A plethora of geophysical, geo-
chemical, and geodynamical observations indicate that
the terrestrial planets have differentiated into silicate
crusts and mantles that surround a dense core. The
latter consists primarily of Fe and some lighter
alloying elements (e.g., S, Si, C, O, and H) [1]¿. The
Martian meteorites show evidence of chalcophile
element depletion, suggesting that the otherwise Fe-Ni-
rich core likely contains a sulfide component, which
influences physical state

Antonangeli, D.

Banerdt, W. B.

Clinton, John F.

Drilleau, M.

Duran, A. C.

García, Raphael

Giardini, Domenico

Gudkova, T.

Horleston, Anna

Huang, Quancheng

Irving, J.

Kawamura, T.

Khan, A.

Kim, Doyeon

Lekic, Vedran

Lognonné, P.

Maguire, Ross

Marusiak, A. G.

Myhill, Robert

Nimmo, F.

Panning, Mark

Plesa, A. C.

Rivoldini, A.

Samuel, Henri

Savas, Ceylan

Schimmel, Martin

Schmelzbach, C.

Schmerr, Nicholas C.

Sollberger, David

Stanley, S.

Stutzmann, E.

Stähler, S. C.

van Driel, M.

Weber, R.

Xu, Zongbo

Zenhäusern, G.

English

Digital.CSIC

SEISMIC DETECTION OF THE MARTIAN CORE BY INSIGHT.  Stähler, S. C.1, Savas Ceylan1, Andrea Cecilia Duran1, Raphaël Garcia2, Domenico Giardini1, Quancheng Huang3, Amir Khan1,4, Doyeon Kim3, Philippe Lognonné5, Ross Ronan Maguire3, Angela G. Marusiak6, Ana Catalina Plesa7, Henri Samuel5, Nicholas C. Schmerr3, Martin Schimmel8, David Sollberger1, Eleonore Stutzmann5, Daniele Antonangeli9, John F. Clinton1, Martin van Driel1, Mélanie Drilleau2, Tamara Gudkova15 Anna Horleston10, Jessica Irving10, Taichi Kawamura5, Ved Lekic3, Robert Myhill10, Francis Nimmo11, Mark Panning6, Attilio Rivoldini12, Cédric Schmelzbach1, Sabine Stanley13, Renee C. Weber14, Zongbo Xu5, Géraldine Zenhäusern1, W. Bruce Banerdt6 1 Institute of Geophysics, ETH Zurich, Sonneggstr. 5, 8092 Zürich, Switzerland, 2 Institut Supérieur de l'Aéro-nautique et de l'Espace SUPAERO, 3 Department of Geology, University of Maryland, College Park, 8000 Regents Dr., College Park, MD, 20782-4211, USA, 4 Physik-Institut, Universität Zürich, 5 Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France, 6 Jet Propulsion Laboratory, California Institute of Tech-nology, Pasadena, CA 91109, USA, 7 Institute of Planetary Research, German Aerospace Center (DLR), 12489 Ber-lin, Germany, 8 Institute of Earth Sciences Jaume Almera – CSIC, Barcelona, Spain, 9 IMPMC, Sorbonne Université, MNHN, CNRS , Paris, France 10 School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK, 11 Dept. of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA, 12 Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium, 13 Johns Hopkins University, 3400 N. Charles St., Baltimore, MD, 21218, USA 14 NASA MSFC, 320 Sparkman Drive, Huntsville, AL 35805, USA 15 Schmidt Institute of Physics of the Earth RAS, B.Gruzinskaya 10-1, Moscow 123242, Russia    Introduction:  A plethora of geophysical, geo-chemical, and geodynamical observations indicate that the terrestrial planets have differentiated into silicate crusts and mantles that surround a dense core. The latter consists primarily of Fe and some lighter alloying elements (e.g., S, Si, C, O, and H) [1]⁠. The Martian meteorites show evidence of chalcophile element depletion, suggesting that the otherwise Fe-Ni-rich core likely contains a sulfide component, which influences physical state.  There is strong evidence from measurements of the tidal deformation of the planet that the core of Mars is presently liquid   [2–4]. Recent experimental studies of phase relations in the Fe-S and (Fe,Ni)-S systems at conditions of the center of Mars are compatible with an entirely liquid core at present [5, 6]; yet, core size and composition are un-certain [7, 8]. Related hereto is the question of the presence or absence of a lower mantle in Mars, i.e., whether bridgmanite-structure silicates are present, which can possibly exert considerable control over the dynamical evolution of mantle and core [9–13]. A small core will tend to be Fe-rich and favor the presence of a lower mantle whereas large cores will tend to be enriched in light elements and inhibit a lower mantle.  The InSight mission aims at constraining these numbers via the RISE radio tracking experiment [14], and the SEIS seismic package [15]. The results of RISE are presented in a separate abstract in the same issue [25], and here we focus on the results of SEIS. Data: We used data recorded by SEIS [26] for high SNR marsquakes between March 2019 and July 2020. The InSight Marsquake Service located these events in the distance range 27-40 degrees, based on identification of P- and S-body waves [16–19]. Later studies identified a number of secondary, surface-reflected phases, which were used to constrain the upper mantle (see LPSC abstract by Khan et al. [27]). We build upon the velocity models derived from these phase picks to constrain the time window in which to look for shear waves reflected from the core mantle boundary. Since shear waves cannot propagate in a fluid medium, the core mantle boundary (CMB) acts as a polarization filter, which fully reflects horizontally polarized shear waves back into the mantle. Shear waves reflected from the CMB, called ScS, are therefore expected to have a predominantly horizontal polarization at the receiver, with an azimuth orthogonal to the source direction. In this distance range, ScS is separated in time from any other body wave phase and therefore well-observable.  Methods: We follow a two-step approach: 1. Confirm seismic arrivals as ScS, based on existing mantle velocity models. 2. Pick precise arrival times and invert those for mantle profiles and core size, 1545.pdf52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)constrained by mineralogy, moment of inertia and average density of the planet.  To enhance body wave arrivals, we remove elliptically and polarized signal to enhance ScS waves. For the events listed above, we sum the signal power in a suitable frequency band around the predicted arrival time for ScS in a set of 5000 mantle models compatible with the travel times of other phases. The stack confirms that significant energy is present in time windows that correspond to reflection from a strong interface at ~1600 km depth associated with the liquid Fe-rich core with a radius of around 1800 km radius. We identified these arrivals for each individual event and picked travel times and measured frequency content. Using these updated depths, we invert all the travel times together with moment of inertia and the mean density of the whole planet. Results: The inversion of travel times constrains the core radius to the upper end of pre-mission geo-physics-based estimates [7, 20]. This value is com-patible with estimates from the geodetic experiment RISE onboard and implies that a lower mantle is unlikely to be present. Moreover, a large core has important implications for core composition. Average retrieved core density is 6 g/cm^3, which implies that for a (Fe-Ni)-S composition, a sulfur content in excess of 18% is required. This is above the eutectic composition observed experimentally with potentially profound implications for the future crystallization of the Martian core [6, 21], subject to further laboratory research of Fe-S data under core conditions [22].  All ScS candidate phases that were observed show significant seismic energy and a relatively flat spectrum above 0.1 Hz, which implies a low seismic attenuation throughout the mantle. The spectral character of direct S-phases for the distant-most events [16] is consistent with that of the ScS-phases for more nearby events, which supports the identification of the arrivals as core-reflected. As expected from typical rheologies [23], the quality factor at seismic periods is significantly higher than the average value found at the period of the Phobos tide of 5.5h (Qµ=85,  [24]). Acknowledgments: The InSight SEIS data is available from the Planetary Data System (PDS).  References:   [1] R.D. Oldham, Quarterly Journal of the Geological Socie-ty, 1906, 62, 456. [2] A. Genova et al., Icarus, 2016, 272, 228. [3] A.S. Konopliv et al., Icarus, 2016, 274, 253. [4] C. F. Yoder et al., Science, 2003, 300, 299. [5] A. Pommier et al., Icarus, 2018, 306, 150. [6] A.J. Stewart et al., Science, 2007, 316, 1323. [7] A. Rivoldini et al., Icarus, 2011, 213, 451. [8] S.E. Smrekar et al., Space Science Reviews, 2019, 215, 3. [9] H. Harder, U.R. Christensen, Nature, 1996, 380, 507. [10] D. Breuer et al., Earth and Planetary Science Let-ters, 1997, 148, 457. [11] P. van Thienen et al., Physics of the Earth and Planetary Interiors, 2004, 142, 61. [12] T. Ruedas et al., Physics of the Earth and Planetary Interiors, 2013, 216, 32. [13] S.E. Smrekar et al., Space Science Re-views, 2019, 215, 3. [14] W.M. Folkner et al., Space Science Reviews, 2018, 214, 100. [15] P. Lognonné et al., Space Science Reviews, 2019, 215, 12. [16] D. Giardini et al., Na-ture Geoscience, 2020, 13, 205. [17] J.F. Clinton et al., Phys-ics of the Earth and Planetary Interiors, 2020, 106595. [18] P. Lognonné et al., Nature Geoscience, 2020, 13, 213.  [19] W.B. Banerdt et al., Nature Geoscience, 2020, 13, 183.  [20] A. Khan et al., Journal of Geophysical Research: Planets, 2018, 123, 1. [21] S.A. Hauck et al., Journal of Geophysical Research E: Planets, 2006, 111, 1. [22] H. Terasaki et al. Journal of Geophysical Research: Planets, 2019, 124, 2272– 2293. [23] A. Bagheri et al., Journal of Geophysical Re-search: Planets, 2019, 124, 2703. [24] F. Nimmo, U.H. Faul, Journal of Geophysical Research E: Planets, 2013, 118, 2558. [25] S. Le Maistre et al., LPSC52, 2021, #2011. [26] P. Lognonné et al., LPSC52, 2021, #1936. [27] A. Khan et al., LPSC52, 2021, #1836  1545.pdf52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

Seismic detection of the martian core by InSight

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Seismic detection of the martian core by InSight

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