METEORITES BY Brian MASON* Outer space is a theme which in its many variations affects all of us today. Plans are made to send manned vehicles to the moon, and ultimately to our neighboring planets. Science fiction authors would extend our range even further, to the distant stars. Adventure seekers now look to interplanetary travel, rather than the Antarctic or Mt. Everest, to satisfy the lure of the unknown and their thirst for fame or notoriety. The firing of man- made missiles into outer space may be considered as a form of retaliation, as the Earth has been bombarded from outer space since the beginning of time. Fortunately, most of these extraterrestrial missiles burn up in the atmosphere and reach the ground as fine dust. However, occasionally a larger piece survives this perilous passage, lands on the Earth, and is eventually collected. Such pieces are known as meteorites, which we collect and preserve as the only tangible samples of the great universe that exists beyond the Earth. We are fortunate that the frequency of fall is very low, otherwise more of us would suffer the unique and undesirable experience of Mrs. HODGES of Sylacauga, Alabama who, on Nevember 30, 1954, was bruised when a meteorite broke through the roof of her house and struck her on what was delicately described as her upper thigh ! There are well-authenticated records of meteorite falls as far back as the fifteenth century, but our ancestors were skeptical about reports of stones falling from the sky. It was not till early last century that scientists were convinced of the fact that stones from outer space do fall on the earth's surface. In the U. S. the fall of more than 100 kilograms of stones near Weston, Connecticut, at 6 : 30 a. m. on December 14, 1807, was extensively reported on by Professors SILLIMAN and KINGSLEY of Yale University. However, skepticism was still rife, because even the then President, Thomas JEFFERSON, ventured the opinion that it was easier to believe that Yankee professors would lie than to believe that stones would fall from heaven ! Once it is admitted that falls actually occur, the question naturally arises : What is the frequency of meteorite falls ? This is not an easy problem to resolve. In most places there is no one to observe a fall when it occurs, and the probability of its being picked up afterwards depends on many chance factors. A study of the frequency of meteorite falls in densely populated areas indicates that the average rate of fall of meteorites on the Earth probably lies between 0.3 and 1 .0 falls per year per million square kilometers which is between about 150 & 500 annually for the whole earth. This would mean between two and eight falls a year within continental United States. No great reliance can be placed on this estimate, although it is probably of the right order of magnitude. Occasionally there have been falls of large meteorites which must have forced themselves on the attention of any observer who was within hundreds of miles of the spot. Such a case was that of the Tunguska meteorite, which fell in an uninhabited region of northern Siberia on June 30, * Chairman , Department of Mineralogy, The American Museum of Natural History, New York ; Journal of Geography (Chigaku Zasshi), No. 723, 1961 43 194 Brian MASON 1908. The impact was felt as an earthquake along the Trans-Siberian railway, 500 miles to the south, and the forest surrounding the point of impact was felled radially outwards for a distance of 60 km. Meteor Crater in Arizona (Fig. 1), 1,300 meters across and 200 meters deep, is evidence of a similar meteorite fall in prehistoric times, and meteorite cra - ters are known from other parts of the world (generally in deserts, where erosion is very slow, and hence the crater survives). The largest meteorite " in captivity " is the Cape York, which weighs 34 tons (Fig. 2) ; it was brought back from Greenland by Admiral PEARY in 1896, and is now in The American Museum of Natural History. The largest known meteorite is the Hoba iron, which still lies where it was found, on a farm in Southwest Africa, (Fig. 3) ; its weight is probably about 70 tons. Fig . 1 Fig . 2 Fig . 3 Meteorites are classified according to their composition into three groups : the irons, the stony-irons, and the stones. There is a possible fourth group, the tektites, which are small glassy objects which have been found in isolated areas in several parts of the Earth (Australia, Southeast Asia, Czechoslovakia, Texas and Georgia) ; they are different from known terrestrial glasses, but since no one has ever seen a tektite fall, their classification as meteorites is still disputed. Meteorites are named after the nearest geographical location where they fell or were found. There are about 1,500 well- authenticated meteorites, using the term meteorite for a single fall-each fall may however comprise many individual pieces (tens of thousands of fragments have been picked up around Meteor Crater, and the Holbrook, Arizona fall of 1912 was made up of over 14,000 individual stones. A common misconception is that all 44 Meteorites 195 or most meteorites are irons. It is true that the largest meteorites are all irons, and that in our museum collections irons generally dominate over stones. However, if we examine the statistics of finds (meteorites found but not seen to fall) versus falls (those collected after having been seen to fall), we see a remarkable reversal in the proportions of different types between the finds and the falls (Tab . 1). The reason is not far to seek. The relative abundance of irons as finds is due to their being easily recognized as meteorites , whereas a stony meteorite unless seen to fall could easily be overlooked as such. If we were to plot on a map the finds of meteorites in the United States we might conclude that the Great Plains have been bombarded whereas New England had been mysteriously spared . The reason is that any stone on the Great Plains attracts attention, whereas in the rocky terrain of lo!e w England it has to be a very unusual stone to attract a second glance. Thus we get a truer indication of the relative abundance of the different meteorite types from the relative proportions of those actually seen to fall ; this shows that the stone meteorites are far more abundant than all other types. Table 1 . Frequency of meteorite finds and falls (From PRIOR & HEY 1953) The iron meteorites consist essentially of an iron-nickel alloy, the average composition being about 91% iron, 8 . 5% nickel, and 0 . 5% cobalt. All miteoritic iron contains nickel, and a test for this metal is usuful for eliminating specimens of cast iron and similar material which are often picked up and sent in to the museum as possible meteorites. The iron meteorites commonly contain small rounded inclusions of troilite (iron sulphide) and schrei- bersite (iron-nickel phosphide). The metal generally shows a definite structure, known as Widmanstatten figures, which is brought out by etching a polished surface with acid (Fig. 4). The Widmanstatten structure is a network of bands crossing one another in two, three, or four directions. The bands are composed of kamacite (a nickel-iron alloy with about 6% of nickel), bordered by thin lamellae of a nickel-rich alloy called taenite. The angular interstices of the network are filled with a fine-grained mixture of kamacite and taenite, known as plessite. This structure is the result of a chemical segregation after the metal crystallized. The nickel-iron must have been originally molten, and then solidified as large crystals of a homogeneous nickel-iron solid solution. However, on further cooling this solid solution was no longer stable ; it broke down into a mixture of two phases--a nickel-poor one (kamacite) and a nickel-rich one, taenite. The regular arrangement of the bands results from their being oriented parallel to the octahedral planes in the original homoge- neous crystal of nickel-iron (Fig. 5). Such a structure is typical of an alloy cooled very slowly after crystallization. The stony-iron meteorites are made up of nickel-iron and iron-magnesium silicate 45 196 Brian MASON Fig . 4 Fig . 5 minerals in approximately equal amounts. The silicate mineral is generally olivine, (Mg, Fe)2 SiO4, occasionally with some pyroxene, (Mg, Fe) SiO3. Commonly the olivine occurs as rather large rounded grains in a sponge-like network of nickel-iron (Fig. 6). It is interesting to note that a meteorite of this type, weighing about 1,500 pounds, was found near Krasnojarsk in Siberia in 1749, and was seen by the Swedish traveller P. S. PALLAS in 1772, who recognized that it must be of extraterrestrial origin. Stony-iron meteorites of this type are known as pallastites, in his honor. The stones are the most abundant meteorites, and also show the greatest variety in composition and structure. They are divided into two groups on the basis of structure, the chondrites and the achondrites. The chondrites are so named because of the presence of chondri or chondrules, which are small (generally about one millimeter in diameter) rounded bodies consisting of olivine or pyroxene (Fig. 7). The achondrites are without chondri, and are much more coarsely crystallized that than the chondrites. Chondrites are far more common than achondrites, making up over 90% of all stones. Fig . 6 Fig . 7 Mineralogically most chondritic meteorites are a mixture of olivine and pyroxene, generally with some nickel-iron (usually between 5% and 20%), troilite (about 5%), and plagioclase, socium calcium aluminosilicate (about 10%).