—Celestial latitude, measured in degrees north or south of the ecliptic.


—The process by which a planetary body becomes heterogeneously mixed, as heavier material sinks to its core and lighter materials float toward its surface.



—The plane of Earth's orbit around the Sun.


—Relating to geological processes of internal origin such as mantle convection, volcanism, or plate tectonics.


—The bottom of the exosphere, the level at which a large fraction of atmospheric atoms and molecules can leave the atmosphere without colliding with another atom or molecule (those with sufficient speed will escape from the atmosphere).


—Relating to geological processes of external origin such as impacts or fluvial erosion.


Galilean satellites

—The four brightest moons of Jupiter—Io, Europa, Ganymede, and Callisto—first observed by the astronomer Galileo.


Hubble Space Telescope, HST

—A 2.4-meter-aperture, low Earth-orbiting optical/ultraviolet telescope developed by NASA and the European Space Agency.


Interstellar medium, ISM

—The gas and dust particles found between stars.


Kuiper Belt

—A region of space containing icy planetesimals distributed in a roughly circular disk in the outer regions of our solar system, 50 to 100 AU from the Sun. Pluto is believed to circumscribe the innermost region of the Kuiper Belt.

Kuiper Belt object, KBO

—A general name for the bodies found in the Kuiper Belt, a region in the outer solar system.


Major summer

—Triton's seasonal cycle is extremely complex due to the peculiarities of its orbital geometry. Its seasons are modulated by the period of its motion around Neptune (14 days), the precession of its orbit (688 years), and Neptune's rotation around the Sun (165 years). The net result is that Triton experiences a series of major and minor seasons as the amplitude of its seasonal cycle is driven by these various modulations.

Material unit

—A generic term used in place of rock unit, sediment, formation, and so on when the exact geological nature of a three-dimensional body of material is unknown.

Mutual events

—A series of occultations and transits in which a number of celestial bodies form apparent alignments along an observer's line of sight.



—The obscuration of one celestial body by another of greater apparent diameter, as occurs, for instance, in the passage of an asteroid or comet in front of a star.

Oort Cloud

—A spherical distribution of comets having semimajor axes between 1,000 and 50,000 AU, typically with low orbital eccentricity.

Orbital resonance

—A phenomenon that occurs when the mutual gravitational interaction between two planetary

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