ORCID

http://orcid.org/0000-0001-7758-9392

Date of Award

Spring 5-15-2020

Author's School

Graduate School of Arts and Sciences

Author's Department

Earth & Planetary Sciences

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Asteroids and Kuiper belt objects are for the most part the remnant planetesimals that have not been able to form into planets. These small bodies’ evolutionary pathways encode information from the early days of the Solar System. Recent space missions such as Dawn and New Horizons have now made detailed geological investigations of small bodies (i. e. , Ceres, Vesta, and Arrokoth) possible. In this thesis, I use geophysical modeling to study small body internal structure and to investigate spin history under the influence of impacts. I analyze the gravity and shape data obtained by Dawn of Ceres and make interpretations on Ceres’ internal density variations, and then expand my geophysical analysis to a comprehensive study of the possible spin evolution of Ceres (a dwarf planet), Vesta (a large asteroid), and Arrokoth (a small Kuiper belt object). Chapter 1 briefly introduces the background of the studied Solar System small bodies in this thesis – Ceres, Vesta, and Arrokoth, with an emphasis on their surface geology/mineralogy, internal structure, and rotational dynamics. I start with discussing geophysical observations of these bodies by their respective spacecraft missions (Dawn and New Horizons) and explain the importance of investigating these worlds in the context of understanding the formation and evolution of the Solar System. Then I focus on the dynamics of spin evolution due to various mechanisms, but ultimately focus on impacts. Chapter 2 analyzes Ceres’ observed degree-2 zonal gravity (J2) and topography obtained by Dawn. The gravity field outside a celestial body can be carefully reconstructed by Doppler tracking the velocity changes of an orbiting spacecraft. For Dawn, the onboard radio tracking system allows precise measurement of its location and velocity with respect to Ceres. The gravity data can then be used to inversely determine the internal structure of this dwarf planet, within certain limits. In order to interpret Dawn’s gravity measurements, I model the shape of Ceres as a two-layer oblate spheroid in hydrostatic equilibrium and calculate its corresponding J2. I confirm that Ceres is a differentiated object with its core density analogues to low density carbonaceous chondrites. Comparing the hydrostatic prediction and the actual observation by Dawn, I find Ceres’ degree-2 gravity is non-hydrostatic at the 10% level. That is, Ceres’ external geoid, derived from its observed degree-2 and degree-4 gravity, is less oblate than its shape; equivalently, Ceres possesses a non-trivial equatorial bulge of 2. 5 km. To rationalize Ceres’ observed J2 and this equatorial bulge, I propose two explanations to reconcile Ceres’ observed J2 and shape. Assuming Ceres’ observed shape is hydrostatic, its current J2 can be explained by a rotation rate that is 7 ± 4% faster than today. The subsequent despinning of Ceres, under this scenario, can be rationalized by one large impact or many smaller ones. Otherwise, Ceres may have deep interior mass anomaly now, which may contribute to its degree-2 zonal non-hydrostatic topography. Via negative admittance at this wavelength, Ceres’ observed J2 implies about 2. 0 ± 1. 2 km of non-hydrostatic zonal topography. Eventually, I conclude that both faster paleospin and deep mass anomaly may act together to reconcile Ceres’ observed J2 and shape.

Language

English (en)

Chair and Committee

William B. McKinnon

Committee Members

Bradley L. Jolliff, Henric S. Krawczynski, Viatcheslav S. Solomatov, Michael E. Wysession,

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