Dorsum

Dorsum (mare ridge)

(glossary entry; plural = Dorsa)

Description

One of 18 different categories of lunar features recognized in the current system of IAU nomenclature. The IAU defines a dorsum as "a ridge". The term is used as prefix to the feature name. The term was introduced in 1973 to recognize a class of features that had not previously been named in the IAU system. The dimension given for a dorsum usually refers to the length of the ridge.
 
 

Additional Information

• See tirade under Catena. Instead of calling a lunar feature a dorsum it is historically proper to call it a wrinkle ridge or a mare ridge. In addition, the IAU definition does not adequately distinguish between the unique morphology of a mare ridge and any other type of ridge on the Moon. There was a name for this class of feature that had been used for more than 100 years, in fact there were two names: wrinkle ridge and more recently mare ridge. At least one specific ridge, first drawn by Schröter, had a marvelously descriptive name: Serpentine Ridge. - tychocrater Jun 20, 2007
• With the exception of the minor feature Dorsum Thera, the Moon's ridges have been named exclusively after earth scientists. This is a conscious decision, which was codified in IAU Transactions XVIB (Third Meeting, Resolution II(8)). Dorsae are the only lunar features whose names are restricted to a particular class of scientists. - Jim Mosher
• The distinction between “dorsum” (singular) and “dorsa” (plural) in the IAU names is extremely vague. The singular is sometimes used for features that appear to consist of many separate ridges (such as Dorsum Buckland); while the plural is sometimes used for a feature that, by comparison, seems much closer to a single ridge (such as Dorsa Tetyaev).

No systematic attempt to name lunar wrinkle ridges?

• Unlike most other categories of lunar features, no systematic attempt has ever been made to name all the major ridges on the Moon. Instead, the IAU list of dorsum names consists almost exclusively of ones introduced in the very small area covered by NASA's highly detailed LTO charts. Most of these were approved by the IAU in 1976 (IAU Transactions XVIB) “as now assigned and printed on the 1:250,000 lunar map series” (the LTO charts). Many of the names can also be found printed on the LM series of lunar charts (although it is the LTO designations that were approved). Unfortunately the authors of the LTO charts never drew lines around the features they were labeling, and since ridges, especially groups of ridges (dorsa), rarely have clearly defined starting and ending points, their intentions are rarely clear. In addition, many dorsa extend over several LTO “provinces”, and features are not always labeled in a consistent or predictable fashion on adjacent charts, As a result, there are numerous apparent conflicts between the IAU-endorsed labels on the LTO's and the dorsa positions and "diameters" listed in the IAU Planetary Gazetteer (whose origin is unclear). Sometimes the IAU diameter is too small to encompass all the features labeled on the LTO's (for example, Dorsum Guettard); other times it includes an area much larger than that labeled on the LTO's (for example, Dorsum Von Cotta) or is centered differently (for example, Dorsa Smirnov. In only an extremely small number of cases does the approval citation in the IAU Transactions explicitly indicate where a named ridge starts and ends (for example, Dorsa Mawson). Because of these ambiguities, there is little agreement among recent maps as to how the dorsum names should be placed. Also many very minor ridges have names, while much more conspicuous ones (in regions not covered by the LTO's) do not. - Jim Mosher

Many confusions

• As an example of the many confusions encountered attempting to relate IAU dorsa names to specific lunar surface features, LM-62 indicates that the name Dorsa Harker consists of a major part to the east and north of the crater Fahrenheit plus a smaller part to its southwest. The region indicated on the LM-62 spans at least four LTO provinces including LTO-44C3 (for which no LTO map was ever prepared), LTO-62B1, LTO-62B4 and possibly LTO-62A3. When the three existing LTO charts are put together, one sees that the main part of the ridge is labeled on LTO-62B1 as a continuous diagonal feature connecting to a labeled extension to the southwest of Fahrenheit, plotted on LTO-62B4. The part of the main ridge that LM-62 shows turning to the south near Mons Usov is not labeled as part of Dorsa Harker on LTO-62B4, nor is there any further label given where the southwest extension in LTO-62B4 continues on into LTO-62A3. So the labeling on the LTO’s fails to explain how what seems to be one continuous system on the LTO's becomes two distinct systems on the LM map. And to add to the confusion the position and diameter given in the on-line IAU Planetary Gazetteer encompasses the eastern ridges labeled on LM-62, but is too small to include the western portion, even though that is labeled as part of Dorsa Harker on both LTO-62B4 and LM-62. This is only an example. Similar ambiguities arise with nearly every other IAU-named dorsum. - Jim Mosher

A new attempt to name all the major wrinkle ridges on the moon

• A systematic attempt to name each one of the moon's wrinkle ridges (and systems of wrinkle ridges) is made by Danny Caes during the years 2015-2016. See below: the page Lunar Dorsa contains both the official IAU list and the much larger unofficial list.

List of Lunar Dorsa (officially and unofficially named wrinkle ridges)

LPOD Articles

A Glorious Serpentine Ridge

Bibliography and atlases

• Every book about the moon which shows photographs or drawings of the sunrise or sunset terminator at certain local areas of the mare regions.
• Antonin Rukl's well-known Atlas of the Moon (the updated online edition, full of names for previously unnamed wrinkle ridges!!!).
• The 21st Century Atlas of the Moon by Charles A. Wood and Maurice J. S. Collins.
• Gerard P. Kuiper's Consolidated Lunar Atlas.
• Perhaps the best online source to discover every wrinkle ridge on the moon's surface is the ACT-REACT QUICK MAP, created to explore the cornucopia of high-resolution photographs made by the Lunar Reconnaissance Orbiter (LRO). The WAC Nearside and Farside Mosaics which show Big Shadows are very entertaining for those who want to see all of the most subtle height-differences at the lunar surface.

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