Related Research Discussions
Low angle-of-incidence (oblique) impacts have only recently been the subject of scrutiny. This may well be due to the identification of them in the imagery returned from planetary missions. Once considered far outliers, new estimates put them at ~5% of all impacts. Experimentation in this area is constrained by the availability of testing platforms, as the most commonly used rig can only go as shallow as 15 degrees from horizontal, effectively eliminating it as a tool to test out-of-orbit trajectories, which can approach tangential.
We are presenting abstracts here which may be of interest for further research.
Understanding Oblique Impacts from Experiments, Observations, and Modeling
E. Pierazzo1 and H. J. Melosh1
1Lunar and Planetary Lab., University of Arizona, 1629 E. University Blvd., Tucson, Arizona, 84721; email: betty@lpl.arizona.edu , jmelosh@lpl.arizona.edu
Annual Review of Earth and Planetary Sciences
Vol. 28: 141-167 (Volume publication date May 2000)
(doi:10.1146/annurev.earth.28.1.141)
Abstract
Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters’ rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle.
Azimuthal impact directions from oblique impact crater morphology
Wallis, D; Burchell, M J; Cook, A C; Solomon, C J and McBride, N (2005). Azimuthal impact directions from oblique impact crater morphology. Monthly Notices of the Royal Astronomical Society, 359(3), pp. 1137–1149.
Monthly Notices of the Royal Astronomical Society: Letters
Volume 359 Issue 3, Pages 1137 - 1149, Published Online: 21 Apr 2005
DOI (Digital Object Identifier) Link: http://dx.doi.org/doi:10.1111/j.1365-2966.2005.08978.x
Abstract
Planetary impact craters have a high degree of radial symmetry. This hampers efforts to identify the azimuthal impact direction for most craters – the radially symmetric component of an impact crater swamps any asymmetries that may be present. We demonstrate how the asymmetric component can be isolated and the direction of the asymmetries quantified using a two-dimensional eigenfunction expansion over a circular domain. The complex coefficients of expansion describe the magnitude and phase (angular alignment) of each term. From the analysis of hypervelocity impact craters formed in the laboratory, with impact angles ranging from 0° to 50° from the surface normal, we show that asymmetries which reveal the impact direction are still present at just 10° from the surface normal, and that the phase of one complex coefficient of expansion, c11, indicates the impact direction. Analysis of the lunar crater Hadley shows bilateral symmetry in the radially asymmetric component, which may be due to oblique impact. The 31-km lunar ray crater Kepler has morphological features that indicate the azimuthal impact direction. Coefficient c11 gives an azimuthal impact direction similar to that expected from the morphology, although post-impact gravitational collapse and slumping obscure the result to some degree. Ray craters may provide a means of testing the method for smaller 'simple' craters when data are available.
IMPACT CRATERING FROM EXPERIMENTS AND MODELS
Lunar and Planetary Science XXXVIII (2007)
This document contains 13 papers from Lunar and Planetary Science XXXVIII (2007) sessions Monday, March 12, 2007. Chairs: G. S. Collins, T. Kenkmann
of particular interst is:
Deformation of Sandstone in Meso-Scale Hypervelocity Cratering Experiments [#1527], Kenkmann T. , Patzschke, M. Thoma K., Schäfer F., Wünnemann K., Deutsch A.
Hydrocode modeling of oblique impacts: The fate of the projectile
E. Pierazzo, H.J. Melosh
Meteoritics and Planetary Science 35(1), 117-130, 2000.
ABSTRACT
All impacts are oblique to some degree. Only rarely do projectiles strike a planetary surface (near) vertically. The effects of an oblique impact event on the target are well known, producing craters that appear circular even for low impact angles (>15° with respect to the surface). However, we still have much to learn about the fate of the projectile, especially in oblique impact events. This work investigates the effect of angle of impact on the projectile.
Sandia National Laboratories' three-dimensional hydrocode CTH was used for a series of high-resolution simulations (50 cells per projectile radius) with varying angle of impact. Simulations were carried out for impacts at 90, 60, 45, 30, and 15° from the horizontal, while keeping projectile size (5 km in radius), type (dunite), and impact velocity (20 km/s) constant.
The three-dimensional hydrocode simulations presented here show that in oblique impacts the distribution of shock pressure inside the projectile (and in the target as well) is highly complex, possessing only bilateral symmetry, even for a spherical projectile. Available experimental data suggest that only the vertical component of the impact velocity plays a role in an impact. If this were correct, simple theoretical considerations indicate that shock pressure, temperature, and energy would depend on sin2(theta), where (theta) is the angle of impact (measured from the horizontal). However, our numerical simulations show that the the mean shock pressure in the projectile is better fit by a sin(theta) dependence, whereas shock temperature and energy depend on sin3/2(theta). This demonstrates that in impact events the shock wave is the result of complex processes that cannot be described by simple empirical rules. The mass of shock melt or vapor in the projectile decreases drastically for low impact angles as a result of the weakening of the shock for decreasing impact angles. In particular, for asteroidal impacts the amount of projectile vaporized as always limited to a small fraction of the projectile mass. In cometary impacts, however, most of the projectile is vaporized even at low impact angles.
In the oblique impact simulations a large fraction of the projectile material retains a net downrange motion. In agreement with experimental work, the simulations show that for low impact angles (30° and 15°), a downrange focusing of projectile material occurs, and a significant amount of it travels at velocities larger than the escape velocity of Earth.
INTERACTIONS BETWEEN AN IMPACT GENERATED E JECTA CURTAIN AND AN ATMOSPHERE
International Journal of Impact Engineering 23 (1999) 5 i-62
OLIVIER S. BARNOUIN-JHA ’ and PETER H. SCHULTZ ”
' The Johns University Applied Physics Laboratory, Johns Hopkins Road, Laurel, MD 20723.6099, USA;
“ Dept. of Geological Sciences, Box 1846, Brown University, Providence, RI 02912
Summary
A theoretical model investigates the interaction between an ejecta curtain and a variety of differing atmospheric conditions in order to determine the ejecta entrainment capacity
of winds generated by an advancing curtain. The model assesses the curtain shape, the position along the curtain where flow separation occurs, the velocity of winds winnowing ejecta out of the effectively impermeable portions of the curtain and the velocity of winds flow separating at its top. Wind velocities allow estimating the size range of ejecta entrained. Tested against laboratory impacts into coarse sand, the model results duplicate observation of curtain shape and size of ejecta entrained. The position where flow separation occurs is duplicated when the curtain porosity is assumed to increase with time. 0 1999 Elsevier Science Ltd. All rights reserved.
Interactions between impact-induced vapor clouds and the ambient atmosphere: 1. Spectroscopic observations using diatomic molecular emission
Author(s)
SUGITA Seiji (1) ; SCHULTZ Peter H. (2) ;
(1) Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo, JAPON
(2) Department of Geological Sciences, Brown University, Providence, Rhode Island, ETATS-UNIS
Abstract
The importance of interactions between impact-induced vapor clouds and an ambient atmosphere has been widely recognized, and theoretical approaches have provided significant insights. Few experiments, however, have been done to observe directly the energy partitioning during the interactions between impact vapor clouds and the ambient atmosphere. The present study attempts to understand the difference between actual and theoretical model impact vapor clouds produced under an atmosphere. A series of hypervelocity impact experiments was conducted using a spectroscopic measurement method. Plastic (polycarbonate) impactors allowed simulating vaporization phenomena associated with natural impactors (e.g., silicates and metals) at high impact velocities into water. Water as the target material served to suppress the effect of fine-grained fragments from the target. Emission spectra of the leading part of downrange-moving impact vapor clouds were captured with high-speed spectrometers as a function of time for various ambient pressures. The emission spectra exhibit strong molecular bands from carbon compounds as well as blackbody continuum radiation. In order to estimate the temperature of the radiation source, we carried out a spectral-form inversion analysis based on diatomic emission theory. Obtained molecular radiation temperatures range from 4500 K to 5500 K with relatively high accuracy (∼2%) and place a number of well-defined constraints for the radiation source. A simple theoretical model that is often assumed for an impact-induced vapor cloud, however, does not readily satisfy the constraints. This strongly suggests that real impact-induced vapor clouds may be more complex than previously thought.
Journal of geophysical research ISSN 0148-0227
2003, vol. 108, noE6, pp. 5.1-5.11 (27 ref.)
American Geophysical Union, Washington, DC, ETATS-UNIS (1949) (Revue)
Effect of impact angle on vaporization
SCHULTZ P. H. (1) ;
(1) Department of Geological Sciences, Brown University, Providence, Rhode Island, ETATS-UNIS
Abstract
Impacts into easily vaporized targets such as dry ice and carbonates generate a rapidly expanding vapor cloud. Laboratory experiments performed in a tenuous atmosphere allow deriving the internal energy of this cloud through well-established and tested theoretical descriptions. A second set of experiments under near-vacuum conditions provides a second measure of energy as the internal energy converts to kinetic energy of expansion. The resulting data allow deriving the vaporized mass as a function of impact angle and velocity. Although peak shock pressures decrease with decreasing impact angle (referenced to horizontal), the amount of impact-generated vapor is found to increase and is derived from the upper surface. Moreover, the temperature of the vapor cloud appears to decrease with decreasing angle. These unexpected results are proposed to reflect the increasing roles of shear heating and downrange hypervelocity ricochet impacts created during oblique impacts. The shallow provenance, low temperature, and trajectory of such vapor have implications for larger-scale events, including enhancement of atmospheric and biospheric stress by oblique terrestrial impacts and impact recycling of the early atmosphere of Mars.
Journal of Geophysical Research ISSN 0148-0227
1996, vol. 101, noE9, pp. 21117-21136 (42 ref.)
