Glossary

Ablation:
The loss of surface material by fusion and vaporization as a meteoroid travels through the atmosphere.

Achondrite:
A stony meteorite that contains no chondrules.

Amino acids:
Organic molecules that are the building blocks of proteins. Amino acids, which are essential to life, have been found in some meteorites.

Asteroids:
Small, irregular-shaped rocky or metallic bodies of varying sizes that orbit around the Sun, usually between Mars and Jupiter. Most meteorites come from asteroids.

Asteroid belt:
The region of the solar system located between the planets Mars and Jupiter where most asteroids orbit.

Ataxite:
An iron meteorite that contains between 13% and 25% nickel. These meteorites, almost exclusively composed of taenite, show no Widmanstätten pattern when etched with acid.

Atmosphere:
The envelope of gases that surround some celestial bodies, such as the Earth. The atmosphere is held in place by the gravitational field of the celestial body.

Basalt:
A volcanic rock formed by the crystallization of lava.

Breccia:
An assemblage of rock fragments cemented together in a fine matrix.

Calcium-aluminum inclusions (CAIs):
Irregular-shaped white inclusions, made of calcium and aluminum and found in carbonaceous chondrites. These inclusions are thought to be composed of the first fragments of rock that condensed in the nascent solar system.

Carbonaceous chondrite:
A meteorite rich in carbon. These meteorites are the most primitive. Some might have originated from comets.

Chondrite:
A stony meteorite made essentially of chondrules. Most meteorites are chondrites.

Chondrule:
A small sphere composed mainly of pyroxene and/or olivine that is found in chondrites.

Comet:
A body of rock and ice traveling through the solar system. Comets are said to look like dirty snowballs. When a comet nears the Sun, some of its material vaporizes and forms a long dust tail. Some meteorites may come from the nuclei of comets.

Core:
The innermost part of a planet.

Cosmic rays:
Very energetic particles emitted mainly by the Sun and other bodies in the galaxy. These particles modify the isotopic composition of the bodies that they strike, such as meteoroids. This enables scientists to evaluate the time elapsed between the moment a meteoroid is creation and when it enters the Earth's atmosphere.

Crust:
The outer rocky layer of a planet.

Density:
The amount of matter in a given volume. Meteorites have a greater density than terrestrial rocks.

Differentiation:
The separation of an initially homogeneous material into several distinct layers. In asteroids, metals are found at the centre while silicates, being lighter, float at the surface.

Fall:
Describes both the fact of a meteorite's fall and, by extension, a meteorite that was seen falling.

Find:
Meteorite found on the ground without its fall being observed.

Fireball:
A luminous meteor brighter than any planet or star usually accompanied by a sonic boom. A fireball generally ends with a meteorite landing on the ground.

Fragmentation:
The breaking up of a meteoroid into fragments due to stresses associated with the meteoroid's passage through the atmosphere.

Fusion crust:
The fine vitrified covering on the surface of meteorites, usually black in colour. A fusion crust is created in the last moments of a meteorite's passage through the atmosphere.

Hexahedrite:
An iron meteorite with a low nickel content (4.5% to 6.5%). The main alloy in this type of meteorite is kamacite.

Impact crater:
A circular depression created when a meteorite hits a planet's surface. There are two main types of craters: simple bowl-shaped craters and complex craters with raised centres.

Impactite:
Terrestrial rock that is transformed (vitrified) by the heat released from the impact of a large meteorite on Earth.

Iron meteorite:
A meteorite composed almost exclusively of iron and nickel.

Isotopes:
Atoms of a single chemical element that have the same number of protons but different numbers of neutrons. In pairs of isotopes, one isotope (the daughter isotope) is produced by the slow decay of the other isotope (the parent isotope). Because the rate of decay is constant, the proportion of the parent isotope compared to that of the daughter isotope can be used as a clock to date various events.

Kamacite:
A nickel-iron alloy that contains less than 7.5% nickel and that is a component of iron meteorites.

Kirkwood Gaps:
Unstable regions in the asteroid belt. The orbits of asteroids that venture into these regions are shifted by the giant planet Jupiter.

Main mass:
Refers to the only fragment of a meteorite or the largest fragment from a multiple fall.

Mantle:
Middle layer of a planet, located between the crust and the core.

Mass extinction:
The dying out of a high percentage of species, which frees ecological spots for the surviving species.

Mesosiderite:
A stony-iron meteorite made up of a fine mixture of nickel-iron and silicates. The origin of these meteorites is still debated in the scientific community.

Metamorphism:
The passage of a rock from a liquid to a solid state due to an increase in temperature and/or pressure. The term “crystallization” is a synonym.

Meteor:
The luminous phenomenon created when a solid object from space enters the Earth's atmosphere.

Meteorite:
The unvaporized part of a meteoroid that passed through the atmosphere and reached the Earth's surface.

Meteorite hunter:
A collector who travels the globe searching for meteorites. Meteorite hunters particularly prize sites such as deserts that have few terrestrial rocks, the areas around craters, and the places where fireballs have been sighted.

Meteoroid:
A solid object less than one kilometre across, in orbit around the Sun. Some meteoroids eventually strike the Earth.

Metric ton:
Unit of weight equal to 2 204.6 pounds or 1 000 kilograms. Not to be confused with the British ton or the North American ton .

Micrometeorite:
An extraterrestrial particle so small that it passes through the Earth's atmosphere and reaches the ground without being burned up.

Near-Earth asteroid (NEA):
An asteroid found in the inner solar system. NEAs are classified into three families. The Aten and Apollo asteroids cross the Earth's orbit. The Amor approach it with orbits exterior to Earth's but interior to Mars.

Neumann lines:
A network of fine lines visible in hexahedrites after they are lightly etched in acid.

Octahedrite:
An iron meteorite containing between 7% and 13% nickel. Kamacite and taenite, which are nickel-iron alloys, are both present in octahedrites.

Olivine:
A natural silicate containing iron and magnesium, olive-green in colour.

Orbit:
The trajectory, influenced by gravity, of a celestial or artificial body around another body.

Pallasite:
A stony-iron meteorite formed of a nickel-iron matrix, and containing numerous rounded olivine crystals. These meteorites come from the core-mantle boundary of a differentiated asteroid.

Parent body:
The celestial object from which a meteorite comes. Most of the time, this is an asteroid. Less frequently, the Moon or Mars is the parent body.

Planet:
A non-luminous celestial body that revolves around a star. Planets are spherical and have a diameter greater than 1000 kilometres.

Pyroxene:
A group of silicated minerals containing iron, calcium, sodium, aluminum, and manganese.

Regmaglypts:
Depressions that resemble thumbprints on the surface of iron meteorites. Regmaglypts are produced during the passage through the Earth's atmosphere.

Silicates:
Any of a large group of minerals that contain silicon and that are widely found in terrestrial rocks.

Shock wave:
A sudden, extreme increase in pressure that travels through the surrounding medium.

SNC:
An acronym used to refer to meteorites from Mars. The three letters are the initials of the first Mars meteorites ever identified: Shergotty (India), Nakhla (Egypt) and Chassigny (France).

Solar nebula:
Disk of gas and dust that surrounded the newly formed Sun and within which the planets developed.

Sonic boom:
A loud sound like an explosion that occurs when an object “breaks through” the sound barrier in the atmosphere.

Stony meteorite:
A meteorite composed mainly of silicated minerals.

Stony-iron meteorite:
A meteorite composed of nickel-iron and silicate minerals in approximately equal proportions.

Strewn field:
The distribution on the ground of fragments from a meteorite shower.

Supernova:
The final phase in the evolution of a star many times more massive than our Sun. New heavy elements are created during the explosion of the outer layers of the star and blasted into interstellar space.

Taenite:
A nickel-iron alloy containing more than 20% nickel and found in iron meteorites.

Tektite:
A natural glass formed at very high temperature and pressure during a meteorite impact.

Thin section:
A polished section of meteorite, thin enough (0.03 millimetre) to allow light to pass through it. Thin sections are observed through a microscope to determine the mineral content of meteorites.

Widmanstätten pattern:
Criss-cross pattern of taenite and kamacite bands inside an octahedrite. The pattern is revealed when the octahedrite is etched in acid.