based on the lecture notes of Prof. Stephen A. Nelson, Tulane University
adapted to HTML
A Meteorite is a piece of rock from outer space that strikes the surface
of the Earth. A Meteoroid is a meteorite before it hits the surface of the
Earth. Meteors are glowing fragments of rock matter from outside the
Earth's atmosphere that burn and glow upon entering the Earth's
atmosphere. They are more commonly known as shooting stars.
Some meteors, particularly larger ones, may survive passage through the
atmosphere to become meteorites, but most are small objects that burn up
completely in the atmosphere. They are not, in reality, shooting
stars. Fireballs are very bright meteors. Meteor Showers - During
certain times of the year, the Earth's orbit passes through a belt of high
concentration of cosmic dust and other particles, and many meteors are
observed. The Perseid Shower, results from passage through one of
these belts every year in mid-August.
Throughout history there have been reports of stones falling from the sky,
but the scientific community did not recognize the extraterrestrial origin
of meteorites until the 1700s. Within recent history meteorites have
even hit humans-
1938 - a small meteorite crashed through the roof of a garage in
1954 - A 5kg meteorite fell through the roof of a house in Alabama.
1992 - A small meteorite demolished a car near New York City.
Meteorite fragments have been found all over the surface of the Earth,
although most have been found in Antarctica. In Antarctica they are
easily seen on the snow covered surface or embedded in ice. The fall of
meteorites to the Earth's surface is part of the continuing process of
accretion of the Earth from the dust and rock of space. When these
rock fragments come close enough to the Earth to be attracted by its
gravity they may fall to the Earth to become part of it. As we will
see the evolution of life on the Earth has likely been affected by
collisions with these space objects, and collisions could affect the Earth
in the future as well.
Composition and Classification of Meteorites
Meteorites can be classified generally into three types:
Stones - Stony meteorites resemble rocks found on and within the
Earth. They are the most common type of meteorite, although because
they resemble Earth rocks they are not commonly recognized as
meteorites unless someone actually witnesses their fall. Stony
meteorites are composed mainly of the minerals olivine, and
pyroxene. Some have a composition that is roughly equivalent to
the Earth's mantle. Two types are recognized:
Chondrites - Chondrites are the most common type of stony
meteorite. They are composed of small round glassy looking
spheres, called chondrules, that likely formed from condensation
from the gaseous solar nebula early in the history of the
formation of the solar system. Most chondrites have
radiometric age dates of about 4.6 billion years.
Achondrites - Achondrites are composed of the same minerals as
chondrites, but lack the chondrules. They appear to have been
heated, melted, and recrystallized so that the chondrules are no
longer present. Most resemble volcanic rocks found on the
Irons - Iron meteorites are composed of alloys of iron and nickel.
They are easily recognized because they have a much higher density
than normal crustal rocks. Thus, most meteorites found by the
general populace are iron meteorites. When cut and polished, iron
meteorites show a distinct texture called a Widmanstätten pattern (see
figure 10.5, p. 254 in your text). This pattern results from
slow cooling of a once hot solid material. Most researchers
suggest that such slow cooling occurred in the core of much larger
body that has since been fragmented. Iron meteorites give us a clue to
the composition of the Earth's core.
Stony Irons - Stony iron meteorites consist of a mixture of stony
silicate material and iron. Some show the silicates embedded in
a matrix of iron-nickel alloy. Others occur as a breccia, where
fragments of stony and iron material have been cemented together by
either heat or chemical reactions.
Origin of Meteorites
Most meteorites appear to be fragments of larger bodies called parent
bodies.These could have been small planets or large asteroids that were
part of the original solar system. There are several possibilities
as to where these parent bodies, or their fragments, originated.
The Asteroid Belt
The asteroid belt is located between the orbits of Mars and
Jupiter. It consists of a swarm of about 100,000 objects called
asteroids. Asteroids are small rocky bodies with irregular
shapes that have a cratered surface. About 4,000 of these
asteroids have been officially classified and their orbital paths are
known. Once they are so classified they are given a name.
The asteroids are either remnants of a planet that formed in the region
between Mars and Jupiter but was later broken up by a collision with
another planetary body, or are fragments that failed to accrete into a
planet. The latter possibility is more likely because the total mass
of the asteroids is not even equal to our moon. It does appear that
some of the asteroids are large enough to have undergone internal
differentiation. Differentiation is a process that forms layering in
a planetary body (i.e. the Earth has differentiated into a core, mantle,
and crust). If these larger asteroids did in fact undergo
differentiation, then this could explain the origin of the different types
of meteorites. Because of the shapes of the asteroids it also
appears that some of them have undergone fragmentation resulting from
collisions with other asteroids. Such collisions could have caused
the larger bodies to be broken up into the smaller objects we observe as
The Asteroids as Parent Bodies of Meteorites
Much evidence suggests that the asteroids could be the parent bodies of
meteorites. The larger ones could have differentiated into a core, mantle,
Fragmentation of these large bodies would then have done two things:
First the fragments would explain the various types of meteorites found on
Earth - the stones representing the mantle and crust of the original
parent body, the irons representing the cores, and the stony irons the
boundary between the core and mantle of the parent bodies. Second, the
collisions that caused the fragmentation could send the fragments into
Some of the asteroids have orbits that bring them close to Earth.
These are called Amor objects. Some have orbital paths that cross
the orbital path of the Earth. These are called Earth-crossing
asteroids or Apollo objects. All objects that have a close approach
to the Earth are often referred to as Near Earth Objects or NEOs.
About 150 NEOs with diameters between 1 and 8 km are known, but this is
only a fraction of the total number. Many NEOs will eventually
collide with the Earth. These objects have unstable orbits because
they are under the gravitational influence of both the Earth and
Mars. The source of these objects is likely the asteroid belt.
Comets as Parent Bodies of Meteorites A Comet is a body that orbits
around the Sun with an eccentric orbit. These orbits are not
circular like those of the planets and are not necessarily within the
same plane as the planets. Most comets have elliptical orbits
which send them to the far outer reaches of the solar system and back
toward a closer approach to the sun. As a comet approaches the sun,
solar radiation generates gases from evaporation of the comet's
surface. These gases are pushed away from the comet and glow in
the sun light, thus giving the comet its tail. While the outer
surface of comets appear to composed of icy material like water and
carbon dioxide solids, they likely contain a more rocky nucleus.
Because of their eccentric orbits, many comets eventually cross the
orbit of the Earth. Many meteor showers may be caused by the
Earth crossing an orbit of a fragmented comet.
The collision of a cometary fragment is thought to have occurred in the
Tunguska region of Siberia in 1908. The blast was about the size of
a 15 megaton nuclear bomb. It knocked down trees in an area about
850 square miles, but did not leave a crater. The consensus among
scientists is that a cometary fragment about 20 to 60 meters in diameter
exploded in the Earth's atmosphere just above the Earth's surface. Only
small amounts of material similar to meteorites were found embedded in
trees at the site.
While the asteroid belt seems like the most likely source of meteorites,
some meteorites appear to have come from other places. Some
meteorites have chemical compositions similar to samples brought back from
the moon. Others are thought to have originated on Mars. These
types of meteorites could have been ejected from the Moon or Mars by
collisions with other asteroids, or from Mars by volcanic eruptions.
When a large object impacts the surface of the Earth, the rock at the site
of the impact is deformed and some of it is ejected into the atmosphere to
eventually fall back to the surface. This results in a bowl shaped
depression with a raised rim, called an Impact Crater. The size of
the impact crater depends on such factors as the size and velocity of the
impacting object and the angle at which it strikes the surface of the
Earth. Meteorite Flux and Size
Meteorite flux is the total mass of extraterrestrial objects that strike
the Earth. This is currently about 107 to 109 kg/year. Much of this
material is dust-sized objects called micrometeorites. The
frequency at which meteorites of different sizes strike the Earth depends
on the size of the objects, as shown in the graph below. Note the
similarity between this graph and the flood recurrence interval graphs we
looked at in our discussion of flooding.
Tons of micrometeorites strike the Earth each day. Because of their small
size, they do not usually burn up when entering the Earth's atmosphere,
but instead settle slowly to the surface. Meteorites with diameters of
about 1 mm strike the Earth about once every 30 seconds. Upon
entering the Earth's atmosphere the friction of passage through the
atmosphere generates enough heat to melt or vaporize the objects,
resulting in so called shooting stars. Meteorites of larger sizes
strike the Earth less frequently. If they have a size greater than
about 2 or 3 cm, they only partially melt or vaporize on passage through
the atmosphere, and thus strike the surface of the Earth.
Objects with sizes greater than 1 km are considered to produce effects
that would be catastrophic, because an impact of such an object would
produce global effects. Such meteorites strike the Earth relatively
infrequently - a 1 km sized object strikes the Earth about once
every million years, and 10 km sized objects about once every 100 million
Velocity and Energy Release of Incoming Objects
The velocities at which small meteorites have impacted the Earth range
from 4 to 40 km/sec. Larger objects would not be slowed down much by
the friction associated with passage through the atmosphere, and thus
would impact the Earth with high velocity. Calculations show that a
meteorite with a diameter of 30 m, weighing about 300,000 tons, traveling
at a velocity of 15 km/sec (33,500 miles/hour) would release energy
equivalent to about 20 million tons of TNT.
Such a meteorite struck at Meteor Crater, Arizona (the Barringer Crater)
about 49,000 years ago leaving a crater 1200 m in diameter and 200 m
deep. The amount of energy released by an impact depends on the size
of the impacting body and its velocity. An impact like the one that
struck the Yucatan Peninsula, in Mexico about 65 million years ago,
thought responsible for the extinction of the dinosaurs and numerous other
species, created the Chicxulub Crater, 180 km in diameter and released
energy equivalent to about 100 million megatons of TNT.
For comparison, the amount of energy needed to create a nuclear winter on
the Earth as a result of nuclear war is about 8,000 megatons, and the
energy equivalent of the world's nuclear arsenal is about 60,000 megatons.
Looking at the surface of the Moon, one is impressed by the fact that most
of the surface features of the moon are shaped by impact craters.
The Earth is subject to more than twice the amount of impacting events
than the moon because of its larger size and higher gravitational
attraction. Yet, the Earth does not show a cratered surface like the
moon. The reason for this is that the surface of the Earth is
continually changing due to processes like erosion, weathering, tectonism,
sedimentation, and volcanism. Thus, the only craters that are
evident on the Earth are either very young, very large, or occurred on
stable continental areas that have not been subject to intense surface
modification processes. Currently, approximately 200 terrestrial
impact structures have been identified, with the discovery rate of new
structures in the range of 3-5 per year (see figure 10.15, page 262 in
The Mechanics of Impact Cratering
When a large extraterrestrial object enters the Earth's atmosphere the
initial impact with the atmosphere will compress the atmosphere, sending a
shock wave through the air. Frictional heating will cause the object
to heat and glow. Melting and even vaporization of the outer parts
of the object will begin, but if the object is large enough, solid will
remain when it impacts the surface of the Earth.
Impact of large meteorites have never been observed by humans.
Much of our knowledge about what happens next must come from scaled
experiments. As the solid object plows into the Earth, it will
compress the rocks to form a depression and cause a jet of fragmented rock
and dust to be expelled into the atmosphere. This material is
called ejecta. The impact will send a shock wave into the rocks
below, and the rocks will be crushed into small fragments to form a
breccia. Some of the ejecta will be hot enough to vaporize, and the
heat generated by the impact could be high enough to actually melt the
rock at the site of the impact. The shock wave entering the Earth
will first move in as a compressional wave (P-wave), but after passage of
the compressional wave an expansion wave (rarefaction wave) will move back
toward the surface. This will cause the floor of the crater to be
uplifted and may also cause the rock around the rim of the crater to bent
upward. Faulting may also occur in the rocks around the crater,
causing the crater to become enlarged, and have a concentric set of rings.
The ejecta will eventually settle back to the Earth's surface forming an
ejecta blanket that is thick near the crater rim and thins outward from
the crater. Rocks below the crater that were not melted by the
impact will be intensely fractured. All of this would happen in a
matter of 1 to 2 minutes.
Meteorite Impacts and Mass Extinctions
The impact of a space object with a size greater than about 1 km would be
expected to be felt over the entire surface of the Earth. Smaller
objects would certainly destroy the ecosystem in the vicinity of the
impact, similar to the effects of a volcanic eruption, but larger impacts
could have a worldwide effect on life on the Earth. We will here first
consider the possible effects of an impact, and then discuss how impacts
may have resulted in mass extinction of species on the Earth in the past.
Regional and Global Effects
Again, we as humans have no firsthand knowledge of what the effects of an
impact of a large meteorite or comet would be. Still, calculations
can be made and scaled experiments can be conducted to estimate the
effects. The general consensus is summarized here.
Massive earthquake - up to Richter Magnitude 13, and numerous large
magnitude aftershocks would result from the impact of a large object
with the Earth.
The large quantities of dust put into the atmosphere would block
incoming solar radiation. The dust could take months to settle back to
the surface. Meanwhile, the Earth would be in a state of
continual darkness, and temperatures would drop throughout the world,
generating global winter like conditions. A similar effect has been
postulated for the aftermath of a nuclear war (termed a nuclear
winter). Blockage of solar radiation would also diminish the
ability of photosynthetic organisms, like plants, to photosynthesize.
Since photosynthetic organisms are the base of the food chain, this
would seriously disrupt all ecosystems.
Widespread wildfires ignited by radiation from the fireball as the
object passed through the atmosphere would be generated. Smoke
from these fires would further block solar radiation to enhance the
cooling effect and further disrupt photosynthesis.
If the impact occurred in the oceans, a large steam cloud would be
produced by the sudden evaporation of the seawater. This water
vapor and CO2 would remain in the atmosphere long after the dust
settles. Both of these gases are greenhouse gases which scatter
solar radiation and create a warming effect. Thus, after the
initial global cooling, the atmosphere would undergo global warming
for many years after the impact.
If the impact occurred in the oceans, giant tsunamis would be
generated. For a 10 km-diameter object the leading edge would
hit the seafloor of the deep ocean basins before the top of the object
had reached sea level. The tsunami from such an impact is
estimated to produce waves from 1 to 3 km high. These could
easily flood the interior of continents.
Large amounts of nitrogen oxides would result from combining
Nitrogen and Oxygen in the atmosphere due to the shock produced by the
impact. These nitrogen oxides would combine with water in the
atmosphere to produce nitric acid which would fall back to the surface
as acid rain, resulting in the acidification of surface waters.
Geologic Record of Mass
It has long been known that extinction of large percentages families or
species of organisms have occurred at specific times in the history of our
planet. Among the mechanisms that have been suggested to have caused
these mass extinctions have been large volcanic eruptions, changes in
climatic conditions, changes in sea level, and, more recently, meteorite
impacts. While the meteorite impact theory of mass extinctions has
become accepted by many scientists for particular extinction events, there
is still considerable controversy among scientists. In this course we will
accept the possibility that an impact with a large object could have
caused at least some of the mass extinction events, as it would certainly
seem possible given the effects that an impact could have, as discussed
above. Still, because of their are many other possibilities for the
cause of mass extinctions, please read your book for the arguments
against the impact theory.
Major extinction events occurred at
the end of the Tertiary Period, 1.6 million years (m.y.) ago.
the end of the Cretaceous Period, marking the boundary between the
Cretaceous and Tertiary periods 65 m.y. ago. (Geologists use the
letter K to stand for Cretaceous Period and the letter T for the
Tertiary Period. Thus this boundary is commonly called the K-T
the end of the Triassic, 208 m.y. ago.
the end of the Permian, 245 m.y. ago (estimated that over 96% of the
species alive at the time became extinct).
the end of the Devonian, 360 m.y. ago
the end of Ordovician, 438 m.y. ago
the end of the Cambrian period, 505 m.y. ago
The mass extinction at the end of the Mesozoic Era, that is the Cretaceous
- Tertiary boundary (often called the K-T boundary) 65 million years ago,
shows much evidence that it was related to an impact with an
extraterrestrial object. This event resulted in the extinction of over 50%
of the species living at the time, including the dinosaurs. In 1978 a
group of scientist led by Walter Alvarez of the University of California,
Berkeley, were able to locate the K-T boundary very precisely in layers of
limestones near Gubbio, Italy. At the boundary they found a thin clay
layer. Chemical analysis of the clay revealed that it contains an
anomalously high concentration of the rare element Iridium (Ir). Ir
has extremely low concentrations in most crustal rocks, however it reaches
very high concentrations in meteorites. The only other possible
source of high concentrations of Ir is basaltic magmas. Over the
next several years, the K-T boundary was located at several other sites
throughout the world, and also found to have a thin clay layer with high
concentrations of Ir. Although a large eruption of basaltic magma
could not immediately be ruled out as the source of the high concentration
of Ir, other evidence began to accumulate that the fallout of impact
ejecta had been responsible for both the thin clay layers and the high
concentrations of Ir. Among the evidence found at different
localities where the K-T boundary is exposed is:
Clay layers at some localities have a high proportion of black
carbon that could have originated as soot produced by wildfires set
off by an impact.
Some of the clay layers contain grains of quartz with a crystal
structure that shows evidence that the quartz was severely strained by
a large shock.
In some clay layers tiny grains of the mineral stishovite is
found. Stishovite is a high pressure form of SiO2 that is not found at
the Earth's surface except around known meteorite impact sites.
The mineral can only be produced as a result of extremely deep burial
in the Earth, or by high pressure generated by an impact.
Other clay layers contain tiny spherical droplets of glass, called
spherules. The glass is not basaltic in composition, but could
represent droplets of melt formed during an impact event.
At the time of these discoveries, there was no known impact structure on
the Earth with an age of 65 million years.
This is not unexpected, since 71% of the Earth's surface is covered by
water, and is largely unexplored. But, in the late 1980s attention
started to be focused on a buried impact site near the tip of the Yucatan
Peninsula, in Mexico. Here oil geologists had drilled through layers
of brecciated rock and found impact melt rock. Further
geophysical studies revealed a circular structure about 180 km in
diameter. Radiometric dating reveals that the structure, called the
Chicxulub Crater, formed about 65 million years ago.
Although the crater itself is now filled and buried by younger rocks,
drilling throughout the Gulf of Mexico has revealed the presence of
shocked quartz, glass spherules, and soot in deposits the same age as the
crater. In addition, geologists have found deposits from the tsunami
that was generated by the impact all along the Gulf of Mexico coast
extending considerable distance inland from the current
shoreline. The size of the crater suggest that the object that
produced it was about 10 km in diameter. While there is still some
debate among geologists and paloebiologists as to whether or not the
extinctions that occurred at the K-T boundary were caused by the impact
that formed Chicxulub Crater, it is clear that an impact did occur about
65 million years ago, and that it likely had effects that were global in
scale. What would happen if another such event occurred while we
humans dominate the surface of the Earth, and what could we as humans do,
if anything to prevent such a catastrophic disaster?
It should be clear that even if an impact of a large space object did not
cause the extinction of humans, the effects would cause a natural disaster
of proportions never witnessed by the human race. Here we first look
at the chances that such an impact could occur, then look at how we can
predict or provide warning of such an event, and finally discuss ways that
we might be able to protect ourselves from such an event.
Risk - It is estimated that in any given year the odds that you will die
from an impact of an asteroid or comet are about 1 in 20,000. The
table below shows the odds of dying in the U.S. from various other
causes. Although 1 in 20,000 seem like long odds, you have about the
same odds of dying in an airplane crash, and somewhat less risk of dying
from other natural disasters likes floods and tornadoes. In fact the
odds of dying from an impact event are much better than the odds of
winning the lottery.
Odds of Dying in the U.S. from Selected Causes
Motor Vehicle Accident
1 in 100
1 in 300
1 in 800
1 in 2,500
1 in 5,000
Asteroid or Comet Impact
1 in 20,000
1 in 20,000
1 in 30,000
1 in 60,000
Venomous Bite or Sting
1 in 100,000
Food Poisoning by Botulism
1 in 3,000,000
Odds of winning the Lottery
1 in 7,000,000
In March, 1989 an asteroid named 1989 FC passed within 700,000 km of the
Earth, crossing the orbit of the Earth. It was not discovered until
after it had passed through the orbit of the Earth. Its size was
estimated to be about 0.5 km. Such a body is expected to hit the
Earth about once every million years or so, and would release energy
equivalent to about 10,000 megatons of TNT, a little greater than the
energy released in a nuclear war, and enough to cause nuclear winter event
(see graph above). Although 700,000 km seems like a long distance,
it translates to a miss of the Earth by only a few hours at orbital
Torino Scale -
In order to develop a better means of communicating the potential hazards
of a possible impact with a space object, scientists have developed a
scale that describes the potential (see - http://neo.jpl.nasa.gov/torino_scale.html).
scale is called the Torino Scale, and is shown below.
Having No Likely Consequences (White Zone)
likelihood of a collision is zero, or well below the chance that
a random object of the same size will strike the Earth within
the next few decades. This designation also applies to any small
object that, in the event of a collision, is unlikely to reach
the Earth's surface intact.
Meriting Careful Monitoring (Green Zone)
chance of collision is extremely unlikely, about the same as a
random object of the same size striking the Earth within the
next few decades.
Concern (Yellow Zone)
somewhat close, but not unusual encounter. Collision is very
close encounter, with 1% or greater chance of a collision
capable of causing localized destruction.
close encounter, with 1% or greater chance of a collision
capable of causing regional devastation.
close encounter, with a significant threat of a collision
capable of causing regional devastation.
close encounter, with a significant threat of a collision
capable of causing a global catastrophe.
close encounter, with an extremely significant threat of a
collision capable of causing a global catastrophe.
collision capable of causing localized destruction. Such events
occur somewhere on Earth between once per 50 years and once per
collision capable of causing regional devastation. Such events
occur between once per 1000 years and once per 100,000 years.
A collision capable of causing
a global climatic catastrophe. Such events occur once per
100,000 years, or less often.
For an object making a close approach to Earth, its categorization on the
Torino Scale is dependent upon its placement within this plot showing
kinetic energy versus collision probability. (One MT = 4.3 x 10^15 J.) The
left-hand scale also indicates approximate sizes for asteroidal objects
having typical encounter velocities. For an object that makes multiple
close approaches over a set of dates, a Torino Scale value should be
determined for each approach. It may be convenient to summarize such an
object by the greatest Torino Scale value within the set.
Prediction and Warning - It is estimated that over 90% of NEOs have
not yet been discovered. Because of this, with our present
knowledge, there is a good chance that the only warning we would have
is the flash of light from the fireball as one of these objects
entered the Earth's atmosphere. Scientists have proposed the
"Spaceguard Survey" to find and track all of the large NEOs. If
such a survey is carried out, we could predict the paths of all NEOs
and have years to decades to prepare for an NEO that could impact the
Mitigation - Impacts are the only natural hazard that we can prevent
from happening by either deflecting the incoming object or destroying
it. Of course, we must first know about such objects and their
paths in order to give us sufficient warning to prepare a
defense. Sufficient time is usually thought to be about 10
years. This would likely give us enough time to prepare a space
mission to intercept the object and deflect its path by setting off a
nuclear explosion. Currently, however, there are no detailed
plans. But, even if we did not have the ability to destroy or
deflect such an object, 10 years warning would provide sufficient time
to store food and supplies, and maybe even evacuate the area
immediately surrounding the expected impact site.