AuthorRichardson Jr., James Edward
asteroid cratering records
AdvisorMelosh, Henry J.
Greenberg, Richard J.
Committee ChairMelosh, Henry J.
Greenberg, Richard J.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractImpact-induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids. In this study, I use a series of linked seismic and geomorphic models to investigate the process in detail. I begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid, and I use this theory to model the global vibrations experienced on the surface of an asteroid following an impact. These synthetic seismograms are then applied to a model of regolith resting on a slope, and the resulting downslope motion is computed for a full range of impactor sizes. Next, this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure, showing how topographic erosion accumulates as a function of time and the number of impacts. Finally, these results are applied in a stochastic cratering model for the surface of an Eros-like body (same volume and surface area as the asteroid), with craters formed by impacts and then erased by the effects of superposing craters, ejecta coverage, and seismic shakedown. This simulation shows good agreement with the observed 433 Eros cratering record at a Main Belt exposure age of $400 \pm 200$ Myr, including the observed paucity of small craters. The lowered equilibrium numbers (loss rate = production rate) for craters less than $\sim 100$ m in diameter is a direct result of seismic erasure, which requires less than a meter of mobilized regolith to reproduce the NEAR observations.This study also points to an upper limit on asteroid size for experiencing global, surface-modifying, seismic effects from individual impacts of about 70-100 km (depending upon asteroid seismic properties). Larger asteroids will experience only local seismic effects from individual impacts.In addition to the study of global seismic effects on asteroids, a chapter is also included which details the impact ejecta plume modeling I have done for the Deep Impact mission to the comet Tempel I. This work will also have direct application to impacts on asteroids, and will be used in the future to refine the cratering history modeling performed thus far.
Degree ProgramPlanetary Sciences
Degree GrantorUniversity of Arizona
Showing items related by title, author, creator and subject.
PHYSICAL CHARACTERIZATION OF ∼2 m DIAMETER NEAR-EARTH ASTEROID 2015 TC25: A POSSIBLE BOULDER FROM E-TYPE ASTEROID (44) NYSAReddy, Vishnu; Sanchez, Juan A.; Bottke, William F.; Thirouin, Audrey; Rivera-Valentin, Edgard G.; Kelley, Michael S.; Ryan, William; Cloutis, Edward A.; Tegler, Stephen C.; Ryan, Eileen V.; Taylor, Patrick A.; Richardson, James E.; Moskovitz, Nicholas; Le Corre, Lucille; Univ Arizona, Lunar & Planetary Lab (IOP PUBLISHING LTD, 2016-11-14)Small near-Earth asteroids (NEAs) (< 20 m) are interesting, because they are progenitors for meteorites in our terrestrial collection. The physical characteristics of these small NEAs are crucial to our understanding of the effectiveness of our atmosphere in filtering low-strength impactors. In the past, the characterization of small NEAs has been a challenge, because of the difficulty in detecting them prior to close Earth flyby. In this study, we physically characterized the 2 m diameter NEA 2015 TC25 using ground-based optical, near-infrared and radar assets during a close flyby of the Earth (distance 128,000 km) in 2015 October 12. Our observations suggest that its surface composition is similar to aubrites, a rare class of high-albedo differentiated meteorites. Aubrites make up only 0.14% of all known meteorites in our terrestrial meteorite collection. 2015 TC25 is also a very fast rotator with a period of 133 +/- 6 s. We combined the spectral and dynamical properties of 2015 TC25 and found the best candidate source body in the inner main belt to be the 70 km diameter E-type asteroid (44) Nysa. We attribute the difference in spectral slope between the two objects to the lack of regolith on the surface of 2015 TC25. Using the albedo of E-type asteroids (50%-60%) we refine the diameter of 2015 TC25 to 2 m, making it one of the smallest NEAs ever to be characterized.
Orbit-Dependent Spectral Trends for the Near-Earth Asteroid PopulationFevig, Ronald Adrey (The University of Arizona., 2006)Results of visible to near-infrared spectrophotometric observations of 55 near-Earth asteroids (NEAs) are reported. The observing techniques, instrumentation, and method of data analysis are described. A new asteroid classification method that directly compares these NEA spectra with spectral features of meteorites is presented. Two major siliceous groups (having discernible "1-micron" absorptions) result from this method, OC-likes which match the spectra of ordinary chondrites and S-types. The dataset shows a preponderance of spectra consistent with ordinary chondrites (23 NEAs), as well as S-types (19), 2 with spectra consistent with black ordinary chondrites, 2 R-types, and 9 that show no 1-micron absorption.The spectral characteristics of the siliceous S-type and OC-like asteroids blend together, providing evidence that S-type asteroids are simply ordinary chondrites whose surface has been modified by weathering. This helps resolve the long standing question of the lack of main belt asteroids having spectra matching ordinary chondrite meteorites. Main belt asteroids have on average much older surfaces while NEAs that exhibit OC-like spectra have younger surfaces.It was found that fresh objects having spectra consistent with ordinary chondrites (1) occupy mostly highly eccentric Apollo orbits which encounter a strong collisional environment in the asteroid main-belt, (2) may have been recently injected into high eccentricity orbits, or (3) have suffered tidal disruption. S-type NEAs reside primarily in orbits that do not cross the asteroid main-belt. This orbit dependent trend is verified by using the larger NEA dataset of Binzel et al. (2004a).Nine NEAs from this survey exhibiting no 1-micron absorption can be associated with extinct comets, iron meteorites or enstatite meteorites. It is shown that most of these NEAs must be extinct comets, implying a considerably larger fraction of comets among the NEA population than previously thought. A correlation of these objects with low inclination orbits is found.This study finds that the NEA population is divided roughly as follows: ~40% fresh ordinary chondrites, ~35% S-types, ~20% extinct comet candidates, and ~5% in minor classes. This work may guide NEA mitigation planning should such an emergency arise.
Dynamical History of the Asteroid Belt and Implications for Terrestrial Planet BombardmentMinton, David A. (The University of Arizona., 2009)The main asteroid belt spans ~2-4 AU in heliocentric distance and is sparsely populated by rocky debris. The dynamical structure of the main belt records clues to past events in solar system history. Evidence from the structure of the Kuiper belt, an icy debris belt beyond Neptune, suggests that the giant planets were born in a more compact configuration and later experienced planetesimal-driven planet migration. Giant planet migration caused both mean motion and secular resonances to sweep across the main asteroid belt, raising the eccentricity of asteroids into planet-crossing orbits and depleting the belt. I show that the present-day semimajor axis and eccentricity distributions of large main belt asteroids are consistent with excitation and depletion due to resonance sweeping during the epoch of giant planet migration. I also use an analytical model of the sweeping of the ν6 secular resonance, to set limits on the migration speed of Saturn.After planet migration, dynamical chaos became the dominant loss mechanism for asteroids with diameters D>10 km in the current asteroid belt. I find that the dynamical loss history of test particles from this region is well described with a logarithmic decay law. My model suggests that the rate of impacts from large asteroids may have declined by a factor of three over the last ~3 Gy, and that the present-day impact flux of D>10 km objects on the terrestrial planets is roughly an order of magnitude less than estimates used in crater chronologies and impact hazard risk assessments.Finally, I have quantified the change in the solar wind 6Li/7Li ratio due to the estimated in-fall of chondritic material and enhanced dust production during the epoch of planetesimal-driven giant planet migration. The solar photosphere is currently highly depleted in lithium relative to chondrites, and 6Li is expected to be far less abundant in the sun than 7Li due to the different nuclear reaction rates of the two isotopes. Evidence for a short-lived impact cataclysm that affected the entire inner solar system may be found in the composition of implanted solar wind particles in lunar regolith.