METEORITICS&
PLANETARY
SCIENCE
Volume 39, Issue 2
February 2004
ABSTRACTS
Please, contact the corresponding author for reprints of all published
articles. © Meteoritical Society, 2004
Dynamic tensile
strength of terrestrial rocks and application to impact cratering
Huirong-Anita AI* and Thomas J. AHRENS
Lindhurst Laboratory of Experimental Geophysics, Seismological Laboratory,
California Institute of Technology,
Pasadena, California 91125, USA
*Corresponding author. E-mail:
ahr@gps.caltech.edu
Dynamic tensile strengths and
fracture strengths of 3 terrestrial rocks, San Marcos gabbro, Coconino
sandstone, and Sesia eclogite were determined by carrying out flat-plate (PMMA
and aluminum) impact experiments on disc-shaped samples in the 5 to 60
m/sec range. Tensile stresses of 125 to 300 MPa and 245 to 580 MPa were
induced for gabbro and eclogite, respectively (with duration time of ~1
µs). For sandstone (porosity 25%), tensile stresses normal to bedding of
~13 to 55 MPa were induced (with duration times of 2.4 and ~1.4 µs).
Tensile crack failure was detected by the onset of shock-induced (damage)
P and S wave velocity reduction.
The dynamic tensile strength of gabbro determined from P and S wave
velocity deficits agrees closely with the value of previously determined
values by post-impact microscopic examination (~150 MPa). Tensile strength
of Coconino sandstone is 20 MPa for a 14 µs duration time and 17 MPa for a
2.4 µs duration time. For Sesia eclogite, the dynamic tensile strength is
~240 MPa. The fracture strength for gabbro is ~250 MPa, ~500 MPa for
eclogite, and ~40 MPa for sandstone. Relative crack-induced reduction of S
wave velocities is less than that of post-impact P wave velocity
reductions for both gabbro and eclogite, indicating that the cracks were
predominantly spall cracks. Impacts upon planetary surfaces induce tensile
failure within shock-processed rocks beneath the resulting craters. The
depth of cracking beneath impact craters can be determined both by seismic
refraction methods for rocks of varying water saturation and, for dry
conditions (e.g., the Moon), from gravity anomalies. In principle, depth
of cracking is related to the equations-of-state of projectile and target,
projectile dimension, and impact velocity. We constructed a crack-depth
model applicable to Meteor Crater. For the observed 850 m depth of
cracking, our preferred strength scaling model yields an impact velocity
of 33 km/s and impactor radius of 9 m for an iron projectile.
Experimental
ejection angles for oblique impacts: Implications for the subsurface
flow-field
Jennifer L. B. ANDERSON,* Peter H. SCHULTZ, and James T. HEINECK
*Corresponding author. Geological Sciences, Brown University, Providence,
Rhode Island 02912, USA
E-mail:
Jennifer_Anderson@brown.edu
A simple analytical solution for
subsurface particle motions during impact cratering is useful for tracking
the evolution of the transient crater shape at late times. A specific
example of such an analytical solution is Maxwell’s Z-Model, which is
based on a point-source assumption. Here, the parameters for this model
are constrained using measured ejection angles from both vertical and
oblique experimental impacts at the NASA Ames Vertical Gun Range. Data
from experiments reveal that impacts at angles as high as 45° to the
target’s surface generate subsurface flow-fields that are significantly
different from those created by vertical impacts. The initial momentum of
the projectile induces a subsurface momentum-driven flow-field that
evolves in three dimensions of space and in time to an excavation
flow-field during both vertical and oblique impacts. A single, stationary
pointsource
model (specifically Maxwell’s Z-Model), however, is found inadequate to
explain this detailed evolution of the subsurface flow-field during
oblique impacts. Because 45° is the most likely impact angle on planetary
surfaces, a new analytical model based on a migrating point-source could
prove quite useful. Such a model must address the effects of the
subsurface flow-field evolution on crater excavation, ejecta deposition,
and transient crater morphometry.
Modeling damage
and deformation in impact simulations
Gareth S. COLLINS,* H. Jay MELOSH, and Boris A. IVANOV
*Corresponding author. Lunar and Planetary Laboratory, University of
Arizona, Tucson, Arizona 85721, USA
E-mail:
gareth@lpl.arizona.edu
Numerical modeling is a powerful
tool for investigating the formation of large impact craters but is one
that must be validated with observational evidence. Quantitative analysis
of damage and deformation in the target surrounding an impact event
provides a promising means of validation for numerical models of
terrestrial impact craters, particularly in cases where the final pristine
crater morphology is ambiguous or unknown. In this paper, we discuss the
aspects of the behavior of brittle materials important for the accurate
simulation of damage and deformation surrounding an impact event and the
care required to interpret the results. We demonstrate this with an
example simulation of an impact into a terrestrial, granite target that
produces a 10 km-diameter transient crater. The results of the simulation
are shown in terms of damage (a scalar quantity that reflects the totality
of
fragmentation) and plastic strain, both total plastic strain (the
accumulated amount of permanent shear deformation, regardless of the sense
of shear) and net plastic strain (the amount of permanent shear
deformation where the sense of shear is accounted for). Damage and plastic
strain are both greatest close to the impact site and decline with radial
distance. However, the reversal in flow patterns from the downward and
outward excavation flow to the inward and upward collapse flow implies
that net plastic strains may be significantly lower than total plastic
strains. Plastic strain in brittle rocks is very heterogeneous; however,
continuum modeling requires that the deformation of the target during an
impact event be described in terms of an average strain that applies over
a large volume of rock (large compared to the spacing between individual
zones of sliding). This paper demonstrates that model predictions of
smooth average strain are entirely consistent with an actual
strain concentrated along very narrow zones. Furthermore, we suggest that
model predictions of total accumulated strain should correlate with
observable variations in bulk density and seismic velocity.
Structural
evidence from shock metamorphism in simple and complex impact craters:
Linking observations to theory
Michael R. DENCE
824 Nesbitt Place, Ottawa, Ontario K2C 0K1, Canada
E-mail:
mrdence@rsc.ca
The structure of Canadian impact
craters formed in crystalline rocks is analyzed using shock metamorphism
and evidence for movement along shear zones. The analysis is based on an
interpretation that, beyond the near field region, shock pressure
attenuates down axis as P ~ R–2, in agreement with nuclear test and
computed results, and as P ~ R–3 near the surface. In both simple and
complex craters, the transient cavity is defined by the limit of
fragmentation due to direct and reflected shock waves. The intersection of
the transient cavity with hemispheric shock isobars indicates that the
transient cavity has a parabolic form. Weakening by dilation during early
uplift
allows late stage slumping of the walls of simple craters. This is
controlled by a spheroidal primary shear of radius rs
≈ 2dt, where dt is
the depth of the transient crater due to excavation and initial
compression. With increasing crater diameter, the size of the transient
cavity decreases relative to the shock imprint, suggesting that
fragmentation and excavation is limited by progressively earlier collapse
of the margins under gravity. Central peak formation in complex craters
may be initiated by relaxation of the shock-compressed central
parautochthone, so the primary shear, lubricated by friction melting,
meets below the crater floor and drives the continuing upward motion.
The importance
of being cratered: The new role of meteorite impact as a normal
geological process
Bevan M. FRENCH
Department of Mineral Sciences, MRC 119, Smithsonian Institution,
Washington D.C. 20560, USA
Mailing address: 7408 Wyndale Lane, Chevy Chase, Maryland 20815, USA
E-mail:
french.bevan@nmnh.si.edu
This paper is a personal (and, in
many ways, incomplete) view of the past development of impact geology and
of the newly recognized importance of impact events in terrestrial
geological history. It also identifies some exciting scientific challenges
for future investigators: to determine the full range of impact effects
preserved on the Earth, to apply the knowledge obtained from impact
phenomena to more general geological problems, and to continue the merger
of the once exotic field of impact geology with mainstream geosciences.
Since the recognition of an impact event at the Cretaceous-Tertiary (K-T)
boundary, much current activity in impact geology has been promoted by
traditionally trained geoscientists who have unexpectedly encountered
impact effects in the course of their work. Their studies have involved:
1) the recognition of additional major impact effects in the geological
record (the Chesapeake Bay crater, the Alamo breccia, and multiple layers
of impact spherules in Precambrian rocks); and 2) the use of impact
structures as laboratories to study general geological processes (e.g.,
igneous petrogenesis at Sudbury, Canada and Archean crustal evolution at
Vredefort, South Africa). Other research areas, in which impact studies
could contribute to major geoscience problems in the future, include: 1)
comparative studies between low-level (≤7 GPa) shock deformation of
quartz, and the
production of quartz cleavage, in both impact and tectonic environments;
and 2) the nature, origin, and significance of bulk organic carbon (“kerogen”)
and other carbon species in some impact structures (Gardnos, Norway and
Sudbury, Canada).
Observations at
terrestrial impact structures: Their utility in constraining crater
formation
Richard A. F. GRIEVE* and Ann M. THERRIAULT
*Corresponding author. Earth Sciences Sector, Natural Resources Canada,
588 Booth Street, Ottawa, Ontario K1A 0Y7, Canada
E-mail:
richard.grieve@nrcan.gc.ca
Hypervelocity impact involves the
near instantaneous transfer of considerable energy from the impactor to a
spatially limited near-surface volume of the target body. Local geology of
the target area tends to be of secondary importance, and the net result is
that impacts of similar size on a given planetary body produce similar
results. This is the essence of the utility of observations at impact
craters, particularly terrestrial craters, in constraining impact
processes. Unfortunately, there are few well-documented results from
systematic contemporaneous campaigns to characterize specific terrestrial
impact structures with the full spectrum of geoscientific tools available
at the time.
Nevertheless, observations of the terrestrial impact record have
contributed substantially to fundamental properties of impact. There is a
beginning of convergence and mutual testing of observations at terrestrial
impact structures and the results of modeling, in particular from recent
hydrocode models. The terrestrial impact record provides few constraints
on models of ejecta processes beyond a confirmation of the involvement of
the local substrate in ejecta lithologies and shows that Z-models are, at
best, first order approximations. Observational evidence to date suggests
that the formation of interior rings is an extension of the structural
uplift process that occurs at smaller complex impact structures. There
are, however, major observational gaps and cases, e.g., Vredefort, where
current observations and hydrocode models are apparently inconsistent. It
is, perhaps, time that
the impact community as a whole considers documenting the existing
observational and modeling knowledge gaps that are required to be filled
to make the intellectual breakthroughs equivalent to those of the 1970s
and 1980s, which were fueled by observations at terrestrial impact
structures. Filling these knowledge gaps would likely be centered on the
later stages of formation of complex and ring structures and on ejecta.
Early
fracturing and impact residue emplacement: Can modelling help to
predict their location in major craters?
Anton KEARSLEY,* Giles GRAHAM, Tony McDONNELL, Phil BLAND, Rob HOUGH,
and Paul HELPS
*Corresponding author. Department of Mineralogy, The Natural History
Museum, Cromwell Road, London SW7 5BD, UK
E-mail:
antk@nhm.ac.uk
Understanding the nature and
composition of larger extraterrestrial bodies that may collide with the
Earth is important. One source of information lies in the record of
ancient impact craters, some of which have yielded chemical information as
to the impacting body. Many deeply eroded craters have no remaining melt
sheet or ejecta yet may contain impactor residue within basement
fractures. The emplacement mechanism for fractionated siderophile residues
is likely to be gaseous, although, melt droplets and some solid materials
may survive. For breccia- and melt-filled fractures to contain
extraterrestrial material, they must form very early in the impact
process. Most current numerical models do not dwell on the formation and
location of early major fractures, although, fractures in and around small
craters on brittle glass exposed to hypervelocity impact in low Earth
orbit have been successfully simulated. Modelling of fracture development
associated with larger craters may help locate impact residues and test
the models themselves.
Mass-velocity
distributions of fragments in oblique impact cratering on gypsum
Naomi ONOSE* and Akira FUJIWARA
The Institute of Space and Astronautical Science, Yoshinodai 3-1-1,
Sagamihara, Kanagawa, 229–8510, Japan
*Corresponding author. E-mail:
onose@planeta.sci.isas.ac.jp
Oblique impact cratering experiments
into gypsum targets were performed, and masses and velocities of the
fragments were measured within the observational limit of 0.1–100 m/s in
velocity and 0.0003–1 g in mass. The fragments observed were divided in
two groups according to ejection time: early fragments ejected conically
within a few msec after the impact followed by late fragments consisting
of hundreds of slow, small fragments ejected almost perpendicular to the
target. The relationship between mass and velocity of early fragments was
observed to follow a power law with an exponent of –0.11 ± 0.06,
consistent with previous studies (e.g., Nakamura and Fujiwara 1991; Giblin
et al. 1998). The cumulative number of fragments heavier or equal to a
given mass versus fragment mass distributions shows a power law exponent
of –1.49 ± 0.09 for late fragments
and steeper than –0.49 ± 0.18 for early fragments. More than 10% of the
mass was ejected from the crater with ejection speed slower than 2 m/s.
Those fragments will reaccumulate on porous (<1500 kg/m3) and small (<4 km
in diameter) asteroids.
Marine-target
craters on Mars? An assessment study
Jens ORMÖ,* James M. DOHM, Justin C. FERRIS, Alain LEPINETTE, and
Alberto G. FAIRÉN
*Corresponding author. Centro de Astrobiología (CSIC/INTA) Instituto
Nacional de Técnica Aeroespacial Ctra de Torrejón a Ajalvir, km 4 28850
Torrejón de Ardoz, Madrid, Spain
E-mail:
ormo@inta.es
Observations of impact craters on
Earth show that a water column at the target strongly influences lithology
and morphology of the resultant crater. The degree of influence varies
with the target water depth and impactor diameter. Morphological features
detectable in satellite imagery include a concentric shape with an inner
crater inset within a shallower outer crater, which is cut by gullies
excavated by the resurge of water. In this study, we show that if oceans,
large seas, and lakes existed on Mars for periods of time, marine-target
craters must have formed. We make an assessment of the minimum and maximum
amounts of such craters based on published data on water depths, extent,
and duration of putative oceans within “contacts 1 and 2,” cratering rate
during the different oceanic phases, and computer modeling of minimum
impactor diameters required to form longlasting
craters in the seafloor of the oceans. We also discuss the influence of
erosion and sedimentation on the preservation and exposure of the craters.
For an ocean within the smaller “contact 2” with a duration of 100,000 yr
and the low present crater formation rate, only ~1–2 detectable
marine-target craters would have formed. In a maximum estimate with a
duration of 0.8 Gyr, as many as 1400 craters may have formed. An ocean
within the larger “contact 1-Meridiani,” with a duration of 100,000 yr,
would not have received any seafloor craters despite the higher crater
formation rate estimated before 3.5 Gyr. On the other hand, with a maximum
duration of 0.8 Gyr, about 160 seafloor craters may have formed. However,
terrestrial examples show that most marine target
craters may be covered by thick sediments. Ground penetrating radar
surveys planned for the ESA Mars Express and NASA 2005 missions may reveal
buried craters, though it is uncertain if the resolution will allow the
detection of diagnostic features of marine-target craters. The
implications regarding the discovery of marine-target craters on Mars is
not without significance, as such discoveries would help address the
ongoing debate of whether large water bodies occupied the northern plains
of Mars and would help constrain future paleoclimatic reconstructions.
Tectonic
influences on the morphometry of the Sudbury impact structure:
Implications for terrestrial cratering and modeling
John G. SPRAY,* Hadyn R. BUTLER, and Lucy M. THOMPSON
*Corresponding author. Planetary and Space Science Centre, Department of
Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, New
Brunswick, E3B 5A3, Canada
E-mail:
jgs@unb.ca
Impact structures developed on
active terrestrial planets (Earth and Venus) are susceptible to pre-impact
tectonic influences on their formation. This means that we cannot expect
them to conform to ideal cratering models, which are commonly based on the
response of a homogeneous target devoid of pre-existing flaws. In the case
of the 1.85 Ga Sudbury impact structure of Ontario, Canada, considerable
influence has been exerted on modification stage processes by late Archean
to early Proterozoic basement faults. Two trends are dominant: 1) the
NNW-striking Onaping Fault System, which is parallel to the 2.47 Ga
Matachewan dyke swarm, and 2) the ENE-striking Murray Fault System, which
acted as a major Paleoproterozoic suture zone that contributed to the
development of the Huronian sedimentary basin between 2.45–2.2 Ga. Sudbury
has also been affected by syn- to post-impact regional deformation and
metamorphism: the 1.9–1.8 Ga Penokean orogeny, which involved NNW-directed
reverse faulting, uplift, and transpression at mainly greenschist facies
grade, and the 1.16–0.99 Ga Grenville orogeny, which overprinted the SE
sector of the impact structure to yield a polydeformed upper amphibolite
facies terrain. The pre-, syn-, and post-impact tectonics of the region
have rendered the Sudbury structure a complicated feature.
Careful reconstruction is required before its original morphometry can be
established. This is likely to be true for many impact structures
developed on active terrestrial planets. Based on extensive field work,
combined with remote sensing and geophysical data, four ring systems have
been identified at Sudbury. The inner three rings broadly correlate with
pseudotachylyte (friction melt) -rich fault systems. The first ring has a
diameter of ~90 km and defines what is interpreted to be the remains of
the central uplift. The second ring delimits the collapsed transient
cavity diameter at ~130 km and broadly corresponds to the original melt
sheet diameter. The third ring has a diameter of ~180 km. The fourth ring
defines the suggested apparent crater diameter at ~260 km. This
approximates the final rim diameter, given that erosion in the North Range
is <6 km and the ring faults are steeply dipping. Impact damage beyond
Ring 4 may occur, but has not yet been identified in the field. One or
more rings within the central uplift (Ring 1) may also exist. This form
and concentric structure indicates that Sudbury is a peak ring or, more
probably, a multi-ring basin. These parameters provide the foundation for
modeling the formation of this relatively large terrestrial impact
structure.
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