Cobolide Impact Extinctions
Ted S. Frost
And the third angel sounded, and there fell from heaven a great star, burning like a torch . . . REVELATION 8:10
Astrobiologists teach us Earth is a superior environment for the development of complex organisms such as us. At the same time, geobiologists point out bad things happen, as evidenced by twenty or so known mass extinctions.
Soon to become extinct Tyrannosaurus Rex witnessing the impact of the KT asteroid. Courtesy of John Sibbick.
The most famous was the day the dinosaurs checked out, 65 million years ago—the Cretaceous Tertiary (KT) extinction. This extinction is noteworthy not only for giving mammals a big break, but also because in the Gulf of Mexico, off the Yucatan Peninsula, there is evidence of a big crater in the sea floor known as Chicxulub that geologists tell us is 65 million years old.
Fractured rocks, fossil beds evidencing massive mayhem, and a world-wide layer of extraterrestrial iridium clinched it: the crater is the impact site of a very large asteroid. An asteroid so large ( ˜ 10 km) energy from its impact would have devastated Earth’s lithosphere, at least as far as multi-cellular organisms are concerned. Fires, floods, dark skies, choking atmospheres, and downpours of acid rain would have ensued. All land animals larger than a turkey disappeared. Only small mammals, birds, and a few cold-blooded reptilian and amphibian orders survived.
Barringer crater, Arizona. From a one km asteroid impacting ≈ 49,000 years ago. NASA photo by D. Roddy
Because the asteroid aspect of KT’s extinction is so well documented, scientists have searched for evidence of similar impacts that might have caused other extinction events. The one engendering the most attention is The Big One—the Permian-Triassic extinction 251 million years ago. At that time, some ninety percent of all animal orders became extinct. The die-off was so extensive that life was nearly reduced to the level of one-celled bacteria.
Did a giant asteroid cause the Permian-Triassic extinction? Unfortunately, the farther back in time we go, the sketchier geological artifacts become and 251 million years is a long, long time. Mechanisms such as plate tectonics, volcanism, erosion, uplifts, and metamorphism smudge the evidence. All of which leave plenty of room for differences of opinion Which is why the Permian extinction has generated a rip-roaring debate in the scientific community.
The chief proponents of the theory that an extraterrestrial impact caused the Permian extinction are Luann Becker and her colleagues. Professor Becker, until a few years ago, was a University of Washington faculty member. She is currently with the University of California, Santa Barbara.Peter Ward and Roger Buick, University of Washington astrobiologists, disagree. Being clobbered by bolides is a fact of life for the Solar System’s inner planets. During Earth’s Hadean period (4.5 to ˜ 3.8 Ga) impacts were so severe that some are calculated to have been large enough to cause the oceans to boil, thereby sterilizing the planet of any life except microbes existing deep underground. Fortunately, major impact events decreased after the Hadean period.
951 Gaspra, a 20 x 10 km asteroid in the inner Asteroid Belt, 1.2 AU’s from Earth. The Galileo Project, NASA
But Earth and its sister planets have continued to be bashed from time to time by asteroids and comets large enough to survive their plunge through planetary atmospheres. Because of Earth’s dynamic geology, much of the evidence has been obliterated. However, one has only to look at the cratered surfaces of Mars and the Moon to see what Earth has endured.
Luann Becker and her colleagues first arrived at their impact theory for the Permian extinction a few years ago while examining ancient soils from Meishan, China. The soil samples were thought to have been laid down during the Permian-Triassic extinction. The Becker team found evidence of geochemical markers indicative of a severe impact event. Their results suggested it could have been the cause for the Permian- Triassic extinction.
The ensuing academic paper1 brought both fame and notoriety to its authors. The results of the Becker team were not duplicated by other researchers and a Japanese scientist even claimed that some of the soil samples were improperly dated and not from the Permian-Triassic extinction boundary at all. But the Becker team stood by their research. Last year, Becker et al claimed to have made another startling discovery—evidence of a giant impact crater off the north shore of Australia known as the Bedout High2. They tentatively dated the crater’s age at 251 Myrs. The data for their conclusion came from analysis of core drilling samples accumulated by oil drilling companies. The samples once again included geochemical markers evidencing an impact. Was this the cause of the Permian-Triassic extinction?
Another spate of publicity ensued as the Becker Team sensed vindication. But criticism of their conclusions came quickly. Some claimed their geochemical markers could have resulted from nonimpact causes. Some pointed out lack of coroborating evidence of an impact. (Where was the iridium layer?)
Some once again questioned dating of the site. Then, early this year, a paper came forth authored by Peter Ward, Roger Buick, and others3 documenting fossil evidence that the Permian extinction was not as sudden as had been supposed. Instead, it was most likely drawn out over hundreds of thousands of years and could have been caused by environmental degradation of volcanic origins. Ward’s paper, like Becker’s, has been criticized, but recently its extinction conclusion has been backed up by research involving fossil beds in China4.
Regardless of whether or not an impact caused the Permian-Triassic extinction, it’s tempting to speculate impacts have been involved in extinctions. An interesting peculiarity of mass extinctions is their periodicity. Analysis of fossil records shows they occur in regular cycles of 26 million years5 and 62 million years6. Some researchers have posited this is due to extra-terrestrial phenomena causing perturbations of asteroids and comets. Phenomena such as our solar system’s periodic passage through an exceptionally massive arm of the Milky Way, or influence from an unknown planet "X" with an exaggerated elliptical orbit cruising beyond the outer planets, an idea given credence by discovery this year of what might be the solar system’s tenth planet—object 2003 UB313, larger than Pluto, which is 97 AU’s away, with a wildly eccentric and inclined orbit.
As the accompanying chart illustrates, there does appear to be a rough relationship between the periodicity of extinction events and known impact craters. Certainly large impacts have occurred and will continue to occur . Plenty of impact ammunition is out there. Between Mars and Jupiter is the Asteroid Belt (2–4 AU’s). Beyond Neptune (35–50 AU’s), Kuiper Belt objects circle the Solar System. And way, way out there (50,000–100,000 AU’s), is the Oort Cloud, home base for many comets. All that is needed for one of these objects to draw a bead on Earth is appropriate perturbations from a large gravitational body, such as a passing star or a planetary object. In addition, between Jupiter and Neptune are dozens of so-called planetoids astronomers term"Centaurs." These are asteroid/comet type objects such as 148 x 208 km Chiron. Centaurs are scary because anything that size would sterilize the planet. However, even bolides that are only one km (about one tenth the size of the KT asteroid) are considered significant threats to human life. And objects 10 km or more are believed to be full-blown extinction threats. Hence the activity seeking to identify large objects with potential Earth crossing orbits.
Since 1995, NASA has sponsored a survey program aimed at discovery of 90% of the one km or more Near Earth Objects (NEO). (‘Near Earth’ means having a perihelion of less than 1.3 AU’s.) Over the past ten years, this program has uncovered 594 previously unknown NEO’s. Most have been discoveries by NASA and U. S. Air Force funded GEODSS7 telescope observatories in New Mexico and Hawaii—the Lincoln Near Earth Asteroid Research project (LINEAR) and the Near Earth Asteroid Tracking project (NEAT). At present, there are approximately 710 known Potentially Hazardous Asteroids (PHAs). A PHA is an asteroid projected to intersect Earth’s orbit and come within .05 AU’s or less. That is, closer than 20 Earth-to- Moon distances, the distance considered too close for comfort.
None of the currently recognized NEO’s or PHA’s are an immanent threat. The most dangerous so far is a 1.1 km asteroid known as 1950 DA that is given a 1/300 chance of hitting Earth by year 2880. However, plenty of undiscovered objects are lurking out there with unknown orbits. Most astronomers agree there is an orbiting rock somewhere with Earth’s name on it. But when and where will it arrive?
And what do we do if we do find one on course for Earth? Blow it up? Nudge it off course? Direct it into the Sun? And if so, how? And who? And how much time will we have? Current thinking is that any NEO in the form of an asteroid will likely swing by many times before it actually hits. But what about comets with wildly eccentric orbits that may come at us from out of nowhere?
These are serious questions. Because one thing we do know for certain is that if a large extra-terrestrial object does arrive, Tyrannosaurus Rex is likely to have company.
References and notes:
1. “Impact Event at the Permian-Triassic Boundary,” Luann Becker et al, Science, Vol. 291 (Feb. 2001).
2. “Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia,” Luann Becker et al, Science, Vol. 304, (Jun 2004 ).
3. “Abrupt and Gradual Extinction Among Late Permian Land Vertebrates in the Karoo Basin, South Africa,” Peter D. Ward et al, Science, Vol. 307, (Feb. 2005). Also, “Repeated Carbon-cycle Disturbances at the Permian- Triassic Boundary Separating Two Mass Extinctions,” R. Buick et al, unpublished.
4. “Two episodes of microbial change coupled with Permo/Triassic fauna mass extinction, “S. Xie et al, Nature, 434, (3/10/05).
5. “Periodicity of Extinctions in the Geological Past,” J. Sepkoski, Jr. et al, Proceedings of Natl. Academy of Sciences, Vol. 81, (2/1/84).
6. “Cycles in Fossil Diversity,” R. Muller et al, Nature 434,(3/10/05).
7. GEODSS (Ground-based Electro-Optical Deep Space Surveillance) is a telescope designed for the Air Force to observe Earth orbital spacecraft.