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COMETARY VERSUS ASTEROIDAL ORIGIN OF
GEORGE W. WETHER/LL
University of California, Los Angeles
Much of what we know about the early history of the solar system has been learned from the study of meteorites. This results from the fact, demonstrated by isotopic age measurements, that all of the various classes of stone and iron meteorites were formed .4.6 X 10° yr ago within a short period of time, probably less than 100 million yr in duration. This is also the age of Earth and the Moon and may be presumed to be the time of formation of the solid bodies in the solar system. Measurements of the products of the decay of the extinct radioactive isotopes 12°xe and ***Pu show, furthermore, that the formation of these solid bodies occurred within 100 million yr of the time of separation of the solar nebula from interstellar matter. Except for physical fragmentation into smaller bodies, the chemical and mineralogical composition of most meteorites has been essentially unaltered since this time during the formation interval of the solar system.
This situation contrasts with that found on Earth, where geological processes have essentially erased the record of the first 25 percent of Earth's history. The Moon is now known to be intermediate between Earth and meteorites in this regard. Although the record of the Moon's early history is preserved to a much greater extent than that of Earth, significant formation of lunar rocks occurred at least as recently as 3.3 × 10° yr ago. Although the best preserved record of the early history of the solar system is to be found in the meteorites, these data are difficult to interpret because, unlike rocks from Earth and the Moon, we have no definite information regarding the sources in the solar system of these rocks that are now colliding with Earth. Were such information to become available, the role of meteorites would become fully equivalent to that of lunar samples in experimental studies of the origin of the solar system.
From the work of Öpik (1951) we know that neither the meteoritic fragment nor its parent body can have been in its present Earth-crossing orbit for the entire history of the solar system. This is because these orbits are stable
with respect to planetary impact or ejection from the solar system for no more than 100 million yr. From the cosmic-ray-exposure ages we also know that the meteorite was broken from a larger body late in the history of the solar system. These facts require us to find some place in the solar system where we can “store” the larger body from which the meteorite was fragmented for most of the solar system's history, and then we must find a way to transfer more recently either the fragment or the parent body itself from its “storage place” into an Earth-crossing orbit, from which further fragmentation and collision with Earth are possible. The problem of identifying the source of meteorites can therefore be approached from the point of view of finding an appropriate storage place. The surfaces of a planetary body such as Mars or the Moon have been proposed, but are very unlikely to be satisfactory. It is hard to see how a fragment of meteoritic size can be accelerated to planetary escape velocities without complete destruction, or without at least experiencing shock metamorphism far exceeding that found in most meteorites. For this reason, as well as others, smaller bodies are more promising candidates. The two principal types of smaller bodies in the solar system are the comets and the asteroids. The associated storage spaces are the cometary cloud of Oort and the asteroid belt, respectively. No other associations of small bodies and storage places are known at present; it is conceivable that in the outer solar system there are additional unobservable families of small bodies of some kind. However, it seems most fruitful to give first consideration to known classes of bodies, rather than to entirely speculative ones. We have insufficient knowledge of the chemical or mineralogical composition, or for that matter, even the mean density of comets and asteroids to permit identification of any class of meteorites with these bodies on the basis of data of these kinds. Also, because the chemistry and mineralogy of the meteorites has been fixed since some time during the formation of the solar system, it is possible that the establishment of this chemistry and mineralogy preceded the time at which the present parent bodies were formed. Consequently, it is very difficult to make even plausible arguments concerning the kind of objects that could be derived from comets or asteroids without far more understanding than we possess regarding the processes by which these objects were formed. It is possible that identification of meteorites with their sources must await in situ analyses and other studies by suitable spacecraft. However, prior to such studies, there is a body of dynamical evidence bearing on this problem that can prove valuable in making plausible inferences regarding this identification and that can provide reasonable hypotheses useful in planning such missions. Application of dynamical data to this problem has been described (Arnold, 1965a,b; Öpik, 1966; Wetherill, 1968a,b; Wetherill, 1969). The purpose of this report is to update this earlier work and describe the progress that has been made in the last few years.