Meteorites, Impacts, & Mass Extinction

On February 15, 2013 a meteor exploded in the sky over Chelyabinsk, southern Russia. Although no people or buildings were hit by the resulting meteorite, the shockwave from the exploding object injured about 1500 people and caused damage to 7200 buildings in the region. The fireball and was caught on video, mainly by dash cameras throughout the region, which were posted on the internet by news organizations individuals. Although the Chelyabinsk meteorite probably weighed about 12,000–13,000 metric tonnes, and measured 17 to 20 m in diameter before it exploded, sientists were quick to point out that it was very small compared to other objects that could potentially hit the earth. The explosion released energy estimated at about 500 kilotons of TNT (about 20 to 30 times more energy than the Hiroshima atomic bomb). The event brought to the world's attention the very real hazards associated with the impact of objects from outer space.
A Meteorite isa piece of rock from outer space
that strikes the surface of the Earth.
AMeteoroid isa
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, and Leonid
shower occurs in mid-November.
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
Illinois
1954 - A 5kg meteorite fell through the roof of a house in Alabama.
1992 - A small meteorite demolished a car near New York City.
2003 - A 20 kg meteorite crashed through a 2 story
house in uptown New Orleans
2003 - A shower of meteorites destroyed several houses
and injured 20 people in India
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 made of olivine, pyroxene, and iron - nickel alloys that are magnetic. They are composed of small round spheres,
called chondrules, made of the minerals olivine and pyroxene. They appear to have formed by rapid melting followed by rapid cooling early in the history 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 igneous rocks found on the Earth.
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. All are magnetic. When cut and polished, iron meteorites show a distinct texture called a
Widmanstätten pattern. This pattern results
from slow cooling of a once hot solid material. Most research 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 meteorites.
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, and crust. 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 Earth-crossing
orbits.
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. Although still controversial, the general 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.
A similar event if it happened over a large city, would be devastating.
Other Sources
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.
Impact Events
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
years.
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.
E = ½ MV2
where E = Energy,
M = Mass (depends on size and density of the object), and
V = 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.
Cratered Surfaces
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.
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 material will remain when it impacts the surface of the Earth.
Impacts 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 (> 1 km in size) 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 tsunami 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.
The Geologic Record of Mass Extinction
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 boundary).
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. (See simulation at http://es.ucsc.edu/%7Eward/chix.mov)
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?
Human Hazards
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 between 1 in 3,000 and 1 in 250,000. The table below
shows the odds of dying in the U.S. from various other causes. Although this seems like long odds, you have a
about the 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 Powerball lottery.
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 velocities.
On March 19, 2004, a 30 m diameter asteroid, named 2004 FH, passed within
26,500 miles (43,000 km) of earth, just beyond the orbit of weather
satellites. The object was small , and likely would have only caused
a local effect if it had hit the earth's atmosphere, but it was discovered
only 4 days before it passed.
In June of 2012, Asteroid 2012 LZ1 passed within about 3 million miles of Earth. Although it was never a threat, the fact that it was discovered only a few days before was alarming. Furthermore, its size was originally estimated to be only 500 m in diameter, as it passed, scientists realized that this was an underestimate. Its size turned out to be about 1 km.
The 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).
The scale is called the Torino Scale, and is shown
below.
Events Having No Likely Consequences
(White Zone)
0
The 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.
Events Meriting Careful Monitoring
(Green Zone)
1
The 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.
Events Meriting Concern
(Yellow Zone)
2
A somewhat close, but not unusual encounter.
Collision is very unlikely.
3
A close encounter, with 1% or greater chance of a
collision capable of causing localized destruction.
4
A close encounter, with 1% or greater chance of a
collision capable of causing regional devastation.
Threatening Events
(Orange Zone)
5
A close encounter, with a significant threat of a
collision capable of causing regional devastation.
6
A close encounter, with a significant threat of a
collision capable of causing a global catastrophe.
7
A close encounter, with an extremely significant threat
of a collision capable of causing a global catastrophe.
Certain Collisions
(Red Zone)
8
A collision capable of causing localized destruction.
Such events occur somewhere on Earth between
once per 50 years and once per 1000 years.
9
A collision capable of causing regional devastation.
Such events occur between once per 1000 years
and once per 100,000 years.
10
A collision capable of causing a global climatic
catastrophe. Such events occur once per
100,000 years, or less often.
Prediction and Warning - In 1998 scientists and Congress approved the Spaceguard Survey which had a goal of identifying 90% of all NEOs with a size greater than 1 km.
In September, 2011, NASA announced that they had identified 93% of all NEOs of this size and that of the total number estimated to exist (989) they had identified 911.
For mid-sized NEOs, with sizes between 100 m and 1 km, 5,200 have been found and are being tracked, but it is estimated that there are still over 15,000 of such bodies that have not yet been discovered.
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.