ablation
Loss of material from the surface of a meteoroid being
heated by friction as it passes through an atmosphere.
accretion
Process of particles sticking together to form larger
bodies; for example, solar nebular dust accreted to form chondrules, and
planetesimals accreted to form planets.
achondrite
A stony meteorite representing differentiated planetary
material. Because differentiation is an igneous process, these are igneous rocks
or breccias of igneous rocks.
asteroid
A fragment of a planetesimal (or possibly an aggregate of
fragments). Most asteroids orbit the sun between Mars and Jupiter, although many
also have orbits which cross that of the Earth and are thus called near-Earth
asteroids (NEA's).
breccia
A rock consisting of many angular fragments, some of which
may have different compositions.
carbonaceous chondrite
A type of primitive chondrite with evidence
of nebular processes.
chondrite
A meteorite containing chondrules and other components
produced in the solar nebula.
comet
A rocky and icy body in orbit around the sun. These objects
contain volatile material (such as hydrogen) which meteorites lack.
crater
A bowl-like depression. On planetary surfaces, both volcanic
craters and impact craters may exist, but because each type of crater has its
own characteristics, scientists can determine whether a crater was produced by
an impact or by volcanism.
crystalline
Indicates a rock is composed of mineral crystals rather
than glass. In general, when igneous melts cool very fast they form glass (like
obsidian), but when they cool slower, mineral crystals have an opportunity to
grow.
differentiated
When a (partially) molten body has been divided into
two or more fractions of dissimilar compositions. In the case of the Earth,
iron-nickel metal was differentiated from silicate material to form the planet's
core.
ejecta
Material thrown out of a crater during an impact event.
enstatite
A type of primitive chondrite that chondrite is dominated
by the silicate mineral enstatite.
fall
A meteorite that was observed to fall and recovered. Because
this type of meteorite is usually collected soon after falling, weathering and
other terrestrial processes do not have an opportunity to degrade the sample.
find
A meteorite that was not seen to fall, but found at some later
date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago.
fireball
A very large meteor.
igneous rock
A rock that was once molten.
impact
A collision between two planetary bodies. In the case when
one is much smaller than the other (like a meteoroid colliding with the Earth),
a crater may be produced on the larger body.
impact melt spherule
Spherules of shock-melted rock ejected from an
impact crater. Most of these objects cool rapidly in the Earth's atmosphere and
solidify to a glassy state. However, some may remain molten until they splash
onto the ground or into water where they are quenched to form solidified
particles of glass.
lava
Molten rock that is erupted onto the surface of a planet and
is hot enough to flow.
magma
Very hot, fluid rock. Magma is used to describe molten rock
both below and on top of the surface of a planet and thus is a more general term
than lava. Magma may contain solid mineral crystals which are suspended in the
melt.
mesosiderite
A type of stony-iron meteorite with silicate material
and iron-nickel metal. These meteorites have been brecciated and metamorphosed,
whereas pallasites, another type of stony-iron meteorite, are igneous rocks.
metamorphic rock
A rock that has been heated and compressed so that
it recrystallizes, but does not melt.
meteor
A bright light produced when a meteoroid is frictionally
ablated as it passes through an atmosphere. A very large and bright meteor is
usually called a fireball.
meteorite
A fragment of one planetary body that lands on another
planetary body. In the case of Earth, a meteorite must survive ablation in the
atmosphere.
meteoroid
A small (<1 km) natural object whose orbit around the
sun may cause it to collide with other planetary bodies. The term meteoroid is
usually used to describe the rocky core of a meteor, while the term meteorite is
used to describe the object once it is on the ground.
micro-meteorite
A tiny particle from one planetary body which lands
on another planetary body. Micrometeorites typically range in size from a
millionth to a thousandth of a meter on Earth. Some micrometeoroids are melted
or burned up completely in the Earth's atmosphere. On planetary bodies with much
thinner atmospheres, such as the Moon, many more micrometeoroids survive.
nodule
A semi-spherical fragment of rock embedded in a matrix with
a different composition.
ordinary chondrite
The most common type of meteorite to fall on
Earth. Some are primitive specimens containing evidence of nebular processes,
while others have been metamorphosed on a planetary body.
pallasite
A stony-iron meteorite in which nodules of olivine (a
silicate mineral) are surrounded by a network of iron-nickel metal.
planetesimal
A rocky and/or icy body, a few to several tens of
kilometers in size, that was produced in the solar nebula.
shock
Unusually high pressures produced briefly by an impact. These
pressures may be sufficiently high to shatter, melt, and vaporize rocky
material.
silicate
Rocky material consisting, in part, of the elements
silicon and oxygen. Most rocks on the surface of the Earth (such as sandstones,
granites, rhyolites, and basalts) are silicate materials. (In contrast, metal
that is separated from silicates during planetary differentiation lacks oxygen
and contains very little, if any, silicon.)
solar nebula
The disk of gas and dust that surrounded the sun when
it was forming.
strewn field
A large area over which impact melt spherules or
tektites are found.
tektite
Spherule of molten rock ejected from an impact crater and
then cooled rapidly to produce glass.