Astronomy
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Item Mechanical evolution of small solar system bodies(2023) Marohnic, Julian Charles; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a series of studies that develop and apply numerical modeling techniques to small bodies in the solar system. We are particularly interested in low-energy deformations, collisions, and disruptions, and our subjectsrange from near-Earth asteroids to Kuiper belt contact binaries in the farthest reaches of the solar system. We use the N-body code pkdgrav to investigate these processes and describe our significant additions to its capabilities. Our first subject is the Kuiper belt object Arrokoth. On January 1, 2019, the New Horizons spacecraft flew within 3,550 km of Arrokoth, returning the first in-situ images of a small body in the classical Kuiper belt. Arrokoth was found to be bilobate, with a distinctive contact binary structure. We use pkdgrav to investigate the origins of Arrokoth's striking shape and find that plausible formation mechanisms are quite limited. We rule out the possibility of a direct impact between two unbound objects and put forward an alternate scenario in which two cometesimals in a close, synchronous orbit gradually spiral in toward one another before meeting in a gentle merger. We conclude by exploring implications for the formation of small Kuiper belt objects more generally. Next, we describe our work modifying pkdgrav to accommodate non-spherical particles. Prior work in granular physics has established that particle shape is an important factor governing the behavior of granular bodies like small solar system objects. Irregular particles tend to interlock with one another, inhibiting bulk motion and adding to the shear strength of a medium. We adapt pkdgrav's existing soft-sphere, discrete element contact physics model to allow for modeling of non-spherical grains. We then apply this new capability in three, small-scale proof of concept studies of spin-up, tidal disruption, and the Brazil nut effect. We find a significant difference in behavior when comparing small rubble-pile bodies composed of spherical particles and those composed of non-spherical particles. Finally, we apply our newly-developed tools to a more comprehensive investigation of particle shape in tidal disruption simulations. We construct small rubble piles from a range of differently-shaped constituents and subject them to simulated tidal encounters with the Earth. We conduct a parameter sweep across different encounter geometries and constituent shapes and conclude that particle shape is a significant contributor to tidal encounter outcomes. The role of particle resolution is also investigated.Item WATER IN THE EARLY SOLAR SYSTEM: INFRARED STUDIES OF AQUEOUSLY ALTERED AND MINIMALLY PROCESSED ASTEROIDS(2017) McAdam, Margaret; Sunshine, Jessica M; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates connections between low albedo asteroids and carbonaceous chondrite meteorites using spectroscopy. Meteorites and asteroids preserve information about the early solar system including accretion processes and parent body processes active on asteroids at these early times. One process of interest is aqueous alteration. This is the chemical reaction between coaccreted water and silicates producing hydrated minerals. Some carbonaceous chondrites have experienced extensive interactions with water through this process. Since these meteorites and their parent bodies formed close to the beginning of the Solar System, these asteroids and meteorites may provide clues to the distribution, abundance and timing of water in the Solar nebula at these times. Chapter 2 of this thesis investigates the relationships between extensively aqueously altered meteorites and their visible, near and mid-infrared spectral features in a coordinated spectral-mineralogical study. Aqueous alteration is a parent body process where initially accreted anhydrous minerals are converted into hydrated minerals in the presence of coaccreted water. Using samples of meteorites with known bulk properties, it is possible to directly connect changes in mineralogy caused by aqueous alteration with spectral features. Spectral features in the mid-infrared are found to change continuously with increasing amount of hydrated minerals or degree of alteration. Building on this result, the degrees of alteration of asteroids are estimated in a survey of new asteroid data obtained from SOFIA and IRTF as well as archived the Spitzer Space Telescope data. 75 observations of 73 asteroids are analyzed and presented in Chapter 4. Asteroids with hydrated minerals are found throughout the main belt indicating that significant ice must have been present in the disk at the time of carbonaceous asteroid accretion. Finally, some carbonaceous chondrite meteorites preserve amorphous iron-bearing materials that formed through disequilibrium condensation in the disk. These materials are readily destroyed in parent body processes so their presence indicates the meteorite/asteroid has undergone minimal parent body processes since the time of accretion. Presented in Chapter 3 is the spectral signature of meteorites that preserve significant amorphous iron-bearing materials and the identification of an asteroid, (93) Minerva, that also appears to preserve these materials.Item Numerical Simulations of Granular Physics in the Solar System(2017) Ballouz, Ronald; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granular physics is a sub-discipline of physics that attempts to combine principles that have been developed for both solid-state physics and engineering (such as soil mechanics) with fluid dynamics in order to formulate a coherent theory for the description of granular materials, which are found in both terrestrial (e.g., earthquakes, landslides, and pharmaceuticals) and extra-terrestrial settings (e.g., asteroids surfaces, asteroid interiors, and planetary ring systems). In the case of our solar system, the growth of this sub-discipline has been key in helping to interpret the formation, structure, and evolution of both asteroids and planetary rings. It is difficult to develop a deterministic theory for granular materials due to the fact that granular systems are composed of a large number of elements that interact through a non-linear combination of various forces (mechanical, gravitational, and electrostatic, for example) leading to a high degree of stochasticity. Hence, we study these environments using an N-body code, pkdgrav, that is able to simulate the gravitational, collisional, and cohesive interactions of grains. Using pkdgrav, I have studied the size segregation on asteroid surfaces due to seismic shaking (the Brazil-nut effect), the interaction of the OSIRIS-REx asteroid sample-return mission sampling head, TAGSAM, with the surface of the asteroid Bennu, the collisional disruptions of rubble-pile asteroids, and the formation of structure in Saturn's rings. In all of these scenarios, I have found that the evolution of a granular system depends sensitively on the intrinsic properties of the individual grains (size, shape, sand surface roughness). For example, through our simulations, we have been able to determine relationships between regolith properties and the amount of surface penetration a spacecraft achieves upon landing. Furthermore, we have demonstrated that this relationship also depends on the strength of the local gravity. By comparing our numerical results to laboratory experiments and observations by spacecraft we can begin to understand which microscopic properties (i.e., grain properties) control the macroscopic properties of the system. For example, we can compare the mechanical response of a spacecraft to landing or Cassini observations of Saturn's ring to understand how the penetration depth of a spacecraft or the complex optical depth structure of a ring system depends on the size and surface properties of the grains in those systems.Item The Development and Implementation of Numerical Tools for Investigation into the Granular Dynamics of Solid Solar System Bodies(2013) Schwartz, Stephen Ross; Richardson, Derek C; Michel, Patrick; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The work advanced in this thesis joins together the disciplines of planetary science and granular physics. Grain dynamics have played a prominent role in the evolution of our Solar System from planetesimal formation billions of years ago to the surface processes that take place today on terrestrial planets, moons, and small bodies. Recent spacecraft images of small Solar System bodies provide strong evidence that the majority of these bodies are covered in regolith. This regolith ranges in size from the fine powder found on the Moon to large rocks and boulders, like the 27 m Yoshinodai boulder on the small asteroid, Itokawa. Accordingly, the processes that take place on the solid bodies of the Solar System vary widely based upon the material properties of the regolith and the gravitational environments on their surfaces. An understanding of granular dynamics is also critical for the design and operations of landers, sampling devices and rovers to be included in space missions. Part of my research is concerned with the development of numerical tools that have the ability to provide explanations for the types of processes that our spacecraft have observed. Granular processes on Earth are incredibly complex and varied, and constitute an enormous field of study on their own, with input taken from across the broad disciplines of engineering and the physical sciences. In micro-gravity, additional forces, which on Earth are relevant only to micron-size particles or smaller, are expected to become important for material up to the size of large rocks, adding further complexity. The numerical tools developed in this work allow for the simulation of grains using an adaptation of the Soft-Sphere Discrete Element Method (SSDEM) along with implementations of cohesive forces between particles into an existing parallel gravity tree code.Item Forming Binary Near-Earth Asteroids From Tidal Disruptions(2006-11-28) Walsh, Kevin John; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present simulations and observations as part of a model of the binary near-Earth asteroid population. The study of binary asteroid formation includes a series of simulations of near-Earth asteroid (NEA) tidal disruption, analyzed for bound, mutually orbiting systems. Discrete and solid particles held together only by self-gravity are employed to model a ``rubble pile'' asteroid passing Earth on a hyperbolic encounter. This is accomplished via N-body simulations, with multiple encounter and body parameters varied. We examine the relative binary production rates and the physical and orbital properties of the binaries created as a function of the parameters. We also present the overall relative likelihoods for possible physical and dynamical properties of created binaries. In order to constrain the shape and spin properties of the bodies that feed the NEA population, an observing campaign was undertaken to observe lightcurves of small Main Belt asteroids (D < 5 km, SMBAs). Observations of 28 asteroids increases the overall number of SMBAs studied via lightcurves to 86. These observations allow direct comparison between NEAs and MBAs of a similar size. The shape and spin for the SMBAs are incorporated into a Monte Carlo model of a steady-state NEA population, along with the binaries created by tidal disruption simulations. Effects from tidal evolution and binary disruption from close planetary encounters are included as a means of altering or disrupting binaries. We find that with the best known progenitor (small Main Belt asteroids) shape and spin distributions, and current estimates of NEA lifetime and encounter probabilities, that tidal disruption should account for approximately 1-2% of NEAs being binaries. Given the observed estimate of an ~15% binary NEA fraction, we conclude that there are other formation mechanisms that contribute significantly to this population.Item Light Scattering Properties of Asteroids and Cometary Nuclei(2005-04-20) Li, Jian-Yang; A'Hearn, Michael F.; McFadden, Lucy A.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The photometric properties of asteroids and cometary nuclei, bodies important for understanding the origin of the Solar System, are controlled by the physical properties of their surfaces. Hapke's theory is the most widely used theoretical model to describe the reflectance of particulate surfaces, and has been applied to the disk-resolved photometric analyses of asteroid 433 Eros, comet 19P/Borrelly, and asteroid 1 Ceres, in this dissertation. Near Earth Asteroid Rendezvous returned disk-resolved images of Eros at seven wavelengths from 450nm to 1050nm. The bidirectional reflectance of Eros's surface was measured from those images with its shape model and geometric data. Its single-scattering albedo, w, was found to mimic its spectrum, with a value of 0.33+/-0.03 at 550nm. The asymmetry factor of the single-particle phase function, g, is -0.25+/-0.02, and the roughness parameter, theta_bar, is 28+/-3 deg, both of which are independent of wavelength. The V-band geometric albedo of Eros is 0.23, typical for an S-type asteroid. From the disk-resolved images of Borrelly obtained by Deep Space 1 (DS1), the maps of its w, g, and theta_bar were constructed by modeling the reflectance of Borrelly terrain by terrain. w varies by a factor of 2.5, with an average of 0.057+/-0.009. g changes from -0.1 to -0.7, averaging -0.43+/-0.07. theta_bar is <=35 deg for most of the surface, but up to 55 deg for some areas, with an average of 22+/-5 deg. The 1-D temperature measurement from DS1 can be well described by the standard thermal model assuming a dry surface, except for one area, where the discrepancy can be explained by a sublimation rate that is consistent with the observed water production rate. HST images through three filters, covering more than one rotation of Ceres, were acquired. Its V-band lightcurve agrees with earlier observations very well. A strong absorption band centered at about 280nm is noticed, but cannot be identified. w of Ceres was modeled to be 0.073+/-0.002, 0.046+/-0.002, and 0.032+/-0.003 at 555nm, 330nm, and 220nm, respectively. The maps of w for Ceres at three wavelengths were constructed, with eleven albedo features identified. Ceres' surface was found to be very uniform.