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Article Dans Une Revue Proceedings of the National Academy of Sciences of the United States of America Année : 2023

First observations of core-transiting seismic phases on Mars

Zongbo Xu
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Résumé

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 to Mars, 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 (PP, SS). SKS travels through the core as a compressional wave, providing information about its 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 = 1780-1810 km) and denser (core density = 6.2-6.3 g/cm 3) core compared to previous estimates, with a P-wave velocity of 4.9-5.0 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 20-22 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. Mars | Core evolution | Planetary structure ous estimation of a relatively low core density has motivated 29 questions about its composition: if only sulfur is considered as 30 an alloying element, an implausibly high core sulfur fraction 31 is required to match the core density whilst satisfying con-32 straints on mass, moment of inertia, and tidal response of the 33 planet (11). Though the observation of seismic waves reflected 34 from Mars' CMB has helped constrain the core radius, and 35 geophysical and cosmochemical inversions have sought to infer 36 its average density and composition (e.g. 37), observations of 37 seismic waves that directly probe core properties have been 38 lacking to date. 39 Seismological investigations of core-transiting waves have 40 been made on Earth for more than a century (38, 39), where 41 both seismometers and hypocenters of large earthquakes are 42 distributed around the globe. Analyses of their travel times 43 have constrained the seismic properties of the liquid outer-44 core (e.g. 40, 41), supporting the presence of light elements 45 (42), and enabling the estimation of its equation of state (e.g. 46 43). With just a single broadband seismometer and seismic 47 sources smaller than those routinely detected on the Earth, 48 comparable observations have proved more challenging on 49 Mars. Here, we present and analyze the implications of new 50 observations, which constitute the first detection of seismic 51 waves transiting the Martian core. 52 Observations and Analysis 53 The events. The InSight mission deployed a very broadband 54 seismometer (44) onto the surface of Mars in late 2018, leading 55 to the identification of numerous seismic events (45-48). To 56 date, only two seismic events have been identified as located 57 on the opposite hemisphere of Mars to the InSight lander (49). 58 These events (Fig. 1 A) are designated S0976a and S1000a, 59 corresponding to the first seismic events detected on Sols 60 (InSight Martian mission days) 976 and 1000, respectively. 61 These events were located using phases identified as PP and 62 SS-waves which travel down to a depth of nearly 1200 km 63 in the Martian mantle, and reflect from the surface of Mars 64 midway along their path. The Mars Quake Service (MQS) 65 estimated the epicentral distances of S0976a and S1000a to be 66 146 • ± 7 • and 128 • ± 19 • , respectively, with Mars-calibrated 67 magnitudes (50) M M a w of 4.2 and 4.1, respectively (47, 49). 68 Uncertainty on marsquake locations is considerable, because 69 they depend on seismic models of Mars' layered crust and 70 mantle and, for source depth determination, the unequivocal 71 identification of depth phases (e.g. 7, 8). Fortunately, orbital 72 imaging combined with data from InSight revealed S1000a to 73 be an impact at a distance of 125.9 • and a backazimuth of 74 ∼ 34 • (51), though the exact time of the impact is not known. 75 We are therefore able to precisely locate the source of S1000a, 76 and fix its depth to zero, removing this source of uncertainty 77 from our work. 78 Phase detection. At the distances corresponding to S0976a 79 and S1000a, SKS, which travels as a shear wave in the crust 80 and mantle and as a compressional wave in the core (Fig. 1 81 A), will be the first core-transiting phase to arrive (Supp.
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hal-04123960 , version 1 (09-06-2023)

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Jessica C E Irving, Vedran Lekic, Cecilia Durán, Mélanie Drilleau, Doyeon Kim, et al.. First observations of core-transiting seismic phases on Mars. Proceedings of the National Academy of Sciences of the United States of America, 2023, ⟨10.1073/pnas.2217090120⟩. ⟨hal-04123960⟩
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