Physicochemical Controls on the Compositions of the Earth and Planets

TL;DR

The study tackles why Earth and other terrestrial planets differ chemically from canonical solar nebula condensates by integrating disk thermodynamics, nebular condensation chemistry, and isotopic provenance. It demonstrates that nebular condensation alone cannot account for the Earth’s bulk Fe/O and volatile inventories, implicating entropy-driven mixing of non-chondritic components and/or post-nebular processes. A central theme is the NC–CC dichotomy, with Earth and Mars showing signatures consistent with inner-disk NC material and limited CI-like contributions, while Moon and Vesta reflect more volatile-depleted, oxidised histories. The findings illuminate the role of disk evolution, oxidation state changes, and selective transport in shaping planetary compositions, offering a framework to interpret exoplanetary systems and guiding future Mercury- and exoplanet studies.

Abstract

Despite the fact that the terrestrial planets formed from the protoplanetary disk, their compositions show marked departures from that of solar nebula condensates. Metallic cores fix oxygen fugacities ($f$O$_2$s) of the planets to 5 (Mercury) to 1 log units below the iron-wüstite (IW) buffer, orders of magnitude higher than the nebular gas. Their oxidised character is coupled with a lack of volatile elements with respect to the solar nebula. Condensates from a solar gas at different temperatures ($T_0$) have Fe/O (by mass) of 0.93 ($T_0$ = 1250 K) to 0.81 ($T_0$ = 400 K), far lower than that of Earth (1.06). Because the reaction Fe(s) + H2O(g) = FeO(s) + H2(g) proceeds <600 K, temperatures at which most moderately volatile elements (MVEs) have condensed, oxidised planets should be volatile-rich, and vice-versa. That this is not observed suggests that planets did not accrete from equilibrium nebular condensates and/or underwent additional volatile depletion/$f$O$_2$ changes. Indeed, MVEs in small telluric bodies (Moon, Vesta) indicate near equilibrium evaporation/condensation at IW-1 and 1400-1800 K. Volatile-depleted elemental yet near-chondritic isotopes of larger telluric bodies (Earth, Mars) reflect mixing of bodies of variable volatile depletion, overprinted by volatile-undepleted material. From the Cr- and Ti isotopes in the BSE, such undepleted matter has been proposed to be CI chondrites. 6% CI added late to an enstatite chondrite-like proto-Earth would match the Earth. However, because Earth is an end-member in isotopic anomalies of heavier elements, no combination of existing meteorites alone can account for its chemical- and isotopic composition. Instead, the Earth is made partially or essentially entirely from an NC-like missing component. If so, the oxidised-, yet volatile-poor nature of inner solar system bodies, including Earth and Mars, is intrinsic to the NC reservoir.

Figures (21)

• Figure 1: Oxygen fugacities, with respect to the iron-wüstite (IW) buffer oneill_pownceby1993 of the planets as a function of a) heliocentric distance (in AU) and b) the log$_{10}$ of their masses. Also shown is the $f$O$_2$ defined by a solar gas at 1300 K and 10$^{-4}$ bar grossman2008redox. Source data is shown in Table \ref{['tab:chemphys_prop']}.
• Figure 2: The Mg # of differentiated (black) and undifferentiated (grey) bodies as a function of their total iron contents. Source data is from wassonkallemeyn1988 for chondritic meteorites.
• Figure 3: The K/U and Rb/Sr ratios, normalised to those in CI chondrites, of differentiated planetary bodies. References: Angrite parent body - dauphas2022alkali, Vesta - sossi2022stochastic, Moon - sossi2024moon, Mars and Earth - dauphas2022alkali.
• Figure 4: Radial distribution of a) temperature in Kelvin and b) pressure in bar in the midplane of the disk for different stellar mass accretion rates ($\dot{M}$) in solar masses/yr. Here $\alpha= 0.0054$. The temperature of the snowline (i.e., the temperature at which the reaction H$_2$O(g) = H$_2$O(s) proceeds to the right) is roughly between 220 -- 150 K, depending on pressure. NB: Temperatures and pressures inward of 1 au are extrapolated. Constructed with data from bitsch2015structure.
• Figure 5: A schematic illustration of the radial motion of the evaporation/condensation front of gas and dust. The top panel shows the situation prevailing in a continuous disk. The radial motion of the gas (mostly hydrogen) towards the central star (dashed black arrows) is faster than the radial displacement of the evaporation/condensation front (thick blue line) that is due to disk cooling over time (cf. eq. \ref{['eq:hartmann_time_acc']}). The dust moves towards the star (solid red arrows) even faster than does the gas. When crossing the evaporation front, the released vapour then moves inward (red dashed arrows) at the same speed as the gas. As a consequence, the vapour/hydrogen ratio is increased with respect to the its initial ratio (i.e. the stellar abundance). Indeed, the red dashed arrows are denser in space than the black dashed lines, which indicates an enhanced density of the evaporated species. The bottom panel shows the case where the dust is trapped at some location beyond the evaporation front, due to the formation of a pressure maximum. In this case, the dust does not reach the evaporation front. Therefore, even if the disk cools, there is no further condensation possible.