The Moon does not possess valleys created by the erosion of flowing water like those found on Earth. Instead, the lunar surface features numerous linear depressions, primarily known as rilles (or rimae in Latin). These formations are scars left behind by massive geological processes, including volcanic activity and crustal movement, indicating the Moon had a highly active past. Studying these features offers insight into the internal workings and evolution of Earth’s nearest celestial neighbor.
Rilles: The Moon’s Primary Linear Depressions
Rilles are the most common features resembling valleys on the Moon, characterized by their long, narrow, and trench-like shapes that can stretch for hundreds of kilometers. Planetary scientists categorize these depressions into three distinct types based on their morphology. Sinuous rilles, the most frequent variety, exhibit a meandering path similar to a terrestrial river, such as the famous Hadley Rille explored by the Apollo 15 mission. These winding channels often begin at a small, deep depression and continue across the flat plains of the lunar maria.
Straight rilles, by contrast, follow linear paths, suggesting they were formed by forces that fractured the lunar crust. The Rima Sirsalis is a prime example, extending approximately 400 kilometers across the Oceanus Procellarum, or “Ocean of Storms.” The final type, arcuate rilles, have a distinct, smooth curve. They are almost exclusively found along the edges of the dark, basaltic plains of the maria, such as the curved Rima Sulpicius Gallus.
Geological Processes That Formed Lunar Rilles
The diverse shapes of rilles are linked to the distinct geological processes that created them. Sinuous rilles are primarily believed to be the remnants of collapsed lava tubes or channels carved by fluid lava flows. As molten rock flowed beneath a solidified crust, the lava drained away, leaving behind a hollow tunnel that eventually caved in, forming a trench. The Hadley Rille, for instance, is a substantial sinuous channel measuring about 1.5 kilometers wide and over 300 meters deep in some places.
Straight rilles, and many arcuate rilles, owe their existence to tectonic forces, forming as fault valleys known as grabens. These occur when the lunar crust experienced extensional stress, pulling sections apart. The rock between two parallel faults then dropped down, creating the flat-floored, linear valley seen from orbit. Arcuate rilles follow the circular boundaries of ancient impact basins, likely forming as the weight of the mare basalts caused the basin floor to sag, creating concentric fault lines.
Larger Scale Linear Features and Impact Valleys
Beyond the smaller rilles, the Moon possesses a class of much larger linear features designated as Vallis (Latin for valley). These features are immense trenches that often represent the largest-scale faulting events in lunar history, rather than simple lava channels. The most striking example is Vallis Alpes, or the Alpine Valley, a spectacular feature that cuts directly through the Montes Alpes mountain range.
Vallis Alpes is an imposing structure, stretching over 166 kilometers long and reaching a width of up to 20 kilometers. It is interpreted as a large-scale graben formed by stresses that radiated outward from the impact that created the nearby Imbrium basin. The valley floor was later flooded by lava and contains a smaller sinuous rille running down its center, demonstrating a complex, multi-stage formation history.