Rille

This article is about lunar features. For other uses, seeRille (disambiguation).

Rimae on the floor of the lunar craterGassendi,fromApollo 16.

Mamers Vallesrille on Mars.

Rima Ariadaeusis categorized as a straight rille (graben) and is over 300 km in length.

Hadley Rille at center is a sinuous rille visited by theApollo 15mission.

Detail of part of Hadley Rille

Rille/ˈrɪl/(German for 'groove') is typically used to describe any of the long, narrow depressions in the surface of theMoonthat resemblechannels.The Latin term isrima,pluralrimae.Typically, a rille can be several kilometers wide and hundreds of kilometers in length. However, the term has also been used loosely to describe similar structures on a number of planets in theSolar System,includingMars,Venus,and on a number ofmoons.All bear a structural resemblance to each other.

Structures

Three types of rille are found on thelunarsurface:

• Sinuous rillesmeanderin a curved path like a mature river, and are commonly thought to be the remains of collapsedlava tubesor extinctlavaflows. They usually begin at an extinctvolcano,then meander and sometimes split as they are followed across the surface. As of 2013,195 sinuous rilles have been identified on the Moon.Vallis SchröteriinOceanus Procellarumis the largest sinuous rille, andRima Hadleyis the only one visited by humans, on theApollo 15mission. Another prominent example isRima Herigonius.
• Arcuate rilleshave a smooth curve and are found on the edges of the darklunar maria.They are believed to have formed when thelavaflows that created amarecooled, contracted and sank. These are found all over the Moon, examples can be seen near the south-western border ofMare Tranquillitatisand on the south-eastern border ofMare Humorum.Rima Sulpicius Gallusis a clear example in southwesternMare Serenitatis.
• Straight rillesfollow long, linear paths and are believed to begrabens,sections of the crust that have sunk between two parallelfaults.These can be readily identified when they pass throughcratersor mountain ranges.Vallis Alpesis by far the largest graben rille, indeed it is regarded as too large to be called a rille and is itself bisected by a linear rille;Rima Ariadaeus,west of Mare Tranquillitatis, is a clearer example.

Rilles which show more than one structure are termed hybrid rilles.Rima HyginusinSinus Mediiis an example, initially formed through a fault and subsequently subject to volcanic activity.

Formation

Precise formation mechanisms of rilles have yet to be determined. It is likely that the different types are formed by different processes. Common features shared by lunar rilles and similar structures on other bodies suggest that common causative mechanisms operate widely in the solar system. Leading theories includelava channels,collapsed lava tubes, near-surface dike intrusion,nuée ardente(pyroclastic cloud), subsidence of lava-covered basin and crater floors, and tectonic extension. On-site examination would be necessary to clarify exact methods.

Sinuous rilles

According to NASA, the origin of lunar sinuous rilles remains controversial.TheHadley Rilleis a 1.5 km wide and over 300 m deep sinuous rille. It is thought to be a giant conduit that carried lava from an eruptive vent far to the south. Topographic information obtained from theApollo 15photographs supports this possibility; however, many puzzles about the rille remain.

See also

• Arachnoid (astrogeology)
• Chaos terrain
• Dark dune spot
• List of features on the Moon
• Lunar lava tube
• Martian spiders (dark dune spots)
• Planetary geology

References

General

• Ewen A. Whitaker,Mapping and Naming the Moon,Cambridge University Press,1999,ISBN0-521-62248-4.
• American Astronomers Report:What Formed the Moon's Sinuous Rilles?,Sky & Telescope, Vol. XXVI, No. 1, July, 1963.
• Atlas of Lunar Sinuous Rilles