Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Geology and Geophysics
First Advisor
Bercovici, David
Abstract
Planetesimals and planetary embryos are building blocks of Earth-sized planets. The formation and evolution of these objects are partly recorded by remnants of the early solar system in the form of (primarily) asteroids and our sampling of them through astronomical observations, meteorite collections and some in situ space-craft measurements. To decipher the information recorded in these remnants is critical for understanding planet formation and early solar system processes. In this thesis, I develop theoretical models for the interiors of planetesimals and planetary embryos at various stages of formation and evolution, and combine the model results with meteorite and asteroid observations to constrain their formation. The decay of short-lived radio-isotope 26Al may cause melting in early-formed objects. In Mars-sized planetary embryos (with Mars-like alkaline-depleted bulk compositions), the liquid phase of partial melt and can migrate rapidly. This migration leads to efficient heat transport, and the mantle of a planetary embryo can stay effectively solid. The mostly solid mantle would have undergone substantial differentiation, and the disparate building blocks would not have been homogenized. Hence, very early silicate differentiation and significant nucleosynthetic isotope heterogeneity would be expected for stranded planetary embryos. As these features are not identified in Martian meteorites, we suggest that the accretion of Mars involved at least one collision between planetary embryos with comparable sizes that caused complete melting and homogenization. In hundred-kilometer-size planetesimals of primitive (alkaline-undepleted) bulk composition, low-degree silicate partial melts are of high viscosities. Melt percolation models suggest that these melts are too immobile to account for the formation of primitive achondrites (i.e., residual solid materials after extraction of low-degree partial melts from planetesimal mantle). Partially molten planetesimals may be shattered by collisions into rubble piles. I model melt migration in such rubble piles and suggest that melts can be squeezed into voids between rock boulders, where they can migrate rapidly for planetesimal-scale silicate differentiation. Therefore, primitive achondrites may record melt migration in partially molten rubble piles. The compositions and metallographic cooling rates of IVA iron meteorites suggest that they are derived from an inward solidifying metallic body with little or no silicate mantle, possibly the result of a mantle stripping collision. Paleomagnetic records suggest that their parent body possessed an internal magnetic field. This is paradoxical since inward solidification is unlikely to generate a magnetic field. I propose a solution to this puzzle by invoking a rubble-pile inner core that may have coalesced from cold collisional fragments after the mantle-stripping impact that formed the metal asteroid. The resulting cold inner core would have extracted heat from the overlying liquid, leading to solidification and light element release to drive convection and power a magnetic field, which was recorded in the cooling and thickening crust. In addition, the IVA iron meteorite observations suggest the existence of large metallic asteroids. Many M-type (metallic) asteroids have high radar albedos and have been thought to be metallic. However, this view has been challenged by the observations that the densities of M-type asteroids are typically around or less than half the density of iron. I develop models for cold compaction of rubble-pile bodies of different compositions. Applying the model for silicate/chondritic rubble piles, I suggest that the size-density correlation for S-type (stony) asteroids can be explained by cold compaction of rubble piles of ordinary chondrite-like boulders. Applying the model for metallic rubble piles, I suggest that they can preserve large (e.g., ~50%) porosities due to cold welding between metal boulders and the high yield strength of metal for either ductile or brittle-like deformation. This implies that M-type asteroids such as (16) Psyche and (216) Kleopatra may be purely metallic. These results may be tested by the Discovery Class NASA mission, Psyche.
Recommended Citation
Zhang, Zhongtian, "The formation and evolution of planetesimals and planetary embryos" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1163.
https://elischolar.library.yale.edu/gsas_dissertations/1163
