Rima

Rima (rille)

(glossary entry; plural = Rimae)

Description

One of 18 different categories of lunar features recognized in the current system of IAU nomenclature. The IAU defines a rima as a "fissure". The term is used as prefix to the feature name.
 

• Quaide (1965), using Lick Observatory photos, recognized (as had others before him) four sub-classes of rilles: "straight" (such as Rima Ariadaeus), "arcuate" (curving around the margins of Maria), "irregularly branching" (as at Rimae Triesnecker and inside floor fractured craters), and "sinuous" (such as Rima Hadley).

Rilles are definitely not "fissures". There is no English word that has been used to describe them accurately, so for more than 100 years the German word rille (or rill in the past) has been widely used. - tychocrater Sep 2, 2007

Additional Information

• Features of this type are commonly referred to as "rilles", a term which, according to Whitaker (p. 106), was introduced by Schröter. They were called "clefts" by the early English selenographers. The terms "rima" and "rimae" seem to have been introduced in the latinizations of the System of Lunar Craters; however there were no "rimae", for mulitple rilles were distinguished, as they had been in Blagg and Müller, by Roman numerals. - Jim Mosher
• The IAU introduced Latin terms for types of features, including rima, in 1961, and although we did not like this change, implemented it in the System of Lunar Craters. - tychocrater Oct 24, 2007
• In 1973, the IAU declared (without further explanation) that "Rimae and Rima systems will receive new and more appropriate designations."
  • One possible interpretation of this statement is that since the IAU was complaining at this time that there were not enough letters and numerals to designate all the features it wanted to name, the individual components of the Roman-numeraled rille systems should receive distinctive non-numeric names.
  • A more likely explanation, hinted at in Ashbrook, 1974, is that the IAU wanted to re-classify the rills, confining the term rima to the "irregular cracks or fissures, such as those often noted inside large craters" and introducing three new descriptor terms for other classes of rilles: fossa (plural fossae] (the Latin word for a trench or ditch) for the relatively straight graben-type riles, anguis (plural angues) (Latin for snake) for sinuous rilles, and catena for rille-like features which on closer examination turned out to be chains of closely-spaced craters.
  • Whichever interpretation is correct, aside from the features reclassified as catena no formal renaming seems ever to have taken place. Instead, for the most part, the sets of rilles that formerly shared a parent name but had individual Roman numerals have been lumped together as a single "rimae" system with the old parent name, while single rilles have maintained the "rima" designation. This has made the identity of the features far less certain than it used to be; and, in many cases, made it impossible to make clear reference to individual rilles (assuming the Roman numerals have been dropped). - Jim Mosher
• In 1976, the IAU added to the confusion by publishing Fossa as a descriptor term for "long, narrow, shallow depressions," but not including it among the names allowed on the Moon. By that time the term had been used extensively on NASA's LTO charts for naming linear graben-type rilles. Since this term was never accepted for use on the Moon. Unless later changed to "rima", "rimae", or possibly "vallis", these are now invalid names. - Jim Mosher
• The on-line IAU Planetary Gazetteer gives an approval date of "1964" for many of the modern "rimae" names. This is apparently a reference to their depictions in the System of Lunar Craters and LAC charts. However, this cannot truly be the approval date, for the rilles all had singular or Roman-numeraled designations at that time. One might think that the modern "rimae" names are simply a way of referring to all the earlier Roman-numeraled features that shared a parent name, but this does not seem to be the case. The "rimae" positions and diameters (of unknown origin) given in the on-line Gazetteer, sometimes seem to exclude rilles that were part of the older nomenclature, and at other times define areas encompassing additional rilles. See, for example, Rimae Prinz. Since no official approval information seems to have ever been published in the IAU Transactions, the intention of many modern "rimae" names is ambiguous. - Jim Mosher

Lists

• List of 92 rilles from Beer & Mädler
• Julius Schmidt's 1865 catalog of 1787(?) rilles (numbered 1-425).
• Schmidt's list of additional rilles observed through 1874.
• Clickable list of current IAU-named Lunar Rilles and rille systems (and also an almost daily updated alphabetically arranged appendix-list of unofficially named rilles on the moon's FARSIDE which were (and are) not officially named by the International Astronomical Union) (why not? one might ask).

LPOD Articles

Rules for Rilles - map of sinuous rilles from article by Guest and Murray

Bibliography

• Guest, J. E. and Murray, J. B. 1976. Volcanic features of the nearside equatorial lunar maria. Journal of the Geological Society. v. 132; no. 3; pp. 251-258.
• Quaide, William. 1965. Rilles, Ridges and Domes-Clues to Maria History. Icarus, vol. 4, pp. 374-389.

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