Ot follow on;
OBSERVATIONS ON ASTRONOMICAL COLLISIONS AND GLOBAL MASS EXTINCTION
Ever since geologists and paleontologists discovered that fossils were once living animals and plants they have speculated about ancient living systems. Many strange animals that are no longer here today including ammonites (cousins to the nautilus), trilobites (ancient marine arthropods), archaeocyathids (affinity uncertain), and dinosaurs that once roamed our planet. What happened to these ancient beasts? Did they gradually die out, a process known as gradualism or did they die in some ancient cataclysmic upheaval, known as catastrophism-a global mass extinction? For our purposes we will shorten the name to mass extinction.
Traditionally, the debate over the demise of these extinct fauna has centered on terrestrial causes such as volcanic eruptions and climate change. These causes seemed to account for most mass extinctions, but not all. A new hypothesis proposed by Luis Alverez pointed to an incoming asteroid that might have wreaked havoc by producing high levels of dust in the atmosphere as a result of the impact. The dust would cause a darkening of the atmosphere for several months (Alvarez, and others, 1979). Denying sunlight to plants would cause their demise and consequently destroy the animals, which ultimately depends upon plants for food. Also, thermal stresses by either warming the oceans or by greenhouse gasses could adversely affect the global bio-system (Emiliani, et al, 1981).
But what is a mass extinction? Generally, mass extinction is defined as a drop in standing diversity of animal/plant families (closely related animals/plants) of 11% above background extinction levels, that is, the gradual dying of animals as recorded in the geologic rock record. For a mass extinction these catastrophic events must occur within 1 to 15 million years. Extinction events taking millions of years are considered terrestrial. They are induced by volcanic eruptions, climate change, and the reduction of living habitats; say, the drying up of lakes or sea level changes. An astronomical event providing the mechanism for a mass extinction would occur in a geologically instantaneous interval of time--a matter of years.
Present data suggests that there have been one major, four intermediate and ten lesser mass extinction events (Sepkoski, 1982) in the Phanerozoic. The Phanerozoic is a time period beginning about 570 million years before present (mybp) extending to the present.
Global Mass Extinctions (Table 1):
TABLE 1
MASS EXTINCTION EVENTS
TIME mybp
GEOLOGIC NAME
MAJOR
INTERMEDIATE
LESSER
3
40
TERTIARY
PLIOCENE
L. EOCENE
65
97
CRETACEOUS
MAASTRICTIAN
CENOMANIAN
150
194
JURASSIC
L. TITHIOIAN
E. TOARCIAN
200
TRIASSIC
NORIAN
230
PERMIAN
L. PERMIAN
CARBONIFEROUS
None
None
None
357
DEVONIAN
FRANSIAN
SILURIAN
None
None
None
437
ORDOVICIAN
ASHGILLIAN
570-500
CAMBRIAN
5 (NOT NAMED)
Time, mybp=million years before present. From Sepkoski, 1982
Major Mass Extinction.
The greatest extinction event in the world’s biosphere occurred in the Late Permian approximately 230 mybp. It is by far, the most devastating event to disrupt the marine environment. This is by far the greatest mass extinction producing a decline of some 50% of animal families. This event however, occurred over an extensive amount of time of some 15 million years. It probably consisted of several smaller events and was produced by the change of environmental habitats resulting from geologic processes forming the super-continent of Pangea.
Intermediate Mass Extinctions.
Maastrictian event. This is the famous mass destruction that occurred 65 mybp. The mechanism is thought to be an asteroid impacting near the Yucatan and the resulting crater has been named Chicxulub. The crater is believed to be between 110 miles to 180 miles wide. It was a catastrophic occurrence and has been said to have ended the reign of the dinosaurs. That’s not quite correct. The dinosaurs were not a flourishing community, and had actually been declining for some seven million years before the asteroid struck. Despite the popular media, in the two to three million years preceding the Chicxulub asteroid there May have been less than twenty dinosaur species left on the earth, occurring mainly in the Western Seaway of North America (Schopf, 1982). Nonetheless, this was a great upheaval in the global bio-system. Plants and animals from all over the earth encountered dooms day.
Norian and Fransian events. These mass extinction events lasted a few million years of duration and are thought to be of a terrestrial origin.
Ashgillian event. It is probably the second most devastating mass extinction in the Phanerozoic. It occurred primarily in tropical seas and was probably a terrestrial event.
Lesser Mass Extinctions.
Pliocene, Cenomamian, Tithonian, Toarcian, and Cambrian events. These are minor mass extinction events and are thought to be a result of terrestrial processes.
Late Eocene event. This event mainly affected marine plankton (one celled animals/plants floating in the sea). Although small, they are important in the control of oxygen (phytoplankton) and carbon dioxide (foraminifera) by incorporating carbon into their shells. Interestingly, the Late Eocene event occurred about 40 million years ago, about the time the Popigai meteorite struck Siberia. It was a large meteorite producing a crater some 60 miles across.
In general, it seems that global mass extinctions are rare with only one known to struck 65 mybp (Chicxulub) and another 40 mybp (Popigai?) all other mass extinctions have a terrestrial origin. These celestial visitors that produce global devastation are rare. If it is assumed that the Popigai bolide (fireball) was large enough to cause a global mass extinction, then I wonder if an object that can produce a crater of 60 miles width or larger might be an indicator of a global mass extinction?
However, it is not these rare monsters that interest me, but their smaller deadly cousins. There are many, many smaller Apollo and Amor objects called ‘earth grazers’, asteroids that come near or within the earth’s orbit. Although relatively small they can be up to five mile across such as 2212 Hephaistos (Wetherill, G., Shoemaker, E., 1981). In order to avoid confusion about the size range of asteroids as to meteoroids (meteorites in space), I shall refer to all objects that make a crater as bolides.
Craters from these objects are known throughout the globe (Table II). As can be seen, these craters do not seem to be randomly distributed. North America (Canada, USA, and Mexico) alone, have 36% of the earth’s craters. It must be kept in mind however, that the present craters are only the ones that erosion has not destroyed. Also, we have little or no knowledge of how many bolides have fallen into the sea, which covers some two-thirds the surface area of the globe. Closer to home (Table III) demonstrates that some twelve craters have been identified in the intracontinental U.S.A. They range in age throughout the Phanerozoic. They range in size from small craters 550 ft in width (Odessa) to the Manson Crater located in Iowa, 19 miles wide.
TABLE II
KNOWN GLOBAL CRATERS
AREA
NUMBER
PERCENTAGE
N. America
37
35.9
S. America
5
4.8
Russia/Siberia
27
26.2
Europe
10
9.7
S.E. Asia
1
0.97
China/Mongolia
1
0.97
Middle East
1
0.97
Australia
11
10.6
Africa
10
9.7
TOTAL
103
99.8 (round-off error)
From King, 1976 and Greive, 1981
TABLE III
CRATERS IN THE INTRACONTINENTAL U.S.A.
LOCATION
CRATER SIZE (miles)
AGE
Meteor Crater, Arizona
0.76
20,000-40,000 years
Haviland, Kansas
6.8
?
Odessa, Texas
0.1
?
Bee Bluff, Texas
1.5
> 40 mybp
Crooked Creek, Missouri
3.5
320 mybp
Decaturville, Missouri
3.7
> 300 mybp
Kentland, Indiana
8
300 mybp
Manson, Iowa
19
> 70 mybp
Middlesburo, Kentucky
3.7
300 mybp
Serpent Mound, Ohio
3.9
300 mybp
Sierra Madera, Texas
8
100 mybp
Wells Creek, Tennessee
8.7
200 mybp
From Grieve, 1981,
What would be the effect of an incoming bolide in the size range of an object that produced the 19 mile wide Manson Crater? By comparing the results from much work that has been performed studying the Ries bolide in Germany we May obtain some idea. The object that struck southern Germany occurred about 15 mybp. The crater it produced is approximately 16 miles wide, a little smaller than the Manson bolide. The bolide was made of an enstatite chondrite (aubrite).
We use the assumed values of a mass of 1.6X1015 gm, density of 3.0 gm/cm, and an impact velocity of 25km/sec and using the relationships to find the result:
D=M/V
R=(3V/4pi).333
K.E.=0.5 MVi2
1KT=4X1019 ergs
1MT=1,000 KT
Where D=density, M=mass, R=radius, V=volume, Vi=impact velocity, pi=constant, K.E.=kinetic Energy, KT=kilotons of equivalent TNT, and MT=megatons of equivalent of TNT.
Changing the metric values to the English system the bolide would have a diameter of 0.6 miles and produce a crater 16 miles in diameter. The blast energy would be in the range of about 125,000 MT (the largest hydrogen bomb exploded on the earth was by the USSR in 1961 and had a yield of 58 MT). This is a truly catastrophic event for a state or country. And the frightening thing is, that there are objects out there similar in size that have recently have come close to the earth. Discovered just this year the asteroid 2002 NY40 is about the same size as the Ries bolide, having a diameter 0.5 miles and passed within 330,000 miles (just outside the moons orbit). Another recent ‘earth grazer’ discovered on July 9th this year is 2002 NT7. It has a diameter of 1.4 miles.
Along with meteors and asteroids one must include comets. In June 1908 a large object collided near the Tungska River in Siberia. Shockwaves were recorded around the world. Distances extending from 19 to 25 miles were affected. Since there have been no meteorites associated with the site, a comet is thought to have been the impacting body.
As a final note, a celestial intruder might not even have to strike the earth to put the human race in jeopardy. The summer of 2002 saw two atomic powers face-off. Pakistan and India exchanged gun and canon fire over contested Kashmir and both countries waved the nuclear saber. During those crucial times, U.S. satellites detected an atmospheric explosion of a meteor producing some 12 kilotons (the atomic bomb dropped over Hiroshima contained 15-kiloltons) of kinetic energy over the Mediterranean Sea. Since neither Pakistan nor India possess sophisticated detection sensors, if that explosion occurred over India rather than the Mediterranean, the resulting confusion and panic May have sparked a nuclear war according to Brig. General Simon Worden (Worden, 2002). Worden also said that in 1996 satellite sensors detected a 100-kiloton burst when a meteor exploded over Greenland. As more countries and terrorists organizations gain the nuclear genie it becomes even more important for mankind to understand the nature and the potential threat of our incoming neighbors from the sky.
References
Alvarez, et al, 1979, Extraterrestrial cause for the Cretaceous-Tertiary extinction: Experiment and theory: Lawrence Berkeley Report LBL-9666.
Emiliani, C., et al, 1981, Sudden death at the end of the Mesozoic, Earth and Planetary Science Letters, v. 55.
Grieve, R., 1981, The record of impact on Earth, in GSA Special Paper 190.
King, E., 1976, Space Geology, John Wiley &Sons, Inc.
Schopf, T., 1982, Extinction of the dinosaurs: A 1982 understanding, in GSA Special Paper 190.
Sepkoski, J., 1982, Mass extinctions in the Phanerozoic oceans: A review, in GSA Special Paper 190.
Wetherill, G., Shoemaker, E., 1981, Collision of astronomically observable bodies with the Earth, in GSA Special Paper 190.
Worden, S., 2002, in Near-Earth objects pose threat, BBC on Line.
meteorite-times.com |
