meteorite impacts

Meteorite impacts

Comparative energiesNo 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 1994impacts hitting Jupiter

Direct observations of meteorite impactsIn 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 impactsPreserved 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

ManicouaganThe 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

St. Lawrence RiverManicouagan

Central uplift

Asteroids and the Asteroid BeltThe 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

AsteroidsAsteroids are rocky fragments (diameter 10m to 1000 km) which either:

failed to consolidate into a planet, or

represent remnants of a fragmented planet

AsteroidsMetallic: some stony types are strong and hard and may hit the Earth.

Weak, friable types likely will explode in the atmosphere at high altitudes.

CometsComets 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

CometsThe 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 1994This comet was first detected on 24 March 1993

It was broken apart by a close pass to Jupiter on 7 July 1992Hubble image,1 July 1993

The sequence of eventsThe 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

EnergiesFragment 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 definitionsMeteor: 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 chondrulesPhoto of a carbonaceous chondrite (carbon-bearing)

Types of meteorites derived from asteroidsAchondrites have a metallic core and stony silicate mantle

As asteroids fragment, both metallic and silicate pieces are producedMetallic coreStony silicate mantle

Iron meteoritesThese consist of nearly pure metallic nickel and iron

This photo shows an iron meteorite named ARISPE

Stony-iron meteoritesThese are a mixture of the previous two types

Often they are fragmental, suggestive of violent processes

This stony-iron meteorite is named ESTHER

Impact events1. Probabilities

2. Nature of the event

3. Consequences

4. Mitigation

1. Probabilities of a collisionWhat 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 - ZebrowskiZebrowski 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 populationFrom Zebrowski (1997)

Probabilities - CourtillotIs 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 destructiveFrom 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 erodedEjecta blanketfracturingBroken rock

Central upliftComplex craters are generated by rebound of the central core

This core, as it decompresses, may meltmelt

There are about 200 large, well-preserved impact craters worldwideBUT>>200 impact events during Earths historyThis 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 2For 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 4All 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 5One 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)

MitigationThe 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 1908Something big seems to have exploded in the atmosphere

The exact cause is uncertain, but we suspect a comet or a meteorAerial 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 altitudeTree blowdown from the explosions; Note parallel alignment of the trees

Big firesIn the central region, forests flashed to fires which burned for weeks

a herd of 600-700 reindeer was incinerated

Aligned treesTrees 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 foundFelled trees aligned parallel to each other

Area of devastation superimposed on a map or Rome. Yellow=charred trees; Green=felled treesThe 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