Lunar rays are striking features visible on the Moon’s surface, appearing as brilliant streaks that radiate outward from certain impact sites. These bright, linear patterns are a direct consequence of energetic cosmic collisions, recording the Moon’s recent geological history. They become most apparent to observers on Earth during the full moon phase when sunlight shines directly onto the lunar surface. Understanding these features provides insight into the physics of impact events and the processes that slowly alter the lunar landscape.
Defining Lunar Rays
A lunar ray appears as a high-albedo streak, meaning it is significantly brighter than the surrounding lunar terrain. These luminous features are not single, continuous lines but are instead vast, radial patterns of scattered material and countless tiny secondary craters. The rays can stretch for enormous distances, often extending for hundreds or even thousands of kilometers away from their central crater of origin. For example, the ray system originating from the prominent crater Tycho extends across almost half the visible face of the Moon.
The visibility of these structures is strongly dependent on the angle of solar illumination. When the Sun is low on the horizon near the lunar terminator, shadows are long and details of topography, like crater walls, are emphasized. However, lunar rays are virtually invisible under these conditions because they lack significant height. Conversely, they “pop” into sharp contrast when the Sun is high in the sky, such as during a full moon, because their elevated reflective quality—or albedo—is maximized against the darker background.
The Mechanics of Ray Formation
The formation of a lunar ray system is a direct result of the immense kinetic energy released when a meteoroid or asteroid impacts the Moon’s surface. This hypervelocity impact instantly vaporizes the target rock, melts a portion of it, and excavates a vast quantity of material called ejecta. This material is thrown out from the impact point at very high velocities, often following ballistic trajectories across the vacuum of space.
The ejected material is scattered in a radial pattern, creating the broad, fan-like structure of the rays. The brightness comes from the fact that this material is fresh, pulverized rock excavated from beneath the surface, which has not yet been exposed to the harsh space environment. Larger chunks of ejecta traveling along these paths strike the surface far from the primary crater, forming swarms of smaller impacts known as secondary craters.
The region immediately surrounding the main impact site is covered by a continuous blanket of ejecta, which is thick and blocky. The rays begin to appear just outside this continuous blanket, where the ejecta is no longer a solid layer but has dispersed into distinct, linear streamers. The lack of an atmosphere on the Moon allows even the finest rock particles to travel great distances without being slowed by air resistance, contributing to the extensive reach of the ray system.
Why Ray Systems Fade
While a newly formed ray system is dazzlingly bright, its high contrast appearance is temporary on a geological timescale. The primary reason rays eventually fade is a process known as space weathering, which constantly alters the physical and optical properties of the lunar surface. The Moon, lacking a protective atmosphere or global magnetic field, is constantly bombarded by solar wind particles, cosmic rays, and tiny micrometeorites.
This steady bombardment gradually darkens the fresh, high-albedo ejecta that makes up the ray. Micrometeorite impacts melt and vaporize small amounts of the lunar soil, or regolith, creating tiny glass-welded particles called agglutinates. Within these particles, submicroscopic specks of metallic iron, known as nanophase iron, are formed. The accumulation of this nanophase iron darkens the lunar surface, causing the bright ray material to slowly lose contrast against the background terrain.
Dating the Moon’s Surface Using Rays
The ephemeral nature of ray systems makes them useful tools for scientists to determine the relative age of features on the lunar surface. Since space weathering causes the bright rays to mature and darken over time, a crater that exhibits a prominent, highly visible ray system must be geologically young. Conversely, a crater of similar size that lacks any visible rays is presumed to be much older, having had its ejecta darkened by long exposure to the space environment.
The principle of relative age dating is supported by the concept of superposition. If a ray originating from one crater crosses over another feature, such as a mare plain or a different crater’s rim, the ray and its source crater must be the younger of the two. This allows planetary geologists to build a timeline of impact events.