Minor Planets and Terrestrial Resources
Michael J. Gaffey, Trude V. V. King
Asteroids are the surviving remnants of planetesimals that originally populated the inner solar system, and which represented the building blocks that accreted to form the terrestrial planets. These minor planets are significant to the geosciences community. (1) Their properties were a major factor in controlling the initial conditions on the primordial Earth. (2) The impact of these bodies onto the Earth represents a significant geologic process that can produce economically important structures and deposits, and that may have influenced extinctions and the evolution of living organisms. (3) Their relatively common samples--the meteorites--provided strong motivation and direction to Bronze Age cultures to develop ferrous metallurgy. (4) The Earth-approaching asteroids of er economically viable sources of raw materials to support large-scale space operations and, perhaps, for the terrestrial marketplace.
The main belt asteroids--located between the orbits of Mars and Jupiter--include several hundred thousands bodies from 1 km in diameter (> 2 billion MT), to nearly 1,000 km. The number of asteroids in a size interval increases proportionally to the inverse square of their diameters. Earth-based reflectance spectroscopy has been used to characterize the surface lithology of these minor planets. They include sedimentary and metamorphic assemblages, uniformly of chondritic bulk composition, that range from carbonaceous shale-analogs (C-type asteroids, with relatively abundant water and carbon, such as organics, graphite, and diamond) through anhydrous olivine-pyroxene-nickel-iron metal assemblages (ordinary chondrites, Q-type asteroids). The inner part of the main belt was apparently ntensely heated, and most of these asteroids (S, V, M, and A types) underwent melting and magmatic differentiation to develop a suite of igneous assemblages including pure nickel-iron metal, stony irons, eruptive and cumulate basalts, dunites, and pyroxenites. The complex water-modulated and multiple-cycle enrichment processes, which are important terrestrial ore-forming mechanisms, were absent or rare on asteroidal objects. In contrast the fractionation of nickel, cobalt, gallium, germanium, platinum group metals, and other siderophile elements into the metallic melts, and the subsequent exposure of the metal cores of these planetesimals by collisional disruption produced a previously unattainable ore class.
In excess of a hundred thousand small (> 100 m, > 2 million MT) asteroids approach or cross the Earth's orbit. Many tiny fragments of this population fall to Earth each year as meteorites. Every few thousand years, an asteroid of sufficient size to form a meteor crater falls to Earth, and every few ten or hundred million years, an object collides with the Earth that produces a Manicouagan (70 km), a Sudbury (140 km), a Puchezk-Katunki (80 km), or a Rieskessel (24 km) sized impact structure. Energetically, many such Earth-approaching asteroids are closer to low Earth orbit than is the Earth's surface. This economical position makes such objects attractive sources of unsophisticated bulk materials (e.g., water and carbon for fuels or bulk mass for shielding), which are needed in q antity for large-scale space operations.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.