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valles or vallis

Sinuous valley.


Widespread lowlands.


The speed and direction of an object.

Second planet from the sun. Named for the Roman goddess of love and beauty.
HREF="../planets/profile.cfm?Object=Venus">More on Venus >>

Full Face of Venus

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.


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.


Produced or discharged from a volcano.


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.)

Volcanic plume from Jupiter's moon Io

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

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Last Updated: 25 Oct 2013