While most planetary impact craters are essentially symmetric, many oblique planetary craters express marked asymmetries. Experimental impacts produce analogous features. This dissertation examines the cause of these asymmetries, starting from the premise that impact structures form in response to impact-induced stresses. Hence, structural asymmetries must reflect shock wave asymmetries. Chapter 1 examines shock asymmetries using peak particle velocity as a measure of shock strength. Standard particle velocity scaling relations result in separate decay trends for different impact speeds. Incorporating an effective projectile radius that varies with impact speed resolves this discrepancy. Nevertheless, particle velocities in different target regions follow distinct trends, with enhanced velocities beneath the impact and downrange. Reconstructed particle velocity curves illustrate important consequences of impact angle, including extended regions of deformation downrange of oblique impacts. Chapter 2 explores the relationship between peak particle velocity, shock rise time, and compressive strain rate. While rise time and strain rate show the expected relationships with peak stress, rise times downrange of and beneath impacts are significantly shorter than those lateral to and uprange of the impact, and strain rates downrange and beneath the impact are significantly elevated. Chapter 3 examines momentum transfer. Stress wave momentum/area is the time integral of stress over the duration of the wave. Momentum contents downrange are significantly greater than those measured uprange, possibly due to extended projectile-target interaction times. Finally, Chapter 4 documents shear waves from experimental oblique impacts. Standard seismic models assume that impacts are more like explosions (no significant shear waves) than earthquakes (shear motions dominate). Here, shear strain measurements demonstrate that in addition to radial deformation (P-waves), oblique impacts also resulted in strong transverse deformation (S-waves). While P/S amplitude ratios for vertical impacts are explosion-like, those from oblique impacts are more earthquake-like. Consequently, oblique impacts may be difficult to discriminate from earthquakes, which may have important consequences for planetary seismic experiments.
Dahl, Jason M.,
"Shock Asymmetries from Oblique Hypervelocity Impacts"
(2016).
Earth, Environmental and Planetary Sciences Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z07D2SK8
