The composition of Mars

Abstract

Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth's composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperature. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ≤7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core.

Autocorrelation Reflectivity of Mars

“…To recover the deeper seismic discontinuities in Mars, we examined low‐frequency filtered autocorrelograms which show reflection signals at times consistent for the Martian olivine‐wadsleyite transition and the Martian core‐mantle boundary. Using different velocity models simulated for various compositional and thermal models (Khan et al, 2018; Panning et al, 2017; Yoshizaki & McDonough, 2020), the olivine‐wadsleyite transition and core‐mantle boundary of Mars would be at 1,110–1,170 and 1,520–1,600 km, respectively.…”

“…That lower-frequency signals from deep within Mars are observable is not too surprising, as Mars is a cold, dry planet, and seismic attenuation is strongly dependent on temperature and on the fluid content of rocks. We use four P wave velocity models (Figure 3b) to depth convert the stacked autocorrelograms, one is the LFAK model (Khan et al, 2018), three are from theoretical calculation of shear wave velocity for various bulk composition and thermal state models (Panning et al, 2017) where we assume a Vp/Vs ratio of 1.82 (Sohl & Spohn, 1997), and one is from a theoretical calculation of Vp (Yoshizaki & McDonough, 2020). Figure 3c shows the depth conversion of the reflectivity series filtered between 0.05 and 0.1 Hz using the four models shown in Figure 3b.…”

What Martian Meteorites Reveal About the Interior and Surface of Mars

“…According to this study, the martian core contains light elements with ≤7 wt.% of S, less than previously suggested (Stewart et al, 2007), but contains O (5.2 wt.%) and H (0.9 wt.%) (Yoshizaki & Mc-Donough, 2020). The martian mantle is more oxidized and has a lower Mg# (= molar MgO/[MgO + FeO]) of ∼0.79 than the Earth's mantle (McDonough & Sun, 1995;Yoshizaki & McDonough, 2020).…”

The Solidus and Melt Productivity of Nominally Anhydrous Martian Mantle Constrained by New High Pressure‐Temperature Experiments—Implications for Crustal Production and Mantle Source Evolution

“…Previous studies also suggested that variation in Mg# [molar 100 × MgO/(MgO + FeO)], though subordinate to alkali variations at lower pressures, can affect the solidus temperature significantly (Herzberg et al, ; Hirschmann, ) at higher pressures as the effect of alkali diminishes because of enhanced compatibility of Na in clinopyroxene. Although all available models from geochemical and geophysical perspectives suggest that Martian mantle has either higher or similar total alkali concentration and much lower Mg# (~75–80) compared to the Earth's mantle (Mg#~90, McDonough & Sun, ; Workman & Hart, ), the Mg# and total alkali (Na 2 O+ K 2 O) concentration among different model Martian mantle compositions vary between 72 and 80 and 0.5 and 1.2 wt.%, respectively (Dreibus & Wanke, ; Khan et al, ; Khan & Connolly, ; Lodders & Fegley, ; Morgan & Anders, ; Ohtani & Kamaya, ; Sanloup et al, ;Taylor, ; Yoshizaki & McDonough, ). Among all the model composition, Dreibus and Wanke (DW) composition is the most widely accepted model for Martian bulk composition because it is derived from elemental correlations in Martian meteorites (Taylor, ).…”