Meteorites and Their Properties - The Origin of Meteorites

II. The Origin of Meteorites
Meteorites are particularly valuable geologic specimens because they
represent samples of planetary bodies (mostly asteroids) which we have not yet
obtained through either manned or unmanned space missions. Neither is it likely
that samples of these bodies will be available by any other means than
fortuitous falls for at least a generation. Thus, as a scientific resource,
meteorites provide us with some of our first glimpses of the diverse array of
planetary material scattered throughout the inner solar system.
The oldest meteorite specimens are remnants of the very first geologic
processes to occur in our solar system 4.6 billion years ago. (The origin of the
solar system should not be confused with the origin of the universe, commonly
known as the Big Bang, which occurred at least 9 billion years ago and possibly
as long ago as 20 billion years.) Our solar system formed when a cloud of
interstellar dust and gas collapsed. Because the interstellar cloud had been
slowly spinning, the result was a nearly flat rotating disk which we refer to as
the solar nebula. Much of the dust and gas in the disk moved to the
center of the nebula where it fed a growing protostar which eventually became
our sun.
Schematic figure of collapsing nebula
A schematic diagram of the collapse of a molecular
cloud core to form the solar nebula. (a) The arrows suggest the incident
angles between the solar nebula and the material that accreted to it. (b)
Once the material was in the solar nebula, most of it was transported
towards the protosun (long horizontal arrows). There was a critical radius
in the outer portion of the nebula beyond which a small amount of material
was transported outward (short horizontal arrows). (c) After the accretion
of interstellar material had decreased, dust in the nebula settled to the
midplane. This created a zone with a high density of dust near the
midplane. A series of aggregational processes swept up this dust to form
planetesimals and eventually planets.
Photomicrograph of a chondrule in thin-section
A microscopic view of a spherical chondrule, 1
millimeter in diameter. The bright, colored regions within its margins are
mineral crystals. The black region between the crystals is glass and
represents once-molten material. This chondrule is in Semarkona, an
unequilibrated ordinary chondrite which fell in the Madhya Pradesh region
of India.
The dust and gas remaining in the nebula was incorporated into primitive
planetary material. Initially, nebular dust stuck together (or accreted) to form
small, loosely-bound dustballs. In some regions of the solar nebula these
dustballs encountered violent, high-temperature events and were melted, forming
molten silicate and metal droplets (like lava). Henry Clifton Sorby, a geologist
of the 1800's and one of the first to examine these droplets with a microscope,
described them as having once looked like fiery rain. Because the
high-temperature events were brief, the molten droplets cooled quickly in the
nebula and formed millimeter-sized spheres of rock called chondrules.
Meteorites containing these objects are called chondrites.
Sometimes the temperatures rose so high in the solar nebula that dust began
to evaporate, leaving behind refractory residues. At other times, the
temperatures became so low that new dust condensed from the nebular gas (like
snow from the air). Over time, chondrules, evaporative residues, and condensates
collided with each other and accreted to form nebular sediments and eventually
larger bodies called planetesimals (a few to several tens of kilometers
in diameter). Our most primitive meteorite specimens are samples of these
complex, yet primitive, mixtures of nebular (pre-planetary) material. In many
cases these meteorites are so primitive they contain traces of interstellar dust
which survived thermal processing in the solar nebula.
The small planetary bodies from which primitive meteorites come formed
throughout a large portion of the inner solar system. Within these vast
distances material was apparently distributed unevenly and affected by different
amounts of thermal processing. Thus, meteoritic parent bodies produced in
different regions of the solar nebula had slightly different chemical and
structural properties. The three principal groups of primitive chondrites
representing these variations are the carbonaceous chondrites,
enstatite chondrites, and unequilibrated ordinary chondrites.
In some cases, rocky (and perhaps icy) planetesimals continued to accrete
into much larger planetary bodies. The temperatures of these bodies began to
increase significantly, in part from the energy deposited by impacts as they
grew by accretion. Perhaps more importantly, the thick accumulation of material
insulated the interiors of these bodies, preventing heat produced by
naturally-occurring radioactivity from radiating into space. They may also have
been heated by interacting with a magnetic field that extended from the sun and
through the inner regions of the nebula. Consequently, temperatures were able to
rise high enough to metamorphose primitive chondritic material (like rocks
buried deep in the Earth's crust), causing minerals to recrystallize and grow
larger. Meteorite samples of these metamorphic rocks are called equilibrated
chondrites, because the metamorphic process also homogenized the disparate
chemical compositions of minerals in these bodies.
Elsewhere, temperatures became so high in planetesimals that primitive
material was completely melted, and thus able to form magma chambers and other
features typical of igneous activity. Meteorites produced by these igneous
processes are called achondrites, which means they do not contain
chondrules. In those cases where planetary bodies became large enough to have
substantial gravitational fields, iron-rich metal, which is denser than the
silicate portions of magma, may have separated from the partially or wholly
molten bodies to form dense iron-rich cores inside shells of silicate material.
This process is similar to that responsible for the differentiation of the
Earth, and thus why iron meteorites are considered analogues for the
Earth's core. Another group of meteorites called pallasites are intimate
mixtures of iron-rich metal and silicate crystals, and appear to represent
regions where achondrite and iron meteorite magmas were incompletely separated,
such as the core-mantle boundaries of planetesimals.
The different types of meteorites described above come from asteroids, which
are fragments of parent planetesimals or mixtures of fragmented planetesimals.
Comets are another potential source of primitive material, but most scientists
think that few, if any, large meteorites come from comets. On the other hand,
comets contribute a significant number of micrometeorites. In addition, although
fragments of comets may not survive as meteorites (in part because they collide
with the Earth with higher velocities than asteroids and are thus more likely to
be vaporized), they may be responsible for some of the larger impact events on
Earth (see Section IV below).
Calcalong Creek, a lunar meteorite
Calcalong Creek is a lunar meteorite which was
recovered in Australia. It is the twelfth lunar meteorite to be found and
is the first discovered outside of Antarctica.
A new and particularly exciting revelation is that some meteorites were
ejected from the Moon's cratered surface by lunar impacts. To date, 12 lunar
meteorites have been discovered. These samples are important because they
may sample a larger region of the moon than that of the six manned Apollo and
three robotic Luna (Soviet) missions. We also suspect some meteorites known as
SNC's (for the principal meteorites in that class: Shergotty, Nakhla, and
Chassigny) were ejected from the surface of Mars, and thus may be our first
samples of a neighboring planet.