Glossary

A

Adiabatic

—Describing a physical process occurring without the loss or gain of heat.

Adiabatic lapse rate

—The rate at which the temperature of a parcel of gas changes with height as it moves vertically in a planetary atmosphere, in the absence of heating.

Albedo

—The fraction of incident light reflected by a body.

Aphelion

—The point at which a body is farthest from the Sun in its orbit.

Astronomical unit, AU

—The mean distance from the Sun to Earth.

Atomic mass unit, amu

—A unit commonly used to measure molecular masses, equivalent to the mass of one proton.

C

Centaur

—A planetary body found in orbit around the Sun between the orbits of Neptune and Saturn, e.g., Chiron. These bodies typically display a variety of characteristics, ranging from asteroidal in appearance to cometlike. The nature of the Centaurs' orbits indicates relatively recent arrival at their present positions, leading scientists to theorize that Centaurs are migrated Kuiper Belt objects.

CHNOPS

—Carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur: the key elements of living organisms.

Coma

—The spherical envelope of gas and dust surrounding the nucleus of an active comet, created when the ambient heat causes the vaporization of comet material.

Comet

—A volatile-rich body that develops a transient atmosphere, or coma, as it approaches the Sun. Most observed comets have highly elliptical orbits, sometimes approaching parabolic.

C-type asteroids

—The largest class of asteroids, grouped according to their spectral characteristics. In general they have low albedos and are believed to have significant mineralogical similarities to the carbonaceous chondrite meteorites.



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--> Glossary A Adiabatic —Describing a physical process occurring without the loss or gain of heat. Adiabatic lapse rate —The rate at which the temperature of a parcel of gas changes with height as it moves vertically in a planetary atmosphere, in the absence of heating. Albedo —The fraction of incident light reflected by a body. Aphelion —The point at which a body is farthest from the Sun in its orbit. Astronomical unit, AU —The mean distance from the Sun to Earth. Atomic mass unit, amu —A unit commonly used to measure molecular masses, equivalent to the mass of one proton. C Centaur —A planetary body found in orbit around the Sun between the orbits of Neptune and Saturn, e.g., Chiron. These bodies typically display a variety of characteristics, ranging from asteroidal in appearance to cometlike. The nature of the Centaurs' orbits indicates relatively recent arrival at their present positions, leading scientists to theorize that Centaurs are migrated Kuiper Belt objects. CHNOPS —Carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur: the key elements of living organisms. Coma —The spherical envelope of gas and dust surrounding the nucleus of an active comet, created when the ambient heat causes the vaporization of comet material. Comet —A volatile-rich body that develops a transient atmosphere, or coma, as it approaches the Sun. Most observed comets have highly elliptical orbits, sometimes approaching parabolic. C-type asteroids —The largest class of asteroids, grouped according to their spectral characteristics. In general they have low albedos and are believed to have significant mineralogical similarities to the carbonaceous chondrite meteorites.

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--> D Declination —Celestial latitude, measured in degrees north or south of the ecliptic. Differentiation —The process by which a planetary body becomes heterogeneously mixed, as heavier material sinks to its core and lighter materials float toward its surface. E Ecliptic —The plane of Earth's orbit around the Sun. Endogenic —Relating to geological processes of internal origin such as mantle convection, volcanism, or plate tectonics. Exobase —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). Exogenic —Relating to geological processes of external origin such as impacts or fluvial erosion. G Galilean satellites —The four brightest moons of Jupiter—Io, Europa, Ganymede, and Callisto—first observed by the astronomer Galileo. H Hubble Space Telescope, HST —A 2.4-meter-aperture, low Earth-orbiting optical/ultraviolet telescope developed by NASA and the European Space Agency. I Interstellar medium, ISM —The gas and dust particles found between stars. K 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. M 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. O Occultation —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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--> bodies causes their orbital periods to have a ratio expressible in small, whole numbers. For example, for every two times that Neptune orbits the Sun, Pluto revolves three times. P Perihelion —The point at which a body's orbital motion takes it closest to the Sun. Phase space —A multidimensional plot showing the positions and velocities of particles in a dynamical system. The trajectory followed by any particular particle represents the evolution of that particle's dynamics as a function of time. Planetesimals —The planetary bodies that formed the building blocks of all the solar system's planets and satellites. R Radiogenic heating —Heating of a celestial body due to the decay of radioactive isotopes. Retrograde motion —The rotational or orbital motion of a planetary body opposite to the dominant direction of the orbiting and rotating of the Sun and planets. In our solar system, retrograde motion is clockwise as viewed from the north pole of the ecliptic. Triton's orbit around Neptune and Venus's rotation are both examples of retrograde motion. S Solar nebula —The cloud of gas and dust from which our Sun, planets, and other bodies in our solar system formed. Spectrum —The characteristic emission or absorption of certain electromagnetic frequencies by elements and compounds. Stratosphere —The region above the troposphere where the atmosphere becomes stably stratified as a result of solar heating. T Thermosphere —The uppermost region of an atmosphere, where the temperature increases with height as a result of strong heating from above and where molecular diffusion of heat plays a major role in vertical heat transport. Tholin —the reddish tarlike organic residue created in simulations of the action of ultraviolet radiation on gases typically found in planetary environments. Tidal heating —The internal heating of a planetary body due to friction caused by the differential gravitational effect of an external body on the mass in question. Transit —The apparent passage of one body across the disk of a larger companion. Tropopause —The top of the troposphere and the base of the stratosphere. Troposphere —The lowermost portion of a planetary atmosphere, in which temperature decreases with height and thermal convection takes place. T Tauri star —A type of irregular variable star whose spectrum shows broad and very intense emission lines. They are believed to be very young stars that have not yet reached the main sequence. V Voyager —A pair of deep-space missions launched by NASA to the outer solar system in 1977. Combined, Voyagers 1 and 2 have conducted close-up observations of Jupiter (1979, 1979), Saturn (1980, 1981), Uranus (1986), and Neptune (1989).