rim of the Noachian age Endeavour crater, where orbital spectral reflectance signatures indicate the presence of Fe(+3)-rich smectites. The signatures are associated with fine-grained, layered rocks containing spherules of diagenetic or impact origin. The layered rocks are overlain by breccias, and both units are cut by calcium sulfate veins precipitated from fluids that circulated after the Endeavour impact. Compositional data for fractures in the layered rocks suggest formation of Al-rich smectites by aqueous leaching. Evidence is thus preserved for water-rock interactions before and after the impact, with aqueous environments of slightly acidic to circum-neutral pH that would have been more favorable for prebiotic chemistry and microorganisms than those recorded by younger sulfate-rich rocks at Meridiani Planum.

The Microscopic Imager on the Spirit rover analyzed the textures of the soil and rocks at Gusev crater on Mars at a resolution of 100 micrometers. Weakly bound agglomerates of dust are present in the soil near the Columbia Memorial Station. Some of the brushed or abraded rock surfaces show igneous textures and evidence for alteration rinds, coatings, and veins consistent with secondary mineralization. The rock textures are consistent with a volcanic origin and subsequent alteration and/or weathering by impact events, wind, and possibly water.

The bulk chemical compositions, mineralogy, and mineral proportions of sands and muds of the Mallacoota Basin in southeastern Australia reflect the composition of weathering profiles mantling source rocks, rather than bedrock. Muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles. Sands are strongly enriched in quartz relative to source rocks, even in the headwaters of the fluvial system, demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system. K-feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or K-feldspar in weathering profiles. Subsequent erosion and sorting produce sands enriched in quartz, with high K-feldspar:plagioclase ratios relative to source rocks. The composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weathering. In weather-ing profiles the anorthite component of plagioclase weathers more rapidly than the albite component so that vestigal plagioclase in the profiles, and in sands derived therefrom, is more albitic than in the source rocks. Traditional point-counting techniques to obtain modal estimates of sediments and sedimentary rocks are not widely applicable because (1) most sedimentary rocks-shales and equivalent mud-grade materials-are too fine-grained for petrographic examination, and (2) rock fragments are rare to non-existant in most \"basement\" source terranes. We use a method that involves identification of mineral species present, information on their compositions, and bulk chemical compositions to calculate modal compositions by an albegraic method (CAM). The technique is equally applicable to source rocks, sediments, and sedimentary rocks of all size grades, so that meaningful mass balances and provenance studies can be carried out. Results of this study show that, where substantial chemical weathering has occurred, Q:F ratios and P:K ratios more closely reflect those of weathering profiles than bedrock from which they were ultimately derived.