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Martian Feature Name Nomenclature

When early astronomers peered at Mars through their telescopes and began to make maps of the planet, features were simply identified as being land or water depending on their albedo and assigned an arbitrary name. As telescopes improved, so did maps and the number of features increased. Because there was no established standard for naming newly observed features, individual astronomers were free to come up with their own naming schemes, resulting in confusion over the "correct" name for any given feature.

It was not until 1958 that the IAU (International Astronomical Union) adopted a set of formal naming standards for Mars. Designation of Martian features are made internationally binding by the International Astronomical Union. The official map they produced had a total of 128 named features (reduced from the 404 taken from Antoniadi's monograph which itself had 558 named features). Of the 128 names used 105 came from Schiaparelli, 16 from Antoniadi, 2 from Lowell, 2 from Flammarion, and 3 were created by the subcommittee assigned the task.

The space age and missions to Mars placed new demands on the naming in system. In response, the IAU formed a Mars nomenclature working group in 1970. The group was assigned the responsibility of assigning names to the plethora of topographic features imaged by the Mariner and Viking missions.

Features on Mars are grouped by their aerological(geological) category or type. These groupings are based on the general physical appearance of the feature in question. However, just because two or more features look alike does not mean that they originated and evolved via the same geologic/aerologic processes. For example, is a large hole in the ground the result of a large impact or volcanic activity or the result of sapping? For a number of features, these questions will remain unanswered until we obtain much more detailed information about the features in question.

The following table lists each feature type along with a brief description of what that type category represents. A feature class may be accessed directly by clicking on its name in the list below.


Mars Feature Definition List

Albedo Feature
Albedo features are those that are observed because of variations in the amount of sunlight reflected by the martian surface from one area to the next. A surface that reflected all the light hitting it would have an albedo of 1.0 while a surface that absorbed all light hitting it would have an albedo of 0.0. Therefore the dark features on Mars have a lower albedo than the bright features. The first observed feature on Mars was Syrtis Major, the darkest region of the planet.

Catena or Catenae
A catena (singular) or catenae (plural) is a linear string of craters, perhaps produced by a train of impactors, much like Shoemaker-Levy 9 did to Jupiter. Alternatively, the chain of crater-like features could be volcanic or tectonic in origin and represent the collapse of surface materials into subsurface lava tubes or fractures. There are 16 features on Mars with the classification of catena, the largest of which is Tractus Catena with a diameter of 897 kilometers.

Cavus or Cavi
A cavus (singular) or cavi (plural) is a crater-like, irregularly shaped depression or hollow. These are not thought to be of impact origin. About half of these features are actually classical albedo features meaning that there is not a structural relationship between the named feature and the surface topography. Rather it is a roundish area darker (lower albedo) than its surroundings. The largest of these is Amenthes Cavi, for which the Amenthes Quadrangle is named. In total there are 15 cavi/cavus on Mars.

Chaos' are one of the more interesting features of Mars. A chaos is an area of jumbled or hummocky terrain that is thought to have been created as a result of the collapse of the surface when subsurface water or ice was removed. These features are named after the nearest named albedo feature. Figure 1 below shows the distribution of chaos on Mars.

Chasma or Chasmata
A chasma (singular) or chasmata (plural) is a large canyon or a steep sided depression or trough. There are 25 such features on Mars the largest of which is Eos Chasma followed closely by Capri Chasma, both being sections of Valles Marineris.

A colles is a smallish hill or knob. A group of such hills is referred to as colles, the plural form of collis. Note that there are no collis features, only colles. There are 16 of these features, the most well known probably being Cydonia Colles.

A crater is a circular depression created by the impact of some body, be it an asteroid or comet. Craters can be subdivided as being (from small to large) simple, complex, central peak basin, peak ring basin, and multi-ring basin. Mars is unique in that is has craters with ejecta deposits that have a unique "flowing" appearance. It is believed that the presence of groundwater or ice is responsible for these features. Large craters are named after deceased scientists who studied Mars or for writers who have contributed to Martian lore. Smaller craters are named after towns with populations under 100,000. While there are almost 1,000 named craters on Mars, the vast majority are identified simply by a multi-character designation which is based on the first 3 letters of the albedo name for their quadrant and on a 2-3 character suffix. This suffix begins with Aa in the lower right hand corner of the quadrangle and ends at Zz in the upper left hand corner of the quadrangle.

Dorsum or Dorsa
A dorsum (singular) or dorsa (plural) is an elongated prominence, ie. a longer than wide area of high ground. The Earth analogue would be a ridge. This term was originally used by the Romans to indicate a mountain ridge. There are 33 dorsum and dorsa on Mars.

A fluctus is a terrain that looks like a flow or a flow-like feature. The term was originally used to describe a feature on Io that looked like a flow. The Latin meaning of fluctus is a wave or billow. There are only two features of this type on Mars - Galaxias Fluctus and Tantalus Fluctus. Both are in the northern hemisphere between 30 and 35 north latitude.

Fossa or Fossae
A fossa (singular) or fossae(plural) is one or more linear depressions most likely created as a the result of faulting. These depressions are long and narrow and resemble a ditch or trench. The Earth analogue is most likely a graben. On Mars, fossa often occur in groups. A group of fossa are referred to as fossae. There are 56 fossa and fossae on Mars, the largest being Sirenum Fossae.

Labes is used to describe those features that appear to be the result of a landslide. The Latin meaning for labes is a "falling in" or "sinking in.". Melas Labes and Ophir Labes are two examples, both being landslides in Valles Marineris. In fact there are only 5 features classified as labes and they are all located within the Valles Marineris canyon system.

Labyrinthus is an area of intersecting liner depressions/canyons/valleys. There are only 5 such named features on Mars. The classic example is Noctis Labyrinthus at the extreme western end of the Valles Marineris canyon system.

Lingula or lingulae
Lingula (singular) or lingulae (plural) is a new addition to the family of martian feature names and is defined as an extension of plateau that has rounded lobate or tongue-like boundary. There are currently only 3 such features.

Mensa or mensae
A Mensa (singular) or mensae (plural) is an elevated area with a flat top and steep sides (think of a cake). The Earth analog would be the mesas of the US southwest. There are 25 mensa or mensae on Mars and they take their name from the nearest albedo feature.

Mons or montes
A mons (singular) is a large isolated mountain, as opposed to a montes (plural) which is a chain of mountains. On Mars, most are of volcanic origin. There are 36 features classified as mons or montes, the most famous of which is Olympus Mons - the great shield volcano. See Figure 2 below for a map of their distribution on Mars.

Patera or paterae
A patera (singular) is a complex crater or an irregularly shaped one with low relief and having scalloped edges and radiating channel-like features. There are 18 patera but no paterae. These features are thought to be volcanic in origin. They are named after the nearest albedo feature. See Figure 2 below for a map of their distribution on Mars.

Planitia or planitiae
A planita (singular) is a plain whose area is at a lower altitude than the surrounding regions. Many Martian planitia are impact basins. There are 9 named planitia on Mars. Planitia are named after the nearest albedo feature.

Planum or plana
A planum (singular) is a reasonably smooth, flat, relatively uncratered area of ground that is higher than the surrounding region and that is bordered by steep sides. The Earth analogue would be a plateau. There are 26 planum on Mars and they are named after the nearest albedo feature.

A rupes is a cliff or scarp that is straight or linear rather than sinuous. There are 22 rupes on Mars and they take there names from the nearest albedo feature. The best known is the giant Olympus Rupes cliff that borders the Olympus Mons volcano.

Scopulus or scopoli
A scopulus (singular) or scopoli (plural) is a cliff or scarp that is irregular or lobate in appearance. There are 12 scopulus and scopoli on Mars and they take their names from the nearest albedo feature. My personal favorites are the pair Scylla Scopulus and Charybdis Scopulus.

Sulcus or sulci
A sulcus (singular) or sulci (plural) is a feature that has the appearance of a furrow, ditch, or wrinkle. The short definition is subparallel furrows and ridges. Subparallel means that the individual furrows or ridges are almost parallel to one another but there is a degree of convergence or divergence. In the case of sulci, these are ridges or furrows that occur in groups. There are no sulcus but there are 13 sulci.

Terra or terrae
An extended areal region or land mass. It is used in reference to the older, cratered highlands. There are 11 terra on Mars the largest of which is Terra Cimmeria which belongs to the older cratered highlands of Mars and is home to Gusev Crater and Ma'adim Vallis. Terra are named for the nearest albedo feature.

Tholus or tholi
A tholus (singular) is an isolated, dome-shaped small mountain or hill. Tholus are thought to be of volcanic origin. The plural form is tholi. There are 13 tholus and tholi on Mars. See Figure 2 below for a map of their distribution on Mars.

Undae (plural) is an area of dunes that are very wave-like in appearance. There are only 3 undae on Mars and they are all located at approximately 80 north latitude. This is in the region known as Vastitas Borealis, a great sea of dunes that encircles the north pole of Mars.

Vallis or valles
Vallis (singular) or valles (plural) is a sinuous valley most of which are probably of fluvial origin. Large valleys are named for foreign language names for Mars while small valleys are named for classical and modern rivers. The largest and most famous valley on Mars is Valles Marineris, named after the Mariner 9 spacecraft that first imaged it. Also see Ma'adim Vallis. There are 127 vallis and valles on Mars.

A vastitas is a very large lowland plain. There is only one feature on Mars to have this designation: Vastitas Borealis, the dune sea that circles the north pole of Mars.



Figure 1. A map of the distribution of chaos features on Mars. Note the map has been darkened to make the location of the chaos more visible.

Global map that shows how chaos features are distributed on Mars

Figure 2. A map of the distribution of mons, patera and tholus features on Mars. Mons are red, patera are yellow and tholus are green.

Map of the distribution of mons, patera and tholus features on Mars

2005-2023 Jim Plaxco,