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/