Hemispheric Dichotomy in Lithosphere Thickness on Mars Caused by Differences in Crustal Structure and Composition

Abstract

Estimates of the Martian elastic lithosphere thickness suggest small values of ∼25 km during the Noachian for the southern hemisphere and a large present‐day difference below the two polar caps (≥300 km in the north and >110 km in the south). In addition, young lava flows suggest that Mars has been volcanically active up to the recent past. We run Monte Carlo simulations using a 1‐D parameterized thermal evolution model to investigate whether a north/south hemispheric dichotomy in crustal properties and composition can explain these constraints. Our results suggest that 55–65% of the bulk radioelement content are in the crust, and most of it (43–51%) in the southern one. The southern crust can be up to 480 kg/m3 less dense than the northern one and might contain a nonnegligible proportion of felsic rocks. Our models predict a dry mantle and a wet or dry crustal rheology today. This is consistent with a mantle depleted in radioelements and volatiles. We retrieve north/south surface heat flux of 17.1–19.5 mW/m2 and 24.8–26.5 mW/m2, respectively, and a large difference in lithospheric temperatures between the two hemispheres (170–304 K in the shallow mantle). This difference could leave a signature in the seismic signals measured by the future InSight mission.

The Thermal State and Interior Structure of Mars

“…All our best-fit models use a 62-km-thick crust with a uniform density of 3,100 kg/m 3 , in excellent agreement with petrological analyses of martian meteorites and surface rocks (Baratoux et al, 2014). A present-day crustal heat production rate of 44.1-49 pW/kg is in good agreement with the gamma-ray measurements (Hahn et al, 2011) and with a recent study (Thiriet et al, 2018), which employed parameterized thermal evolution models to investigate the thickness and enrichment of the southern and northern martian crusts. The present-day mantle would then contain only about 30-35% of the current bulk HPE inventory.…”

“…These studies assumed a uniform crustal thickness, while ours includes a suite of spatially varying crustal thickness models based on geophysical data. We find the interior of Mars to have a high reference viscosity representative of a dry mantle rheology, in agreement with other recent petrological and geodynamical studies Filiberto, Gross, et al, 2016;McCubbin et al, 2016;Thiriet et al, 2018). The thick crust covering the southern hemisphere together with a significant enrichment in HPE as suggested by gamma-ray spectroscopy data allows a thin elastic lithosphere during the Noachian even for a dry mantle.…”

“…Indeed, geological evidence suggests that the bulk of the crust has been built early during the planetary evolution (Greeley & Schneid, 1991;Nimmo & Tanaka, 2005). A present-day crustal heat production rate of 44.1-49 pW/kg is in good agreement with the gamma-ray measurements (Hahn et al, 2011) and with a recent study (Thiriet et al, 2018), which employed parameterized thermal evolution models to investigate the thickness and enrichment of the southern and northern martian crusts. A present-day crustal heat production rate of 44.1-49 pW/kg is in good agreement with the gamma-ray measurements (Hahn et al, 2011) and with a recent study (Thiriet et al, 2018), which employed parameterized thermal evolution models to investigate the thickness and enrichment of the southern and northern martian crusts.…”

Magma Ascent and Eruption Triggered by Cratering on the Moon

“…Indeed, smaller values of ∼25 to 75 km, characteristics of hotter lithosphere and earlier times, are obtained by comparing tectonic observations and models of basin loading by lava infilling (McGovern & Litherland, 2011;Solomon & Head, 1979). Spatial differences in elastic thickness are also likely on the Moon, since heat producing elements appear concentrated in the Oceanus Procellarum Kreep Terrane, which might explain part of this discrepancy (Thiriet et al, 2018), though many FFCs are located close to this province. Spatial differences in elastic thickness are also likely on the Moon, since heat producing elements appear concentrated in the Oceanus Procellarum Kreep Terrane, which might explain part of this discrepancy (Thiriet et al, 2018), though many FFCs are located close to this province.…”

“…An elastic thickness of T e = 50 km is however inconsistent with FFC data since many FFCs would plot above the zero-pressure limit (Figures 3 and S3). Spatial differences in elastic thickness are also likely on the Moon, since heat producing elements appear concentrated in the Oceanus Procellarum Kreep Terrane, which might explain part of this discrepancy (Thiriet et al, 2018), though many FFCs are located close to this province.…”