The Moon presents a surface that is complex, marked by billions of years of geological activity and constant bombardment. While it may appear to be a simple gray sphere, telescopic observation reveals a surface covered in distinct linear features. These lines fall into three main categories—topographical channels, bright surface deposits, and mysterious albedo patterns.
Linear Channels (Rilles)
The most common topographical “line” is the rille, a long, narrow depression that often resembles a dried riverbed. Rilles, which can measure several kilometers wide and extend for hundreds of kilometers, are categorized into three distinct types based on their shape and suspected formation mechanism. The most prevalent are sinuous rilles, which meander and twist like a mature river on Earth; Vallis Schröteri is a prominent example.
Sinuous rilles are thought to be the remnants of ancient volcanic activity, either open lava channels or collapsed lava tubes. The Moon’s low gravity and the high temperature of its ancient lavas allowed them to flow swiftly over great distances, carving out long channels. Over time, the roofs of these subterranean lava tubes may have collapsed, leaving behind the trench-like features, such as the Hadley Rille explored by the Apollo 15 mission.
Straight rilles are flat-floored and follow remarkably linear paths. These features are commonly understood to be grabens, which are sections of the lunar crust that have dropped down between two parallel faults. This faulting is evidence of crustal stress, where the Moon’s surface was pulled apart by tectonic forces.
The third type, arcuate rilles, are smooth, curved depressions typically found along the edges of the dark, flat maria, or “seas.” These formed as the masses of basaltic rock that created the maria cooled, contracted, and sagged under their own weight, causing the edges of the plains to pull apart.
Bright Streaks (Ejecta Rays)
Bright streaks are another prominent linear feature that radiate outwards from some younger impact craters, such as the rays emanating from Tycho and Copernicus. These features are not valleys or channels like rilles, but are instead surface deposits known as ejecta rays. They are formed during a high-velocity impact event when pulverized rock and debris are thrown out in distinct, radial patterns.
This ejected material is lighter-colored, fresh rock that was excavated from beneath the surface. The rays appear bright because they have not yet been darkened by space weathering, surface alteration caused by the solar wind and micrometeorite impacts. Over millions of years, this space weathering creates a dark, mature layer. The presence and brightness of a ray system thus serve as an indicator that the parent crater is young, as the rays fade and disappear as the fresh material darkens over time.
Winding Patterns (Lunar Swirls)
Lunar swirls are the most enigmatic linear features, such as the Reiner Gamma formation. These swirls are features of albedo, meaning they represent differences in brightness, but they impart no observable topography, lying flat over craters and other geological features. The bright sections have the optical characteristics of “immature” regolith, which is material that has not yet undergone the full darkening effects of space weathering.
Swirling patterns correlate with localized patches of remnant magnetism in the lunar crust, known as magnetic anomalies. The prevailing hypothesis suggests that these magnetic fields act as miniature shields, deflecting the charged particles of the solar wind away from the surface below. Since the solar wind is a primary cause of the darkening effect of space weathering, the protected areas remain bright, while the surrounding, unprotected material continues to darken.
This shielding mechanism creates the contrast between the bright, preserved material and the darker, weathered surrounding, forming the distinctive patterns. The Reiner Gamma swirl, located on the near side in Oceanus Procellarum, is a prominent example, with tendrils extending for hundreds of kilometers. These magnetic anomalies are a puzzle because the Moon currently lacks a global magnetic field, suggesting that the magnetism was either locked into the crust during an earlier period when the Moon had an active core dynamo, or formed through other mechanisms, such as large impacts.