Meteorite impacts Comparativ e energies No human in
Meteorite impacts Comparativ e energies No human in past 1,000 years has been killed by a meteorite
Direct observations of meteorite impacts Tunguska, Siberia, 30 June 1908a big bang above the Earths surface Shoemaker-Levy 9, July 1994 impacts hitting Jupiter
Direct observations of meteorite impacts In 1954, a 5-kg meteorite crashed through a house in Alabama the object bounced off a radio and hit the owner in the head Effects upon children
Indirect evidence of meteorite impacts Preserved craters on the continents, mainly the oldest parts (shields) Lac cratre in northern Qubec is a simple crater its rim diameter is 3.4 km, it is 250 m
deep, and it is 1.4 Ma in age Location map of some impact craters seen at the surface Lac cratre Meteor crater in Arizona is another
simple crater showing rim ejecta Manicouagan The Manicouagan crater in Qubec is a spectacular example of a complex crater Its original rim has been removed by erosionthe current diameter is 100 km It has an uplifted central core and outer rings, which are
filled by a lake Its age - 210 Ma - coincides approximately with a large extinction at the end of the Triassic period Manicouagan St. Lawrence River
Central uplift Asteroids and the Asteroid Belt The Asteroid Belt lies between Mars and Jupiterthere are about 4,000 objects As asteroids collide
with one another, they fragment and send pieces into near-Earth orbits Asteroids Asteroids are rocky fragments (diameter 10m to 1000 km) which
either: failed to consolidate into a planet, or represent remnants of a fragmented planet Asteroids Metallic: some stony types are strong and hard and may hit the
Earth. Weak, friable types likely will explode in the atmosphere at high altitudes. Comets Comets come from the far reaches of the Solar System (outer solar system, kuiper Belt and in the Oort Cloud). They mainly consist of frozen water, carbon dioxide, or
both with admixed small rock fragments and dust, thus are referred to as dirty icebergs or dirty snowballs They have highly elongate, elliptical orbits which bring them close to the Sun Comets The tail of the comet is produced as
ices melt and gases and dust particles are shed from the object. Generally explode in the atmosphere at high altitudes. Comet West, 9 March 1976
Comet P/Shoemaker-Levy 9, July 1994 This comet was first detected on 24 March 1993 It was broken apart by a close pass to Jupiter
on 7 July 1992 Hubble image, 1 July 1993 The sequence of events The collision of the comet with Jupiter occurred over several days, 16-22 July 1994
It was the first collision of 2 solar system bodies ever observed At least 20 fragments hit Jupiter at speeds of 60 km/second Energies Fragment A struck with energy equivalent to 225,000 megatons of TNT, the plume rising to
1000 km Fragment G was the biggie, with 6,000,000 megatons TNT energy and a plume rising to 3,000 km Fragment G (and K, L) created dark impact sites whose diameters were at least that of Earths radius
Other definitions Meteor: light through the sky. Most meteors are destroyed in Earths atmosphere. Meteoroid: matter revolving around the Sun or any object in planetary space too small to be called an asteroid or a comet Meteorite: a meteoroid which reaches the surface of the Earth without being vaporized
Stony meteorites (94% of all meteorites) Two types: Chondritescontain chondrulesthey are very old and primitive
Achondritesno chondrules Photo of a carbonaceous chondrite (carbon-bearing) Types of meteorites derived from asteroids
Achondrites have a metallic core and stony silicate mantle Metallic core As asteroids fragment, both
metallic and silicate pieces are produced Stony silicate mantle Iron meteorites These consist of nearly pure
metallic nickel and iron This photo shows an iron meteorite named ARISPE Stony-iron meteorites These are a mixture of
the previous two types Often they are fragmental, suggestive of violent processes This stony-iron meteorite is named ESTHER
Impact events 1. Probabilities 2. Nature of the event 3. Consequences 4. Mitigation 1. Probabilities of a collision What are the chances of a large meteorite
hitting Earth? As of 2003, ~700 objects with diameters > 1 km known to have orbits which intersect that of Earth And 30 new objects are discovered each year, with the search only 8% complete! Probabilities - Zebrowski
Zebrowski shows that, on average, collisions of 1 kmdiameter objects occur every 250,000 years Such an impact is sufficient to wipe out most of the human
population From Zebrowski (1997) Probabilities - Courtillot Is Zebrowskis estimate too high? Courtillot suggests it is
about 1 Ma between events In any case, you can see that these events are both very rare and very destructive From Courtillot (1999)
2. Nature of the event Impact cratering is an important process in the history of Earth and other planets 107 to 109 kg of meteoritic flux strikes Earth each year, mostly in the form of
dust Impact events The cratering process is very rapid Since the objects travel so fast (4-40 km/second), a huge amount of energy is transferred upon impact
Cratering A blanket of ejecta is dispersed around the crater rock is fractured, crushed, and broken In large impact events, the rock can even be vaporized (depending on the type of rock)
Cratering (continued) Very high pressures are reached, resulting in shock metamorphism (pressure-temperature increases) After the initial compression comes decompression, which may cause the rock to melt
Ejecta blanket Broken rock fracturing
Simple craters are basically simple bowls With time, the ejecta blanket outside the crater is eroded melt Central uplift
Complex craters are generated by rebound of the central core This core, as it decompresses, may melt There are about 200 large, well-preserved impact
craters worldwideBUT>>200 impact events during Earths history This map shows both SURFACE and SUB-SURFACE examples Consequences of a large impact event
These would apply for an object of about 1 km or larger Actually, you may not want to hear the list of death and destruction (or maybe you do)... Consequences 1
A base surge, similar to a volcanic pyroclastic flow, will be generated by the impact For a terrestrial impact, rock will be pulverized and/or vaporized, sending up huge amounts of dust into the stratosphere
Consequences 2 For an oceanic impact: huge amounts of water will be vaporized Global tsunamis will be generated, which will ravage the Earths coastlines Consequences 3
In the short term, global wildfires will be generated by the impact event These fires will burn uncontrollably across the globe, sending more soot, dust, and gas into the stratosphere Consequences 4 All this suspended dust and soot will cause global winter and global darkness
Acid rains will fall Crops will fail catastrophically The end result will be MASS EXTINCTIONS Consequences 5 One last interesting point: The impact likely will trigger devastating quakes around the globe, especially where
tectonic stresses are high (i.e., plate margins) Volcanism (flood basalts) may occur on the opposite side of the globe from the impact, as a result of shock waves travelling through the center of the Earth From Murck et al. (1996)
Mitigation The problem is the possibility of little or no warning There are proposals to use nuclear weapons and satellites to shoot down or destroy such killer objects For further edification, rent Armegeddon from Blockbuster (1998)
Good subject for a paper ! Two case studies Tunguska 1908, Russia The Cretaceous-Tertiary extinction, 65 Ma
Tunguska, Russia, 30 June 1908 Something big seems to have exploded in the atmosphere The exact cause is uncertain, but we
suspect a comet or a meteor Aerial view of Tunguska Natural Reserve What happened? The objects entry appeared to be at
an angle of 30-35 The object shattered in a series of explosions at about 8 km altitude
Tree blowdown from the explosions; Note parallel alignment of the trees Big fires In the central region, forests flashed to fires which burned
for weeks a herd of 600700 reindeer was incinerated Aligned trees Trees were felled in a radial sense
About 2,000 km2 were flattened by the blasts What happened? Our best scientific guess is that it was part of a comet 20-60
meters in diameter no crater was found and no meteoritic debris has been found Felled trees aligned parallel to each other
Area of devastation superimposed on a map or Rome. Yellow=charred trees; Green=felled trees The lack of a crater suggests disintegration above the surface
of the Earth The lack of solid debris implies a comet rather than an asteroid A global view
Soot from the fires circled the globe, producing spectacular sunrises and sunsets for months afterward The Tunguska event was the largest known comet/asteroid event in the history of civilization Impact events and mass
extinctions In the Phanerozoic (570-0 Ma), there have been two great extinctions of fauna and flora: 1) end of the Permian Period at about 250 Ma 2) end of the Cretaceous Period at 65 Ma These extinctions serve to divide geologic time in the Phanerozoic into three main eras
The Cretaceous-Tertiary (KT) extinction at 65 Ma End of the dinosaurs and other species In fact, about two-thirds of all species wiped out 80% of all individuals killed off Thereafter, mammals took over What caused the extinction?
The two main theories are: (1) a meteorite impact (2) flood basalt volcanism Some important questions Was the extinction of the dinosaurs rapid or prolonged? Or both? In other words, prolonged followed by
abrupt? Did a meteorite impact trigger volcanism? Note location of the Chicxulub crater to the Deccan basalts Was it a meteorite? Evidence for meteorite impact
High iridium at the K-T boundary Unique to the K-T boundary? 9 parts per billion (ppb) Ir in clay at the boundary Background in area <<1 ppb Earths crust < 0.1 ppb Some metallic meteorites ~500 ppb Iridium and the dinosaurs
The high iridium is coincident with the disappearance of the dinosaurs, as seen in the fossil record No dinosaur fossils above the K-T boundary, whereas there are lots below, as old as 165 Ma The iridium
The iridium may have come from impact of a metallic meteorite Circulation and settling of Ir-rich dust would result in global distribution of Ir at the K-T boundary Global effects
The atmospheric dust and gas from the impact event would cause global cooling (compare with nuclear winter) Global wildfires also would have been ignited by the fireball Other meteorite evidence
Spherulesthese represent melt droplets dispersed globally from the impact Shocked quartz this requires high pressures
Shocked quartz under the microscope The impact crater Located in the Yucatan Peninsula of Mexico, it is called Chicxulub It is completely buried, and was located
by petroleum geologists The size of the crater implies a meteorite about 10 km in diameter Chicxulu b crater Approx 300 km
Some incidental facts Many of the rocks associated with Chicxulub are evaporite sedimentary rocks (gypsum, anhydrite, etc.) containing sulfur (CaSO4) This sulfur may have been vaporized to produce sulfate aerosols in the atmosphere, contributing to global cooling
Incidental facts (ctd.) Other rocks in the vicinity are limestones (CaCO3) Vaporization of evaporites and limestone would inject sulfur dioxide and carbon dioxide into the atmosphere Sulfur dioxide causes cooling, CO2 causes
warming Climate change Short-term global cooling from: Dust from impact Soot from wildfires Injection of sulfur
Longer-term global warming from: Injection of CO2 Age of Deccan volcanism Interestingly, the Deccan Traps recently have been dated at 63-67 Ma And most of the volcanism occurred during
a 500,000 year period at 65 Mawhich is the K-T boundary This is basically a geological instant in time Some concluding remarks: meteorites vs. volcanoes Ir from a meteorite? From the Earths mantle via eruptions? The iridium anomaly is found not only at the
K-T boundary, but also extends several meters on either side Has the Ir been redistributed from an originally thin layer at the K-T boundary? Or is it a record of more than a single event? Globally speaking... A meteorite impact into the Chicxulub
region would produce: dust from the impact soot from global fires sulfur gases from evaporite rocks CO2 from limestone Basaltic volcanic eruptions would produce abundant sulfur, and probably CO2 also
Points in favour of a meteorite High iridium global distribution of spherules global distribution of shocked quartz Points in favour of volcanic
eruptions The ecological crisis began 105 years before the Ir-rich horizon and appeared to continue for a period of time afterward (~105 years?) Other mass extinctions appear to show some correlation with flood basalt events
5 major extinctions during the Phanerozoic (570-0 Ma) End Ordovician, 440 Ma end Devonian, 350 Ma end Permian, 250 Ma (Paleozoic-Mesozoic boundary)
end Triassic, 200 Ma end Cretaceous, 65 Ma Cenozoic boundary) (K-T event) (Mesozoic- An interesting aside
The K-T extinction is the only one for which there is good evidence for a meteorite impact Meteorite impacts - readings Alvarez, W., 1997. T. Rex and the crater of doom. Princeton, Princeton University Press.
Alvarez, L.W., W. Alvarez, F. Asaro, H. Michel, 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, v. 208, pp. 1095-1108. Frankel, C., 1999. The end of the dinosaurs. Cambridge, Cambridge University Press. Grieve, R.A.F., 1990. Impact cratering on the Earth. Scientific American, v. 262, pp. 66-73.
Meteorite impacts - web Two general sites of interest: http://neo.jpl.nasa.gov/neo/ http://www.nearearthobjects.co.uk/ Shoemaker-Levy: http://seds.lpl.arizona.edu/sl9/sl9.html
Canadian sites on terrestrial impact craters: http://gsc.nrcan.gc.ca/meteor/index_e.php http://www.unb.ca/passc/ImpactDatabase/
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