On 4 May 2022, the seismometer on Mars observed the largest marsquake (S1222a) during its operation. One of the most specific features of S1222a is the long event duration lasting more than 8 hr, in ...addition to the clear appearance of body and surface waves. As demonstrated on Earth, by modeling a long‐lasting and scattered surface wave with the radiative transfer theory under the isotropic scattering condition, we estimated the scattering and intrinsic quality factors of Mars (Qs and Qi). This study especially focused on the frequency range between 0.05–0.09 Hz, where Qs and Qi have not been constrained yet. Our results revealed that Qi = 1,000–1,500 and Qs = 30–500. By summarizing the Martian Qi and Qs estimated so far and by comparing them with those of other celestial bodies, we found that, overall, the Martian scattering and absorption properties showed Earth‐like values.
Plain Language Summary
Since February 2019, NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) has been conducting quasi‐continuous seismic observation for more than three years. The seismic data from Mars has contributed significantly to a better understanding of the interior structure and the seismicity of the red planet. On 4 May 2022 (1222 Martian days after landing), another key event occurred, called S1222a. The event showed the largest seismic moment release (magnitude 4.7) and extremely long duration (>8 hr) with intense seismic scattering. As demonstrated on Earth, the long‐lasting scattered waves are useful for retrieving information about the structural heterogeneity within a planet. In this study, by applying the radiative transfer theory—which considers the energy transportation from the seismic source to the observation point—to Mars, we evaluated the energy decay rate due to seismic scattering and energy absorption by a medium. By comparing our results with those of other solid bodies, we found that the Martian scattering and absorption features were closer to the terrestrial ones than to the lunar ones.
Key Points
We modeled the scattering effect of the largest marsquake (S1222a) using radiative transfer theory on a spherical Mars
The inversion revealed that the intrinsic and scattering quality factors below 0.1 Hz are 1,000–1,500 and 30–500, respectively
We summarized the Martian quality factors derived so far and found that they are relatively Earth‐like rather than Moon‐like
Knowledge of Martian crust and uppermost mantle aid us studying the planet's evolution. NASA's InSight mission provides seismic data being used to reveal the interior structure. Most studies have ...focused on the crustal structure beneath InSight lander, but the seismic structure of other regions has remained poorly known. We use surface‐wave data to investigate the crustal structure of a large region along the Medusa Fossae Formation and the dichotomy. We adopt the largest‐magnitude marsquake (S1222a) that has been recorded, which provides both Rayleigh‐ and Love‐wave signals. We measure and jointly invert these surface‐wave fundamental‐mode group velocities from ∼15 to 40 s to estimate the average 1D isotropic velocity models. These models includes a high‐velocity layer at ∼7‐km depth, which could be due to a regional basaltic activity or regional stress. Our models also indicate that a common intra‐crustal structure (∼12–40 km depth) may exist in this region along the dichotomy.
Plain Language Summary
NASA's Mars exploration mission, InSight, brought a seismometer module that recorded numerous marsquakes. These marsquake recordings reveal the crustal structure beneath the InSight lander. To study the crustal structure of other regions, one can utilize a type of seismic waves, the surface waves. The largest marsquake event observed during InSight provides surface‐wave signals with a high signal‐to‐noise ratio. By analyzing these signals, we investigate the average crustal structure between the epicenter and the InSight landing site in a region near the equator and along the planet's dichotomy. We find a high‐velocity layer at about 7 km depth compared to the layers above and below, which could be due to a regional flood basalt or the regional crustal compressional stress. Our result also exhibits the similar crustal structure (from around 12 to 40 km depth) as the structure beneath the InSight lander, which indicates the possibility of a similar intra‐crustal structure existing along the Martian dichotomy.
Key Points
Joint inversion of the S1222a Rayleigh‐ and Love‐wave group velocities provides 1D isotropic velocity models of the regional Martian crust
These models indicate a high‐velocity layer at ∼7 km depth which could be due to the regional basaltic activity or the regional stress field
Except for the high‐velocity layer, these models are similar to the crustal structure beneath InSight lander from ∼12 to 40 km depth
Estimating the rupture process of a quake constrains the tectonic setting of its source region. In terrestrial seismology, the rupture process can be described with a classical ω2 model. In this ...study, we focus on one key source parameter of S1222a, the corner frequency, to investigate its origin. However, estimating the corner frequency of S1222a is complicated by S1222a's non‐classical spectra which deviate from the ω2 model. To explain S1222a's spectra, we take into account the site effect at the InSight landing site. Through numerical simulation, we demonstrate that the spectral fitting is greatly improved after considering the site effect. We further estimated the source spectrum and stress drop of S1222a. The obtained stress drop resembles the values for regional tectonic earthquakes and shallow moonquakes but differs from the Cerberus Fossae marsquakes, which favors a different tectonic origin from the Cerberus Fossae marsquakes for S1222a.
Plain Language Summary
The source of a quake is a key piece of information to uncover the dynamic process at the source region. In terrestrial seismology, the source of a quake can be described with a classical model in which the spectra decay with frequency at a certain speed. In this study, we focus on one key source parameter of S1222a, the corner frequency, to explore the generating process of S1222a. However, S1222a exhibits non‐classical spectra which cannot be explained by the classical model, making it difficult to estimate S1222a's source spectrum. To explain S1222a's spectra, we hypothesized that the seismic records are contaminated by wave reverberations in the subsurface layers, namely site effect. We validated our hypothesis with a numerical method. After considering site effect, S1222a's source spectrum and stress drop can be better estimated. The obtained stress drop value is close to those for regional tectonic earthquakes and shallow moonquakes but differs from the values observed for some marsquakes located at Cerberus Fossae, a seismically active region on Mars. Thus, we conclude that S1222a has a different tectonic origin from the Cerberus Fossae marsquakes.
Key Points
We constructed a numerical model to simulate the site effect at the InSight landing site to better explain the spectra of S1222a
We estimated the corner frequency of S1222a to be 0.76 (0.58–1.07) Hz, which corresponds to a stress drop of 3.62 (0.80–20.26) MPa
The corner frequency and stress drop estimation favor that S1222a has a different tectonic origin from the Cerberus Fossae marsquakes
We have observed both minor‐arc (R1) and major‐arc (R2) Rayleigh waves for the largest marsquake (magnitude of 4.7 ± 0.2) ever recorded. Along the R1 path (in the lowlands), inversion results show ...that a simple, two‐layer model with an interface located at 21–29 km and an upper crustal shear‐wave velocity of 3.05–3.17 km/s can fit the group velocity measurements. Along the R2 path, observations can be explained by upper crustal thickness models constrained from gravity data and upper crustal shear‐wave velocities of 2.61–3.27 and 3.28–3.52 km/s in the lowlands and highlands, respectively. The shear‐wave velocity being faster in the highlands than in the lowlands indicates the possible existence of sedimentary rocks, and relatively higher porosity in the lowlands.
Plain Language Summary
The largest marsquake ever recorded occurred recently and waves propagating at the surface, called surface waves, have been observed. Owing to the relatively large magnitude (i.e., 4.7 ± 0.2) of this event, surface wave energy is still clearly visible after one orbit around the red planet. The shortest path taken by the wave propagating between the source and the receiver is located in the northern lowlands, near the boundary with the southern highlands (called dichotomy). The surface wave traveling in the opposite direction, following the longer distance between the quake and the seismic station, mostly passes through the highlands. Analyses of these two paths reveal that the average shear‐wave velocity is faster in the highlands than in the lowlands near the dichotomy boundary. This lower velocity in the lowlands may be due to the presence of thick accumulations of sedimentary rocks and relatively higher porosity.
Key Points
Analyses of the minor‐arc and major‐arc Rayleigh waves reveal different Martian crustal structures across the dichotomy boundary
The average shear‐wave velocity is faster in the highlands than in the lowlands near the dichotomy boundary
The lower shear‐wave velocity in the lowlands may be due to the presence of sedimentary rocks and relatively higher porosity
Prior to the 2018 landing of the InSight mission, the InSight science team proposed locating Marsquakes using multiple orbit surface waves, independent of seismic velocity models, for events larger ...than MW4.6. The S1222a MW4.7 of 4 May 2022 is the largest Marsquake recorded and the first large enough for this method. Group arrivals of the first three orbits of Rayleigh waves are determined to derive the group velocity, epicentral distance, and origin time. The mean distance of 36.9 ± 0.3° agrees with the Marsquake Service (MQS) distance based on body wave measurements of 37.0 ± 1.6°. The origin time from surface waves is systematically later than the MQS origin time by 20 s. Backazimuth estimation is similar to body wave estimations from MQS although suggesting a shift to the south. Backazimuth estimates from R2 and R3 are more scattered, but do show clear elliptical motion.
Plain Language Summary
Waves that move along the surface all the way around the planet of Mars can be used to figure out where a Marsquake occurred without knowing in advance how fast the waves move through the planet, because we know how big the planet is. Before InSight got to Mars, we predicted that we would be able to see these waves if an event was big enough, and on 4 May 2022, we finally saw a Marsquake large enough to test this approach. Based on the timing of the arrivals of these waves, we were able to figure out the distance and timing of the Marsquake. The results agreed well with the approach we had been using for smaller events, giving us additional confidence in our tools for figuring out where Marsquakes have happened.
Key Points
The MW 4.7 S1222a event is the first Marsquake large enough for multi‐orbit surface wave location independent of a priori seismic velocity
Using measurements of R1, R2, and R3 Rayleigh waves, we determine an epicentral distance consistent with that estimated from body waves
Elliptical particle motion is observed for Rayleigh wave arrivals broadly consistent with the backazimuth identified from body waves
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/cm
) 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.
Acoustic waves in planetary atmospheres couple into the solid surface, producing ground displacements that can be measured using seismometers. On 26 November 2018, the InSight mission successfully ...landed on Mars. Its objectives include studying Mars' interior using the seismometer SEIS (Seismic Experiment for Interior Structures) and the atmosphere through the weather station APSS (Auxiliary Payload Sensor Suite). Because InSight is the first mission capable of studying infrasound on Mars, we investigate the signature of infrasound both in terms of air pressure and ground velocities. Using numerical simulations, we characterize (1) the acoustic propagation pattern in Martian dusk and (2) the mechanical atmosphere‐to‐ground coupling under acoustic waves. Then, using SEIS data, we demonstrate that two low‐frequency monotone events (S0133a and S0189a) are in fact infrasound trapped in the atmospheric nocturnal surface waveguide. We base our demonstration on the following facts. (1) Seismic signals rarely produce, at a given station, a single frequency varying from one event to the other. (2) No clear seismic phases have been identified for such events. (3) The observed SEIS signals present the characteristics expected for trapped infrasound observed through their compliance effects (specific frequency response, more energy on the vertical component, ±90° phase shift between vertical and horizontal components, and no detection on pressure sensor at these low amplitude levels). Our simulations of the nocturnal waveguide's response are however subject to uncertainties because (1) it relies on the sol‐to‐sol variability of the atmosphere, and (2) subsurface properties are not properly known at this time.
Plain Language Summary
The InSight mission landed on Mars the 26 November 2018 and was designed to study the interior of the Red Planet. The noise level of its seismometer SEIS routinely stays under 1 nm/s2, a precision unmatched by other planetary seismometers. InSight also features a meteorological station, APSS, able to measure the atmosphere's absolute pressure, wind, and temperature. In this work, we use the seismometer and meteorological station to investigate acoustic waves (sound) in the Martian atmosphere. Those acoustic waves are pressure perturbations and consequently cause the ground to move: Positive perturbations push the soil down, whereas negative ones lift it up. InSight's SEIS and APSS constitute the perfect instruments for investigating ground‐coupled acoustic waves. We start by performing numerical simulations of the coupled solid‐atmosphere system. In particular, we investigate how low‐frequency sound (infrasound) travels in Mars' atmosphere. We then derive key properties of the ground deformations due to passing infrasound. These arguments are then applied to InSight's data: ground motion measurements from SEIS and pressure records from APSS. Finally, we show that 3‐axis ground motion can help discriminate acoustic waves from meteorological perturbations. We also demonstrate that some SEIS events are caused by infrasound trapped close to the surface by nocturnal winds.
Key Points
Our numerical modeling predicts both pressure variations and ground movements induced by Martian acoustic waves
Regional propagation of dispersed infrasound is expected in the Martian nocturnal surface waveguide
Some monotone events detected by SEIS are consistent with acoustic waves trapped in this waveguide
Since landing on Mars, the NASA InSight lander has witnessed eight Phobos and one Deimos transits. All transits could be observed by a drop in the solar array current and the surface temperature, but ...more surprisingly, for several ones, a clear signature was recorded with the seismic sensors and the magnetometer. We present a preliminary interpretation of the seismometer data as temperature‐induced local deformation of the ground, supported by terrestrial analog experiments and finite‐element modeling. The magnetic signature is most likely induced by changing currents from the solar arrays. While the observations are not fully understood yet, the recording of transit‐related phenomena with high sampling rate will allow more precise measurements of the transit times, thus providing additional constraints for the orbital parameters of Phobos. The response of the seismometer can potentially also be used to constrain the thermoelastic properties of the shallow regolith at the landing site.
Plain Language Summary
The geophysical lander, InSight, has been operating on the surface of Mars since November 2018. Since then, the Martian moons Phobos and Deimos have been partially blocking the Sun, as seen from the InSight landing site, multiple times. Multiple InSight instruments have been measuring the effect of those transits; this surprisingly includes the seismometer and the magnetometer. We conclude that temperature‐induced deformation and tilt are responsible for the seismic measurements. The change observed in the magnetometer measurements are most likely the result of a drop in the solar array currents. We do not observe atmospheric modulations with InSight's weather station during the transit. These observations help constrain orbital parameters of the Martian moons, and the seismometer signal might allow investigating thermoelastic properties of the shallow Martian material.
Key Points
Multiple geophysical InSight instruments observe unexpected signals during Phobos transits
Local ground deformation due to surface temperature drops explain the tilt signals seen by the seismometer
The drop in the solar array currents results in a change of the magnetic field
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