Core formation and geophysical properties of Mars

Abstract

The chemical and physical properties of the interiors of terrestrial planets are largely determined during their formation and differentiation. Modeling a planet's formation provides important insights into the properties of its core and mantle, and conversely, knowledge of those properties may constrain formational narratives. Here, we present a multi-stage model of Martian core formation in which we calculate core-mantle equilibration using parameterizations from high pressure-temperature metal-silicate partitioning experiments. We account for changing core-mantle boundary (CMB) conditions, composition-dependent partitioning, and partial equilibration of metal and silicate, and we evolve oxygen fugacity (fO2) self-consistently. The model successfully reproduces published meteorite-based estimates of most elemental abundances in the bulk silicate Mars, which can be used to estimate core formation conditions and core composition. This composition implies that the primordial material that formed Mars was significantly more oxidized (0.9-1.4 log units below the iron-wüstite buffer) than that of the Earth, and that core-mantle equilibration in Mars occurred at 42-60% of the evolving CMB pressure. On average, at least 84% of accreted metal and at least 40% of the mantle were This manuscript has been accepted for publication in Earth and Planetary Science Letters. 2 equilibrated in each impact, a significantly higher degree of metal equilibration than previously reported for the Earth. In agreement with previous studies, the modeled Martian core is rich in sulfur (18-19 wt%), with less than one weight percent O and negligible Si. We have used these core and mantle compositions to produce physical models of the presentday Martian interior and evaluate the sensitivity of core radius to crustal thickness, mantle temperature, core composition, core temperature, and density of the core alloy. Trade-offs in how these properties affect observable physical parameters like planetary mass, radius, moment of inertia, and tidal Love number k2 define a range of likely core radii: 1620-1870 km. Seismic velocity profiles for several combinations of model parameters have been used to predict seismic body-wave travel times and planetary normal mode frequencies. These results may be compared to forthcoming Martian seismic data to further constrain core formation conditions and geophysical properties.

Timing of Martian Core Formation from Models of Hf–W Evolution Coupled with N-body Simulations

“…There is a limited range (approximately IW-1.35 to IW-1.10) in which any analogs match Mars, regardless of other parameter values; even modestly 322 more reducing conditions result in a wide distribution of analog properties that extends to high 323 f Hf/W and ε182W, overshooting Mars. Fortunately, the average fO2 of core formation on Mars is constrained by its mantle FeO content (i.e., Equation 3), and previous studies have shown that the FeO-derived fO2 of Mars agrees with the permissible range found here (e.g., Righter & Drake, 1996;Rai & van Westrenen, 2013;Rubie et al, 2015;Brennan et al, 2020). Brennan et al (2020) also used mantle trace elements to constrain the values of Pfrac (0.4-0.6), kcore (0.84-1.0), and kmantle (0.4-1.0).…”

“…Note that this approach is not self-consistent regarding the number of O atoms present in each protoplanet, but this is a negligible effect due to the lithophile character of O in Mars-sized bodies (e.g., Rubie et al, 2004;Steenstra & van Westrenen, 2018;Brennan et al, 2020). Ni was partitioned identically to Fe ( Ni = Fe ), S was approximated as perfectly siderophile, all other major elements (plus Hf) were assumed to be perfectly lithophile, and W partitioned between the core and mantle with its partition coefficient calculated as:…”

Equation of State of TiN at High Pressures and Temperatures: A Possible Host for Nitrogen in Planetary Mantles

“…If TiN is petrologically stable in regions of Earth's mantle, it would not be a buoyant phase, although it may be subject to entrainment (Lin & van Keken, 2006). We also compare the density of TiN to proposed density profiles for Venus (Aitta, 2012) and Mars (Brennan et al, 2020), finding that it is always denser than the mantles of both planets but less dense than their cores (Figure 4a). Compared to a selection of other naturally occurring nitrides (Fe 7 N 3 and Si 3 N 4 ), we find that TiN has an intermediate compressibility at room temperature (Figure 4b).…”

“…(a) TiN along a geotherm (J. Brown & Shankland, 1981) is significantly denser than Earth's mantle (Dziewonski & Anderson, 1981), as well as the mantles of Mars and Venus (Aitta, 2012; Brennan et al., 2020). TiN is less dense than planetary cores, however.…”

Mantle–melt partitioning of the highly siderophile elements: New results and application to Mars

“…Highly and moderately siderophile elements (HSE and MSE, respectively) and Os isotopes are used to constrain processes such as accretion, mantle differentiation, crustal recycling, and core-mantle mixing, and the timing and depth of differentiation of Mars (Brandon et al, 2000;Walker 2009;Righter et al 2015). Righter et al (2015), Righter and Chabot (2011), Brennan et al (2020), Rai and van Westrenen (2013), and Yang et al (2015) showed that the MSE, HSE, and volatile contents of the Martian mantle (Brandon et al 2012;Yang et al 2015) could have been established by deep metal-silicate equilibrium in early Mars (Fig. 1).…”