<p>Many surface and dynamical processes affect the evolution of asteroids in our solar system today. The spectral slopes of S and Q-type asteroids are altered by the weathering of their surfaces due to solar wind interactions and micrometeorite impacts, as well as any processes that work to remove that weathered material. These processes of space weathering and asteroid resurfacing compete with each other to determine the spectral slope of each asteroid, with space weathering raising the spectral slope</p> <p>and resurfacing lowering it. By considering the distribution of spectral slopes with respect to orbital location and size, we can determine which potential resurfacing processes are the most dominant. I show that the distribution of spectral slopes with respect to size is present in all populations of S and Q-type asteroids in the inner solar system, regardless of orbit. I also show that the spectral slopes of S and Q-type Near-Earth Asteroids (NEAs) decrease with decreasing perihelion, but only for perihelia q < 0.9 AU.</p> <p>By building Monte Carlo and models N-body simulations of asteroids, I test which resurfacing mechanisms are consistent with these trends in spectral slopes. I find that spin-up and failure from the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect is an important resurfacing mechanism that creates the observed weathering trends with size. I also show that resurfacing asteroids due to close encounters with the terrestrial planets cannot explain the spectral slope vs. perihelion trend at q .</p> <p>0.9 AU, but that resurfacing asteroids due to thermally induced surface degradation, by assuming a power law relationship between the resurfacing timescale and the solar distance, gives much more consistent results.</p> <p>I also explore the creation rate of asteroid pairs, which are asteroids that have very similar orbits but are not gravitationally bound. The majority of pairs are formed by YORP spin-up and fission, followed by a separation of the two members. Asteroid pairs are then disassociated over time as their orbits become less similar due to chaos, resonances, and the Yarkovsky effect. I simulate both the formation of asteroid pairs in the inner main belt via YORP and their subsequent disassociation. By comparing the distribution of orbital similarity distances from observations and from our model, I estimate that asteroids fission and create an asteroid pair every 8 − 13 YORP cycles, where a YORP cycle is twice the time it takes the YORP effect to change the spin rate of an asteroid from zero to its critical spin rate. I argue that the rate of fissioning via the YORP effect is not substantially limited by any stagnation or stochastic evolution, and that losing mass via rotational fission is much less effective than collisional disruption, even for small asteroids.</p>

<div><div><div><p>Double ridges are one of the most ubiquitous surface features on Europa. Double ridges are pairs of linear topographic highs on the order of 100 m in topographic relief that are divided by a narrow trough. The double ridges are narrow, with widths less than 5 km. They span 100s of km, overlap with one another, and cover much of Europa’s surface. The ubiquity of double ridges implies that the process forming them can occur globally rather than in a single region with unusual properties. Constraining the formation of ridges may provide constraints on possible shell thicknesses and thermomechanical states globally on Europa. However, the mechanism responsible for ridge formation is still uncertain. This thesis discusses tests of the viability of incremental ice wedging to create topography like that observed at Europa’s ridges through finite element modeling. This work also narrows the range of depths at which a wedge could create a double ridge. The results indicate that shallow wedges less than 500 m from the surface can create deformation similar to observed double ridges.</p></div></div></div>

The positive correlation between planet detection rate and host star iron abundance lends strong support to the core accretion theory of planet formation. However, iron is not the most significant mass contributor to the cores of giant planets. Since giant planet cores grow from silicate grains with icy mantles, the likelihood of gas giant formation should depend heavily on the oxygen and silicon abundance of the planet formation environment. Here we compare the silicon and oxygen abundances of a set of 60 planet hosts and a control sample of 60 metal-rich stars without giant planets. We find a 99% probability that planet detection rate depends on the silicon abundance of the host star, over and above the observed planet-metallicity correlation. Due to our large error bars on oxygen abundances, we do not yet observe any correlation between oxygen abundance and planet detection rate. We predict that a correlation between planet occurrence and oxygen abundance should emerge when we can measure [O/Fe] at 0.05 dex precision. Since up to 20% of the carbon in the universe may be in refractory grains, we also predict that planet detection rate should correlate positively with host star carbon abundance for any population of planets formed by core accretion. / text

Thesis (Ph. D.)--University of Wisconsin--Madison, 1962. / Typescript. Includes abstract, computer source code, and vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaf 26).

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This dissertation is composed of two parts. The first part is a description of the Mid-Infrared Array Camera (MIRAC), a new camera for ground-based astronomy. The second part of this dissertation is an infrared study of planetary nebulae utilizing observations with the new camera. MIRAC is a collaborative effort among the University of Arizona, Smithsonian Astrophysical Observatory, and Naval Research Laboratory. It currently utilizes a Hughes 20 x 64 Si:As IBC detector array, which is sensitive to infrared (IR) radiation from 2 to 26 μm. The camera is equipped with 10% bandwidth filters at 2.2, 3.8, 4.6, 8.8, 9.8, 11.7, and 12.5 μm, and a wide band 8.0 to 12.8 μm "N" filter. There is also a 20% filter at 20.5 μm, and a 8-14 μm CVF with a resolution of 1.8%. The MIRAC electronics provides timing signals and coadds successive frames at a maximum rate of 10 KHz for the full array, and higher rates for a partial array readout. The data are transferred via a serial interface to a PC for storage and further processing. The camera recently achieved a NEFD of.010 Jy/arcsec² at 8.8, 11.7, and 12.5 μm for a 900 second on-source integration on the Steward Observatory 1.5 m telescope. Planetary Nebulae (PN) are formed when a star is in the post-Asymptotic Giant Branch stage of evolution. The ejection of circumstellar material is an important enrichment mechanism for the interstellar medium. In many PN, there is an excess of emission in the IR, indicating the presence of dust. There are several different components seen in the IR emission, including a family of unidentified IR (UIR) emission features at 3.3, 6.2, 7.7, and 11.3 μm. Images in the near- and mid-IR are presented here for the following PN: IC 418, BD + 30°3639, J 900, NGC 2392, NGC 6543, AFGL 2688, and M 2-9. In IC 418 and BD + 30°3639, the SiC and UIR emission is seen to be spatially distinct from the IR continuum. In NGC 2392 and NGC 6543, evidence for excess emission is seen in the distribution of the near-IR flux. In the bipolar nebulae AFGL 2688 and M 2-9, structures in the IR emission are seen that could be related to the equatorial density enhancements that have caused the bipolar morphology.

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