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 curtam 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.)
