Origins of crustal layering beneath the InSight landing site (and elsewhere)

Analyses of the InSight seismic data indicate that there are three major seismic discontinuities beneath the lander where the seismic velocity abruptly increases. In addition to the crust-mantle interface at a depth of about 39 km, two major intracrustal discontinuities are observed at depths near 8 and 20 km (Lognonné et al. 2020, Knapmeyer-Endrun et al. 2021, Kim et al. 2021, Duran et al. 2022). There are many possible explanations for the existence of large-scale layering within the crust, and most of these involve a change in either chemical composition or porosity (see Figure 1). Here, we will assess these hypotheses using improved models of the crust that have been made possible by the InSight mission.

 

Figure 1. Interpretations of crustal layering beneath the InSight landing site. In the first two schematics, the crustal layering is a result either of stepwise changes in porosity (left) or composition (center). Our preferred interpretation combines elements of both of these end-member models. The depth of each seismic interface below the surface is denoted on the left schematic.

As summarized in Wieczorek et al. (2022), there are a large number of hypotheses for the origin of the three-layered nature of the Martian crust. One or more layers could be composed of volcanic or sedimentary deposits. One of the discontinuities could represent the removal of pore space either by viscous deformation of the host rock or by the precipitation of cements in an ancient aquifer. One layer could represent thick impact basin ejecta deposits, whereas another could represent thick sequences of magmatic intrusions. One of the discontinuities could represent a change in crystallinity, such as is observed in the oceanic crust of Earth. Lastly, one or more discontinuities could be generated by the fractional crystallization of a giant impact melt pool associated with the Borealis impact. Importantly, each of these hypotheses has different implications for the expected thickness of the crustal layer, its composition, and its associated seismic velocity.

Based on the currently available information, we have constructed a plausible model for the observed crustal layering beneath the landing site. As shown in Figure 1, we interpret the uppermost layer as being a result of thick sequences of volcanic materials that were deposited in the early Hesperian, Noachian, and pre-Noachian periods. To account for the low seismic velocities of this layer, these materials would need to be heavily fractured, perhaps being similar in nature to the pyroclastic deposits that make up the nearby Medusae Fossae formation. Given the long duration over which these materials were emplaced, intercalated sedimentary deposits could be common and these materials might also have undergone substantial aqueous alteration at a later date. Ancient impact ejecta deposits from the Utopia basin are likely to be found below the layer of volcanic and sedimentary deposits, potentially in the middle layer between 8 and 20 km depth. The increase in velocity at 20 km depth is likely to be a consequence of the complete viscous closure of all remaining pore space at about 4 Ga when the crustal temperatures were elevated. The deepest layer, from depths of 20 to 39 km, likely corresponds to the initial crust that formed during the differentiation of the Borealis impact melt sheet.

The expected stratigraphy of the southern highlands is less certain. Nevertheless, we speculate that a 20 km seismic discontinuity would be found there as well that represents the transitions from porous to non-porous materials. Another major discontinuity that is likely to be present in the southern highlands would be the base of thick ejecta deposits derived from the ancient Borealis impact event.

It is important to recognize that the InSight landing site is located in the northern lowlands of Mars, and that the layering that is observed beneath the lander might only be indicative of local geologic structure. Future observations of surface waves, as well as analyses of crustal structure at the bounce point of PP waves, will allow us to assess how the thickness of the crustal layers varies across the planet.

References:

Durán, C. et al. (2022). Seismology on Mars: An analysis of direct, reflected, and converted seismic body waves with implications for interior structure. Phys. Earth Planet. Inter., 325, 106851, doi:10.1016/j.pepi.2022.106851

Kim, D. et al. (2021). Improving constraints on planetary interiors with PPs receiver functions. J. Geophys. Res. Planets, 126, e2021JE006983, doi:172810.1029/2021JE006983

Knapmeyer-Endrun, B. et al (2021). Thickness and structure of the martian crust from InSight seismic data. Science, 373, 438-443, doi:10.1126/science.abf8966

Lognonné, P., et al. (2020). Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data. Nature Geosci., 13, 213-220, doi:175910.1038/s41561-020-0536-y

Wieczorek, M. et al. (2022). InSight constraints on the global character of the Martian crust, J. Geophys. Res., doi:10.1029/2022JE007298