New Evidence for Wet Accretion of Inner Solar System Planetesimals from Meteorites Chelyabinsk and Benenitra

Abstract

We investigated the hydrogen isotopic compositions and water contents of pyroxenes in two recent ordinary chondrite falls, namely, Chelyabinsk (2013 fall) and Benenitra (2018 fall), and compared them to three ordinary chondrite Antarctic finds, namely, Graves Nunataks GRA 06179, Larkman Nunatak LAR 12241, and Dominion Range DOM 10035. The pyroxene minerals in Benenitra and Chelyabinsk are hydrated (∼0.018–0.087 wt.% H2O) and show D-poor isotopic signatures (δDSMOW from −444‰ to −49‰). On the contrary, the ordinary chondrite finds exhibit evidence of terrestrial contamination with elevated water contents (∼0.039–0.174 wt.%) and δDSMOW values (from −199‰ to −14‰). We evaluated several small parent-body processes that are likely to alter the measured compositions in Benenitra and Chelyabinsk and inferred that water loss in S-type planetesimals is minimal during thermal metamorphism. Benenitra and Chelyabinsk hydrogen compositions reflect a mixed component of D-poor nebular hydrogen and water from the D-rich mesostases. A total of 45%–95% of water in the minerals characterized by low δDSMOW values was contributed by nebular hydrogen. S-type asteroids dominantly composed of nominally anhydrous minerals can hold 254–518 ppm of water. Addition of a nebular water component to nominally dry inner solar system bodies during accretion suggests a reduced need of volatile delivery to the terrestrial planets during late accretion.

Reduced Late Bombardment on Rocky Exoplanets around M Dwarfs

“…The redox state of planetesimals and planetary embryos, usually measured by the FeO concentration in a sample, is crucially affected by the abundance of water, which can raise the overall oxidation state of planetary materials by reaction with other compounds (Elkins-Tanton & Seager 2008a, 2008bLichtenberg et al 2022). The increased oxidation state of magmatic iron meteorites (Bonnand & Halliday 2018;Hilton et al 2022), evidence for degassing from their parent planetesimals (Lichtenberg et al 2021b;Hirschmann et al 2021), fluid flow (Lewis & Jones 2016) and hydrogen incorporation (Piani et al 2020;Jin et al 2021) in ordinary and enstatite chondrites, accretion of water onto achondrites (Sarafian et al 2017) and the devolatilization trend in planetary materials (Wang et al 2019a(Wang et al , 2019b, both in carbonaceous and noncarbonaceous meteorites (Alexander 2019a(Alexander , 2019b, all suggest a substantial initial abundance of highly volatile elements (H, C, N) in inner planetary systems during the disk phase. In addition, uncertainties in high-pressure metal-silicate partitioning allow the earliest accretion phase to be dominated by oxidized materials, fulfilling present-day constraints on Earth's mantle composition by a transition from oxidized to reduced (Siebert et al 2013;Huang et al 2020).…”

“…Only for G dwarfs, such as the Sun, do appreciable quantities of late bombardment hit potentially habitable planets beyond the pre-main-sequence phase. Stephant et al 2021;Jin et al 2021), and somewhat wetter but volatile depleted (0.1 wt%) outside. Carbonaceous asteroids hosting a few wt% of water on average (Alexander et al 2018;Alexander 2019b) motivate our outer compositions.…”

Reduced late bombardment on rocky exoplanets around M-dwarfs

“…The redox state of planetesimals and planetary embryos, usually measured by the FeO concentration in a sample, is crucially affected by the abundance of water, which can raise the overall oxidation state of planetary materials by reaction with other compounds (Elkins-Tanton & Seager 2008a,b;Lichtenberg et al 2022). The increased oxidation state of magmatic iron meteorites (Bonnand & Halliday 2018;Hilton et al 2022), evidence for degassing from their parent planetesimals (Lichtenberg et al 2021b;Hirschmann et al 2021), fluid flow (Lewis & Jones 2016) and hydrogen incorporation (Piani et al 2020;Jin et al 2021) in ordinary and enstatite chondrites, accretion of water onto achondrites (Sarafian et al 2017) and the devolatilization trend in planetary materials (Wang et al 2019a,b), both in carbonaceous and non-carbonaceous meteorites (Alexander 2019a,b) initial abundance of highly volatile elements (H, C, N) in inner planetary systems during the disk phase. In addition, uncertainties in high-pressure metal-silicate partitioning allow the earliest accretion phase to be dominated by oxidized materials, fulfilling present-day constraints on the Earth's mantle composition by a transition from oxidized to reduced (Siebert et al 2013;Huang et al 2020).…”

“…We discuss the physical motivation for each model and the results of the respective bombardment simulations below. of the Solar System after the gas disk phase: relatively dry, volatile-poor (0.001 wt%) bodies inside the asteroid belt (Jin & Bose 2019;Alexander 2019a;Piani et al 2020;Jin et al 2021;Stephant et al 2021), and somewhat wetter but volatile-depleted (0.1 wt%) outside. Carbonaceous asteroids hosting a few wt% of water on average (Alexander et al 2018;Alexander 2019b) motivate our outer compositions.…”

An experimental study of proton implantation in olivine

“…When dusts in the proto-solar disk are irradiated by the solar wind, their compositions will be modi ed, and consequently the composition of planets made from the dusts will also be modi ed by the solar wind. Recently studies on an asteroid (Itokawa; Daly et al 2021) and some meteorite (Jin et al 2021) suggested a role of the solar wind on the water (hydrogen) content in these materials and possible contribution of the solar wind as a source of water on Earth.…”