Cecilia Duran

The structure and nature of Earth’s transition zone, which is delineated by the transformation of olivine to its higher-pressure polymorphs, exerts a strong influence on material transfer between upper and lower mantle. Mars, however, because of its relatively large core, is only expected to exhibit the equivalent of Earth’s uppermost transition zone seismic discontinuity. We searched the InSight seismic data for marsquakes and impacts located in an epicentral distance range favorable for detection of seismic phases that have interacted with Mars’s olivine transition (midmantle) discontinuity. Through application of careful data selection criteria and processing schemes, we found 13 events in the distance range in which body waves are expected to refract through the midmantle of Mars. Although triplicated body waves are potentially present in seven events, the distance distribution is insufficient to allow for unambiguous detection of the triplicated waveform pattern associated with the midmantle discontinuity. Comparison of travel times of the observed waveforms with predictions from recent Mars models indicates the possible presence of a midmantle discontinuity located between 987 and 1052 km or 1075 and 1122 km depth, in which the uncertainty comes from our inability to reliably distinguish first from secondary arrivals.

The Marsquake Service (MQS) [1] is finalizing the release of the reference catalogue of Martian seismicity recorded by NASA’s InSight mission. Previous catalogue versions [2] listed over 1300 events classified by their observed frequency content into i) low frequency (LF) and broadband (BB) - the key events for constraining interior structure, ii) 2.4 Hz and high frequency (HF) - representing the majority of events with weak signals and enigmatic origin, and iii) very high frequency (VF) events, including several nearby impacts [3].Since the last release in April 2023, directly following the mission end, MQS has reviewed the entire data set, also using denoised data sets [4,5] to refine phase picks, back azimuths, and ensuring consistency across the catalogue. For distance computation, MQS has adopted an updated suite of interior models, now applied to all event types [6, 7]. Previously 2.4 Hz, HF, and VF events were located using constant crustal velocities. However, new insights from observations of impacts and surface waves [8,9,10] suggest these events propagate through deeper interior layers, leading to significantly larger distances.MQS has revised the frequency-based event classification, incorporating spectral analysis [11] and other parameters such as distance and seasonality to improve event characterization. Events are grouped into three interpretation types:i) Tectonic events, mostly located ~30° east of InSight in Cerberus Fossae and in few other regions with deeper sources, include most LF and BB events with low corner frequencies. ii) Swarm events are a subset of HF and 2.4 Hz events clustered in ~44° distance with shallow seasonal sources, though their source region remains speculative; this group has been significantly expanded through deep learning techniques [5]. iii) Meteorite impacts, characterized by high corner frequencies, encompass VF events, several large BB events, and HF/2.4Hz events located outside of the swarm region. The reference catalogue includes over 1900 events with improved locations and new interpretation types, providing a more comprehensive view of Martian seismicity observed by InSight.[1] Clinton et al (2018), 10.1007/s11214-018-0567-5[2] Ceylan et al. (2022), 10.1016/j.pepi.2022.106943[3] Garcia et al (2022), 10.1038/s41561-022-01014-0[4] Scholz et al (2020), 10.1029/2020EA001317[5] Dahmen et al. (2024), 10.1093/gji/ggae279[6] Khan et al. (2023), 10.1038/s41586-023-06586-4[7] Samuel et al. (2023), 10.1038/s41586-023-06601-8[8] Posiolova et al. (2022), 10.1126/science.abq7704[9] Panning et al. (2023), 10.1029/2022GL101270[10] Charalambous et al. (in press)[11] Stähler et al. (in prep.)

Mars’s atmosphere has theoretically been predicted to be strong enough to continuously excite Mars’s background-free oscillations, potentially providing an independent means of verifying radial seismic body-wave models of Mars determined from marsquakes and meteorite impacts recorded during the Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission. To extract the background-free oscillations, we processed and analyzed the continuous seismic data, consisting of 966 Sols (a Sol is equivalent to a Martian day), collected by the Mars InSight mission using both automated and manual deglitching schemes to remove nonseismic disturbances. We then computed 1-Sol-long autocorrelations for the entire data set and stacked these to enhance any normal-mode peaks present in the spectrum. We find that while peaks in the stacked spectrum in the 2–4 mHz frequency band align with predictions based on seismic body-wave models and appear to be consistent across the different processing and stacking methods applied, unambiguous detection of atmosphere-induced free oscillations in the Martian seismic data nevertheless remains difficult. This possibly relates to the limited number of Sols of data that stack coherently and the continued presence of glitch-related signal that affects the seismic data across the normal-mode frequency range (∼1–10 mHz). Improved deglitching schemes may allow for clearer detection and identification in the future.

The InSight mission collected four years of seismic data from Mars, providing unprecedented data on the planet’s interior. Two seismic events – one marsquake and one meteorite impact – took place on the opposite side of Mars to InSight’s broadband seismometer [1]. These two far-side events were initially located using mantle-transiting PP and SS waves, but we demonstrate that their waveforms contain additional seismic arrivals which are sensitive to the properties of the Martian core. Using multiple seismic methods, we obtain SKS differential travel times for S0976a, a distant marsquake, and S1000a, a distant impact. SKS travels through Mars’ core as compressional waves and is therefore sensitive to its elastic properties. We use these differential travel times to build the first seismically informed models of Mars’ core [2]. The core velocity of Mars is low: 4.9-5.0 km/s at the CMB. We use our seismic results to estimate the fraction of light elements in Mars’ core, finding a high fraction of sulphur is needed, together with lesser amounts of oxygen, carbon and hydrogen. In addition to the SKS signals observed, the waveforms from S1000a contain evidence for a seismic wave which diffracts along a molten silicate layer at the base of the Martian mantle and also reflects from the core-mantle boundary. The presence of a basal silicate molten layer has implications for the areotherm, and could reduce both estimates of metallic core’s radius and its fraction of light elements [3]. The SKS detections reported provide information about the physical properties of the liquid core of Mars. A solid inner core at the centre of Mars is unlikely; we assess what structures inside the Martian core can be excluded based on existing data and models. [1] Horleston et al., 2022. Seism. Rec., 2(2), 88-99 [2] Irving et al., 2023. PNAS, 120, e2217090120 [3] Samuel et al., 2023, Nature, 622, 712–717

Seismic recordings made during the InSight mission¹ suggested that Mars’s liquid core would need to be approximately 27% lighter than pure liquid iron^2,3, implying a considerable complement of light elements. Core compositions based on seismic and bulk geophysical constraints, however, require larger quantities of the volatile elements hydrogen, carbon and sulfur than those that were cosmochemically available in the likely building blocks of Mars⁴. Here we show that multiply diffracted P waves along a stratified core–mantle boundary region of Mars in combination with first-principles computations of the thermoelastic properties of liquid iron-rich alloys³ require the presence of a fully molten silicate layer overlying a smaller, denser liquid core. Inverting differential body wave travel time data with particular sensitivity to the core–mantle boundary region suggests a decreased core radius of 1,675 ± 30 km associated with an increased density of 6.65 ± 0.1 g cm⁻³, relative to previous models^2,4–8, while the thickness and density of the molten silicate layer are 150 ± 15 km and 4.05 ± 0.05 g cm⁻³, respectively. The core properties inferred here reconcile bulk geophysical and cosmochemical requirements, consistent with a core containing 85–91 wt% iron–nickel and 9–15 wt% light elements, chiefly sulfur, carbon, oxygen and hydrogen. The chemical characteristics of a molten silicate layer above the core may be revealed by products of Martian magmatism.

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.

After ~4 years of deployment on the martian surface monitoring the planet’s ground motion, the InSight seismometer is now retired. Here, we review the procedures and methods the Marsquake Service (MQS) used to curate the seismic event catalog and describe the content of the catalog. The marsquake catalogue is different from normal catalogues on Earth as it aims to provide the authoritative catalog for the mission, covering the entire planet, using only a single station. As of January 1st, 2023, the MQS catalog contains 1319 seismic events of which 6 are known meteorite impacts. We have also identified 1383 superhigh frequency events that are interpreted as thermal cracking nearby the InSight lander. Late in the project large distant events occurred that allowed MQS to detect surface waves. Multiple events have been associated as impacts using orbital imaging, confirming the MQS single station location procedures. All of these new seismic phases have contributed to advance our understanding of the internal structure of Mars. The marsquake S1222a, the largest event recorded during the mission (MW 4.7) occurred in March 2022 and is also documented in our latest MQS catalog, V13, with many associated seismic phases including both Rayleigh and Love waves, their first-order overtones, and multi-orbiting surface waves that have not been identified in other marsquake records from our previous catalogues. The InSight mission is now closed but the MQS operation continues to analyze the ~4 years of seismic recordings on Mars and a final catalog, including event-specific products such as filter banks, and spectra, is in preparation. This final catalog will inform capabilities and field strategies in geophysical explorations for future martian science missions.

The InSight mission collected an astounding seismic dataset from Mars during more than four years (1450 sols) of operation until it was retired on 21 December 2022.The Marsquake Service MQS detected more than 1300 events of seismic origins. Two of these events (S1000a and S1094b) were later confirmed as distant impacts (Figure 1), with magnitudes of MWMa=4.0 and 4.2 and crater diameters of 130 and 150 m, respectively. Finally, the largest marsquake (S1222a, MWMa=4.6) that occurred during InSight's lifetime was recorded on May 4, 2022.Here, we present the current understanding of the Martian seismicity and the different types of events we observed on Mars, based on the data collected over the whole mission.Low-frequency (LF) and broadband (BB)The LF family of events include energy predominantly below 1 Hz. They are similar to teleseismic events observed on Earth, and clear P and S waves are often identified. The hypocenter is known for about half of the recorded LF-BB events, owing to the difficulty of determining back-azimuth and in some cases also distance for the smaller events. The following elements are now understood:Seismicity appears to be located only in few spots around Mars (Figure 2) and no tectonic events were located within 25° from the InSight station. A large number of LF-BB events are located 26–30° from the station, interpreted to be associated with the active dynamics of the volcanic Cerberus Fossae area. A group of events show only a weak S-wave energy and are aligned using the P-wave and length of its coda to around 46°. Their tectonic origin is yet unknown. A few events are located around 60° with relatively emergent P- and S-wave energy. Two large events (S0976a and S1000a) lie beyond the core shadow and have PP and SS phases; S0976a in the Valles Marineris region 146° away from InSight, and S1000a as the result of a meteoritic impact. A number of events of uncertain location are clustered in the same distance, around 100-120° distance. LF events have the largest magnitudes with S1222a reaching MWMa=4.6 and a few others at or above MWMa=3.5. High-frequency (HF) The HF family of events are predominantly at and above the 2.4 Hz, local subsurface resonance. The HF events have magnitudes below MWMa 2.5 and originate from a distance range of 25–30°, likely a single area in the central Cerberus Fossae region, as very shallow events associated to active volcanic dykes. Very high frequency (VF):A small number of HF events are characterized by higher frequency content, up to 20–30 Hz with a notable amplification on the horizontal components at very high frequency, and are termed VF events. The amplification is plausibly explained by the local subsurface structure. These events are observed only close to the lander. Remote imaging of recent craters and the presence of a distinctive acoustic signal confirmed that the closest events were produced by meteoric impacts. Investigations are being conducted to understand if other VF events can be confirmed as impacts, too.

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.

NASA's InSight has detected a large magnitude seismic event, labeled S1222a. The event has a moment magnitude of 4.7, with five times more seismic moment compared to the second largest event. The event is so large that features are clearly observed that were not seen in any previously detected events. In addition to body phases and Rayleigh waves, we also see Love waves, minor arc surface wave overtones, and multi‐orbit surface waves. At long periods, the coda event exceeds 10 hr. The event locates close to the North‐South dichotomy and outside the tectonically active Cerberus Fossae region. S1222a does not show any evident geological or tectonic features. The event is extremely rich in frequency content, extending from below 1/30 Hz up to 35 Hz. The event was classified as a broadband type event; we also observe coda decay and polarization similar to that of very high frequency type events.