rille

Rilles are long, narrow depressions on the Moon’s surface that resemble channels or trenches. They are most prominent in the lunar maria but also occur in other regions, and they can extend from several tens to several hundred kilometers. Rilles typically appear as shallow, walled troughs with widths from hundreds of meters to a few kilometers. They are classified into several types, the main ones being sinuous, straight, and arcuate rilles.

Sinuous rilles form winding, meandering paths that resemble dry river valleys. They are generally interpreted as ancient lava channels carved by flowing magma, with some later collapse or cooling leaving a trough. Straight rilles are linear or gently curved troughs that often run tens to hundreds of kilometers; they are thought to reflect tectonic processes such as crustal extension and faulting that pulled the lunar crust apart. Arcuate rilles are curved, segmented depressions that may result from localized subsidence related to the collapse of lava tubes or other volcanic or tectonic activity.

Most rilles are associated with mare regions, where large basaltic plains were once flooded by lava. The prevailing view is that sinuous rilles formed as lava tubes or surface channels that later drained or collapsed, while straight and some arcuate rilles mark zones of crustal extension or faulting. Some rilles may be remnants of collapsed lava tubes beneath the surface.

Prominent examples include Hadley Rille, a deep sinuous channel studied near the Apollo 15 landing site, and Rima Ariadaeus, a long straight rille that crosses the Moon’s near side. Rilles offer clues to the Moon’s volcanic and tectonic history and are visible in lunar images from orbit and, with sufficient illumination, Earth-based observations.

The term rille derives from Latin rima, fissure. In lunar nomenclature, the singular is rima, the plural rimae; the English form rille is widely used.