Index: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z.


A sol (homogeneous suspension, more fluid than a gel) in which the dispersion medium is a gas (e.g. air) containing solid particles or liquid droplets.

Reflectivity of an object, expressed as the ratio or percentage of reflected to incident (incoming) light or electromagnetic radiation.

Used to refer to the side which is facing away from Jupiter.

A small rocky celestial body in orbit around the sun which shows no evidence of an atmosphere or other types of activity associated with comets.

asteroid belt
A region of the solar system between the orbits of Mars and Jupiter (2.2 - 3.3 A.U.) in which most of the asteroids are located.

agents of change
The constructive and destructive processes that reshape a planetary surface.


The variety and variability among living organisms and the ecological complexes in which they occur.

projectile; a meteor or meteorite.

bright terrain
A type of surface on Ganymede which is lighter than other regions on this moon which is often associated with grooves (see also dark terrain).


A large volcanic crater produced by the collapse of underground lava reservoirs.

Having to do with the science and art of constructing maps and charts.

The science and art of constructing maps and charts.

charge-coupled device (CCD)
An electronic device that consists of a regular array of light sensitive elements that emit electrons when exposed to light.

Chicxulub crater
A very large impact crater near the Yucatan Peninsula in Mexico. The effects of this particular impact may have been responsible for the extinction of the dinosaurs 65 million years ago.

Groups of atoms or molecules that are responsible for pigmentation (color).

A comet is a planetesimal in orbit around the sun. Comets are believed to be composed of dust and volatile ices. When close to the sun, comets become heated enough to produce a coma of gas and dust. As this gas and dust moves outward from the comet, it is "blown" away by the solar wind and forms the comet's tail.

A "packing" or reduction in the amount of data being transmitted, using a mathematical formula similar to averaging, in order to optimize the transmission of data from the spacecraft. Once received, these data are "unpacked" (decompressed) to reconstruct the full image. However, if noise (see Radiation) is present it may cause incorrect values for the picture elements it was averaged with, which also affects the resulting image.

The transfer of heat from a region of high temperature to a region of lower temperature by the displacement of the cooler molecules by the warmer molecules.

A typically bowl-shaped pit, depression, cavity or hole, generally of considerable size and with steep slopes, formed on a surface or in the ground by the explosive release of chemical or kinetic energy.

crater density
The number of craters on a surface per unit area.

crater size distribution
The relative numbers of craters of given sizes on a surface.

cross-cut (principle of cross-cutting relationships)
An interruption of a geologic feature by another, which can give an indication of the relative ages of these geologic features/events. (e.g. a fault cutting across an impact crater would be younger than the crater.)

The outermost layer of a planet, composed of relatively low-density materials.


dark terrain
A type of surface on Ganymede and Callisto which is darker than other regions on these moons (see also bright terrain). On Ganymede dark terrain is often associated with furrows.

digital signals
Data that is converted to numerical form so that it can be stored and used in a computer.


The material thrown from an impact crater during its formation.

A set of points, the sum of whose distance from two fixed points (the foci) is constant. An ellipse is essentially a circle that has been stretched out of shape. When describing ellipses, the eccentricity defines how "stretched out" it is.

The process of physically removing weathered materials; the wearing away of soil and rock by weathering, mass wasting, and the action of streams, glaciers, waves, wind and groundwater.

A change in the gene pool of a population over time.

The failure of a taxonomic group to produce direct descendants, causing its worldwide disappearance from the record at a given point in time.


A circular to subcircular region which is brighter than the surrounding area.

A break or crack.

The rubbing of one surface or thing against another.

In the case of Ganymede furrows refer to depressions which occur in the dark terrain.


Galilean satellites
- the four largest satellites of Jupiter, first seen by Galileo in 1610: Io, Europa, Ganymede, Callisto

gas giants
- the outer solar system planets: Jupiter, Saturn, Uranus, and Neptune, composed mostly of hydrogen, helium and methane, and have a density less than 2 grams per cubic centimeter.

geodetic control network
An ellipsoid or sphere-shaped "net" that is used as a type of base-map or control to correlate co-ordinates on a flat image with their actual location on the spherical planetary body in order to obtain accurate measurements from images.

geologic activity
The expression of the internal and external processes and events that affect a planetary body.

geologic time
The time extending from the end of the formative period of the Earth to the beginning of human history.

geologic unit
A body of rock (or ice, in Europa's case) that has a distinct origin and consists of dominant, unifying features that can be easily recognized and mapped.

The acceleration produced by the mutual attraction of two masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass.

gravity gradient
Refers to the difference in the acceleration of gravity from one side of an object to another, because gravity is a force that changes with distance between two points.

great circle
The line of intersection of the surface of a sphere and any plane which passes through the center of the sphere.

In the case of Ganymede grooves refer to depressions which occur in the bright terrain, often in parallel sets.

ground truth
The on-site gathering of data at a particular location, for the purpose of calibrating and interpreting remotely acquired data.


high sun
The condition of the sun being directly overhead (analogous to noon) which eliminates shadows on the surface. When planetary images are obtained in high sun conditions they are examined for albedo information rather than morphologic information (see Terminator). Areas imaged under both high and low sun conditions can be compared to link albedo and morphologic features.


ice as a rock
Water-ice (frozen H20) is an important geologic material on many of the outer planet satellites. Under various conditions ice can have viscosities and flow properties similar to different types of molten rock (basaltic - rhyolitic), and therefore may also result in the formation of similar geologic structures.

Ice that is at near freezing temperatures (273 K) and low pressure (<1 kbar) --Ice I-- has different physical properties than ice that is at lower temperatures and higher pressures. (e.g. Ice II is stable at 200 K and 6 kbar, while Ice VI is stable at 200 K and 15 kbar.) These phases of ice have different densities and flow properties that may be expressed through tectonic and volcanic processes on icy satellites. When these ices contain small amounts of "impurities" (e.g. ammonia) they are known as 'clathrates', and have a lower melting temperature than pure water-ice-- which may be a factor in volcanic processes on these satellites.

ice-volcanic melt
The semi-fluid or fluid material associated with ice volcanism, and like molten rock, can have a wide variety of viscosities and other flow properties.

ice volcanism
The eruption of molten ice or gas-driven solid fragments onto the surface of a planetary body.

impact crater
A crater formed on a surface by the collision of a projectile.

An object which strikes the surface of a celestial body

Electromagnetic radiation with wavelengths longer than those perceived by the eye (visible light) but shorter than radio waves; objects between room temperature and 700 degrees Celsius emit infrared radiation.


The Jet Propulsion Laboratory in Pasadena, California.


The fundamental unit of temperature. It is not calibrated in terms of the freezing and boiling points of water, but in terms of energy itself. The number 0 K is assigned to the lowest possible temperature, called absolute zero.

Kepler, Johannes (1571-1630)
German astronomer and mathematician. Considered a founder of modern astronomy, he formulated the famous three laws of planetary motion.

kinetic energy
The energy of motion; equivalent to one-half an objects mass multiplied by its velocity squared.


The angular distance north or south from the equator.

leading hemisphere
The hemisphere that faces forward, into the direction of motion of a satellite that keeps the same face toward the planet.

liquid crystal
A substance that behaves like both a liquid and a solid.

The edge of the apparent disk of a celestial body.

The angular distance east or west from the prime meridian.


The property of a material to attract iron, cobalt, or nickel.

The asymmetrically shaped volume around a magnetic body in which charged particles are subject to the bodies magnetic field rather than the magnetic field of another body such as the Sun or Jupiter.

mass wasting
The downslope movement of rock, regolith, and/or soil under the influence of gravity.

A unit of distance equivalent to one-millionth (10E-6) of a meter; a micrometer. (E. coli, a common bacteria, is 0.5 microns in length.

mid-ocean ridge
A continuous mountain range with a central valley, located on the ocean floor where two tectonic plates move away from each other allowing molten rock from the Earth's interior to move toward the surface.

The scientific study of form, and of the structures and development that influence form. In geology, the external structure, form, and arrangement of rocks in relation to the development of landforms.


The National Aeronautics and Space Administration.

An instrument that uses measurements of scatterred light from a laser beam to detect and study cloud particles.


A configuration in which a celestial body is in the opposite direction (180 degrees) from the sun as seen from another body located along the line between them. (e.g. when Mars, Earth and the Sun are located along a straight line, Mars is in opposition as seen from Earth.)

opposition surge
An increase in brightness as an observer approaches the line between the sun and a target. The strength of the surge is an indicator of small-scale surface texture.

The path followed by an object in space as it goes around another object.

orbital period
The time required for an object to make a complete revolution along its orbit.

orbital resonance
A relationship in which the orbital period of one body is related to that of another by a simple integer fraction.

organic compounds
Complex chemical compounds that contain carbon.


A roughly circular albedo spot on icy satellites that is presumed to mark the site of a crater and its rim deposit. Little, if any, of the topographic structure exists, but visual distinction from adjacent crust remains.

phase angle
The angle between the Sun, target, and the Galileo spacecraft.

Low-phase is a small angle between the Sun, target, and the spacecraft.
A low phase angle provides high sun illumination, similar to taking a picture from a high altitude at noon (with the sun directly overhead). Such illumination emphasizes the brightness contrasts of light and dark areas.

High-phase is a large angle between the Sun, target, and the spacecraft.
A high phase angle provides low sun illumination, similar to taking a picture from a high altitude at sunset or sunrise. Such illumination emphasizes the topography of the terrain.

The accurate quantitative measurement of the amount of light received from an object or area.

Contraction of "picture element"; the area on the ground represented by each digital number in a digitized image; an individual element in a detector.

Primordial bodies of intermediate size that accreted into planets or asteroids.

plate tectonics
A geological model in which the Earth's lithosphere (crust and uppermost mantle) is divided into a number of more-or-less rigid segments which move in relation to one another.

The measurement and study of the polarization of light reflected off of a surface.

A measure of the amount of work done per second, expressed in Watts.



Energy that is emitted in the form of electromagnetic waves. The electronics of Galileo's camera system are sensitive to radiation, and Jupiter's radiation can affect the images Galileo sends - similar to "snow" on a television with poor reception - which is referred to as "noise."

raw data
The original data we receive from the spacecraft before it has been processed or corrected.

The phenomenon of the trailing hemisphere of a planetary body being darker at shorter wavelengths ("redder") than the leading hemisphere. This effect may be due to magnetospheric bombardment acting preferentially on the trailing hemisphere and impact gardening on the leading hemisphere. Of the Galilean satellites, Europa displays this effect most prominently, and Ganymede to a lesser extent.

The bending of light due to a change in its velocity as it passes the boundary between two materials (e.g. a pencil inserted into a glass of water will look bent due to refraction.)

refractive index (index of refraction)
The ratio of the velocity of light in a vacuum to the velocity of light with in a material.

The layer of rocky or icy debris and dust made by meteoritic impact that forms the uppermost surface of planets, satellites and asteroids.

relative age
The age of one thing, such as a geologic unit, in comparison to another. Relative ages are usually determined by crosscutting relationships and the number of impact craters on planetary surfaces.

Ability to distinguish visual detail, usually expressed in terms of the size of the smallest features that can be distinguished.

Creation of a new surface on a planetary body by volcanic or tectonic processes.

The motion of one body around another (e.g. the motion of the planets in their orbit around the Sun).

rift valley
A valley that has developed along a tectonic rift.

The turning or spinning of a body about an axis running through it.


Any object, man-made or natural, that orbits another body.

Condition in which a surface becomes completely covered with craters, such that the addition of new craters does not increase the overall crater density.

A line of cliffs produced by faulting or erosion; a relatively straight, cliff-like face or slope of considerable linear extent, breaking the general continuity of the land by separating surfaces lying at different levels.

seafloor spreading
The movement of two oceanic plates away from each other, resulting in the construction of a mid-ocean ridge.

A group of minerals constituting about 95% of the Earth's crust, and containing silicon and oxygen combined with one or more other elements.

Shoemaker-Levy 9 (S-L9)
The comet that broke up and fell into Jupiter in June 1994.

An instrument that measures the wavelength of electromagnetic radiation.

The measure of how fast an object is moving; the rate of change of position per change in time.

Solid State Imaging. The SSI Team is the group of people that operate the camera on the Galileo spacecraft.

The layering of rock or ice strata, from which information on succession, age relations, and origin can be deduced.

The layer of the atmosphere above the troposphere and below the mesosphere where the temperature increases with height.

subduction zone
A place on the surface of the Earth where two plates move toward each other, and the oceanic plate plunges beneath the other tectonic plate.

When a substance changes directly from a solid state to a gaseous state without becoming liquid.

The point on a body that is directly beneath the Sun. The Sun's rays will hit this point at 90° to the surface.

A complex area of subparallel furrows and ridges.


The processes of faulting, folding or other deformation of the lithosphere of a planetary body, often the result of large-scale internal movements below the lithosphere.

The line of sunrise or sunset on a planet or its satellite. At dawn and dusk when the sun is lowest in the sky (low sun), topographic features cast their longest shadows. This reveals information about the size and shape of the objects casting the shadows. Therefore, features near the terminator are imaged in order to obtain morphologic information. (see High sun)

The surface features of an area of land.

The inner solar system planets: Mercury, Venus, Earth, Mars, composed of rocky materials and iron, with a density between 4.0 and 5.5 grams per cubic centimeter.

The general appearance or character of a rock. (e.g. the size, shape, and arrangement of its constituent elements)

The general configuration of a land surface.

A torus is a shape that resembles a doughnut or the inner-tube of a tire. In context with the Galileo mission, torus usually refers to the doughnut-shaped cloud of plasma surrounding Jupiter near Io's orbit, also known as the "Io plasma torus".

trailing hemisphere
The hemisphere that faces backwards, away from the direction of motion of a satellite that keeps the same face toward the planet.

Common name for a class of extinct ocean arthropods that lived during the Paleozoic Era, about 540 million to 245 million years ago.

The layer of the atmosphere from the surface to where the temperature stops decreasing with height.



The speed and direction of an object.

The measure or property of a material to be resistant to flow; the internal friction of a material.

Materials with a high viscosity are more resistant to flow, while those with a low viscosity are more fluid.

viscous relaxation
The process whereby topographic features become subdued over time due to the flow of the surrounding geologic material.

The eruption of molten material or gas-driven solid fragments onto the surface of a planetary body. On icy moons the phenomenon is sometimes called cryovolcanism to distinguish it from silicate or sulfur volcanism.

A mountain formed from the eruption of igneous matter through a source vent.


A unit of power, equivalent to one joule of energy per second.

The distance between successive crests or troughs in a wave.

The chemical and physical alteration of materials exposed to the environment on or near the surface of a planetary body.




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