DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

helium (for which the early tracers were metals
like Ca,Fe, and Na). The solar value is about
1.7%, or Z = 0.017. Observed values range
from 10 −4 of solar metallicity (for the oldest
stars in the halos of our own and other galaxies)
to perhaps 2 to 3 times solar (for stars in
the centers of large elliptical galaxies and the
ionized gas near the centers of quasars). Frequently
metallicity is given as the log 10 of the
ratio of iron/hydrogen of the star divided by
iron/hydrogen in the sun. This is abbreviated
[Fe/H]. See elliptical galaxies, halo, quasar.
metamorphism If sedimentary rocks are
buried in a sedimentary basin, the temperature
can become sufficiently high so that chemical
reactions can occur. These reactions can transform
a sedimentary rock into a metamorphic
rock.
meteor Any phenomenon in the air (a cloud
is a humid meteor), but especially the glowing
track of a meteoroid as it travels through the air.
Such meteors also are often called shooting
stars or falling stars. Most of the debris creating
meteors is extremely small (dust to sand-sized)
and burns up during the atmospheric passage.
If the material survives the passage through the
atmosphere and lands on the surface, it is called
a meteorite. A meteor shower occurs when large
numbers of meteors are seen radiating from the
same area of the sky.
meteorite Meteoroids that survive to impact
onto a body’s surface. Meteorites are classified
as stones, meteorites that are composed primarily
of silicates and are similar to terrestrial rocks;
stony-ironmeteoriteswhichhaveapproximately
equal amounts of iron and silicates; iron meteorites,
composed almost entirely of nickel-iron;
and further subdivided according to compositional
details. Stony-irons are the least common.
Most meteorites show thermal processing, and
arebelievedtobefragmentsoflargerparentbodies,
probably asteroids, although Martian meteorites
and lunar meteorites have also been identified.
Meteorites are also often characterized
as falls (seen to fall and recovered shortly thereafter)
and finds, which are found long after they
havelanded. Stonymeteoritesarethemostcommon
types of falls and are thus believed to rep-
© 2001 by CRC Press LLC
meteor shower
resent the majority of meteoritic material. Irons
are the most common types of finds simply because
they do not erode as quickly, nor are they
easily confused with terrestrial rocks. See meteorite
parent body, SNC meteorites, and under
the different classes of meteorites.
meteorite parent body Different classes of
meteorites have been differentiated to various
degrees, and have been subjected to elevated
temperatures and pressures. These processes
presumably occurred while the meteorite was
still part of a parent body. The breakup of such
parent bodies resulted in the various meteortite
classes observed. Particular asteroid classes
have been identified with parent bodies of various
meteorite types based on spectral similarities.
meteoroid A metallic or rocky small body
orbiting the sun in space; small particles that
may derive from comets; larger (1 cm or larger)
particles that probably derive from the asteroid
belt. Meteoroids seen passing through an atmosphere
are called meteors, and if they survive to
land on the surface, they are called meteorites.
Small meteoroids are also often called interplanetary
dust particles (IDPs). Sunlight reflecting
off this material gives rise to the zodiacal light
and the gegenschein.
meteor shower A large number of meteors
radiating from the same area of the sky. Meteor
showers are caused when the Earth passes
through an area of space with an enhanced
amount of small debris. Most meteor showers
are associated with cometary debris left behind
in the comet’s orbit. Meteors in a meteor
shower appear to radiate from the same area of
the sky since the material causing the shower is
located in a relatively small region of space. The
constellation encompassing the area of the sky
from which the meteors appear to radiate provides
the name of the meteor shower (e.g., the
Perseids appear to radiate from the constellation
Perseus, the Leonids come from the constellation
Leo, etc.). Debris large enough to survive
passage through the atmosphere and land on the
surface as a meteorite is very rare during meteor
showers.
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