Following the trail of falling meteors through folklore and fact
Cosmic Debris: Meteorites in History, by John G. Burke. Berkeley, Calif.: University of California Press. 420 pp. Illus. $45. Meteors are those transitory streaks one sees occasionally in the sky at night. Meteorites are the rocks that actually make it to Earth and are here for the finding - free samples from somewhere for us to examine.
Aristotle described them, and just about everything else, in ``Meteorologica''; Franklin's friend John Perkins from Boston thought shooting stars were bolts of electricity from one part of the atmosphere to another.
Newton apparently ruled out their existence because he felt only the great bodies - fixed stars, planets, and comets - existed outside the earth. If material existed outside of the great bodies, he reasoned, it would interfere with the steadiness and consistency of their motion. Newton thought meteors were the result of sulfurous streams fermenting with nitrous acid in the air. And some people claimed stones fell from heaven.
In 1793, most scientists anyway felt that rocks did not fall from heaven. Rocks did not fall from the sky because it was impossible to make rocks from the elements in the air. This view, however, changed in the next 10 years (1794 to 1803). In ``Cosmic Debris,'' John G. Burke describes one of the pivotal events:
``About 7 p.m. on 16 June , a cloud at very high altitude appeared from the north, discharging sparks and emitting smoke. Flashes of lightning, very red and slower than normal, issued from the cloud, and suddenly there was a loud explosion. A shower of stones then rained down east and south of the city, several of which, witnessed by English ladies, fell in a meadow at Cosona some twenty miles away. Some fragments fell into a pond, which was subsequently drained, and those recovered were sold at high prices to English travelers. Hearing of this, the natives attempted to simulate the originals by artificial means, selling them to the unwary. How many stones fell is not known. One report estimated there were about 200, several of which weighed as much as two to three pounds.''
The Siena fall just described provoked a lively controversy, and eventually led to the acceptance of falls and a more serious investigation of the phenomena of meteorites. Ambrogio Soldani, a ``local philosopher,'' thought the stones were formed in the cloud; Giorgio Santi initially thought they had been ejected from Vesuvius, but later changed to thinking they were from an undetected volcano.
But Ernst Chlandi, in a book published in 1794, stated his belief that meteorites consisted of compact and heavy matter that came from cosmic space. Chlandi related meteorites to shooting stars, which he felt were the result of the stones passing through the atmosphere and momentarily emitting light. His ideas came about in part through his long study of the phenomena and his interactions with some of the brightest thinkers of his day. Thus progress in this small but important field began.
``Cosmic Debris'' is not a textbook on meteorites, but a history of the study of meteorites. It brings out the importance of the development of chemistry, geology, and mineralogy in the development of the study of meteorites, and the importance of these strange and diversified objects on the developments in those fields.
One example might illustrate the interrelationships among these varied fields. When the most predominant class of iron meteorites, called octahedrites, are cut, polished, and etched, they reveal a very interesting pattern.
This pattern is thought to result from cooling over hundreds to perhaps billions of years, at temperatures between 350 and 700 degrees C. It is a pattern due to the separating out of different compounds of iron and nickel along octahedral planes of the original solid solution.
Taking the historical perspective in his book, Burke covers the myths, the folklore, the collectors, the mistakes, and the progress. The result is a colorful image of the past and lines of inquiry up to the present. Modern meteor experts have set for themselves the goal of determining the origin and history of the solar system.
In the late 1940s, Harrison Brown and Claire Patterson published a series of papers on the composition of meteorites. The notion of a single parent body of about 6,000-kilometer diameter seemed to be feasible from the data they had used. Harold Urey classified the available data in a different way and concluded that more than one body was involved.
A number of workers developed schemes to classify meteorites, each attempting to glean more of the history of the formation of the solar system. Key in these investigations are reasonable estimates of the age of the solar system, and meteorites, the duration of the exposure of meteorites to the background cosmic radiation, and the terrestrial age of the meteorite.
In the 1950s Claire Patterson announced from a careful study of several meteorites that the age of the solar system was 4.55 billion years (plus or minus 0.07 billion years). This number is still quite accurate, even though different classes of meteorites indicate slightly different ages for formation. By 1963 John Wood in a status report for the field indicated that there were almost as many theories about meteorites as there were researchers. Fortunately, Burke tells us, in the last 20 years the number of hypotheses has not increased with the number of researchers.
One of the exciting things about research in meteorites today is the fact that so much is still under active investigation. A fresh fall will attract a number of workers, and new information and puzzles result.
For example, the Allende fall in Mexico in 1969 brought to light new information about a formation called calcium-aluminum-rich inclusions. The Allende fall convinced researchers almost overnight that there was a high-temperature phase in the formation of the solar system.
There is still much to be learned. The Apollo mission to the moon, coupled with Antarctic collections of meteorites, shows clearly that some meteorites came from the moon. The whole puzzle of the formation of the solar system has yet to be solved.
Burke's book gives some notion of how far researchers have come. Who knows how far they have yet to go? So if you find a rock that has fallen from heaven (perhaps a sample from Mars), contact your local college's geology department, and watch the search continue.
Paul A. Robinson Jr. is a staff scientist at the Jet Propulsion Laboratory in Pasadena, Calif.