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FOSS Planetary Science
Glossary (10/5/04)
Absolute age: The exact age of an object (such as a rock or artifact), found
by techniques, such as radiocarbon dating.
Analysis: An investigation of the parts of a whole and their relationships to
each other and the whole.
Angle: The space between two lines or planes that intersect or join.
Anorthosite: A purple-gray igneous rock composed mostly of the mineral
feldspar.
Arc: A portion of a circle or something curved.
Asteroid: A medium-sized rocky object orbiting the Sun smaller than a
planet and larger than a meteoroid.
Asthenosphere: The hot top of Earth’s mantle, just under the crust.
Evidence suggests that the Moon also has an asthenosphere.
Astrometry: A careful, precise measurement of the positions and motions
of objects in the sky.
Astronaut: A person trained to travel and perform tasks in space.
Astronomy: The scientific study of the universe, especially of the motion,
position, sizes, composition, and behavior of objects in the sky.
Asymmetrical: Not the same on each side.
Atmosphere: The mass of air surrounding Earth or other planets.
Barycenter: The center of mass of a system of masses, such as the
barycenter of the Earth-Moon system; sometimes called the center of mass.
Basalt: A dark, fine-grained volcanic rock composed mostly of iron and
magnesium.
Breccia: A coarse-grained rock composed of angular fragments of
preexisting rock.
Caldera: A large, roughly circular depression usually caused by volcanic
collapse or explosion.
Calibrated: Checked against or adjusted to a known standard.
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Circumference: The distance around something, such as a circle or a
crater.
Comet: A small, icy body that orbits the Sun in a long, loopy orbit, marked
by a tail of gas and dust when it nears the Sun.
Complex: Complicated; made of various interconnected parts.
Crater: A circular depression created by a volcanic eruption; a bowl-shaped
depression in a surface caused by a meteorite impact; a large hole in the
ground caused by an explosion.
Crust: The thin outermost layer of Earth, another planet, or satellite.
Debris: Fragments of rock broken by a powerful natural force, such as a
volcanic explosion; fragments of something that has been destroyed or
broken down.
Diameter: The longest distance across something, especially something
circular, such as a crater.
Doppler effect: The apparent change in wavelength of sound or light
caused by the motion of the source, observer, or both.
Eject: To push or throw out with great force.
Ejecta: Material thrown out of an impact crater.
Equator: The imaginary circle around Earth or a celestial object that is
everywhere the same distance from the poles.
Eratosthenes: A Greek mathematician of the Third century B.C.E. who
used geometry to determine that Earth is round and calculated that its
circumference is 250,000 stadia (46,250 kilometers).
Erosion: The wearing away and transport of soil and rock by weathering,
mass wasting, and the action of water, wind, and ice.
Eruption: Ejection of material (including gases, ash, volcanic fragments,
and lava) through a central vent, fissure, or group of fissures onto the
surface because of volcanic activity, either as an explosion or quiet lava flow.
Escape velocity: The speed required for an object to escape the
gravitational pull of a planet or other body.
Evidence: Observations that give support for an idea or hypothesis.
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Flyby: Travel past an object in the sky without entering its atmosphere or
landing on its surface, but close enough to get scientific information; a
spacecraft that makes a flyby.
Fragment: A piece broken off of something else.
Galaxy: A very large cluster of stars (tens of millions to trillions of stars),
bound together by gravity.
Gas giant: Any of the four outermost planets in the Solar System. These
formed much later than Earth and have a gaseous composition.
Geologic time: The period of time from Earth’s formation to the present.
Gravity: The mutual force of attraction between all particles or bodies that
have mass; the attraction that Earth or other celestial bodies have on other
objects on or near their surfaces.
Gravity assist: Use of a planet’s gravity to help a spacecraft speed up, slow
down, or change direction as it nears the planet.
Highlands: The oldest exposed areas of the Moon’s surface, extensively
cratered, chemically different, and higher in elevation than the maria.
Horns: Two sharp points on the crescent Moon, formed by the terminator.
Impact: The action of one object hitting another; to strike something with
force.
Indirect: Happening as a delayed or unintended effect; not in a direct line,
course, or path.
International date line: The imaginary line halfway around Earth from the
prime meridian. By international agreement, the international date line is
where the new day begins.
Interpretation: An explanation of the meaning or significance of
something.
Iridium: An element found in Earth’s crust in small quantities. It is much
more common in certain meteorites.
Latitude: The distance measured on Earth’s surface north or south of the
equator.
Lava tube: A tunnel formed when the surface of a lava flow cools and
solidifies and the molten material drains away.
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Lithosphere: The solid portion of Earth.
Local noon: The time when the Sun is at the highest point in the sky at a
particular spot on Earth.
Longitude: The distance measured on Earth’s surface east or west of the
prime meridian.
Lunar regolith: Loose rock, mineral, and glass fragments produced by
impacts and covering the Moon’s surface.
Magnetosphere: The area around a planet most affected by its magnetic
field. Its boundaries are set by the solar wind.
Mare (plural: maria): Dark area on the Moon covered by basalt lava.
Mascon: An area with high gravity on the Moon.
Mass extinction: The dying-out of several plant and animal groups over a
brief period of geological time; for example, the dinosaur extinction.
Meteor: A piece of rock from space that hits Earth’s atmosphere; also called
a shooting star.
Meteorite: A piece of rock from space that reaches Earth’s surface.
Meteoroid: Mass of rock in space that becomes a meteor when it enters
Earth’s atmosphere and a meteorite when it reaches the surface.
Micrometeoroid: An extremely small dust particle found in space.
Milky Way: The spiral galaxy in which Earth and the Solar System are
found.
Molten: Melted; changed into liquid form by heat.
Moon: Earth’s natural satellite; a natural satellite that orbits a larger
celestial body.
Navigation: The science of plotting and following a course from one place
to another and determining the position of a moving ship, aircraft,
spacecraft, or other vehicle.
Orbit: The path that a celestial body, such as a planet, moon, or satellite
follows around a larger celestial body.
Organic: Related to or characteristic of living things.
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Organism: A living thing, such as a plant, animal, or bacterium.
Orientation: The positioning of something, or the position or direction in
which something lies.
Origin: A starting point or place.
Parallel: When lines, planes, or curved surfaces are always the same
distance apart and never meet.
Pattern: A regular or repeating form, order, or arrangement.
Phase: The changing appearance of a celestial body as different amounts of
its disk are seen to be illuminated by the Sun.
Planet: An astronomical body that orbits a star and does not shine with its
own light.
Planetary system: A star and the planets, moons, and other objects and
materials that orbit that star.
Plate tectonics: A theory supported by evidence that Earth’s crust and
upper mantle are composed of several thin, relatively rigid plates that move
relative to one another, causing earthquakes and volcanism.
Prime meridian: An imaginary line on Earth between the north and south
poles through the town of Greenwich, England.
Primordial: Existing at the beginning of time.
Projectile: An object that can be fired or launched; for example, a rocket.
Protractor: An instrument shaped like a semicircle and marked with the
degrees of a circle; used to measure or draw angles.
Random: Occurring without a specific pattern, plan, or connection.
Ray: A narrow beam of light from the Sun or an artificial light source; white
lines that extend in all directions from some craters.
Reconnaissance: A preliminary survey to gain information.
Remote: Situated far away; distant in time.
Resolution: The ability to distinguish separate objects; the number of
pixels per area (such as a square inch or centimeter) on a computergenerated display. The greater the resolution, the better the picture.
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Revolution: The path of a celestial object around another object.
Ridge: A long narrow hilltop or range of hills.
Rille: A structure on the Moon’s surface that looks like a canyon or
streambed.
Rotation: The turning of an object on its axis.
Satellite: An object put into orbit around Earth or other planet in order to
relay communication signals or transmit scientific data; a celestial body that
orbits a larger one.
Seismometer: An instrument used to measure vibrations caused by an
earthquake.
Shadow: A darkened shape or absence of light on a surface behind
somebody or something blocking the light.
Simple: Having few parts; not complex or complicated.
Solar nebula: The large cloud of gas and dust from which the Sun and
planets may have formed over 4.5 billion years ago.
Solar System: The Sun and all the planets, satellites, asteroids, meteors,
and comets that are subject to its gravitational pull.
Space probe: A satellite or other spacecraft that is designed to explore the
solar system and transmit data to Earth.
Sputnik: The world’s first artificial satellite, launched by the Soviet Union on
October 4, 1957.
Stadium (plural: stadia): An ancient measure, equal to 185 meters.
Star: A celestial body usually visible as a small point of light in the night
sky; a gaseous mass in space such as the Sun, ranging in size from that of a
planet to larger than Earth’s orbit, which makes energy by thermonuclear
reactions.
Sun: The star in the center of the Solar System around which Earth and
other planets revolve.
Terminator: The line separating the illuminated portion of the Moon from
the dark part.
Terraced: Having a series of nearly level, fairly narrow ledges.
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Terra (plural: terrae): A light-colored, rough area of the Moon.
Terrestrial: Relating to Earth, rather than any of the other planets.
Uniformly: Being the same as another or others; always the same in
quality, degree, character, or manner.
Volatile: Characterized by sudden change; changing into a vapor at a
relatively low temperature.
Volcanic: Relating to or coming from a volcano.
Volcano: An opening in Earth’s crust through which molten lava, ash, and
gases are ejected; a similar opening on the surface of another planet; a
mountain formed by the materials ejected from a volcano.
Volcanic ash: Fine-grained lava that erupts in a gas cloud from a volcano
before settling on the ground.
Volcanism: The processes involved in the formation of volcanoes, and in
the transfer of magma and volatile material from the Earth’s interior to its
surface.
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