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Science 101: Geology
Earth the Planet
Planet Earth is part of the solar system, and this solar system is literally formed of star dust. Cataclysmic explosions of early-generation stars distributed the heavy elements that were created by fusion inside the star. As these materials were strewn into the universe they gradually began to congregate once again, thanks to gravitational attraction. Along with less-dense matter, such as hydrogen and helium, heavy elements helped to form the rotating cloud of gas and dust called the presolar nebula. Heavy elements such as iron and nickel tended to stick together, and in sizes ranging from dust particles to moon-sized asteroids these refractory materials found themselves orbiting the massive center of the nebula—that is, if they escaped falling into its center. The rocky planets that orbit our Sun represent local gravity wells: in other words, accumulations of solid mass that attract other massive solids. Earth, then, seems to have grown bit by bit in a process that continues to the present each time a meteor or speck of cosmic dust enters the atmosphere. In the early days, however, there was nothing gradual about the collisions of meteors that must have fused and melted the earliest rotating glob of material that we could have identified as Earth.
In "The Method of Multiple Working Hypotheses," a charming and now classic article published in 1890, geologist Thomas Chrowder Chamberlin (18431928) argues that scientists should keep a family of hypotheses in mind rather than fixating prematurely on a preferred explanation in any case ofscientific uncertainty. This admonition did not prevent Chamberlin himself from eventually arriving at a preferred hypothesis of his own, along with American astronomer F. R. Moulton (18721952): the Chamberlin-Moulton hypothesis for the planetesimal origin of Earth. Chamberlin's method of multiple working hypotheses seems to have served him well, for the Chamberlin-Moulton hypothesis is now widely accepted as the best explanation for the primary process that formed our planet. There remains, however, some uncertainty about how this planetesimal formation could have occurred. Two major questions are associated with the planetesimal accretion theory: First, how did the planetesimals form in the first place? Second, once planetesimals grew large and began to collide, what prevented the great amounts of energy released on impact from blowing everything to bits? Actually, these questions have closely related answers.
At the first Apollo 11 Lunar Science Conference in January 1970, S. K. Asunmaa, S. S. Liang, and G. Arrhenius, fresh from their studies of the newly arrived lunar rocks provided by NASA, grappled with the first question: Namely, how did primordial accretion occur to form the planetesimals? Somehow, the primary grains needed to coalesce into large pieces, but the gravitational attractions between the little grains were so small as to be negligible. Electrical charges might bring the particles closer together, but by themselves these forces were not strong enough to fuse the particles into a solid mass, and, in any case, electrostatic forces are too weak to keep grains together with relative velocities of greater than about one meter per second. Asunmaa, Liang, and Arrhenius concluded that the grains must have been welded together in the period of time after a high-velocity impact when relative velocity was decreasing to the point that the hot grains (with still molten surfaces) could weld together. Using scanning electron microscopy to view particles of lunar soil, they identified a variety of "coatings and bridging structures between individual particles, thereby developing an aggregate structure with increasing cohesion."
The Accretion Event
Radiometric dating (using rubidium-strontium and neodymium-samarium dating methods) of the most ancient meteorites suggests that plan-etesimals were formed more than four and a half billion years ago. Once formed by the welding-accretion process, the planetesimals came together by a process of impact and collision. Such great amounts of energy were released in these impact events that one might expect that the impact explosion would overcome any gravitational or electrostatic attractions and blast everything (both impactor and its target) back into space as a hot dust (where the welding process would begin anew). Indeed, this does seem to have been at least partly the case during the formation of Earth's Moon. The leading hypothesis suggests that the early Earth was hit by a giant planetesimal approximately the size of Mars. This impact led to the ejection of a huge amount of the early Earth's mantle into space, to form what for a short time at least must have resembled a Saturn-like ring of superheated rock and dust. The welding process led to the formation of the Moon in fairly short order, giving Earth a natural satellite that was considerably closer to Earth than it is now (it has been moving away ever since). Smaller planetesimals, in contrast, did not have such a dramatic effect on the early Earth itself. Nevertheless, the energy delivered with each impact was immense. But instead of being Earth-shattering, calculations have shown that the energy from these smaller collisions would mostly be transformed into thermal energy that was largely retained within the early Earth.
The Four Heat Sources
The heating from planet-esimal impacts was (aside from radioactivity) the first major source of heat present in the early Earth. The larger the planetesimal, the greater amount of heat delivered to the growing planet for two reasons. First, a larger rock body at a given incoming speed brings with it a larger amount of kinetic energy that can be transformed into thermal energy or heat. Second, there will be a difference in the heat retained by the impact of a large body in comparison to that of a small body. A rock body such as a small asteroid will deliver most of its energy to the planetary surface, where much of the thermal energy will escape back into space. A large planetesimal, on the other hand, would penetrate into the early Earth, and the incoming energy would be effectively trapped by the surrounding rock of the . . .Science 101: Geology. Copyright © by Mark McMenamin. Reprinted by permission of HarperCollins Publishers, Inc. All rights reserved. Available now wherever books are sold.