Fri, 31 Jul 2009 01:42 UTC
Writing in Science Express, the online edition of the journal Science, researchers acknowledged that while most scientists agree that an asteroid collision 65 million years ago caused the mass extinction of the dinosaurs, they tend to differ in opinion on how many other mass extinctions have resulted from similar events.
Researchers used computer models to simulate the formation of comet clouds in the solar system for 1.2 billion years. They pinpointed a body called the Oort Cloud as the source for many long-period comets that find their way into Earth's path.
Formed 4.5 billion years ago from the nebula that formed our solar system, the Oort Cloud spans from about 93 billion miles from the sun to about three light years away. Scientist said the Oort Cloud could contain literally billions of comets, many of which are so small and distant to be seen.
"It was thought the long-period comets we see just tell us about the outer Oort Cloud, but they really give us a murky picture of the entire Oort Cloud," said Nathan Kaib, a University of Washington doctoral student in astronomy and lead author.
Kaib worked alongside Thomas Quinn, astronomy professor at the University of Washington. They found that only two or three significant comet collisions are likely to have occurred on Earth for 500 million years.
They based their calculations on the assumption that the inner Oort Cloud was the primary source of long-period comets, although they admit that the actual number of comets is indefinable.
"For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth," Quinn said.
"We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir."
Their findings could explain a minor event known as the late Eocene extinction 40 million years ago, although Kaib and Quinn noted: "if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began."
"That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events," Kaib said.
He added that the Earth has benefited from the gravitational pulls of Jupiter and Saturn, which act as comet deflectors.
"We show that Jupiter and Saturn are not perfect and some of the comets from the inner Oort Cloud are able to leak through. But most don't," Kaib said.
The study was funded by NASA and the National Science Foundation.
Comment: Regarding the claim that "the gravitational pulls of Jupiter and Saturn act as comet deflectors", let's see what Clube and Napier, British astronomers and writers of The Cosmic Serpent, have to say:
The giant comets normally reside far beyond the planets, in a spherical cloud surrounding the Sun, called the Oort cloud. There is also evidence for a flattened disk of comets closer to the inner solar system, called the Edgeworth/Kuiper belt. What prompts members of either of these comet repositories to enter the realm of the planets? Clube and Napier suggest a galactic influence. The solar system periodically passes through the plane of the galaxy as the Sun (and the solar system with it) orbits the galactic center. Each passage may dislodge giant comets and divert them closer to the Sun. The outer planets, particularly Jupiter, may then perturb some of these giant comets into orbits which enter the inner solar system. These comets, stressed both by gravity and by heat from the sun, may fragment into a cloud of smaller objects with dynamically similar orbits.
Chiron offers a good example of a giant comet as called for by Clube and Napier's giant comet hypothesis. Chiron is somewhere between 148 and 208 kilometers in diameter. Currently Chiron's unstable "parking orbit" lies mostly between Saturn and Uranus. Chiron may end up injected into the inner solar system within a hundred thousand years, or ejected from the solar system on a similar time scale. It is also possible that Chiron has already visited the inner solar system.
The Taurid complex and the Kreutz sungrazer group are two families of objects which most likely represent the fragmented remains of two giant comets in the current era. SOHO has recently discovered many new members of the Kreutz group which were previously unknown.
The Kreutz progenitor was injected into a retrograde orbit and attained the sungrazing state at a high inclination to the ecliptic. Hence the debris of its "children" does not pose a threat to the Earth. The Taurid progenitor on the other hand ended up in a short-period low-inclination prograde orbit. This is why the Earth can encounter its debris with potentially calamitous results.
What would happen should the Earth pass through the orbit of a disintegrating giant comet just before or after the comet passes that same point? Since larger fragments tend to cluster close to the nucleus of the comet, chances would increase that the Earth would be bombarded by these larger fragments. The severity of this comet fragment shower would far exceed any ordinary meteor shower. Not only would "shooting stars" and bright fireballs caused by small debris appear, but so too would large airbursts and possibly ground impacts. These would result in significant destruction should they occur over an inhabited area. If a large enough fragment struck in the ocean -- say, 200 meters or so in diameter -- it would raise tsunamis even at a great distance that would sweep away coastal habitations.
Duncan Steel, a colleague of Clube and Napier, refers to this process as coherent catastrophism. Widespread destruction derives from the coherent arrival of many impactors within a few days, as opposed to the sporadic arrival of objects spread randomly in space. The shower repeats for a period of years until the cometary orbit precesses so that the Earth no longer encounters the dense part of the debris field. (Of course, sporadic debris unrelated to the disintegrating comet may impact at any time as well.)