A holistic approach to understand Earth formation

  grant N. 101019380



The first results of the project start to come in, revealing important constraints on the formation of Earth, but also new conundrums that we will have to address.

Burkhardt et al. Terrestrial planet formation from lost inner solar system material (Science advances, 2021) analyzes the isotopic dichotomy of meteorites using the largest possible number of elements and shows that there is no common trend of non-carbonaceous (NC) meteorites towards carbonaceous (CC) meteorites, unlike what had been originally proposed (e.g. Schiller et al., 2018) by looking at a more restricted sample of isotopes, like Ca, Ti and Cr only. Also, the Earth does not appear intermediate between the NC and CC group. These measurements constrain the amount of CC material accreted by the Earth (and equivalently by Mars). This must be less than 10% by mass, with a more probable fraction of 4%.

Steller et al. Nucleosynthetic zinc isotope anomalies reveal a dual origin of terrestrial volatiles (Icarus, 2022) reports measurements of the zinc (Zn) isotope ratios 66Zn/64Zn and 68Zn/64Zn in meteorites and Earth.  It shows that CC and NC meteorites exhibit distinct isotopic ratios, which until now had only been shown for several non-volatile elements such as Ti, Cr, Ca, Mo, but never for a moderately volatile element such as Zn. This result is important because it shows that the NC-CC isotopic dichotomy is unlikely to have been caused by the condensation/evaporation of different chemical carriers, given that it holds for elements with vastly different condensation temperatures, ranging from ~700K (Zn) to ~1,600K (Ca, Ti, Mo). The other important result is that Earth, which is similar to NC meteorites for all non-volatile element isotopes, is found to be intermediate between NC and CC meteorites in Zn isotopes. This result demonstrates that 30% of terrestrial Zn was acquired from the accretion of CC material, the rest being acquired from NC materials. Because CC meteorites are richer in Zn and other volatile elements than NC meteorites, this implies that our planet accreted 5-6% of its bulk mass from CC material, which is consistent with the results of Burkhardt et al. (2021). The small amount of CC material accreted by Earth rules out the hypothesis that the terrestrial planets may have grown substantially by pebble accretion, because pebbles would have drifted from the outer disk, carrying a strong CC signature.

The inefficiency of pebble accretion in terrestrial planet formation is explained in Batygin and Morbidelli A Self-Consistent Model for Dust-Gas Coupling in Protoplanetary Disks (Astronomy & Astrophysics, 2022). This work shows that the small size of rocky grains and the high density of gas in the inner solar system result in pebbles having a very small Stokes number, which implies that the pebble layer is almost as vertically thick as the gas-disk. Pebble accretion is very sensitive on the thickness of the pebble layer. In particular, in the 3D regime, where the thickness of the layer is larger than the Bondi or Hill radius of the growing planet, pebble accretion becomes very slow. Collisions among local planetesimals thus become the dominant growth process for rocky planets (in contrast with the outer disk, beyond the snowline, where pebble accretion dominates and can produce the massive cores of giant planets). Terrestrial planet growth from local planetesimals is consistent with Earth having predominantly a NC composition, as revealed by the aforementioned isotopic analyses. 

Morbidelli et al. Contemporary formation of early solar system planetesimals at two distinct radial locations (Nature Astronomy, 2022) shows that the first planetesimals, related to the iron meteorite parent bodies, may have formed at the sites of the snowline and the silicate sublimation line, at about 5 and 1 au respectively. Assuming that the disk exhibits an isotopic radial gradient due to the accretion of distinct materials at different times (as proposed by Nanne et al., 2019), the formation of early planetesimals at two distinct sites explains the isotopic dichotomy of iron meteorites, revealed in Kruijer et al. (2017). The formation of one of the two groups of planetesimals at the snowline also explains why the cores of the iron meteorite parent bodies isotopically related to carbonaceous chondrites (CC) had a smaller core than those related to non-carbonaceous chondrites (NC), which suggest that the former were more oxidized (i.e. contained more water).

The formation of early planetesimals in a ring around the silicate sublimation line at first sight seems to give strong support to the idea that terrestrial planets fromed from a ring of small objects (Hansen, 2009; Nesvorny et al., 2021). However, a few problems arise.

First, in Woo et al. Terrestrial planet formation from a ring (in preparation, preliminary results prsented at 2022 DPS meting) we simulate self-consistently the formation of planetary embryos from a ring of planetesimals during the gas-dominated era of the disk. We use the code GENGA, which allows simulating the mutual interactions and collisions among the planetesimals. We account for gas drag and migration forces acting on planetesimals and growing embryos. We find that, while they form, the planetary embryos spread radially under the effect of mutual scattering, dynamical friction and eccentricity damping from the disk of gas. At the end of the gas-disk lifetime we obtain a large number of embryos whose orbits are so radially separated from each other that no further collisions occur, even during the giant planet instability. Thus, these simulations fail forming Earth and Venus but lead to a spread-out systems of multiple small planets. Planetesimal formation in a ring is therefore not a sufficient condition for the successful formation of the terrestrial solar system planets. We show that the formation of Earth and Venus analogues can be obtained if the gas surface density distribution is sufficiently peaked near 1 au, causing convergent migration of the embryos. Convergent migration has been already invoked to explain terrestrial planet formation starting from a radially extended disk of planetesimals or embryos (Ogihara et al., 2018; Broz et al.). In Woo et al. we show that it is also needed in the planetesimal-ring hypothesis.  

Second, in Sossi et al. Stochastic accretion of the Earth (Nature Astronomy, 2022) we show that Earth should have accreted a variety of planetesimals, formed at different temperatures in the protoplanetary disk. These planetesimals should have had a step-like volatile depletion pattern, like asteroid Vesta, i.e. they should have been strongly depleted in all elements that are gaseous at the temperature at which they formed. The combination of objects depleted of volatiles at different temperatures produced the smooth volatile depletion pattern characteristic of our planet. This result suggests that Earth accreted planetesimals from a wide range of heliocentric distances, so to probe a variety of formation temperatures, in contrast with the idea that it formed from a narrow ring. A potential solution of this conundrum is that the temperature characterizing the volatile depletion pattern of each planetesimal was determined by its internal thermal evolution and not by its radial location in the disk where it formed. In this case, even a planetesimal population formed in a narrow ring could exhibit a variety of volatile-depletion temperatures and produce a planet with a smooth volatile depletion pattern as Earth. However, volatile-rich and volatile-poor planetesimals would be accreted at random times, in contrast with the constraint coming from the Pd-Ag radiogenic system indicating that volatile-rich bodies have been accreted towards the end of Earth's growth history (Schönbächler et al., 2010). 

Thus identifying how the terrestrial planets formed in a way that satisfies all available constraints remains problematic. The idea that planetesimals existed only in a narrow ring is too radical. Our next efforts will focus in constraining the radial mass distribution in the disk, using both ab-initio planetesimal formation models and terrestrial planet formation simulations.

Please visit the publication page to access the papers from our group described here.