2 March 2000
Ring Around the Earth?
by Kate Melville
At the dawn of the Eocene epoch, when Metasequoia trees towered in arctic latitudes and primitive horses thrived in the humid underbrush, summers were hot and winters were brief and comfortably cool.
Colder winters and the extinction of hundreds of species including horses in Europe and plankton in the Caribbean marked the end of the Eocene, occurring approximately 35 million years ago. Oddly, Eocene summers remained hot. Many scientists believe an asteroid impact--with a signature crater now buried under the Chesapeake Bay in Virginia, U.S.A.--may have caused the Eocene extinctions and associated climate change. But unlike the K-T extinctions, the Eocene deaths are not as extensive.
Now, according to a theory by former NASA astronomer Dr. John A. O'Keefe, of Garretson, S.D., we may have to look elsewhere in space for the Eocene killer.
O'Keefe, the maverick astronomer who discovered Earth's slight pear-shape in 1958 (using U.S. Vanguard satellite data) and who helped establish Project Apollo's lunar geology program in the early 1960s (he is credited with NASA's recruiting the late astrogeologist and comet discoverer Eugene Shoemaker), believes the Chesapeake Bay impact was too small to have caused the Eocene extinctions. Instead, he theorizes, a temporary, Saturn-like ring of tektites around the Earth may be to blame. Such an unstable ring--which might have lasted a million years or more--could have cast a shadow across the North and South Hemispheres as the seasons changed.
The ex-NASA scientist, now an independent researcher, calculates that such a ring would have screened out nearly a third of the sunshine falling to Earth--about as much as a kitchen window screen.
Tektites are glassy stones found in so-called "strewn fields" worldwide (their sizes range from the tiny, called microtektites, to large, lava-like chunks called 'Muong Nongs' or layered tektites). They are nearly free of water and other volatiles.
Tektite strewn fields differ widely in age, but most tektites, regardless of age, are meteorite-like and some show evidence of atmospheric entry--a phenomenon called ablation. Eocene-age tektites found in North America appear to have fallen in a vast arc from Massachusetts to Texas. They are clearly not associated with the Chesapeake crater, O'Keefe notes.
"Conventional wisdom suggests that asteroids produce tektites from sedimentary rock on Earth," says O'Keefe. "I don't believe this. Impact craters, larger and smaller than Chesapeake Bay, can be found that have no tektites associated with them. Besides, the chemical composition of tektites resembles that of high-silica materials such as granite or its glassy equivalent, volcanic rhyolite, not sedimentary material. The origin of tektites by meteorite or comet impact on Earth cannot be squared with known physical principles of how natural glass forms. They don't resemble impact glass."
O'Keefe has demonstrated that tektites--bone-dry stones ranging in color from dark green to black--differ significantly from other natural glasses found at impact sites. The subject of tektites is a complex one; there's no easy telling of how the various shapes form and cool, O'Keefe says. A tektite, he notes, is largely free of impurities; the so-called splashform tektites (shaped like teardrops and dumbbells) took seconds to cool in the vacuum of space while the larger, layered tektite chunks took many minutes to form and cool. Conversely, all natural impact glass forms within seconds of an impact and contains many impurities; these glasses are not tektites.
If tektites aren't formed by impact, as O'Keefe believes, then where do they come from?
"I believe tektites point to a non-impact phenomena, probably a non-terrestrial volcano. The closest logical source for tektites is the Moon," he says. O'Keefe believes that highly explosive, silicic volcanic eruptions on the Moon could blow molten blobs of glass--at lunar escape velocity--far into space. The Earth's gravity would eventually capture the material to form an O'Keefe ring.
O'Keefe points to one smoking gun regarding the origin of tektites. The astronomer sites an extensive study of an Apollo 12 specimen (designated No. 12013) which strongly suggests a lunar origin for at least one family of tektites. Analysis shows that the unusual Apollo moon rock is eerily like the Southeast Asian tektites in major chemical element composition.
Petrologist Darryl Futrell, a natural glass expert from Los Angeles, Calif., agrees with O'Keefe's interpretation of tektites. "Tektites are too water-free to be from the Earth plus they cannot have formed by impact," Futrell says. "It simply takes too long-many hours or even days--for tektite glass to lose its volatiles. The igneous chemical trends in tektites points to a volcanic origin."
So what happened to O'Keefe's hypothetical ring? "It would have been blown away by sunshine," he says. "In the frictionless environment of space that's enough to sweep away a ring in a million years or so."
O'Keefe also suggests that button-like tektites, found in Australia and Indonesia and part of a vast strewn field stretching from China to Australia, may have formed a second, younger terrestrial ring. No known impact crater has ever been found associated with the 700,000-year-old Southeast Asian tektite strewn field.
Story courtesy Louis Varricchio