IMPACT! – BOLIDES, CRATERS,
AND CATASTROPHES
Impacts everywhere!
Even the bolides themselves are cratered!
Galileo image of
asteroid Gaspra
(ca 18 km from end to
end). Image P-40450-C
courtesy of NASA/J et
Propulsion laboratory
W. Uwe Reimold1 and Fred Jourdan2
1811-5209/12/0008-0019$2.50 DOI: 10.2113/gselements.8.1.19
I
t is now universally accepted that the impact of planetesimals, asteroids,
and comets has been a fundamental process throughout the Solar System.
Catastrophic impact events have been instrumental in developing the
early history of the planets and have caused environmental disasters
throughout Earth history. A major mass extinction at the Cretaceous–
Paleogene boundary has been confidently related to an impact event
(Chicxulub, Mexico). While the study of impact cratering is a multidisciplinary
field, mineralogical and geochemical investigations have been central since
the beginning, focusing on the nature of impact-generated rocks and of the
extraterrestrial projectiles as well as their interaction with geological materials. Chemical and isotopic techniques have allowed the dating of impact
events and the identification of traces of meteoritic projectiles in impactformed rocks on Earth and the Moon.
in the recent past, judging by its
youthful appearance and the
excellent preservation of iron
meteorite fragments (Earth Impact
Database 2011). On Earth, only
very young impact structures that
have not yet fallen prey to erosion
or are covered with younger sediment have fresh geometries; older
structures are variably degraded
(see also Turtle et al. 2005; Collins
et al. 2012 this issue).
We now know that impact
cratering is a fundamental process
in the Solar System (B ox 1). The
accretion of tiny particles, then of
Keywords : impact cratering, impactites, shock metamorphism, shatter cones,
planetesimals, and finally of
projectile identification, mass extinctions
planets involved repeated impacts.
Impact events contributed to the
primordial kinetic energy required
IMPACT CRATERING
for the internal differentiation of
IN THE SOLAR SYSTEM
planetesimals and later of planet-sized bodies; they shaped
Some 50 years of space exploration have demonstrated that the earliest crustal fragments, and they were fundamental
all solid surfaces of planetary bodies in the Solar System— in the formation of early crustal lithologies. Time and time
the terrestrial planets (Fig. 1), the moons, the minor again, impact events led to the destruction or formation
planets—and even the cores of comets are peppered with of planetary bodies through gigantic collisions (the most
famous example being the impact event related to the
the results of cosmic bombardments. A range of crater
geometries, such as simple bowl shapes (Fig. 1a, d), complex genesis of the Moon; Canup and Asphaug 2001). The role
of impact cratering during the earliest stages of life develgeometries with a central uplift (Fig. 1a, b) or central peak
opment on our planet is widely debated: Did the giant
ring (Fig. 1c), and multi-ring impact structures, is preserved
impacts of comets bring life-sustaining water to the planet?
on these surfaces (e.g. Reimold and Koeberl 2008). The
Did the seemingly hostile interiors of impact craters harbor
largest, very old impact structures known in the Solar
beneficial microclimates and chemistries suitable for
System measure thousands of kilometers in diameter (e.g.
spawning primitive organisms? Did the intense impact
the 3000 km diameter Valhalla basin on Callisto, a moon
of Jupiter). At the other end of the spectrum, the smallest activity during the first half billion or so years of Earth’s
history frustrate the development of any primitive forms
impact craters known are found on the surfaces of glass
of life prior to the formation of the oldest-known terrestrial
beads in the lunar regolith and on microtektites.
life about 3.8 billion years (Ga) ago.
When fragments of comet Shoemaker-Levy 9 impacted the
Large-scale impact events were prominent in the early
atmosphere of planet Jupiter in July 1994, the world took
stages of Solar System development, as amply demonstrated
note that catastrophic impact events in the Solar System
are not just a phenomenon of the distant past. Recently, by the cratered highlands of the Moon (Stöffler et al. 2006),
two small impact craters were discovered at Carancas, Peru, Mars, and Mercury (Fig. 1a) (see reviews in Koeberl 2006a,
b; Grieve et al. 2006). In contrast, evidence for early impact
and Kamil, Egypt. The Peruvian crater was formed on 15
events is rare on Earth. Only a small number of distal
September 2007, and the Egyptian crater was also created
impact deposits, with ages of ~3.5 and ~2.5 Ga (Glass and
Simonson 2012 this issue), are known from the first half
of the history of our planet, and the oldest-known impact
1 Museum für Naturkunde – Leibniz Institute
structure is Vredefort (South Africa) at 2.02 Ga. This lack
at Humboldt University Berlin
Invalidenstrasse 43, 10115 Berlin, Germany
of evidence for early impacts is clearly the result of plate
E-mail: [email protected]
tectonics on our dynamic planet, which has caused the
2 Western Australian Argon Isotope Facility
obliteration of numerous impact structures and their
Department of Applied Geology & JdL Centre, Curtin University
deposits through erosion, deposition of a sedimentary
GPO Box U1987, Perth, WA 6845, Australia
cover, tectonic dismemberment, or subduction.
E-mail: [email protected]
E lements , V ol . 8,
pp.
19–24
19
F ebruary 2012
A
B
C
D
sensing and field work operations are hampered, the search
for impact structures has been seriously hindered. On the
other hand, there still is much potential for discovering
impact structures in the desert regions of the Middle East
and Africa, in Asia, and in Antarctica.
(A) High-resolution image mosaic of a densely
cratered surface on Mercury (Messenger spacecraft,
October 2008, flyby2_20081007) showing simple bowl-shaped
impact structures, complex structures with central uplifts, and peak
ring structures. The largest crater structure at the top of the image
is 133 km in diameter. (B) The complex impact crater Verdi (150 km
diameter) on Mercury. Credit For (A) and (B): NASA/Johns Hopkins
U niversity A pplied Physics L aboratory/C arnegie Institution of Washington
(C) Mosaic of Clementine UV-VIS images of the Schrödinger impact
basin (312 km diameter) on the Moon. Source : www.lroc. asu.edu ;
credit : NASA/GSFC/A rizona State U niversity (D) Aerial photograph of
a simple, bowl-shaped crater (Wolfe Creek, Australia, 0.9 km diameter). Courtesy of A. B evan
Figure 1
A single impact structure (Mjølnir) is known on oceanic
crust, which covers about two-thirds of Earth’s surface. As
the entire oceanic crust can be recycled in only 150 to 250
million years, its earlier impact record has been lost.
The study of impact structures on the ground can contribute
excellent subsurface stratigraphic information. A notable
example is the deeply eroded Vredefort structure in South
Africa, which has provided insight into the nature of the
mid-crust and the Kaapvaal craton’s geological evolution
over nearly 3.4 billion years. The high-resolution imagery
of Martian impact structures has also provided important
information on the subsurface of that planet.
Of the 180 or so confirmed impact structures known on
Earth (Fig. 2), only three are suspected to be of the multiring-basin type (Grieve et al. 2008): Vredefort, at 250 km
diameter the largest-known terrestrial impact feature
(Gibson and Reimold 2008); Sudbury, Canada (200–250
km, ~1.85 Ga); and Chicxulub, Mexico (180 km, ~66
million years, Ma). Only a few structures have Precambrian
ages, and the impact age population is strongly biased
towards the last 250 million years (Jourdan et al. 2012 this
issue). The spatial distribution of known impact structures
is also uneven, with most known structures located in
North America, northern Europe, and Australia. In heavily
vegetated regions, such as the rainforest belt where remote
E lements
Based on the early studies of impact structures, mostly in
North America and Russia, much has been learned about
the highly dynamic impact process (Melosh 1989; Collins
et al. 2012). Extraterrestrial projectiles approach their
targets at hypervelocity speeds (on average, 15–20 km/s
for asteroids and 30–35 km/s for comets). The considerable
20
F ebruary 2012
IMPACTITES AND SHOCK METAMORPHISM
Box 1
A series of distinct rock types is generated by the destructive forces of impact, beginning with compression and
followed by decompression from peak shock pressures, with
concomitant heat generation. These impactite types
(nomenclature recommended by Stöffler and Grieve 2007)
range from purely clastic breccias composed of fragments
of target rock to impact melt rocks composed of a groundmass of melt containing clasts of target rock and minerals.
The transitional rock type is known as suevite. Note that
the recommendations have not remained unchallenged
(e.g. the nature of suevite groundmass and the genesis of
pseudotachylitic breccia; Reimold et al. 2008).
IMPACT CRATERING:
A FUNDAMENTAL PLANETARY PROCESS
ƒƒ Impact cratering has been an accretionary process
throughout planetesimal/planet evolution.
ƒƒ It is a fundamental agent shaping the surfaces of all
solid planetary bodies (planets, asteroids, comet nuclei).
ƒƒ Impact events had a decisive, yet poorly understood, role
in Archean geological evolution.
ƒƒ Impact crater counting provides a means for relative
surface chronology on planetary bodies accessible only
to remote sensing.
ƒƒ Impact craters provide important targets for the study
of properties (e.g. presence of water, near-surface
stratigraphy, sedimentary processes) on otherwise
inaccessible bodies.
The irreversible effects on rocks and minerals that are
caused by the progression of a shock front through the
target rock (i.e. compression) and by decompression immediately thereafter are generally referred to as shock metamorphism. In the 1960s, a number of impact structures
were studied in great detail, and in cases where it was
possible to investigate comprehensive profiles from the
center of a structure outward, it was recognized that the
degree of shock deformation decreases with radial distance
from the center. During this period, the principles of
progressive shock deformation were established. Specific
petrographic indicators can be related to different shock
pressure levels (Fig. 3). In order of increasing shock pressure, these include planar fractures, planar deformation
features (PDFs), diaplectic mineral glasses (produced
without fusion), fused mineral glasses, and whole rock
melts. For many rock-forming minerals, shock experiments
in the <5 to >100 gigapascal (GPa) pressure range have
calibrated the pressures at which specific shock effects
occur.
ƒƒ Asteroids and comets can be studied through surface
analysis (chronology, strength analysis) or artificial
impact experiments. These studies yield information
on target composition and behavior.
ƒƒ Catastrophic impact events have had a role throughout
Earth history in the development of life and have been
implicated widely in the debates about the causes of
mass extinctions and major radiation events.
ƒƒ The risk of future impacts on Earth should be assessed.
ƒƒ Economic resources have been accumulated as a result
of impacts (ore deposits, fossil fuel reserves, water
reservoirs), and many impact structures represent sites
of high educational, and sometimes also recreational,
value.
kinetic energy of such bolides is transferred via a shock
front into the target rock and causes irreversible deformation and transformation (Langenhorst and Deutsch 2012
this issue). Three stages of impact cratering are distinguished: (1) the contact and shock compression stage; (2)
the excavation stage, leading to the formation of the transient crater; and (3) the collapse or modification stage,
whereby the transient crater succumbs to gravity (Melosh
1989; French 1998).
Distinctive shock effects occur in a P–T regime entirely
separate from that of normal crustal metamorphism
(Fig. 3), which explains why PDFs and other features are
unique characteristics of impact (e.g. Stöffler and
Langenhorst 1994; Grieve et al. 1996; French and Koeberl
2010). Some of these shock features can be recognized with
an optical microscope, but often more sophisticated
approaches, such as transmission electron microscopy or
microRaman spectrometry, are required (Langenhorst and
Distribution of the approximately 180 confirmed
impact structures known on Earth as of February 2011
(Earth Impact Database 2011).
Figure 2
E lements
21
F ebruary 2012
Deutsch 2012). There is also a series of shock metamorphic
calibrations for felsic and mafic target rocks, into which
the experimental findings for several target-rock minerals
have been integrated (Stöffler and Grieve 2007).
3000
2000
Stishovite
Coesite
Coe
Quartz
40
400
00
600
8
800
00
-1
SC
PDF
w
wa
wave
ave number
(cm )
“Normal
Crustal
Metamorphism”
0.1
0.5
1
5
10
Pressure [GPa]
Pressure (P) –temperature (T ) diagram, showing the
different regimes for “normal” crustal metamorphism
and shock metamorphism. Various P–T regions characterized by the
development of specific shock-metamorphic effects are indicated.
Also shown are the stability fields for quartz (Qz), coesite (Coe),
and stishovite (Stv), typical Raman spectra for these minerals, and
liquidi for the various SiO2 polymorphs. SC = shatter cone; PDF =
planar deformation feature. D iagram by Jörg Fritz, M useum für
Naturkunde B erlin
Figure 3
Shock deformation in quartz (e.g. PDFs) is most important,
because this mineral occurs in numerous crustal rock
types. Zircon, which is particularly resistant to weathering
and metamorphic overprint, is very useful for the investigation of the presence of shock deformation and for determining age (Jourdan et al. 2012), especially of old, strongly
overprinted rocks.
platinum-group elements (PGEs) and of several isotopic
methods (Re–Os and Cr isotope systematics) has become
prominent. These methods have allowed the detection of
very small amounts (<0.2%) of components derived from
the extraterrestrial impactor and, in some cases, the identification of the nature of the impactor.
THE TOOLS OF
IMPACT-CRATERING STUDIES
Box 2
RECOGNITION CRITERIA FOR IMPACT
STRUCTURES AND DEPOSITS
Over the last 50 years, we have made great progress in
understanding the physical and chemical processes related
to this complex, catastrophic, and highly dynamic (ultrahigh shock pressures and temperatures, very high strain
rates) phenomenon. However, there are still significant gaps
in our knowledge. For example, the generation of the
common impact breccia known as suevite is not sufficiently
understood.
First indications may be a circular structure that is
anomalous in the regional geology, or a circular or annular
geophysical anomaly; neither is diagnostic and an impact
association must be confirmed by the presence of the
following criteria:
ƒƒ Shatter cones – To date they have only been described
from impact structures (and nuclear explosions), where
they are formed in the shock pressure range of ~2 to
30 GPa.
At the very least, five vital aspects of impact cratering
require further exploration:
ƒƒ Diagnostic shock-metamorphic effects – planar
deformation features; planar fractures in zircon;
diaplectic glass; high-pressure mineral polymorphs such
as diamond, coesite (in upper crustal rocks), stishovite,
and ringwoodite (Langenhorst and Deutsch 2012;
Collins et al. 2012)
1. Our understanding of the basic process must be
improved to have a better comprehension of impact
energy generation and distribution.
2. The existing data do not adequately allow the identification of rocks that were impact-deformed in the lowshock-pressure regime, below the onset of PDF formation
(ca 8–10 GPa). Notably, rocks subjected to such low
shock pressure represent the major part of the entire
affected target-rock volume.
ƒƒ Chemical or isotopic traces of an extraterrestrial
projectile (Koeberl et al. 2012)
Impact melt rock has the added potential to contain traces
of the meteoritic projectile (Koeberl et al. 2012). Upon
impact, most of the mass of the projectile is vaporized and
only a small amount may be incorporated into impact melt
rock, which is generated close to the point of projectile
disintegration. Sophisticated analysis, at the highest
possible resolution, of the abundances of elements that
might distinguish projectile and target rock is required.
Much of the past work has focused on the analysis of siderophile elements such as Ni and Co, but lately, the use of the
3. The interaction of shock waves (i.e. shock metamorphism) with some geological materials is not well understood (e.g. basaltic rocks, despite their ubiquity in the
Solar System).
4. We need to know if, besides shatter cones, there are
other macro- or mesoscopic criteria that could unambiguously confirm the presence of impact structures in
the field.
22
F ebruary 2012
50
Melting
200
2
00
Vaporisation
1000 Qz
Diaplectic
Glass
In the interest of the continuing improvement of the terrestrial impact-cratering record, and particularly of the
ancient impact record, recognition criteria for the products
of impact are required (Box 2) (French and Koeberl 2010;
Koeberl et al. 2012 this issue). The geometry and
morphology of suspected impact structures observed by
remote sensing, circular and annular geophysical anomalies, and seismic information can all provide valuable first
hints for the presence of an impact structure; however, in
each case, confirmation by ground truthing is required
(Reimold and Koeberl 2008). Where a structure is known
from remote sensing data only and is not accessible at
surface, drilling is mandatory to obtain rock samples and
evidence of shock metamorphism. Field work is required
to search for shatter cones (see inset in Fig. 3), which have
been described only in impact structures and in man-made
nuclear explosions. Rock breccias, especially those with
melt, are important, as they may contain shock-metamorphosed lithic or mineral inclusions covering a wide range
of shock pressures and may be suitable for dating (Jourdan
et al. 2012).
Stv
Intensity (a.u.)
Temperature [°C]
Liquid
RECOGNITION CRITERIA FOR IMPACT
STRUCTURES AND DEPOSITS
E lements
“Shock
Metamorphism”
Raman Spectra
100
5. The Earth’s impact record through time must be further
improved to fully understand the role of impact catastrophes, in particular with respect to the evolution of
the planet and of life (e.g. Pierazzo and Artemieva 2012
this issue).
importance of impact events and, in recent years, chapters
about impact cratering have regularly appeared in new
textbooks. In IMPACT! we share with readers of Elements
our enthusiasm for this burgeoning branch of Earth and
planetary sciences.
Impact-cratering and impactite studies have been conducted
using four methodologies: (1) multidisciplinary analysis of
impact structures; (2) laboratory studies of impact-affected
rocks and minerals; (3) experimental calibration of shockmetamorphic stages and experimental cratering; and (4)
numerical-modeling approaches designed to investigate,
in great detail, the influence of key parameters on the
cratering process, but also to constrain the environmental
effects of large-scale, catastrophic impact (Pierazzo and
Artemieva 2012).
ACKNOWLEDGMENTS
This article benefited from insightful reviews by Richard
Grieve and Sandro Montanari. We are particularly grateful
to the authors of the articles in this issue for their expert
work and their great support during the various phases of
preparation of the issue. Elements editors Pierrette Tremblay
and Hap McSween were instrumental in keeping us on our
toes and made valuable input throughout the process. The
following reviewers enhanced the standard of the papers:
Gareth Collins, Richard Grieve, Peter Haines, Friedrich
Hörz, Christian Koeberl, Sandro Montanari, Marc Norman,
Birger Rasmussen, Birger Schmitz, Claudia Trepmann,
Mario Trieloff, Axel Wittmann, Kai Wünnemann, and an
anonymous referee. We also thank all colleagues who made
their material available for inclusion in these articles.
Impact-cratering studies are a vibrant, integrated, multidisciplinary branch of science. They have brought together
scientists from astronomy and planetary exploration,
geology, geophysics, remote sensing, mineralogy, geochemistry, geochronology, life sciences, and other fields. At the
same time, other disciplines have now taken note of the
Dedication of this issue to Elisabetta (Betty) Pierazzo
On 15 May 2011, during the finalization of the papers in this issue, one of our
contributors and a close friend, Dr. Elisabetta (Betty) Pierazzo, passed away at the
age of only 47 after a short but serious illness. The authors and editors of this issue
would like to honor Betty by dedicating IMPACT! to her and her scientific achievements. We all have lost a very close friend and esteemed colleague in the impactcratering field.
Betty Pierazzo in her element –
doing fieldwork at Wetumpka
impact crater (Alabama, USA)
Betty was a senior scientist at the Planetary Science Institute, Tucson (USA), and an
expert in the fields of impact modeling and planetology, work that certainly took
her around the Solar System. She made major contributions to the understanding
of the Chicxulub impact and its environmental consequences and to cratering
processes during the formation of Meteor Crater. She placed constraints on the
thickness of ice on Jupiter’s satellite Europa, and she investigated the possible delivery
of organics to planets and Europa by comets, as well as hydrothermal systems related
to large impacts on Mars. She led a benchmark effort to cross-validate the main
numerical-modeling software and methods, was passionate about science education
and public outreach, and loved playing soccer. Betty is sorely missed by us—
a cherished friend and dear colleague.
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GLOSSARY
Shatter cone – a conical fracture phenomenon in rock,
with narrow ridges (striae, strahlen) emanating from a
small apex area (see inset to Fig. 3). Shatter cones occur
at scales from centimeters to meters; they are only known
from impact structures and, thus, are considered to be
the only known mesoscopic impact-diagnostic criterion.
The shatter cone–forming process is still largely
unknown.
Asteroid – a rocky or metallic body, ranging in size from
meters to hundreds of kilometers, in orbit around the
Sun. Many asteroids are found in the asteroid belt
between the orbits of Mars and Jupiter.
Comet – a minor body that is composed mainly of water
ice and some silicate (rocky) material. Comets originate
in the Kuiper-Edgeworth Belt and the Oort Cloud, both
far outside the Solar System.
Shock front – the instance of change from material properties just before to material properties under compression.
The duration of this change is ultrashort for relatively
small-scale laboratory-based shock experiments, but is
significantly longer in natural impact events.
Impact breccia – a type of impactite formed entirely of
fragments of rock, minerals, or melt
Shock metamorphism (also known as impact metamorphism) – the irreversible mineral and rock deformation resulting from material compression caused by a
shock wave. Whereas conditions of normal crustal metamorphism range from ambient temperature to about
1000 °C and from atmospheric pressure to perhaps
10 kbar, shock metamorphism ranges up to hundreds of
gigapascals (1 GPa = 10 kbar ≈ 10,000 atm).
Impact crater/structure – the geological structure
resulting from the impact of an extraterrestrial body.
Degradation of a crater feature due to erosion, sedimentary cover, or tectonic dismemberment can seriously
change the original crater geometry, and one should then
use the term impact structure.
Impactite – a rock formed from the impact of a large extraterrestrial projectile
Shock wave – a large-amplitude plastic compression wave.
The amplitude of the shock wave in a given material is
in excess of the elastic limit of a material; therefore,
deformation effects resulting from shock wave deformation are, at relatively low shock pressures, limited to
mechanical deformation (fracturing, cataclasis), but
beyond the elastic limit, they involve plastic deformation
or even phase transformations.
Impact melt rock – Following target penetration by the
projectile, shock pressures are so high that associated
postshock temperatures may exceed the melting temperature of the target rock, producing bulk or partial melts.
Lithic impact breccia – an impact breccia that is
composed entirely of mineral and rock (lithic) fragments
derived from the target rock. One distinguishes monomict
(only one rock type) and polymict varieties.
Suevite – an impact breccia formed from rock and mineral
fragments derived from the target rock(s) plus cogenetic
particles of impact melt. The type locality for suevite is
the Ries crater (Germany).
Diaplectite glass (including maskelynite) – a glass-like
(i.e. isotropic) material produced by shock metamorphism. The precursor mineral has not been fused but
has attained a glass-like state in which the original
crystal shape and defects (inclusions, fractures) are
preserved; maskelynite is the diaplectic glass phase after
plagioclase.
Target rock – the rock type(s) affected by the impact of
an extraterrestrial projectile. Fragments of the target
material are found in impact breccia, and the relative
abundance of different rock fragments can possibly be
used to infer the pre-impact geological composition of
the impact-affected rock volume.
Mass extinction – the demise of a large number of biological species in a relatively short geological time, thereby
greatly exceeding the normal “background” extinction
rate
Tektites – small (millimeters to a few centimeters) glassy
bodies of ballistically transported impact melt formed
from uppermost target material. Some regional-scale
tektite strewn fields are unambiguously linked to specific
impact structures (e.g. moldavites in central Europe are
related to the Ries Crater, and the Ivory Coast tektites to
the Bosumtwi crater in Ghana). Microtektites are less
than 1 mm in size.
Planar deformation features (PDFs) – microscopic
features in rock-forming minerals that originate from
elevated shock metamorphism. PDFs are parallel,
extremely narrow, lamellar, isotropic features that have
crystallographically controlled orientations.
Platinum-group elements (PGEs) – the elements ruthenium, rhodium, palladium, rhenium, osmium, iridium,
and platinum. They are all strongly siderophile and are
highly enriched in meteorites in comparison to rocks of
the Earth’s crust.
E lements
24
F ebruary 2012