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 IAU will accept proposals to name boulders or rocks on small planetary bodies, under the following conditions: a) The named object must be larger than 1% (one percent) of the mean diameter of the body b) The named object must have exceptional scientific interest, such as defining a specific longitude in the coordinate system, large size relative to the asteroid diameter, etc.
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 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, before a proposal may be submitted.
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 X2. 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.
While there should be no size limit below which a Jovian satellite must not be named, a Jovian satellite with an absolute magnitude H_V fainter than 18 should only be named if it is of special scientific interest.
In addition to these general rules, each task group develops additional conventions as it formulates an interesting and meaningful nomenclature for individual planetary bodies. Most of these conventions are self evident from study of the appendixes that follow.
Names for all planetary features include a descriptor term, with a few exceptions. 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 channels (valles) on Mars and Venus, and craters on the Moon, Mars, and Venus; naming conventions for these features differ according to size. The categories for naming features on each planet or satellite (and the exceptions) are listed in Categories for Naming Features on Planets and Satellites. 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 also used to delineate a broad geographic region.
Named features on bodies so small that coordinates have not yet been determined are identified on drawings or images 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 Rules 2 and 3 above. As for the larger objects, official names for any such small features would have to conform to established IAU rules and categories.
When a satellite has been discovered through the efforts of a large scientific team, the list of individual team members may be too long to include all contributors. In such cases, credit for the discovery will go to the science team.
Grammar, Spelling, and Editorial Conventions
All official IAU nomenclature should be capitalized, including the unique name and the descriptor term. The single exception is the term crater; the unique name should be capitalized but the word crater is not (unless as part of a title in which all other words are capitalized).
Correct spelling of each name should be used, and may be checked using the Gazetteer's search function.
If a surface feature does not have an IAU-approved name, the IAU descriptor terms should not be used. Other informal morphological terms may be used, such as mountain, valley, hill, cliff, and plain. It is preferred that this informal descriptor term be lowercase. If any unique name is used along with the informal descriptor term, a parenthetical statement indicating its informal status should be given at the first occurrence of the name in the text. The IAU welcomes name requests for features being discussed in peer-reviewed journal articles, conference abstracts, geologic maps, and other publications in which a feature is discussed by the scientific community.
Terms for geological formations or other small features (such as outcrops and individual rocks named by Mars rover teams) are not designated or reviewed by the IAU, so may take whatever form the author, mission team, or journal prefer. The IAU descriptor terms are not intended to imply any specific geological process or formation process; they are strictly morphological in nature.