Bulk composition and early differentiation of Mars

Abstract

We report the concentrations of K, Th, and Fe on the Martian surface, as determined by the gamma ray spectrometer onboard the 2001 Mars Odyssey spacecraft. K and Th are not uniformly distributed on Mars. K ranges from 2000 to 6000 ppm; Th ranges from 0.2 to 1 ppm. The K/Th ratio varies from 3000 to 9000, but over 95% of the surface has K/Th between 4000 and 7000. Concentrations of K and Th are generally higher than those in basaltic Martian meteorites (K = 200–2600 ppm; Th = 0.1–0.7 ppm), indicating that Martian meteorites are not representative of the bulk crust. The average K/Th in the crust is 5300, consistent with the Wänke‐Dreibus model composition for bulk silicate Mars. Fe concentrations support the idea that bulk Mars is enriched in FeO compared to Earth. The differences in K/Th and FeO between Earth and Mars are consistent with the planets accreting from narrow feeding zones. The concentration of Th on Mars does not vary as much as it does on the Moon (where it ranges from 0.1 to 12 ppm), suggesting that the primary differentiation of Mars differed from that of the Moon. If the average Th concentration (0.6 ppm) of the surface is equal to the average of the entire crust, the crust cannot be thicker than about 118 km. If the crust is about 57 km thick, as suggested by geophysical studies, then about half the Th is concentrated in the crust.

Martian surface heat production and crustal heat flow from Mars Odyssey Gamma-Ray spectrometry

“…Heat production varies significantly across the Martian surface (Figure 1), ranging from 2.5 ± 0.2 × 10 −11 W · kg −1 in the Hellas Basin and Solis Planum regions to 7.5 ± 0.5 × 10 −11 W · kg −1 in the Acidalia Planum region. Of the elemental abundances measured by GRS, K and Th most closely correlate with one another compared to other measured elements (e.g., Fe, Cl, Si, or H) [Taylor et al, 2006b]. In addition, Th is presently the largest heat source among the radiogenic isotopes ( Figure 2).…”

“…[8] It has been estimated that as much as 50% or more of the Martian planetary budget of heat producing elements has seen sequestered into the crust during planetary differentiation due to their incompatibility in igneous processes [Taylor et al, 2006b;McLennan, 2001]; a process that mostly took place very early in Martian geological history [Carr and Head, 2010]. As such, the crustal component of heat flow represents as much as half of the total planetary radiogenic heat generation.…”

“…Elemental abundance data for K and Th have been compiled into smoothed maps with 5°× 5°per pixel resolution [Boynton et al, 2007;Taylor et al, 2006aTaylor et al, , 2006b. At this time, it is not possible to generate a useful map of U abundances at comparable resolution (due to U concentrations too low to provide the necessary gamma-ray detection counting statistics).…”

Claritas rise, Mars: Pre-Tharsis magmatism?

“…We use a surface temperature of 220 K, the present-day mean surface temperature (Kieffer et al, 1977), and a thermal conductivity of the crust of 2 W m − 1 K − 1 (e.g., Grott et al, 2005). For the crustal heat production rate, we use potassium and thorium abundances of 3630 and 0.70 ppm, respectively, average values deduced from GRS data for the ancient southern highlands (Taylor et al, 2006). For uranium abundance, we use a Th/U ratio of 3.6 deduced from SNC meteorite geochemistry (McLennan, 2003).…”

“…Thus, we calculate surface heat flows from the best-fit effective elastic thicknesses and crustal densities obtained for Claritas rise (Fig. 15), as well as heat-producing elemental abundances deduced from Mars Odyssey Gamma Ray Spectrometer (GRS) data (Taylor et al, 2006).…”