Science notebook--News from Antarctica and beyond
Just over 2 million years ago, claim the authors of an article in the 27 November 1997 issue of Nature, an asteroid splashed to Earth off the coast of West Antarctica, creating a blast the size of a 10-megaton bomb and firing a column of water 5 kilometers into the air. The impact site was first discovered in the 1960s when the research vessel Eltanin recovered cores that bore the characteristic signature of asteroids: iridium. Iridium, a metal element, is rare in the Earth's crust but common in certain types of asteroids and meteorites. The asteroid was subsequently named for the ship.
In 1995, Rainer Gersonde from the Alfred-Wegener-Institute for Polar and Marine Research in Bremerhaven, Germany, and his colleagues, working aboard the FS Polarstern, returned to the impact site to explore it. The Nature article contains their findings and speculations about the event. By analyzing the geologic record, they determined that the asteroid must have been at least 1 kilometer in diameter and was perhaps as large as 4 kilometers. Its size places the Eltanin asteroid at the threshold believed to have global consequences.
Gersonde and his multinational team believe that when the Eltanin asteroid crashed to Earth, it created devastating tsunamis that swamped the coasts of South America and Antarctica. Sediment blasted from the ocean bottom by the impact probably spread up to 4,000 kilometers away. Dust, vapor, and salts most likely were carried aloft around the world, and the debris and hot vapor emitted from the blast could well have damaged the Earth's ozone layer. Climate change is probable, but the authors say that whether it persisted or lasted only a few years is unknown. No evidence has been found that the climate change caused the extinction of any species.
Of the 140 known impact sites on Earth, the Eltanin asteroid site, at 4,000 meters depth, is the only one in the deep ocean. Researchers hope that what they learn from studying this impact site will help them locate other deep-ocean impact sites. Logic dictates that because the Earth's surface is 70 percent water, impact sites should be more numerous on the ocean floor than on land.
For a brief description of the Nature article, see the 4 December Nature science update, "Earth: Ocean splashdown."
"It is a completely normal part of science," writes geologist Ralph P. Harvey, of Case Western Reserve University, on the Antarctic Search for Meteorites (ANSMET) Web site, "for researchers to 'try on' various theories and interpretations, and at this stage [in the ongoing study of ALH84001] utterly natural that groups might hold to contradictory interpretations. Normally, the public doesn't see science at this stage."
Scientists agree on one thing: because trapped gases within the potato-sized rock match the composition of gases in the atmosphere of Mars, the meteorite, which was found in the Allan Hills region of Antarctica in 1984, did originate on the surface of Mars. But within the Mars rock, researchers have found carbonate globules, and it is the origin of the carbonates that has led to a parting of the ways.
Some researchers studying the rock believe that the carbonates formed as a result of biotic processes and contend that within the globules there are tiny vestiges of ancient microbial life, which they call "nanobacteria." National Aeronautics and Space Administration geologists Everett K. Gibson, Jr., David S. McKay, and their team make their case for this interpretation in the December 1997 issue of Scientific American.
Others--including Ralph Harvey and his fellow researcher H.Y. McSween, Jr., of the University of Tennessee-Knoxville writing in the 4 July 1996 issue of Nature--believe that the carbonates formed right from the host volcanic rock itself during reactions taking place because of physical changes--heating, cooling, being pressurized, or subjected to fluid--to the rock. J. William Schopf, Professor of Paleobiology, Department of Earth and Space Sciences, University of California, Los Angeles, agrees. The nanobacteria the NASA team saw within the carbonate of the rock were all too symmetrical and orderly, in his opinion, to be biological organisms--had a few been flattened or had some been clumped together Schopf might have been more convinced that they are, in fact, the microscopic organisms that Gibson and McKay and their team claim they are.
"Both our study," writes Harvey on the ANSMET Web site, "and that of the McKay group are only small parts of the tremendous amount we still have to learn about ALH84001." Gibson concurs that much yet is to be learned. He is hopeful that when NASA conducts its sample-return mission to Mars in 2005, the Martian rocks brought back will provide scientists with the kind of data they need to determine conclusively whether life came into being on Mars and, in turn, to begin to unlock the mystery of the prevalence of life in the Universe.
The debate between the two groups also is played out in the December issue of Nature (see the 18 December Nature science update), where each side was given 1,000 words to state its case.