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aberration of starlight  The angular shift in the apparent direction of a star caused by the orbital motion of the Earth.
aerosol  Liquid droplets and solids suspended in the atmosphere of a planet or satellite.
aesthenosphere  A layer of plastic, deformable rock located in the upper mantle of a planet directly below the lithosphere.
aurora australis  Light emitted by atoms and ions in the upper atmosphere near the south magnetic pole. The emission occurs when atoms and ions are struck by energetic particles from the Sun.
aurora borealis  Light emitted by atoms and ions in the upper atmosphere near the north magnetic pole. The emission occurs when atoms and ions are struck by energetic particles from the Sun.
basalt  An igneous rock often produced in volcanic eruptions.
bow shock  The region where the solar wind is slowed as it impinges on the Earth’s magnetosphere.
core  The innermost region of the interior of the Earth or another planet.
Coriolis effect  The acceleration that a body experiences when it moves across the surface of a rotating body. The acceleration results in a westward deflection of projectiles and currents of air or water when they move toward the Earth’s equator and an eastward deflection when they move away from the equator.
crust  The outermost layer of the interior of a planet or satellite.
differentiation  The gravitational separation of the interior of a planet into layers according to density. When differentiation occurs inside a molten body, the heavier materials sink to the center and the light materials rise to the surface.
dynamo  A process in which electric currents within a rotating, convective body produce a magnetic field.
exosphere  The outer part of the thermosphere. Atoms and ions can escape from the exosphere directly into space.
greenhouse effect  The blocking of infrared radiation by a planet’s atmospheric gases. Because its atmosphere blocks the outward passage of infrared radiation emitted by the ground and lower atmosphere, the planet can not cool itself effectively and becomes hotter than it would be without an atmosphere.
igneous rock  A rock formed by solidification of molten material.
ionosphere  The lower part of the thermosphere of a planet in which many atoms have been ionized by ultraviolet solar photons.
lava  Molten rock at the surface of a planet or satellite.
lithosphere  The rigid outer layer of a planet or satellite, composed of the crust and upper mantle.
magma  Molten rock within a planet or satellite.
magnetopause  The outer boundary of the magnetosphere of a planet.
magnetosphere  The outermost part of the atmosphere of a planet, within which a very thin plasma is dominated by the planet’s magnetic field.
magnetotail  The part of the magnetosphere of a planet stretched behind the planet by the force of the solar wind.
mantle  The part of a planet lying between its crust and its core.
mesopause  The upper boundary of the mesosphere layer of the atmosphere of a planet.
mesosphere  The layer of a planet’s atmosphere above the stratosphere. The mesosphere is heated by absorbing solar radiation.
metamorphic rock  A rock that has been altered by heat andpressure.
mineral  A solid chemical compound.
oblateness  A departure from spherical shape of a body in which the body’s polar diameter is smaller than its equatorial diameter.
outgassing  The release of gas from the interior of a planet or satellite.
plate  A section of the Earth’s lithosphere pushed about by convective currents within the mantle.
plate tectonics  The hypothesis that the features of the Earth’s crust, such as mountains and trenches, are caused by the slow movement of crustal plates.
primeval atmosphere  The original atmosphere of a planet.
seafloor spreading  The splitting of the oceanic crust where magma forces the existing crust apart, creating new ocean floor.
secondary atmosphere  The atmosphere that forms after a planet has lost any original atmosphere it had.
sedimentary rock  A rock formed by the accumulation of small mineral grains carried by wind, water, or ice to the spot where they were deposited.
seismic wave  Waves that travel through the interior of a planet or satellite and are produced by earthquakes or their equivalent.
seismometers  Sensitive devices used to measure the strengths and arrival times of seismic waves.
silicate  A mineral whose crystalline structure is dominated by silicon and oxygen atoms.
solar wind  The hot plasma that flows outward from the Sun.
stratosphere  The region of the atmosphere of a planet immediately above the troposphere.
subduction  The process through which lithospheric plates of a planet or satellite are forced downward into the mantle.
thermosphere  The layer of the atmosphere of a planet lying above the mesosphere. The lower thermosphere is the ionosphere. The upper thermosphere is the exosphere.
transform fault  The boundary between two of the Earth’s crustal plates that are sliding past each other.
tropopause  The upper boundary of the troposphere of the atmosphere of a planet.
troposphere  The lowest layer of the atmosphere of a planet, within which convection produces weather.
Van Allen belts  Two doughnut-shaped regions in the Earth’s magnetosphere within which many energetic ions and electrons are trapped.
zone of convergence  According to plate tectonics, a plate boundary at which the crustal plates of a planet are moving toward one another. Crust is destroyed in zones of convergence.
zone of divergence  According to plate tectonics, a plate boundary at which the crustal plates of a planet are moving away from one another. Crust is created in zones of divergence.







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