The Moon’s surface features many striking geological formations beyond the familiar impact craters. Among the most intriguing are the lunar rilles, which appear as winding trenches or deep grooves etched into the dark plains. These features are a testament to the Moon’s ancient volcanic and tectonic history. They offer planetary scientists valuable clues about the subsurface structure and evolution of our nearest celestial neighbor.
Defining Rilles: Appearance and Scale
Lunar rilles are long, narrow depressions that resemble channels or valleys on the Moon’s surface. They appear as distinct, negative relief structures cutting across the lunar landscape. Rilles are predominantly found within the dark, smooth plains of the lunar maria, which are vast regions of solidified basaltic lava flows.
The scale of these depressions is immense, ranging from a few hundred meters to several kilometers in width. A typical rille can span hundreds of kilometers in length, with the largest sinuous examples exceeding 500 kilometers. Sinuous rilles can be up to 4.3 kilometers wide and reach depths of over 500 meters, creating vast, visible troughs. Unlike impact craters, rilles are elongated channels that indicate processes originating from beneath the surface.
The Three Major Types of Rilles
Scientists categorize lunar rilles into three distinct types based on their shape: sinuous, arcuate, and straight.
Sinuous rilles are the most common type, tracing a meandering, snake-like path across the mare surface, much like a terrestrial river. They often begin abruptly at a small, crater-like depression, interpreted as the eruptive vent or source. Vallis Schröteri, found in the Oceanus Procellarum region, is a prominent example.
Arcuate rilles have a smooth, curved shape, appearing as broad arcs or partial concentric circles within the immense lunar basins. They are typically located along the inner edges of the dark maria, following the circular structure of the basin itself. Rimae Hippalus, along the borders of Mare Humorum, is an example of this type.
Straight rilles are long, linear trenches that cut across the terrain with little curvature. These features present as flat-floored, trough-like depressions. Rima Ariadaeus, which stretches for over 300 kilometers, is a well-known example of this linear morphology.
Geological Processes of Rille Formation
The three distinct types of rilles point to different underlying geological processes, primarily involving volcanism and tectonic stresses.
Sinuous rilles are strongly associated with ancient volcanic activity and the flow of highly fluid, low-viscosity basaltic lava. The two leading theories suggest they are either open lava channels where the flowing lava thermally eroded the substrate, or they are the remnants of collapsed lava tubes. The collapsed tube hypothesis suggests a roof of cooled lava formed over a river of molten rock; once the lava drained, the roof eventually caved in, leaving the characteristic trench.
Arcuate and straight rilles are primarily the result of tectonic forces acting on the lunar crust. Arcuate rilles formed after the massive impact basins were filled by successive lava eruptions to create the maria. As this immense plug of basalt cooled and contracted, the heavy material subsided toward the basin’s center, causing the surrounding crust to crack in the concentric arc pattern.
Straight rilles are classic examples of grabens, which are down-dropped blocks of crust bounded by two parallel normal faults. These long, linear features form when the lunar surface is pulled apart by tensional stress, causing a section of the crust to drop vertically between the fault lines. Both arcuate and straight rilles provide evidence of past crustal movement and stress.