A rille is a valley or trench on the surface of the Moon, carved by ancient volcanic activity or tectonic forces. These features range from narrow channels just a few hundred meters wide to vast valleys stretching hundreds of kilometers across the lunar landscape. The term was likely coined around 1800 by the German astronomer Johann Schröter, who first documented lunar rilles through his telescope in 1787. The Latin term “rima,” meaning fissure, is also used for these features.
Three Types of Lunar Rilles
Rilles come in three distinct forms, each shaped by different geological processes: sinuous, straight (linear), and arcuate. They look dramatically different from one another, and recognizing the type tells scientists what was happening beneath the Moon’s surface billions of years ago.
Sinuous rilles are the most visually striking. They wind across the lunar surface like dried-up riverbeds, and that comparison isn’t far off. These channels formed when massive volumes of lava erupted and flowed across the surface, melting and eroding the rock beneath. The lava was hotter than the ground it flowed over, so it literally dissolved the substrate and carved a channel as it moved. Many sinuous rilles begin at a distinct depression, essentially a collapsed lava pond that served as the eruption source. The largest sinuous rille on the Moon was discovered by Schröter himself.
Straight and arcuate rilles formed through an entirely different process. These are tectonic features, essentially long, narrow valleys created when the Moon’s crust pulled apart under tension. The stress fields that caused them were typically associated with the formation of massive impact basins or the weight of dense volcanic rock pressing down on the crust. Their walls are steep, formed by inward-dipping faults at roughly 60-degree angles. Unlike sinuous rilles, which meander around obstacles, straight and arcuate rilles cut directly across both the flat volcanic plains (maria) and the rugged highlands without changing direction. They tend to get wider as they pass through highland terrain.
How Big Rilles Actually Are
The size range is enormous. Sinuous rilles span from about 2 kilometers to nearly 700 kilometers in length, though most are much shorter. The average length is around 32 kilometers, with a median of 16 kilometers, meaning a few exceptionally long rilles pull the average up considerably.
Widths range from about 250 meters to over 5 kilometers, with the typical sinuous rille measuring roughly 580 meters across. Depths vary from just a few meters to over 600 meters, though the median depth is about 41 meters. To put that in perspective, the Grand Canyon averages about 1,600 meters deep and 16 kilometers wide, so most rilles are far smaller, but the longest ones rival its length.
How Lava Carved the Moon’s Surface
The process behind sinuous rilles, called thermal erosion, works differently from how water carves valleys on Earth. When lava erupts at temperatures above the melting point of the surrounding rock, it doesn’t just flow over the surface. It melts the ground beneath it and absorbs that melted rock into the flow itself, progressively deepening the channel. The longer and hotter the eruption, the deeper the rille becomes.
Some sinuous rilles originated from lava fountains so dense they formed opaque curtains of molten rock. The erupted material fell back into a growing lava pond at the source, which then overflowed and carved the downstream channel. Thermal erosion happened both at the base of the pond and along the entire length of the flow. This is why sinuous rilles typically get shallower as they extend farther from their source: the lava cooled as it traveled, losing its ability to melt the ground beneath it. Depth profiles from multiple rilles confirm this consistent pattern of decreasing depth toward the downstream end.
Rilles Beyond the Moon
Similar features exist on other worlds. Sinuous rilles are the most common type of volcanic channel observed on Venus, where conditions allowed lava to carve even more impressive features. Baltis Vallis, the longest known channel on Venus, stretches at least 6,800 kilometers, roughly ten times the length of the Moon’s longest rille. There is also evidence of possible sinuous rilles on Mars, though the picture there is complicated by the presence of valley networks that may have been shaped by a mix of tectonic, volcanic, and even fluid erosion processes.
The depth profiles of sinuous rilles on Venus match what theoretical models predict for thermal erosion, showing the same decreasing depth pattern seen on the Moon. This consistency across different worlds reinforces the lava erosion explanation and suggests these features form through fundamentally the same process regardless of differences in gravity and atmospheric conditions.
Why Rilles Matter for Lunar Exploration
Sinuous rilles are closely linked to lava tubes, underground tunnels left behind when the outer surface of a lava flow solidified while molten rock continued flowing inside. Some collapsed rilles may expose sections of intact lava tubes beneath them. These tubes are of serious interest for future human settlement on the Moon because they offer natural shielding from radiation, micrometeorite impacts, and the extreme temperature swings on the lunar surface, where conditions shift by hundreds of degrees between day and night. A habitat built inside a lava tube would bypass many of the engineering challenges that make surface construction so difficult, essentially letting the Moon’s own geology serve as a protective shell.