Chasma
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In planetary nomenclature, a chasma is a deep, elongated, steep-sided depression. As of 2020, the IAU has named 122 such features in the Solar System, on Venus (63), Mars (25), Saturn's satellites Mimas (6), Tethys (2), Dione (8) and Rhea (5), Uranus's satellites Ariel (7), Titania (2) and Oberon (1) and Pluto's satellite Charon (3). An example is Eos Chasma on Mars.
Melas Chasma, as seen by THEMIS. Click on image to see relationship of Melas to other features.
Cliff in Candor Chasma Plateau, as seen by THEMIS. Click on image to see relationship with other features in Coprates quadrangle.
Cliff in northern wall of Ganges Chasma, as seen by THEMIS. Click on image to see relationship with other features in the Coprates quadrangle.
Ophir Chasma Wall, as seen by HiRISE.
Planetary nomenclature
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Planetary nomenclature, like terrestrial nomenclature, is a system of uniquely identifying features on the surface of a planet or natural satellite so that the features can be easily located, described, and discussed. Since the invention of the telescope, astronomers have given names to the surface features they have discerned, especially on the Moon and Mars. To found an authority on planetary nomenclature, the International Astronomical Union (IAU) was organized in 1919 to designate and standardize names for features on Solar System bodies.
The heart-shaped Tombaugh Regio, on Pluto, is named after the planet's discoverer Clyde Tombaugh.
Examples of crater nomenclature on Phoebe