rilles

Rilles are trench-like depressions on the Moon’s surface, typically found in mare regions where basaltic lava once flowed. They vary in length from a few tens to several hundred kilometers and range in width from tens to a few hundred meters. Scientists categorize rilles into three major types based on morphology: sinuous, straight, and arcuate.

Sinuous rilles are winding, channel-like grooves with curves and intersections that resemble river valleys. They are commonly interpreted as the surface expression of lava conduits (lava tubes) that formed as magma moved beneath a cooled crust. When the lava supply waned or the tube roof collapsed, a surface channel may remain.

Straight rilles are long, narrow, and linear depressions, often with a sharp, knife-like profile. They are generally attributed to tectonic faulting and extensional cracking in the lunar crust, sometimes forming as grabens when the crust stretched during cooling of widespread basaltic lava flows.

Arcuate rilles are curved, crescent-shaped segments that tend to outline irregular arcs. They are thought to reflect subsidence along curved faults or the collapse around the edges of former lava tubes, producing arc-shaped trenches.

Rilles record the Moon’s volcanic and tectonic history, helping scientists infer the emplacement of mare basalts, crustal structure, and the dynamics of ancient lava flows. They are objects of interest for future exploration, as intact lava tubes could offer natural shelter and stable environments for in-situ resource use.