On the origin of meteorites.

This paper develops and extends the arguments in favor of the hypothesis that the meteorites had their origin in the disruption of an ancient planet that once revolved about the sun in an orbit lying between the orbits of Mars and Jupiter. In addition it reconstructs a detailed description or model for this proposed ancient planet from a study of the composition and structure of the meteorites. There is shown to be a complete absence of a correlation between the falls of meteorites and the occurrence of meteor showers that are associated with comets. On the other hand, there are evidences that the meteorites are related to the minor planets which revolve about the sun in orbits between the orbits of Mars and Jupiter. The many evidences that the meteorites solidifled within a large body are discussed and extended. Formerly, the extreme differences in the ages of the metallic meteorites presented a serious difficulty to the hypothesis that they had all originated within the metallic core of a single body. However, it has recently been recognized that cosmic radiation produces helium in meteorites. Therefore, their ages, calculated on the assumption that the helium was derived only from the decay of uranium and thorium, are greater than the true times since their solidification. An allowance for the effects of cosmic radiation eliminates much of the scatter in the ages and makes it appear possible that the metallic meteorites solidified while part of the core of an ancient planet. This parent planet of the meteorites was disrupted after it had com- pletely solidified, probably several hundred million years ago. This fact is important because it indicates at once that this planet was considerably smaller than the earth. The model proposed for the ancient planet has a nickel-iron core similar to that believed to exist at the center of the earth. Within this metallic core the nickel is concentrated toward the center. Surrounding this metallic core is a transition layer of which the stony-iron meteorites are representative. Outside of this transition layer is a stony shell. Within the stony shell the model specifies that the percentage of free metal decreases with distance from the metallic core. The small number of stony meteorites in which there is no free metal represents a thin outer mantle that is similar to the rocks observed near the surface of the earth. Specifically, the achondrites are placed near the outside surface of the planet while the stony material in which the chondri occur is at a greater depth in the stony shell. This model has the advantage that, in going outward along a radial line, it not only gives a continuous change in the per cent of free metal present, but it also gives a continuous change in the per cent of nickel in this free metal and arranges the meteorites in a continuous sequence with respect to their structure. In addition, this model satisfies other requirements and has a logical explanation in terms of the physical processes that occur. Observatory, University of Michigan, Ann Arbor, Mich.