albedo (al-BEE-doh) — Ratio of the intensity of light reflected from a surface relative to the intensity of light incident on the surface. Albedos are expressed as numbers between 0 and 1 where 0 indicates total absorption (a blackbody) and 1 represents total reflection (a perfect mirror).

angstrom (ANG-strom) — A unit of length equal to 10-10 m. Visible light has wavelengths between 4000 and 7000 angstroms.

Apollo (uh-POL-loh) — Name of the manned spacecraft missions to the Moon undertaken by the United States. Six Apollo missions landed on the Moon between 1969 and 1972.

asteroid (ASS-tuh-royd) — A small object that orbits the Sun rather than a planet. Ceres, the largest asteroid, is about 1020 kilometers (610 miles) in diameter, but most asteroids are much smaller than 100 kilometers (60 miles) in diameter. Asteroids are most abundant between the orbits of Mars and Jupiter, but some have orbits that cross the orbits of the inner planets. Asteroids are assumed to be rocky objects, the source of most meteorites, but recently it has been recognized that some asteroids may be the inert nuclei of comets that no longer develop a tail.

basalt (buh-SOLT) — A dark volcanic rock that results from the solidification of fluid lava associated with a volcanic eruption.

caldera (kawl-DAIR-uh) — An irregular volcanic depression caused by subsidence of the crust due to withdrawal of support at depth.

comet (KOM-it) — An object orbiting the Sun that loses volatiles such as ice by vaporization when it gets close to the Sun. The dust and gas that escape from the nucleus form the tail of the comet, attaining lengths up to tens of millions of kilometers. The nucleus of a comet becomes very dark as grains of rocky material accumulate on its surface as the ice is lost.

crater — Hole created by the explosive removal of material from a solid surface. Most craters visible on planetary surfaces are the result of the impact of an object from space at very high speed, but volcanic processes can also produce certain types of craters.

inner solar system — The planets Mercury, Venus, Earth, and Mars, which are rocky in composition, in contrast to the gaseous planets of the outer solar system.

mons (MONZ) — Term applied to large mountains (usually central volcanos) on planetary surfaces. Plural form is montes (mon-TEZ).

moon — A natural object in orbit around a planet. Moons vary greatly in size, with the largest moon of Jupiter (Ganymede) being larger than the planet Mercury. In this booklet references to Earth's Moon are capitalized.

occultation (uh-KUL-tay-shun) — Blockage of light by the intervention of another object. For example, a planet can occult (block) the light from a distant star. Occultations of light from stars of of radio signals from spacecraft are very useful for obtaining information about the size of the occulting object or the properties of its atmosphere, if one is present.

outer solar system — The planets Jupiter, Saturn, Uranus, Neptune, and Pluto. These planets are composed primarily of gases and their visible surfaces are clouds or hazes in the atmosphere. Pluto is probably an icy object but its placement as the ninth planet makes it part of the outer solar system.

palimpsest (PAL-im-sest) — A circular feature on the surface of dark icy moons such as Ganymede and Callisto lacking the relief associated with craters. Palimpsests are thought to be impact craters where the topographic relief of the crater has been eliminated by slow adjustment of the icy surface. The term is derived from the printing process where a faint remnant of earlier, partially erased writing is still preserved.

photosphere — The visible surface of the Sun. The photosphere is the upper surface of a convecting layer of gases in the outer portion of the Sun whose temperature causes it to radiate light at visible wavelenths.

planet — An object orbiting the Sun and visible by reflected sunlight. There is no official lower limit to the size of a planet, but the name has not been applied to small objects such as comets or asteroids. The name comes from the Greek word for "wanderer" and was applied to the visible planets that were observed to move relative to the fixed stars. In contrast to comets and asteroids, planets tend to be in quite regular orbits that are usually close to a single plane.

planitia (pla-NEE-she-ah) — Term applied to broad plains that occupy lowlands on planetary surfaces.

prominence (PROM-i-nence) — An eruption of hot gases about the photosphere of the Sun. Prominences are most easily visible close to the limb of the Sun, but some are also visible as bright streamers on the photosphere.

rupes (ROO-peez) — Term applied to scarps (locations of pronounced vertical offset) on planetary surfaces. Many scarps are thought to be the surface expression of faults within the crust of the planetary object.

Skylab — Name of the United States space station occupied in Earth orbit by astronauts during 1973 and 1974. Skylab consisted of a specially modified upper stage of the rocket used to send Apollo missions to the Moon. Skylab reentered Earth's atmosphere and was destroyed in 1977.

tidal forces — Gravitational pull on planetary objects from nearby planets and moons. When the tidal forces of a planet and several moons are focused on certain moons, particularly if the orbits of the various objects bring them into alignment on a repeated basis, the tidal forces can generate a tremendous amount of energy within the moon. The intense volcanic activity of Io is the result of the interaction of such tidal forces.

ultraviolet — Electromagnetic radiation at wavelengths shorter than the violet end of visible light (wavelengths less than 4000 angstroms). The atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.

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