Mercury as the Relic of Earth and Venus Outward Migration

In spite of substantial advancements in simulating planet formation, the planet Mercury’s diminutive mass and isolated orbit and the absence of planets with shorter orbital periods in the solar system continue to befuddle numerical accretion models. Recent studies have shown that if massive embryos (or even giant planet cores) formed early in the innermost parts of the Sun’s gaseous disk, they would have migrated outward. This migration may have reshaped the surface density profile of terrestrial planet-forming material and generated conditions favorable to the formation of Mercury-like planets. Here we continue to develop this model with an updated suite of numerical simulations. We favor a scenario where Earth’s and Venus’s progenitor nuclei form closer to the Sun and subsequently sculpt the Mercury-forming region by migrating toward their modern orbits. This rapid formation of ∼0.5 M ⊕ cores at ∼0.1–0.5 au is consistent with modern high-resolution simulations of planetesimal accretion. In successful realizations, Earth and Venus accrete mostly dry, enstatite chondrite–like material as they migrate, thus providing a simple explanation for the masses of all four terrestrial planets, the inferred isotopic differences between Earth and Mars, and Mercury’s isolated orbit. Furthermore, our models predict that Venus’s composition should be similar to the Earth’s and possibly derived from a larger fraction of dry material. Conversely, Mercury analogs in our simulations attain a range of final compositions.

Chromium Stable Isotope Panorama of Chondrites and Implications for Earth Early Accretion

We investigated the stable isotope fractionation of chromium (Cr) for a panorama of chondrites, including EH and EL enstatite chondrites and their chondrules and different phases (by acid leaching). We observed that chondrites have heterogeneous δ 53Cr values (per mil deviation of the 53Cr/52Cr from the NIST SRM 979 standard), which we suggest reflect different physical conditions in the different chondrite accretion regions. Chondrules from a primitive EH3 chondrite (SAH 97096) possess isotopically heavier Cr relative to their host bulk chondrite, which may be caused by Cr evaporation in a reduced chondrule-forming region of the protoplanetary disk. Enstatite chondrites show a range of bulk δ 53Cr values that likely result from variable mixing of isotopically different sulfide-silicate-metal phases. The bulk silicate Earth (δ 53Cr = –0.12 ± 0.02‰, 2SE) has a lighter Cr stable isotope composition compared to the average δ 53Cr value of enstatite chondrites (–0.05 ± 0.02‰, 2SE, when two samples out of 19 are excluded). If the bulk Earth originally had a Cr isotopic composition that was similar to the average enstatite chondrites, this Cr isotope difference may be caused by evaporation under equilibrium conditions from magma oceans on Earth or its planetesimal building blocks, as previously suggested to explain the magnesium and silicon isotope differences between Earth and enstatite chondrites. Alternatively, chemical differences between Earth and enstatite chondrite can result from thermal processes in the solar nebula and the enstatite chondrite-Earth, which would also have changed the Cr isotopic composition of Earth and enstatite chondrite parent body precursors.

A magma ocean origin for Mercury's earliest exosphere

<p>MESSENGER observations used to constrain surface composition suggest a global magma ocean formed on early Mercury [1, 2]. Our study models the coupled evolution of the Hermean magma ocean and atmosphere, and determines the extent of exospheric loss from an atmosphere formed by evaporation of a magma ocean. Using our framework to couple the interior and exterior chemical reservoirs, we evaluate a range of possible atmospheric evolution scenarios for early Mercury. These include the possibility that Mercury was fully-accreted before a mantle stripping event caused by a giant impact [3] led to degassing, or alternatively that the building blocks of Mercury were already relatively volatile-free.</p> <p>Assuming an initial surface temperature of 2500 K and an oxygen fugacity fixed at 1 log unit below the Iron-Wüstite buffer, we find that the Hermean magma ocean cooled to 1500 K (the rheological transition) in around 400 to 9000 yrs, depending on the efficiency of radiative heat transfer in the atmosphere. We investigate the behaviour of two endmember cases: (1) a present-day sized Mercury that is volatile free and thus cools in a manner similar to that of a blackbody, and (2) a larger Mercury that is sufficiently volatile-rich to provide a greenhouse atmosphere that delays cooling. During the magma ocean stage, evaporation and sublimation of oxide components from the molten silicate magma contribute to the growth of the atmosphere, in addition to volatile outgassing. For the endmember case with initial volatile abundances based on enstatite chondrite-like precursors, the atmosphere is dominated by CO, CO<sub>2</sub>, H<sub>2</sub>, and H<sub>2</sub>O, with minor abundances of SiO, Na, K, Mg, and Fe gas species. For the endmember case without major volatiles (C, H), the atmospheric composition is dominated by metal- and metal oxide gas species only.</p> <p>We apply a Monte Carlo (MC) atmospheric loss model [4] to calculate exospheric losses for all pathways and find that photoionization is the dominant loss mechanism for early Mercury. Cases both with and without major volatiles reveal that the atmosphere lasts between 100 and 250 years before the near-surface of Mercury becomes solid. Preliminary MC results show that photo-dissociation considerably alters the atmospheric composition, efficiently breaking SiO<sub>2</sub> into SiO<sup>+</sup> and O. Solar wind and magnetospheric plasma leads to rapid evacuation of the ionized species from the neutral exosphere whereas oxygen is lost efficiently by Jeans escape.</p> <p>Using our coupled framework, future modelling efforts will aim to understand if and how the evaporation of the magma ocean of early Mercury has modified its surface composition, with a view to interpreting BepiColombo observations. Hence our work provides an important step to connect Mercury’s formation, earliest evolution, and upcoming observations.</p> <p>References: [1] Vander Kaaden, K. E. and McCubbin, F. M. (2016). Cosmochim. Ac., 173, 246–263. [2] Berthet, S., Malavergne, V., and Righter, K. (2009). Geochim. Cosmochim. Ac., 73(20), 6402–6420. [3] Benz, W., et al. (2008). Mercury, pp. 7–20. Springer. [4] Wurz, P., and Lammer, H. (2003). <em>Icarus</em>, <em>164</em>(1), 1–13</p> <p> </p> <p> </p>

Earth’s water may have been inherited from material similar to enstatite chondrite meteorites

The origin of Earth’s water remains unknown. Enstatite chondrite (EC) meteorites have similar isotopic composition to terrestrial rocks and thus may be representative of the material that formed Earth. ECs are presumed to be devoid of water because they formed in the inner Solar System. Earth’s water is therefore generally attributed to the late addition of a small fraction of hydrated materials, such as carbonaceous chondrite meteorites, which originated in the outer Solar System where water was more abundant. We show that EC meteorites contain sufficient hydrogen to have delivered to Earth at least three times the mass of water in its oceans. EC hydrogen and nitrogen isotopic compositions match those of Earth’s mantle, so EC-like asteroids might have contributed these volatile elements to Earth’s crust and mantle.

Partial melting of an enstatite chondrite at 1 GPa: Implications for early planetary differentiation

<p>We present new time series partial melting experiments performed on a natural enstatite chondrite (EL6), aimed at investigating the textural and geochemical changes induced by silicate-metal equilibration during early planetary differentiation. The starting material of our experiments consisted of small fragments (ca. 50 mg) obtained from the interior of the enstatite chondrite MCY 14005 (MacKay Glacier, Antarctica), collected during the XXX° Italian Expedition in Antarctica (PNRA). Experiments were performed in graphite capsules at a pressure of 1 GPa, at temperature ranging from 1100 to 1300 °C, with run durations from 1 to 24 h. The initial phase assemblage of the enstatite chondrite, mostly composed by granular enstatite and Fe-Ni metal (up to 400 µm in size) with minor amounts of sulphides and plagioclase, undergoes significant changes with increasing temperature and run duration. At 1100 °C, no silicate melt is produced and subsolidus reactions occur at the contact between the metal and silicate phases. At 1200 °C, small amounts of silicate melt are produced at the grain boundaries and enstatite grains in contact with the melt grow Fe-enriched rims. The metal portions are characterized by two immiscible liquid phases that exhibit rounded shapes when in contact with the silicate melt, whereas smaller (micrometric) liquid metal spheres occur isolated within the silicate melt throughout the experimental charges. These features are already observed in the 1 h experiment but become increasingly evident with increasing run duration, and at higher temperatures. In the experiments performed at 1300 °C, the amount of silicate melt increases and new silicate minerals form (olivine and low-Ca-pyroxene).</p><p>Enstatite chondrites are characterized by an oxygen isotope composition similar to that of the bulk Earth and Moon, and are considered to have initially formed in the terrestrial planetary zone of the solar nebula. For this reason, they represent a suitable material to investigate the early planetary differentiation processes that occurred in the proto-Earth system. Preliminary results from our experiments indicate that, at the investigated oxygen fugacity (1-2 log units below the IW buffer), the Fe-Si exchange between the metal and silicate phases allows the formation of silicate melt and silicate phases such as olivine and low-Ca-pyroxene. At the same time, the change in shape of the metal grains (increasingly circular/spherical with increasing temperature) and the overall reduction of their number density with increasing experimental time point to rapid aggregation of the metal phase and, possibly, to fast silicate-metal differentiation in small planetesimals.</p>