METEORITES by Brian MASON*

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

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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 homogeneous 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

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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%). Many of the achondrites are similar to the chondrites in composition, except that they contain little or no nickel-iron some, however, contain a considerable amount of plagioclase, and are known as the basaltic-type achondrites, since their composition is similar to basalt, a common volcanic rock.

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There is one small group of chondrites, however, that are remarkably different in their chemistry and mineralogy. These are known as the carbonaceous chondrites, since they contain ceveral percent of free carbon, which gives them a black sooty appearance. Whereas all other meteorites contain essentially no combined water, the carbonaceous chondrites contain ten to twenty percent. They also contain small amounts of organic compounds, free sulphur, and calcium and magnesium sulphates. Instead of olivine and pyroxene, their silicate mineral is largely serpentine, a hydrated iron-magnesium silicate. They contain no metallic nickel-iron. Only nineteen are known ; all of them were seen to fall and were picked up soon after. Indeed, if they were not collected immediately after falling they would not survive very long ; of one of them it is recorded that when placed in a glass of water, it disintegrated and gave off a nasty smell ! (This was probably sulphuretted hydrogen, produced from easily decomposed sulphides). The first one known fell at Alais in France on March 15, 1806 ; it was sent to the famous chemist BERZELIUS for analysis, who expressed great doubt as to its meteoritic origin, since it was so different from all other meteorites. However, the great differences between the carbonaceous chondrites and what we may call the olivine-pyroxene chondrites have obscured one remarkable similarity. Dr. WIIK, a Finnish chemist who has analysed many meteorites, pointed out in 1956 that if analyses of these two types of chondrites are recalculated on a water, carbon, oxygen and sulphur-free basis the elemental composition is then very similar. In other words, the chemical composition of the common olivine-pyroxene chondrites is that of a carbonaceous chondrite from which water, sulphur, carbon, and some oxygen has been removed. The conversion of a carbonaceous chondrite to an olivine-pyroxene chondrite could be produced by a natural heating process. Serpentine decomposes on heating above about 500°C to give olivine and pyroxene ; if carbon is present a natural smelting will take place, reducing some of the combined iron to metallic iron, which is a constituent of the olivine-pyroxene chondrites. Having described the principal characteristics of meteorites, we may now consider their origin. H. A. NEWTON, as long ago as 1886, summarized the numerous theories as follows : " The y came from the moon ; they came from the Earth's volcanoes ; they came from the sun ; they came from Jupiter and the other planets ; they came from some destroyed planet ; they came from comets ; they came from the nebulous mass from which the solar system has grown ; they came from the fixed stars ; they came from the depths of space. " Truly a plethora of possibilities. Some, however, can reasonably be eliminated. An origin from the earth's volcanoes is inconsistent with their composition, which is quite different from all volcanic rocks, except for the small group of basaltic-type achondrites ; and volcanoes, although powerful, do not produce sufficient energy to send rocks into orbit ! It is difficult, if not impossible, to derive meteorites directly from the Sun, the Moon, or Jupiter and the other planets. However, meteorites presumably originated in the solar system ; since, like all other bodies in this system, they revolve about the sun. For many years it has been generally believed that meteorites are fragments of a disrupted planet which once existed between Mars and Jupiter. This region of the solar system is occupied by innumerable small objects (up to a few hundred miles in diameter), the asteroids. The origin of the meteorites and the asteroids from a disrupted planet implies that the disruption probably took place early in the evolution of the solar system and that most of the fragments were swept up soon after. Such a disruption would probably have bombarded the other planets with fragments very much larger than the average meteorite of the present time. No evidence of bombardment on such a scale has yet been recognized in the rocks of the

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Earth's crust, some of which date back over three billion years. The craters on the moon are thought to be due to the impact of meteorites ; in the absence of an atmosphere no weathering or erosion takes place on the moon, and these craters have thus survived unchanged since their formation. The irons, the stony-irons, and the achondrites can readily be explained as originating from the disruption of a planetary body. Such a body, like the Earth, would presumably be differentiated by the force of gravity into an iron core and a stony mantle, the stony- irons coming from a transition zone. Certain features of the chondrites are not so easily accounted for, however. The composition of the carbonaceous chondrites shows that they have never been above 500°C and have probably always been at much lower temperatures. The texture and structure of both the carbonaceous chondrites and the olivine-pyroxene chondrites indicate that they have not originated in any body with a considerable gravitational field. Many of them are very porous and friable, some to a degree that they can be crumbled in the hand, indicating that they were not consolidated under pressure. The nickel-iron in the chondrites is intimately mixed with low-density silicate minerals and shows no sign of gravitational segregation. One of the discoveries of astronomy in recent years has been that outer space is dusty. It is now believed that in the universe more material exists in the form of dust than is aggregated into stars. The composition of this dust is unknown, but it is presumably make up of the same chemical elements that make up the larger bodies. Perhaps the carbonaceous chondrites represent this primordial dust. We have seen that heating processes would convert carbonaceous chondrites into olivine-pyroxene chondrites. The solar system may have originated as an immense cloud of primordial dust, which gradually cohered into moderate-sized masses, of which the chondritic meteorites are a sample. These masses, planetesimals as they were called by Chamberlin and Moulton, aggregated to form the planets. Aggregation into large bodies produced heat sufficient to melt them, at least partially. This melting resulted in a gravitational separation of nickel-iron from the silicate. Breakup of a planet, presumably situated between Mars and Jupiter, produced the asteroids, some of which stray into the Earth's gravitational field and eventually land as the irons, stony-irons, and achondrites.