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valles or vallis
Sinuous valley.

vastitas
Widespread lowlands.

velocity
The speed and direction of an object.

Venus
Second planet from the sun. Named for the Roman goddess of love and beauty.
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viscosity
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.

volatile
As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes hydrogen, helium, water, ammonia, carbon dioxide and methane.

volcanic
Produced or discharged from a volcano.

volcanism
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. (Image: A volcano blasts material off the surface of Jupiter's moon Io.)

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