The Psyche Light Elements Investigation

Abstract

Light elements, such as C, S, Si, O, C, and H, are thought to be present in Earth's liquid-Fe outer core. These elements lower melting temperatures, thereby allowing the core to remain in liquid state at high pressure and influencing magnetic and geodynamic processes. However, the identity and abundance of the light elements in the cores of terrestrial planets and how they were delivered to these cores is not well known. The NASA Psyche mission will travel to and explore (16) Psyche, which may be the metal-rich core of a differentiated planetesimal exposed by collisional stripping. If so, the Psyche mission could provide a direct assessment of the light element content of an asteroidal core, allowing comparisons to the inferred composition of planetary cores and the parent bodies of the magmatic iron group meteorites. In particular, Earth's high-pressure core formed gradually (over ∼100 Myr), in a multistage process, under increasingly oxidizing conditions, whereas the cores of planetesimals formed quickly (within 10 Myr) at low pressure, likely in chemical equilibrium with their mantles. The trace element systematics and mineral composition of magmatic iron meteorites indicate the presence of C, P, and S in planetesimal cores prior to solidification. Such elements would have played a role in core dynamics, including dynamo generation. Their low solubility combined with the immiscibility of their mineral precipitates would have resulted in their separation from Fe upon crystallization and their eruption onto the surface of a stripped core (via ferrovolcanism). The Psyche spacecraft will detect their elemental, mineral, and magnetic signatures with the payload instruments, which include a Gamma Ray and Neutron Spectrometer, a Multispectral Imager, and a Magnetometer. Additional constraints on interior composition and processes influenced by light elements will be provided by Psyche's gravity and geomorphology investigations. We provide a brief introduction to the topic of light elements along with prospects for (16) Psyche. While we emphasize core formation processes, we also consider other possibilities for the origin and evolution of this metal-rich body.

Keywords: Asteroid (16) Psyche; Ferrovolcanism; Iron meteorites; Light elements; Metal-rich chondrite meteorites; NASA Psyche mission; Planetary cores.

Fig. 1

Comparison of sulfur and Ni contents in planetary cores. Estimates include: the liquid outer core of Earth by Hirose et al. (2021); Earth’s bulk core (Fischer and McDonough 2025); two recent estimates of the martian core (Yoshizaki and McDonough ; Khan et al. 2022); the molten cores of the parent bodies of magmatic iron meteorites (IIIAB iron core labeled) (Chabot and Zhang ; Zhang et al. 2024); (4) Vesta’s core based on geochemical models, constraints from HED meteorites, and data acquired by Dawn (Toplis et al. 2013); a rough estimate for (16) Psyche’s core from (this study, see the Appendix and Fig. 4); and bulk metal+sulfide of different chondrite types (CB chondrites labeled) (Jarosewich 1990, 2006). (The chondrites are effectively plotted where cores on their bodies would plot had they differentiated; they are thus directly comparable to the iron meteorite and planetary core compositions shown, but different than where bulk chondrites would plot.) The grey band on the (16) Psyche estimate is the uncertainty derived from the modeling done here. The magmatic iron meteorites are thought to have formed as molten materials cooled within their parent bodies, allowing equilibration with their silicate mantles. This process likely occurred on (4) Vesta; however, (16) Psyche’s remnant core may have cooled in the absence of a mantle following collisional stripping of the mantle (e.g., Elkins-Tanton et al. 2020, 2022), possibly influencing the light element composition of the liquid. Differences between the light element content of Earth, which is thought to include S, Si, O, C, and H, and the cores of magmatic planetesimals, known to include S, O, C, and P from the iron meteorites, are a result of many factors, including accreted materials, the complexity of core-mantle evolution, and the pressure, temperature, and redox conditions during core growth. Abbreviations: CC – carbonaceous chondrites; EC – enstatite chondrites; OC – ordinary chondrites; CC irons – magmatic iron meteorites of the carbonaceous isotopic cluster; NC irons – magmatic irons of the noncarbonaceous isotopic cluster. For the chondrites, values are wt% of the metal+sulphide system, not of the bulk chondrite

Fig. 2

Comparison of trace element systematics for exemplar magmatic (IIIAB) and non-magmatic (IIE) iron meteorite groups. A simple fractional crystallization model based on experimentally determined partition coefficients for the pure Fe-Ni system (Chabot et al. 2017) cannot reproduce the IIIAB iron trend. When partition coefficients that consider the effects of S and P are used in the model, the IIIAB data can be moderately well fit. IIIAB data and model are from Chabot and Zhang (2022); IIE data are from Malvin et al. (1984), Wasson and Wang (1986) and Wasson et al. (1989)

Fig. 3

(4) Vesta from the outside in. On the left, the surface morphology observed by Dawn’s Framing Camera is shown. Superimposed in blue is the distribution of H determined by Dawn’s Gamma Ray and Neutron Detector (Prettyman et al. 2012), which is similar to the distribution of hydrated minerals found by Dawn’s Visible to Infrared Mapping Spectrometer (De Sanctis et al. 2012) and low albedo regions on (4) Vesta’s surface (McCord et al. ; Jaumann et al. 2014). The average concentration of H was 200 $\mu$g/g, which implies accreted carbonaceous chondrite materials comprise less than about 5% of (4) Vesta’s crust (Mittlefehldt and Prettyman 2025). Otherwise, the elemental composition and mineralogy of (4) Vesta’s surface is consistent with that of howardite, which consists of igneous minerals, including Fe-rich pyroxene and Na-poor plagioclase (e.g., Ammannito et al. 2013). On the right is the interior structure and composition inferred from gravimetry and thermodynamic modeling (after Toplis et al. 2013). (4) Vesta’s original basaltic crust was assumed to be derived from melts with the same composition as the primitive, main group eucrite, Juvinas. The current crust is a complex mix of basalts and cumulative lithologies. The mantle consists of olivine and pyroxene, and the core is assumed to consist of Fe,Ni metal surrounded by a metal-sulfide cotectic mix

Fig. 4

The ternary plot shows the posterior distribution of parameters used to model Psyche’s density. Bayesian inversion with Markov Chain Monte Carlo (MCMC) was used to fit the density of (16) Psyche given prior information from spectral reflectance observations and meteorite studies. The MCMC samples are shown as points, with sample density indicated by contours. The density of the asteroid was modeled as mixture of core material (Fe,Ni metal and troilite), lower mantle material (olivine), and void (porosity). The Ni fraction is allowed to vary, but was limited to <30 wt.%, consistent with the range for iron meteorites. The non-metal component is a mixture of troilite, the only S-bearing mineral in our model, and silicates. Silicates are modeled as a mixture of forsterite (Mg-olivine) and fayalite (Fe-olivine). The sampled points yield densities consistent with the most recently reported density of asteroid (16) Psyche using high-precision astrometry of 4172 ± 145 kg/m³ (Farnocchia et al. 2024). The sampled distribution is consistent with the schema presented by Elkins-Tanton et al. (2020). Marginal distributions for each parameter are shown in the Appendix and the 16th, 50th, and 84th percentiles are used to estimate parameters and uncertainties. This approach gives, 0.35 ± 0.06, ${0.32}_{-0.10}^{+0.11}$, and ${0.31}_{-0.06}^{+0.05}$, respectively, for the volume fraction of metal, nonmetal, and void. The estimated sulfur content of the asteroid is ${7.2}_{-3.0}^{+3.7}$ wt.%. Pure troilite was not sampled (36 wt.% S)

Fig. 5

Section of the IIC iron meteorite Ballinoo (image courtesy Smithsonian National Museum of Natural History). The section includes large, troilite nodules (medium yellowish grey inclusions, lower right) that precipitated as the molten Fe,Ni alloy crystallized. The dark circular feature at upper left is a hole in the sample. The dimensions of the section are 19 × 9 cm as listed in Buchwald (1975)

Fig. 6

Corner plot summarizing MCMC samples of the non-metal, metal, and porosity fractions and their correlations. The MCMC samples are the individual points in the correlation charts below the diagonal. The diagonal contains the marginal distributions for the parameters, which we use to estimate their values. Each marginal distribution is marked with the 16th (triangle down), 50th (diamond), and 84th (triangle up) percentiles. The range between the 16th and 84th percentiles provides a measure of uncertainty, with the median taken to be a representative value. Parameter estimates, which appear above each marginal distribution, use the standard notation for asymmetric errors, with the upper uncertainty given by the difference between the 84th percentile and the median, and the lower uncertainty given by the difference between the median and the 16th percentile

Fig. 7

Marginal distribution for the mass fraction of S (from FeS) in the non-porous asteroid material. The marginal distribution of S is marked with the 16th (triangle down), 50th (diamond), and 84th percentiles (triangle up). The range between the 16th and 84th percentiles, which we use as an estimate of uncertainty, is 4.2 wt.% to 11 wt.% with the median of 7.2 wt.% taken to be a representative value