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.
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