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In the case of a comet, the tail is a long link of gas and dust that flow away from the nucleus and coma of the comet.

Diagram of a comet - nucleus, coma and tail.

Deformation forces acting on a planet's crust.


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 dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk. 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.


An extensive land mass.


The surface features of an area of land.

terrestrial planets

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.


Tile; terrain formed of polygonal pattern


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


small domical mountain or hill.

tidal heating

Frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites.


The general configuration of a land surface.


A shape that resembles a doughnut or the inner-tube of a tire.

trailing hemisphere

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


An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points: 588 Achilles, 624 Hektor, and 911 Agamemnon. Saturn's satellites Helene, Calypso and Telesto are also sometimes called Trojans.


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

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Last Updated: 25 Oct 2013