Glossary main content.



The spherical mass, largely made of metallic elements, at the center of Earth and the other terrestrial planets.


A depression on a planet, moon, or meteorite created by the impact of an external body or by volcanic activity.


The outermost solid layer of the Earth or of similar bodies.dataInformation, often in the form of measurements or observations, which can be analyzed.deformationWhen rock changes shape.


The average mass per unit volume of a substance.


The force of attraction acting between any two masses.

laser altimeter

An instrument that measures the height or elevation of the terrain from an aircraft or a satellite. It uses a technique similar to radar but with laser light instead of radio waves.


Magma when it erupts on a planet's surface.


The outer layer of solid rock that includes the crust and uppermost mantle. This layer, up to 100 km thick, forms Earth's tectonic plates.


Molten or partially molten underground rock.

magnetic field

The region of space influenced by a magnetized body. Magnetic fields, due to motion involving charged or magnetic material, are found within the Universe at every scale—from the atomic to the galactic.


An instrument used to measure a magnetic field.mantleThe layer of the Earth (or other rocky body) lying between the outer crust and the central core.


A class of elements that have characteristic physical properties. Metals are generally good conductors, and are reflective, malleable, and ductile.


A naturally-occurring, homogeneous inorganic element or compound having a definite chemical composition and orderly internal structure, crystal form, and characteristic chemical and physical properties.


An immense cloud-like mass of interstellar gas and dust.orbitThe path of one celestial body moving around another under the force of gravity.

protoplanetary disk

A disk of dust, gas, and perhaps developing planets orbiting a young star. A transitional stage between a solar nebula and a solar system.

solar nebula

The cloud of gas and dust that formed the young Sun and the surrounding planets.


A scientific instrument that identifies the chemical elements in a soil or rock sample by subjecting the sample to energy (infrared, xray, etc.) and then measuring the different energies, or wavelengths, of light given off by the minerals in response.tectonic plateOne of several large, interlocking, mobile pieces of Earth’s lithosphere.


Having to do with the Earth or other rocky/metallic planets. Thermal-relating to heat or heat transfer. Topography-the surface elevation of land and its variations.


The path of a body in motion through space.


The gaseous form of substances that are normally in the solid or liquid state, and that can be changed to these states either by increasing the pressure or decreasing the temperature.


Any of the processes by which magma and its associated gases rise to the crust from the interior of a terrestrial planet and are discharged on or above the surface.