The Seismic Effect of Impacts on Asteroid Surface Morphology
Name:
azu_etd_1164_sip1_m.pdf
Size:
9.168Mb
Format:
PDF
Description:
azu_etd_1164_sip1_m.pdf
Issue Date
2005Keywords
impact crateringasteroid sttructures
asteroid seismology
asteroid geomorphology
asteroid cratering records
Advisor
Melosh, Henry J.Greenberg, Richard J.
Committee Chair
Melosh, Henry J.Greenberg, Richard J.
Metadata
Show full item recordPublisher
The University of Arizona.Rights
Copyright © 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.Abstract
Impact-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.Type
textElectronic Dissertation
Degree Name
PhDDegree Level
doctoralDegree Program
Planetary SciencesGraduate College
Degree Grantor
University of ArizonaCollections
Related items
Showing items related by title, author, creator and subject.
-
The Nucleus of Active Asteroid 311P/(2013 P5) PANSTARRSThe unique inner-belt asteroid 311P/PANSTARRS (formerly P/2013 P5) is notable for its sporadic, comet-like ejection of dust in nine distinct epochs spread over similar to 250 days in 2013. This curious behavior has been interpreted as the product of localized, equatorward landsliding from the surface of an asteroid rotating at the brink of instability. We obtained new Hubble Space Telescope observations to directly measure the nucleus and to search for evidence of its rapid rotation. We find a nucleus with mid-light absolute magnitude H-V = 19.14 +/- 0.02, corresponding to an equal-area circle with radius 190. +/-. 30 m (assuming geometric albedo p(V) = 0.29). However, instead of providing photometric evidence for rapid nucleus rotation, our data set a lower limit to the light-curve period, P >= 5.4 hr. The dominant feature of the light curve is a V-shaped minimum, similar to 0.3 mag deep, which is suggestive of an eclipsing binary. Under this interpretation, the time-series data are consistent with a secondary/ primary mass ratio, m(s)/m(p) similar to 1:6, a ratio of separation/primary radius, r/r(p) similar to 4 and an orbit period similar to 0.8 days. These properties lie within the range of other asteroid binaries that are thought to be formed by rotational breakup. While the light-curve period is long, centripetal dust ejection is still possible if one or both components rotate rapidly (less than or similar to 2 hr) and have small light-curve variation because of azimuthal symmetry. Indeed, radar observations of asteroids in critical rotation reveal " muffin-shaped" morphologies, which are closely azimuthally symmetric and which show minimal light curves. Our data are consistent with 311P being a close binary in which one or both components rotates near the centripetal limit. The mass loss in 2013 suggests that breakup occurred recently and could even be on-going. A search for fragments that might have been recently ejected beyond the Hill sphere reveals none larger than effective radius r(e) similar to 10 m.
-
Homogeneous internal structure of CM-like asteroid (41) DaphneContext. CM-like asteroids (Ch and Cgh classes) are a major population within the broader C-complex, encompassing about 10% of the mass of the main asteroid belt. Their internal structure has been predicted to be homogeneous, based on their compositional similarity as inferred from spectroscopy and numerical modeling of their early thermal evolution. Aims. Here we aim to test this hypothesis by deriving the density of the CM-like asteroid (41) Daphne from detailed modeling of its shape and the orbit of its small satellite. Methods. We observed Daphne and its satellite within our imaging survey with the Very Large Telescope extreme adaptive-optics SPHERE/ZIMPOL camera and complemented this data set with earlier Keck/NIRC2 and VLT/NACO observations. We analyzed the dynamics of the satellite with our Genoid meta-heuristic algorithm. Combining our high-angular resolution images with optical lightcurves and stellar occultations, we determine the spin period, orientation, and 3D shape, using our ADAM shape modeling algorithm. Results. The satellite orbits Daphne on an equatorial, quasi-circular, prograde orbit, like the satellites of many other large main-belt asteroids. The shape model of Daphne reveals several large flat areas that could be large impact craters. The mass determined from this orbit combined with the volume computed from the shape model implies a density for Daphne of 1.77 +/- 0.26 g cm(-3) (3 sigma). This density is consistent with a primordial CM-like homogeneous internal structure with some level of macroporosity (approximate to 17%). Conclusions. Based on our analysis of the density of Daphne and 75 other Ch/Cgh-type asteroids gathered from the literature, we conclude that the primordial internal structure of the CM parent bodies was homogeneous.
-
The Dynamical Complexity of Surface Mass Shedding from a Top-shaped Asteroid Near the Critical Spin LimitThe regolith transport near the surface of an asteroid is inherently sensitive to the local topography. In this paper, conditions of surface mass shedding and the subsequent evolution of the shedding material are studied for the primary of 65803 Didymos, serving as a representative for a large group of top-shaped asteroids that rotate near their critical spin limits. We considered the influences of an asymmetric shape and a non-spherical gravity, and demonstrate that these asymmetries play a significant role in the shedding process as well as in the subsequent orbital motion. The mass shedding conditions are given as a function of the geological coordinates, and show a clear-cut dependency on the local topographic features. We find that at different stages of the Yarkovsky-O'Keefe-Radzievskii-Paddack spin-up, the bulged areas exhibit a uniform superior advantage of enabling mass shedding over the depressed areas. "Dead zones" free from mass shedding are found around the polar sites. Numerical simulations show that the orbital motion of the shedding material experiences a drastic change as the spin rate is approaching the critical limit. The "mass leaking" effect is reinforced as the spin rate increases; the lower spin rates correspond to a higher capability of trapping the lofted particles in the vicinity of the asteroid, which statistically improves the probability of collisional growth in orbit. We also find that the topological transition of the equilibrium point can in practice lead to rapid clearance of the shedding material and transport of their orbits to larger distances from the surface.
