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.

IAU approval procedure When images are first obtained of the surface of a planet or satellite, a theme for naming features is chosen and a few important features are named, usually by members of the appropriate IAU task group (a commonly accepted planet-naming group). Later, as higher resolution images and maps become available, additional features are named at the request of investigators mapping or describing specific surfaces, features, or geologic formations. Anyone may suggest that a specific name be considered by a task group. If the members of the task group agree that the name is appropriate, it can be retained for use when there is a request from a member of the scientific community for a name of a specific feature. Names that pass review by a task group are submitted to the IAU Working Group for Planetary System Nomenclature (WGPSN). Once approved by the WGPSN, names are considered official and can be used on maps and in publications. They are also listed in the Gazetteer of Planetary Nomenclature.

IAU rules and conventions Names adopted by the IAU must follow various rules and conventions established and amended through the years by the Union. These include:

Nomenclature is a tool and the first consideration should be to make it simple, clear, and unambiguous. In general, official names will not be given to features whose longest dimensions are less than 100 meters, although exceptions may be made for smaller features having exceptional scientific interest. The number of names chosen for each body should be kept to a minimum. Features should be named only when they have special scientific interest, and when the naming of such features is useful to the scientific and cartographic communities at large. Duplication of the same surface feature name on two or more bodies, and of the same name for satellites and minor planets, is discouraged. Duplications may be allowed when names are especially appropriate and the chances for confusion are very small. Individual names chosen for each body should be expressed in the language of origin. Transliteration for various alphabets should be given, but there will be no translation from one language to another. Where possible, the themes established in early solar system nomenclature should be used and expanded on. Solar system nomenclature should be international in its choice of names. Recommendations submitted to the IAU national committees will be considered, but final selection of the names is the responsibility of the International Astronomical Union. Where appropriate, the WGPSN strongly supports an equitable selection of names from ethnic groups, countries, and gender on each map; however, a higher percentage of names from the country planning a landing is allowed on landing site maps. No names having political, military or (modern) religious significance may be used, except for names of political figures prior to the 19th century. Commemoration of persons on planetary bodies should not normally be a goal in itself, but may be employed in special circumstances and is reserved for persons of high and enduring international standing. Persons being so honored must have been deceased for at least three years. When more than one spelling of a name is extant, the spelling preferred by the person, or used in an authoritative reference, should be used. Diacritical marks are a necessary part of a name and will be used. Ring and ring-gap nomenclature and names for newly discovered satellites are developed in joint deliberation between WGPSN and IAU Commission 20. Names will not be assigned to satellites until their orbital elements are reasonably well known or definite features have been identified on them. Accessible and authoritative sources, including Internet sources, are required for adopted names. Wikipedia is not sufficient as a source, but may be useful for identifying appropriate sources.

In addition to these general rules, each task group develops additional conventions as it formulates an interesting and meaningful nomenclature for individual planetary bodies.

Naming conventions

Names for all planetary features include a descriptor term, with the exception of two feature types. For craters, the descriptor term is implicit. Some features named on Io and Triton do not carry a descriptor term because they are ephemeral.

In general, the naming convention for a feature type remains the same regardless of its size. Exceptions to this rule are valleys and craters on Mars and Venus; naming conventions for these features differ according to size.

One feature classification, regio, was originally used on early maps of the Moon and Mercury (drawn from telescopic observations) to describe vague albedo features. It is now used to delineate a broad geographic region.

Named features on bodies so small that coordinates have not yet been determined are identified on drawings of the body that are included in the IAU Transactions volume of the year when the names were adopted. Satellite rings and gaps in the rings are named for scientists who have studied these features; drawings that show these names are also included in the pertinent Transactions volume. Names for atmospheric features are informal at present; a formal system will be chosen in the future.

The boundaries of many large features (such as terrae, regiones, planitiae and plana) are not topographically or geomorphically distinct; the coordinates of these features are identified from an arbitrarily chosen center point. Boundaries (and thus coordinates) may be determined more accurately from geochemical and geophysical data obtained by future missions.

During active missions, small surface features are often given informal names. These may include landing sites, spacecraft impact sites, and small topographic features, such as craters, hills, and rocks. Such names will not be given official status by the IAU, except as provided for by Rule 2 above. As for the larger objects, official names for any such small features would have to conform to established IAU rules and categories.

Descriptor terms (feature types) Feature Pronunciation Description Designation Albedo feature An area which shows a contrast in brightness or darkness (albedo) with adjacent areas. This term is implicit. AL Arcus, arcūs Arc: curved feature AR Astrum, astra , Radial-patterned features, currently not used for any official surface features AS Catena, catenae , A chain of craters e.g. Enki Catena. CA Cavus, cavi , Hollows, irregular steep-sided depressions usually in arrays or clusters CB Chaos, chaoses A distinctive area of broken or jumbled terrain e.g. Iani Chaos. CH Chasma, chasmata , Deep, elongated, steep-sided depression e.g. Eos Chasma. CM Collis, colles , A small hill or knob. COCollum, colli , A thin section between the two lobes of a contact binary, only used on Arrokoth and DonaldjohansonCL Corona, coronae , An oval feature. Used only on Venus and Miranda. CR Crater, craters A circular depression (in most cases created by impact event). This term is implicit. AA Dorsum, dorsa , Ridge, sometimes called a wrinkle ridge e.g. Dorsum Buckland. DO Eruptive center An active volcano on Io. This term is implicit. ER Facula, faculae , Bright spot FA Farrum, farra , Pancake-like structure, or a row of such structures. Used only on Venus. FR Flexus, flexūs Very low curvilinear ridge with a scalloped pattern FE Fluctus, fluctūs , Terrain covered by outflow of liquid. Used on Venus, Io, Titan, and Mars. FL Flumen, flumina , Channel on Titan that might carry liquid FM Fossa, fossae , Long, narrow, shallow depression FO Fretum, freta , Strait of liquid connecting two larger areas of liquid. Used only on Titan. FT Insula, insulae , Island (islands), an isolated land area (or group of such areas) surrounded by, or nearly surrounded by, a liquid area (sea or lake). Used only on Titan. IN Labes, labēs , Landslide debris. Used only on Mars and Ceres. LA Labyrinthus, labyrinthi , Complex of intersecting valleys or ridges. LB Lacuna, lacunae , Irregularly shaped depression having the appearance of a dry lake bed. Used only on Titan. LU Lacus, lacūs A "lake" or small plain on Moon and Mars; on Titan, a "true lake" of dark liquid hydrocarbons or a small, dark plain with discrete, sharp boundaries; on Pluto, a small isolated glacier LCLanding site name Lunar features at or near Apollo landing sites LFLarge ringed feature Cryptic ringed features LG Lenticula, lenticulae , Small dark spots on Europa LE Linea, lineae , Dark or bright elongate marking, may be curved or straight LI Lingula, lingulae , Extension of plateau having rounded lobate or tongue-like boundaries LN Lobus Lobes of contact binaries. Currently used only on Arrokoth, Selam, and Donaldjohanson LO Macula, maculae , Dark spot, may be irregular MA Mare, maria , A "sea": on the Moon, a low albedo, relatively smooth plain, generally of large extent; on Mars, dark albedo area, e.g. Mare Erythraeum; on Titan, large expanses of dark materials thought to be liquid hydrocarbons, e.g. Ligeia Mare. ME Mensa, mensae , A flat-topped prominence with cliff-like edges, i.e. a mesa. MN Mons, montes , Mons refers to a mountain. Montes refers to a mountain range, e.g. Olympus Mons, Maxwell Montes. MO Oceanus Very large dark area. The only feature with this designation is Oceanus Procellarum. OC Palus, paludes , "Swamp"; small plain. Used on the Moon, Mars, and Pluto. PA Patera, paterae , Irregular crater, or a complex one with scalloped edges e.g. Ah Peku Patera. Usually refers to the dish-shaped depression atop a volcano. PE Planitia, planitiae , Low plain e.g. Amazonis Planitia, Isidis Planitia. PL Planum, plana , A plateau or high plain e.g. Planum Boreum, Lakshmi Planum. PM Plume, plumes A cryovolcanic feature on Triton. This term is currently unused. PU Promontorium, promontoria , "Cape"; headland. Used only on the Moon. PR Regio, regiones , Large area marked by reflectivity or color distinctions from adjacent areas, or a broad geographic region RE Reticulum, reticula , reticular (netlike) patterns, currently not used for any official surface features RT Rima, rimae , Fissure. Used only on the Moon and 21 Lutetia. RI Rupes, rupēs , Scarp RU Satellite feature A feature that shares the name of an associated feature, for example Hertzsprung D. SF Saxum, saxa , Boulder or rock SA Scopulus, scopuli , Lobate or irregular scarp SC Serpens, serpentes , Sinuous feature with segments of positive and negative relief along its length SE Sinus, sinūs "Bay"; small plain on Moon or Mars, e.g. Sinus Meridiani; On Titan, bay within bodies of liquid. SI Sulcus, sulci , Subparallel furrows and ridges SU Terra, terrae , Extensive land mass e.g. Arabia Terra, Aphrodite Terra. TA Tessera, tesserae , An area of tile-like, polygonal terrain. This term is used only on Venus. TE Tholus, tholi , Small domical mountain or hill e.g. Hecates Tholus. TH Undae, undae A field of dunes. Used on Venus, Mars and Titan. UN Vallis, valles , A valley e.g. Valles Marineris. VA Vastitas, vastitates , An extensive plain. The only feature with this designation is Vastitas Borealis. VS Virga, virgae , A streak or stripe of color. This term is currently used only on Titan. VI

Categories for naming features on planets and satellites

Mercury