meteorites and extinctions
Meteroites and Extinctionsbased on the lecture notes of Prof. Stephen A. Nelson, Tulane University adapted to HTML
...see also Meteorites
Minerals in Meteorites
Meteorite Cratering on Earth
Tektites - Australia's unusual meteorite
The Tunguska Event - eyewitness reports translated into English
Composition and Classification of Meteorites
Origin of Meteorites
Meteorite Impacts and Mass Extinctions
Geologic Record of Mass Extinction
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
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-
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
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
Earth has likely been affected by collisions with these space objects,
and collisions could affect the Earth in the future as well.
Meteorites can be classified generally into three types:
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 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.
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.
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
|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
|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
most of the surface features of the moon are shaped by impact
craters. The Earth is subject to more than twice the amount
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
the Earth is continually changing due to processes like erosion,
weathering, tectonism, sedimentation, and volcanism. Thus,
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 your text).
|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
melted by the impact will be intensely fractured. All of this
would happen in a matter of 1 to 2 minutes.
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
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.
extinction events occurred at
|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:|
|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?
|Odds of Dying in the U.S. from Selected Causes|
|Motor Vehicle Accident||1 in 100|
|Murder||1 in 300|
|Fire||1 in 800|
|Firearms Accident||1 in 2,500|
|Electrocution||1 in 5,000|
|Asteroid or Comet Impact||1 in 20,000|
|Airplane Crash||1 in 20,000|
|Flood||1 in 30,000|
|Tornado||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 velocities.
|Events Having No
|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.|
|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.|
|A somewhat close, but not unusual encounter. Collision is very unlikely.|
|A close encounter, with 1% or greater chance of a collision capable of causing localized destruction.|
|A close encounter, with 1% or greater chance of a collision capable of causing regional devastation.|
|A close encounter, with a significant threat of a collision capable of causing regional devastation.|
|A close encounter, with a significant threat of a collision capable of causing a global catastrophe.|
|A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe.|
|A collision capable of causing localized destruction. Such events occur somewhere on Earth between once per 50 years and once per 1000 years.|
|A 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.