InSight Constraints on the Global Character of the Martian Crust

Abstract

Analyses of seismic data from the InSight mission have provided the first in situ constraints on the thickness of the crust of Mars. These crustal thickness constraints are currently limited to beneath the lander that is located in the northern lowlands, and we use gravity and topography data to construct global crustal thickness models that satisfy the seismic data. These models consider a range of possible mantle and core density profiles, a range of crustal densities, a low-density surface layer, and the possibility that the crustal density of the northern lowlands is greater than that of the southern highlands. Using the preferred InSight three-layer seismic model of the crust, the average crustal thickness of the planet is found to lie between 30 and 72 km. Depending on the choice of the upper mantle density, the maximum permissible density of the northern lowlands and southern highlands crust is constrained to be between 2,850 and 3,100 kg m −3 . These crustal densities are lower than typical Martian basaltic materials and are consistent with a crust that is on average more felsic than the materials found at the surface. We argue that a substantial portion of the crust of Mars is a primary crust that formed during the initial differentiation of the planet. Various hypotheses for the origin of the observed intracrustal seisimic layers are assessed, with our preferred interpretation including thick volcanic deposits, ejecta from the Utopia basin, porosity closure, and differentiation products of a Borealis impact melt sheet. Plain Language SummaryThe crust, mantle and core are the three major geochemical layers that make up a planet. Before NASA's InSight mission, the thickness of the crust of Mars was inferred using indirect techniques, including analyses of gravity data collected from orbit and the composition of surface rocks. Estimates for the average thickness using these techniques spanned the range from 27 to 118 km. Analyses of data collected by the InSight seismometer have provided us with the first direct seismic measurement of the thickness of the crust, but this measurement is only for beneath the lander that is located in the northern lowlands where the crust is expected to be thinner than average. In this work, gravity and topography data are used to construct global crustal thickness models that satisfy the new seismic constraints. The average crustal thickness is found to be somewhere between 32 and 70 km, and the average density of the crust can be no larger than 3,100 kg m −3 . This bulk crustal density is lower than most typical Martian WIECZOREK ET AL.

Interior Dynamics and Thermal Evolution of Mars -- a Geodynamic Perspective

“…The observed crustal thickness at the InSight landing site (Knapmeyer-Endrun et al, 2021) indicate two possible crustal thickness models: a two layer crust based on two strong seismic discontinuities or a three layer crust including a third weaker discontinuity recorded in the data. The extrapolation to global crustal thickness using gravity and topography data (Wieczorek and Zuber, 2004;Wieczorek et al, 2022) suggests an average crustal thickness between 24 and 72 km (Knapmeyer-Endrun et al, 2021), with the three layer crust showing values between 30 and 72 km (Wieczorek et al, 2022). We note that the range of crustal thicknesses of Wieczorek et al (2022) is larger than the range of 39 to 72 km presented in Knapmeyer-Endrun et al (2021).…”

“…Geodynamic models have been employed in previous studies to investigate the effects of crustal thickness variations, as derived from gravity and topography data, on the surface heat flow variations and the thermal state of the lithosphere (Plesa et al, 2016(Plesa et al, , 2018b. The crust in these studies does not change with time, but varies spatially according to the chosen crustal thickness model Wieczorek and Zuber (2004); Wieczorek et al (2022). Results show that the crustal thickness variations and the crustal enrichment in HPEs control the surface heat flow distribution at present day, the thickness of the lithosphere and the lithospheric temperature variations.…”

“…Crustal thickness models that can explain the gravity and topography data are non-unique. While an anchor point given by a crustal thickness value at a known location can help to constrain these models, one major assumption that remains is the density difference between mantle and crust (Wieczorek et al, 2022). Depending on the assumed crust-mantle density contrast, the difference between the crustal thickness of the southern and northern hemisphere can be small (Fig.…”

“…The extrapolation to global crustal thickness using gravity and topography data (Wieczorek and Zuber, 2004;Wieczorek et al, 2022) suggests an average crustal thickness between 24 and 72 km (Knapmeyer-Endrun et al, 2021), with the three layer crust showing values between 30 and 72 km (Wieczorek et al, 2022). We note that the range of crustal thicknesses of Wieczorek et al (2022) is larger than the range of 39 to 72 km presented in Knapmeyer-Endrun et al (2021). This is due to the fact that Wieczorek et al (2022) considered cases with a density dichotomy between the northern and southern hemisphere, while in Knapmeyer-Endrun et al (2021) only cases with a homogeneous crustal density were discussed.…”

Second Seismic Anchor Point of the Martian Crustal Structure Away From the InSight Landing Site

“…Accordingly, the absence of a shallow layer characterized by low seismic wave speed (and thus low density) could be due to different processes depending on the precise location of the bounce point. If the bounce point is to the south of the dichotomy boundary, in areas underlain by Noachian crust, any largescale impact-induced porosity that might be expected in the upper crust would have to be reduced by processes such as viscous closure or thermal annealing during burial and/or occlusion of pore space by secondary minerals and/or ices (e.g., Wieczorek et al, 2022). On the other hand, if the bounce point is to the north of the dichotomy boundary or in an area resurfaced by Hesperian-Amazonian volcanism to the south, any pre-existing porosity in the uppermost crust may have been obscured or occluded by later, higher density magmatism (e.g., Wilson and Head, 1994).…”