Meteorite impacts. Comparative energies No human in past 1,000 years has been killed by a meteorite.
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Transcript of Meteorite impacts. Comparative energies No human in past 1,000 years has been killed by a meteorite.
Meteorite impacts
Comparative energies
No human in past 1,000 years has been killed by a meteorite
Direct observations of meteorite impacts
Tunguska, Siberia, 30 June 1908…a big bang above the Earth’s 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 cratére in northern Québec 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 cratère
Meteor crater in Arizona is another simple crater showing rim ejecta
Manicouagan
The Manicouagan crater in Québec is a spectacular example of a complex crater
Its original rim has been removed by erosion…the 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
St. Lawrence RiverManicouagan
Central uplift
Asteroids and the Asteroid Belt
The Asteroid Belt lies between Mars and Jupiter…there 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 Earth’s radius
Other definitions
Meteor: light through the sky. Most meteors are destroyed in Earth’s 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:
Chondrites…contain chondrules…they are very old and primitive
Achondrites…no chondrules
Photo of a carbonaceous chondrite (carbon-bearing)
Types of meteorites derived from asteroids
Achondrites have a metallic core and stony silicate mantle
As asteroids fragment, both metallic and silicate pieces are produced
Metallic core
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 km-diameter 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 Zebrowski’s 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
Simple craters are basically simple bowls
With time, the ejecta blanket outside the crater is eroded
Ejecta blanket fracturing
Broken rock
Central uplift
Complex craters are generated by rebound of the central core
This core, as it decompresses, may melt
melt
There are about 200 large, well-preserved impact craters worldwide…BUT…>>200 impact events during Earth’s 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 Earth’s 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 object’s 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 600-700 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 (K-T) 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 ppbEarth’s crust < 0.1 ppbSome 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
Spherules…these 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
Approx 300 kmChicxulub crater
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 Ma…which 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 Earth’s 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 (K-T event) (Mesozoic-Cenozoic boundary)
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/