Autocorrelation reflectivity of Mars

Sizhuang Deng; Alan Levander; Valerie Payre

Determining the Martian interior structure will contribute to understanding the early formation and evolution of the inner planets in our solar system. The Martian subsurface structures have been studied at different scales during past decades by gravity anomaly inversion (Zuber et al., 2000; Neumann et al., 2004; Keifer et al., 1996), solar tide detection (Yoder et al., 2003), high-pressure experiments (Stewart et al., 2007; Bertka et al., 1997; Fei, 2013) and mineralogic model calculation (Sohl and Spohn, 1997; Zharkov et al., 2009; Rivoldini et al., 2011; Khan et al., 2018). Mars, sharing the similar interior structures as Earth, can be divided into crust, mantle and core (Smrekar et al., 2018). The Viking 2 seismic recordings provided modest information on the interior structure of the planet (Anderson et al, 1977) which has led to numerical and theoretical estimation of seismic parameters prior to the InSight mission (Panning et al., 2017; Lognonne et al., 2019). Estimates of crustal thickness have been made from gravity observations (Zuber, 2001). Evidence that Mars' core is largely liquid comes from the large Love number (Yoder et al., 2003), and from high-pressure experiments using assumed core properties (Stewart et al., 2007), while the existence of a solid inner core is still unknown. The data recorded by the SEIS instrument will provide an understanding of the interior seismic structure, hence providing information on tectonic history and evolution of Mars. Claerbout (1968) showed that the reflectivity series of a layered acoustic medium can be recovered by taking the autocorrelation of the surface recording of the normal-incidence transmission response, which was extended to elastic media and non-normal incidence angles by Frasier (1970). Autocorrelation reflectivity imaging has been used successfully to investigate the reflectivity response of the crust and Moho discontinuity (Gorbatov et al., 2013; Oren and Nowack, 2017), lithosphere-asthenosphere boundary (LAB) (Kennett, 2015; Kennett and Sippl, 2018) and core-mantle boundary (Wang et al., 2015) on Earth. In this study, we calculated and stacked the autocorrelograms of the available SEIS data, approximately 900 hours, to construct the reflection response beneath the InSight lander. We identify noise that is approximately periodic with the Martian sol, some of which we associate with lander operation and some to the daily variation in Martian weather. The data are cut into approximately 450 2-hour-long windows, and temporally balanced to remove the dominant features related to the lander and weather conditions. To identify signals at different depths, the autocorrelation reflectivity for internal structure is filtered in different bands of 2 octave width. We have identified signals in the autocorrelations that we tentatively identify as the base of a surface layer, the Martian Moho, a deep mantle discontinuity at a depth appropriate for the olivine-spinel transition, and prominent signals at the expected depth of the core-mantle boundary. Bootstrap calculations suggest the signals we are identifying are robust.

Autocorrelation reflectivity of Mars

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