RILLE

A rille is any long narrow depression on the surface of an astronomical body. It was originally used to categorize depressions on the Moon, but is now used when describing the same formation on other solar system bodies such as Mercury. The latin form of the word is rima (pl. rimae). Apollo 15 landed near Hadley Rille located on the eastern edge of Mare Imbrium. One of the leading theories is that the 80 mile long depression is a collapsed lava tube. The astronauts spent portions of the first and third EVA’s studying the rille which was estimated to be 1,200 feet deep at the landing site.