Theses and Dissertations from UMD
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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 Modeling Optically Thick Molecular Emission Spectra of Comets Using Asymmetric Spherical Coupled Escape Probability(2014) Gersch, Alan Michael; A'Hearn, Michael F; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Comets are frozen remnants from the formation of the Solar System. As such, their chemical composition is of great significance to understanding the origin of the planets and the distribution of important molecules, including water and other volatiles, throughout the Solar System. Recent observations, in particular those of the Deep Impact and EPOXI Missions, have provided better spectra of a cometary coma than were previously available. These observations include spectra with high spatial resolution very near to the nucleus. The purpose of this research is to better understand the abundances, distributions and creation mechanisms of various volatiles observed in cometary comae, in particular those of comet 9P/Tempel 1, the target of the Deep Impact Mission, and 103P/Hartley 2, the subject of the EPOXI mission. In order to do so, I have built a computer model of the spectrum of the comet's coma which includes the difficult and often ignored problem of accurately including radiative transfer to account for the potentially optically thick coma (or regions of the coma) near the nucleus. I have adapted Coupled Escape Probability, a new exact method of solving radiative transfer problems, from its original plane-parallel formulation for use in asymmetrical spherical situations. My model is designed specifically for use in modeling optically thick cometary comae, although not limited to such use. By providing for asymmetric geometry in the coma, the model is able to include the morphology of the near nucleus coma, as observed by the Deep Impact spacecraft for Tempel 1 and Hartley 2, and include this in the modeling of radiative transfer. This method enables the accurate modeling of comets' spectra even in the potentially optically thick regions nearest the nucleus, such as those seen in Deep Impact observations of 9P/Tempel 1 and EPOXI observations of 103P/Hartley 2. This model will facilitate analyzing the actual spectral data from the Deep Impact and EPOXI missions to better determine abundances of key volatile species, including CO, CO2 and H2O, as well as remote sensing data on active comets.Item Infrared Spectroscopy of Parent Volatiles in Comets: Chemical Diversity and a New Fluorescence Model for the Ethane nu5 Band(2010) Radeva, Yana Lyubomirova; A'Hearn, Michael F; Mumma, Michael J; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work investigates the chemical and dynamical diversities of comets, and explores the clues they hold to understanding the formation and evolution of the Solar System. This research is based on analysis of high-resolution infrared spectroscopic data obtained with the Near Infrared Echelle Spectrograph on the Keck II telescope. Gas production rates of parent volatile species released from cometary nuclei are measured, and the relative enrichment of organics in comets, with respect to the dominant volatile - H2O - is determined. These measurements require fluorescence models for each species, as well as derivation of an accurate rotational temperature. A major contribution of this work is the development of a theoretical model of the fluorescence of the infrared C2H6 nu5 band in comets (at 2896 cm-1), which can be used to derive an accurate rotational temperature for this parent volatile (unlike the C2H6 nu7 band at 2985 cm-1). As a symmetric hydrocarbon C2H6 is uniquely observed in the infrared, and now brings the number of molecules for which we can derive a rotational temperature to four (along with H2O, HCN and CO). Also, C2H6 nu5 is observed simultaneously with H2CO, OH, CH4, HCN, C2H2 and H2O, which eliminates many systematic effects. The C2H6 nu5 model is applied to cometary spectra, and it used to extract ethane rotational temperatures, production rates and mixing ratios. The rotational temperatures derived from C2H6 nu5 agree with those measured for H2O (and other species). Mixing ratios from the C2H6 nu7 band are also confirmed by the nu5 band - agreement is within 1-sigma (2-sigma in one case). Analysis of the depleted organic composition of the Oort cloud comet C/2000 WM1 (LINEAR) is presented, along with the ecliptic comet 2P/Encke, and their compositions are compared with those of other comets. The results from this dissertation contribute to understanding physics in the inner cometary coma, and on a grander scale - to the exploration of cometary origins in terms of Solar System formation and evolution.