Edward Anders and Gordon G. Goles
Un~versityof Chicago Chicago, Illinois
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Theories on the Origin of Meteorites
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I t is generally agreed that meteorites are fragments of larger bodies. There are, however, profound differences of opinion on the properties and dimensions of these parent bodies. A central point in this controversy is the nature of the processes t,hat led to the structural and compositional features of t,he meteorites. The chemist may have much to say concerning these problems. Some of the most crucial points, such as the dating of events in meteoritic history by the use of radioactive decay (1, g), and the investigation of elemental abundanees to infer both the primordial composition of the solar system (3, 4) and its chemical history (5, 6),are best treated by chemical techniques. Chemical insight is also valuable in interpreting many of the observations on the detailed structure of meteorites. The laws of physical chemistry, derived from observations on the Earth's surface, may be fruitfully applied to treat events occurring in the outer reaches of the solar system as long as 4.5 aeons' ago. It is important to keep in mind, however, that t,hese measnrement,~and observations on the met,eorites relate t,o a sample which is only a minut,e fraction of the hypothetical parent body-perhaps 10-l5 to Furthermore, it seems likely that this sample is biased in favor of material which would not easily be destroyed during the break-up of the parent body and which could survive passage through the atmosphere. Possibly the sample also is weighted in favor of material which mas near t,he surface of the parent body. Any ronclusions about the nature of the parent bodies drawn from experiments on this sample necessarily represent a long extrapolation. Theories of the Origin of Meteorites
I n one respect, all the attempts at understanding the origin of the meteorites begin on common ground: it is now generally believed that the arcumulation of the parent bodies from the solar nebula occurred at temperatures below 600°K (8, O), and the chemical nature of this starting material has been investigated in some detail (10, 11). The chief differences in the three principal theories of meteorite origin lie in the dimensions proposed for the parent bodies. These are postulated to have been (1) of planetary size; ( 2 ) two successive generations, one of lunar size and one of asteroidal size; and (3) of asteroidal size only. All Presented as part of the Symposium "Geochemistry: Analysis and Synthesis" before the Divisions of Inorganic Chemistry, Physical Chemistry, and Chemioal Education at the 137th Meee ing of tho ACS, Cleveland, Ohio, April 1960. This work was supported in part by the U. S. Atomic Energy Commission. 1 An aeon is defined as 100 years, which is a convenient unit of time for discussion of the history of the solar system (7).
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Journol o f Chemicol Educofion
of these theories have certain advantages and disadvant,ages in their application to explaining the observations 011 meteorit,es. I t is probably best to regard them all as working hypotheses, subject to further experimental tests. Plonefory-Sized Porenf Body
A number of investigators, among whom Ringwood (12) and Lovering (IS, 14) may be cited as recent,
examples, have expressed t,he view that the meteorites evolved in a single body, of a size intermediate between that of the moon and that of the earth. The chief advanbges of this hypothesis are that it is apparently simple, with only one or a t most two parent bodies required; that many analogies with the presumed composit,ion and structure of the earth's interior can be drawn; and that the occurrence of diamonds in meteorites may be at,t,rihuted t,o the graphite-to-diamond transformat,ion at high gravitational pressures. Also, a source of heat for met,eorite synthesis is available in the long-lived radioactivities which now heat the earth's interior, and would certainly have been more intense heat sources some four-and-a-half aeons ago. There are, however, very serious disadvantages connected with these point,s. Breaking up a body of planet,ary size is e~t~remely difficult and no one has proposed a t,ruly satisfact,ory solution to the problem. I t is necessary to suggest a plausible way to perform this disruption so as to eject materials from as deep as t,he core (which is postulated to be the place of origin of the iron meteorites) and a t the same time to avoid the vaporization or profound metamorphosis of most of the ejected materials, hot,h silicates and metal. One should note t,hat the energy released in allowing two planets to fall together would be enormous, more than enough to vaporize them both if efficiently converted to heat (16). The mean energy per gram in the collision of two objects of equal radius a and equal mean densit,y pis on the order of:
where G is the gravitational constant. If each were the size of the moon, with a density of 3.3 g/cma, (E) would be -640 cal/g, while for objects the size of the Earth, having the same density as that quoted, (8) 8500 cal/g. If all of this energy were converted to heat, the increases in the mean temperature for the two limiting cases would be -3200°C and -42,00OoC, assuming a mean heat capacity of 0.2 cal/g/deg. Clearly, some of this energy must appear as kinetic energy of the escaping fragments; also, there is no simple way of estimating what the true mean heat capacity would be. Nevertheless, heating must have
t,ransformat,ion temperature of iron-nickel alloys t,o the point where t,he diffusioi~rates would be small, and t,hus the formation times of the Widmanstatten pattern become exccediugly long. Furthermore, even if the interior of t,he primary object were initially as cool as 450°C, duc to efficient loss of heat from near-surface chemical reactions, it would soon begiu to heat up from the decay of I
