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.