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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.

degree
Measure of angle. A right angle equals 90 degrees.

density
measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3.

differentiation
The separation of planetary material according to density.

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

disk
The visible surface of the sun or any other heavenly body projected against the sky.

Doppler Effect
The apparent change in wavelength of sound or light caused by the motion of the source, observer or both.

Doppler shift
"Doppler shift" refers to the stretching of waves (known as "redshifting") from a source moving away from the observer, and the compression of waves (called "blueshifting") from a source approaching the observer. Redshifted waves have a longer wavelength (or lower frequency) than they otherwise would, and blueshifted waves have a shorter wavelength (or higher frequency). This works with radio waves, light, and all other forms of electromagnetic radiation. It's also noticeable in sound waves-it causes a siren coming toward a listener to have a higher pitch than a siren moving away. Just as police equipped with radar guns use Doppler shift to determine the speed of a car, scientists can use the phenomenon to determine the speed at which a spacecraft or celestial object is moving toward or away from Earth.

dorsum
A ridge.

DSN
Acronym for the Deep Space Network, a global network of communications systems that allows mission controllers to keep in constant communication with spacecraft throughout the solar system.