altimeter An instrument for measuring altitude.
aurora A luminous phenomenon that consists of streamers or arches of light appearing in the upper atmosphere of a planet’s magnetic polar regions.
concretion A mass of mineral matter found generally in rock of a composition different from its own.
convulsion A violent disturbance.
density The mass of a substance by unit volume.
elliptical In the shape of an ellipsis or oval.
greenhouse effect Warming of the surface and lower atmosphere of a planet (as Earth or Venus) that is caused by conversion of solar radiation into heat which remains trapped in the planet’s atmosphere, warming its surface further.
inferior planet Either of the planets Mercury and Venus whose orbits lie within that of Earth.
magnetic field The portion of space near a magnetic body in which the magnetic forces can be detected.
mantle The part of the interior of a terrestrial planet that lies beneath the crust and above the central core.
phase A particular appearance or state in a regularly recurring cycle of changes.
plate tectonics A theory in geology that the lithosphere of Earth is divided into a small number of plates that float on and travel independently over the mantle.
retrograde motion Motion in a direction contrary to that of the general motion of similar bodies and especially east to west among the stars.
scarp Long, steep cliff.
seismic Of or relating to a vibration on a celestial body comparable to a seismic event on Earth, caused by an explosion or impact of a meteorite.
spectrometer An instrument used for measuring wavelengths of light.
terrestrial Belonging to the class of planets that are like Earth (as in density).
topography The configuration of a surface including its relief and the position of its natural features.
transit An eclipse that occurs when a planet moves directly between Earth and the Sun.
wavelength The distance in the line of advance of a wave from any one point to the next point of corresponding phase.