|On Deathstars, Cosmic Dust, and Moonstones|
|Paul W. Healy|
Issue #40 (December 1984)
Does the sun have a dark companion that approaches the solar system close enough to perturb the "Oort cloud" (the nursery of the huge "dirty snowballs" that later become comets) and cause mass extinctions every 26 million years? Are there pieces of the moon (other than those brought back by the Apollo voyagers) lying about on the earth's surface? Have pieces of Mars [quoteright]been blasted from its surface with enough force to reach the earth? These and similar questions were posed, and tentative answers to them given, at the 47th annual meeting of the Meteoritical Society held in Albuquerque, New Mexico, over three days ending August 2, 1984.
The sudden (well, over a period of several tens of thousands of years) disappearance of the dinosaurs from the face of the earth took place at the end of the Cretaceous period--about 65 million years ago. Not just the dinosaurs but also many marine and land species of animals and plants perished at the same time. Recent studies have shown that such extinctions--though not usually on so massive a scale as the one when the dinosaurs perished--have occurred about every 26 million years. These periodic events caused some astronomers to postulate the existence of a dark companion for our sun (after all, most stars in the universe are double or otherwise plural) with a period of about 26 million years, which would perturb the sphere of unborn comets (the Oort cloud) out beyond the orbit of Pluto, causing comets to appear in great numbers, along with corresponding falls of giant meteorites. This, in turn, would cause sufficient dust in the upper reaches of the atmosphere to change the climate of the earth for a considerable period, thereby causing the mass extinctions. But Dr. E.M. Shoemaker, a scientist with the U.S. Geological Survey, pointed out that careful Monte Carlo calculations on the existence of such a star for the time in question yielded a probability of not more than one in a thousand. On the other hand, it does seem more likely that the Oort cloud is perturbed by giant molecular clouds through which the sun periodically passes. Additional evidence that it is indeed the falls of giant meteorites that trigger the extinctions is found in the fact that there are clusters of fossil impact craters in geological strata dated at or near the times of mass extinctions.
K. Yanai and H. Kojima, scientists from the National Institute of Polar Research, Tokyo, presented compelling evidence that two of the more than 5000 meteorites the Japanese have recovered from Antarctica came from the moon. Their composition and appearance are nearly identical to those of rocks brought from the moon by astronauts, and their composition is quite different from that of other meteorites in the same general class. In another paper, it was pointed out that one recovered meteorite seemed to have a composition similar to Martian rocks--at least so far as information from the Viking landers can be used to determine Martian rock structure. Here, however, another difficulty arises: at present, we know of no mechanism by which a rock could be hurled from the Martian surface with enough force to carry it to earth without being vaporized in the process.
The Giotto Mission to Halley's Comet was described by Dr. A.D. Johnstone (England) and R. Reinhard (Netherlands). Most Americans present were quite chagrined that the United States, alone of the major powers, is sending no spacecraft to intercept Halley's Comet-although the European scientists are generously allowing us to place experiments aboard their vehicles. The mission takes its name from the Italian painter Giotto di Bodone, who, in 1304 A.D., depicted Halley's Comet as the Star of Bethlehem in a nativity scene on the wall of a chapel in Padua. Because the comet's orbit is retrograde (it goes around the sun counterclockwise, if we consider the orbits of all the planets as clockwise) the flyby velocity will be quite high--69 kilometers/second--so the instruments must gather their data quickly. It is unfortunate indeed that this apparition of the comet is the poorest in 2000 years and that the four locations for really good viewing are in the high Andes, central Australia, and South Africa. (The Bay Area may be a wonderful place to live, but it will not afford residents much of a view of Halley's Comet.) NASA is studying a proposal to use a second Giotto spacecraft, made by ESA (European Space Agency), to intercept another periodic comet and return a sample of the coma (head) to earth.
The doyen of the Meteoritical Society, H.H. Nininger, founder and owner of the American Meteorite Museum (now, alas, a ruined shell on old US. 66) urged younger members to adopt his method for recovering meteorites. Anyone who looks at a map of the distribution of meteorite falls in the US. will observe a heavy concentration in Kansas. Now, Kansas was no more subject to showers of meteorites than any other state. But there was one important difference: H.H. Nininger lived there.
Each fall, after the crops were in, he took off in his jalopy and visited as many farms as he could manage. At each one, he engaged a farmer in conversation and showed him specimens of a variety of meteorites. Then he said, "Next spring, when you are plowing, keep an eye out for rocks like these. If you find one, put it aside. I'll be back next year, and if it is a meteorite, I'll pay you a dollar a pound." A dollar a pound for a rock seemed like found money to farmers in the Depression--you can be sure they looked at every rock they turned up carefully--and Nininger accumulated an extensive meteorite collection. In his paper, he urged meteoriticists to visit Bishopville, in Sumpter County, South Carolina. There, just at dusk on March 25, 1843, a meteor streaked across the sky, witnessed by residents in a 40-mile circle. One six-kilogram stone was seen to strike the earth. No ground search was ever made for other fragments--there surely must have been some--and Nininger recommended to young members the search for those, using his own method in Kansas. He pointed out that the recovery of pieces of the stone after 141 years would be a valuable contribution to science.
Anyone passing through Albuquerque would find the Institute of Meteoritics of the University of New Mexico, the only one of its kind in this country, well worth a visit.